Doug Lea, the father of concurrency
I don’t know Doug Lea in my life, and it’s useless to learn about 
concurrency. The core of Java concurrent programming lies in the java. Shared acquisition, etc., and the abstraction of this behavior is based on AbstractQueuedSynchronizer, referred to as AQS. AQS defines a set of synchronizer frameworks for multi-threaded access to shared resources, and is a state-dependent synchronizer.
Features of AQS:
- blocking wait queue
- fair/unfair
- exclusive/shared
- reentrant
- Allow interrupts
For example, the implementation of synchronizers in Java.concurrent.util, such as Lock, Latch, Barrier, etc., are all based on the AQS framework.
- Generally, AQS is inherited by defining the internal class Sync
- Map all calls of the synchronizer to Sync corresponding to the method
AQS internal maintenance attribute volatile int state (32 bits)
- state indicates the available state of the resource
State three access methods
getState(), setState(), compareAndSetState()
AQS defines two resource sharing methods
- Exclusive-exclusive, only one thread can execute, such as ReetrantLock
- Share-shared, multiple threads can execute at the same time, such as Semaphore/CountDownLatch
Two queues defined by AQS
- Conditional Waiting Queue
- synchronous waiting queue
Different custom synchronizers contend for shared resources in different ways. When the custom synchronizer is implemented, it only needs to realize the acquisition and release of the state of the shared resource. As for the maintenance of the specific thread waiting queue (such as enqueuing/awakening out of the queue after failing to acquire resources, etc.), AQS has already been implemented at the top level. When implementing a custom synchronizer, the following methods are mainly implemented:
- isHeldExclusively(): Whether the thread is holding resources exclusively. It is only necessary to implement it when the condition is used.
- tryAcquire(int): Exclusive mode. Attempts to fetch the resource, returning true on success and false on failure.
- tryRelease(int): Exclusive mode. Attempts to release the resource, returning true on success and false on failure.
- tryAcquireShared(int): sharing method. Try to fetch resources. A negative number indicates failure; 0 indicates success with no resources remaining; a positive number indicates success with resources remaining.
- tryReleaseShared(int): sharing method. Attempts to release the resource, if it is allowed to wake up the subsequent waiting node after release, it returns true, otherwise it returns false.
The synchronous waiting queue
in the QS is also called the CLH queue. The CLH queue is a queue based on a doubly linked list data structure invented by Craig, Landin, and Hagersten. It is a FIFO first-in-first-out thread waiting queue. The CLH queue in Java It is a variant of the original CLH queue, and the thread is changed from the original spin mechanism to the blocking mechanism.
Conditional waiting queue
Condition is a tool class for coordinating communication between multiple threads, so that one or some threads wait for a certain condition (Condition) together. Only when the condition is met, these waiting threads will be awakened, so as to re-compete Lock
Fair lock
Queues for both fair and unfair locks are based on a doubly linked list maintained inside the lock, and the value of the table node Node is each thread requesting the current lock. The fair lock is to get the value from the team leader in turn every time.
The implementation of the lock is based on the following points:
- The volatile keyword of the table node Node and the state state.
- The core code of the atomic operation of sum.misc.Unsafe.compareAndSet (see appendix)
:
/**
* 公平锁
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
/**
* 重写aqs中的方法逻辑
* 尝试加锁，被AQS的acquire()方法调用
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
/**
* 与非公平锁中的区别，需要先判断队列当中是否有等待的节点
* 如果没有则可以尝试CAS获取锁
*/
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
//独占线程指向当前线程
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
Unfair lock
In the process of waiting for the lock, if any new thread attempts to acquire the lock, there is a high probability that the lock will be acquired directly.
- ReentrantLock locks will not interrupt the thread unless the developer sets the interrupt bit himself.
- ReentrantLock acquires a lock that looks like a spin code, but it is not a spin lock.
- ReentrantLock fair and unfair locks are exclusive locks.
Core code:
/**
* 非公平锁
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* 加锁行为
*/
final void lock() {
/**
* 第一步：直接尝试加锁
* 与公平锁实现的加锁行为一个最大的区别在于，此处不会去判断同步队列(CLH队列)中
* 是否有排队等待加锁的节点，上来直接加锁（判断state是否为0,CAS修改state为1）
* ，并将独占锁持有者 exclusiveOwnerThread 属性指向当前线程
* 如果当前有人占用锁，再尝试去加一次锁
*/
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
//AQS定义的方法,加锁
acquire(1);
}
/**
* 父类AbstractQueuedSynchronizer.acquire()中调用本方法
*/
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
Reentrant lock Reentrant
lock is also called recursive lock, which means that after the outer function of the same thread acquires the lock, the inner recursive function still has the code to acquire the lock, but it is not affected.
Core code:
public class LockTemplete {
private Integer counter = 0;
/**
* 可重入锁,公平锁
* 公平锁，
* 非公平锁
* 需要保证多个线程使用的是同一个锁
*
*
* synchronized是否可重入？
* 虚拟机，在ObjectMonitor.hpp定义了synchronized他怎么取重入加锁 ..。hotspot源码
* counter +1
* 基于AQS 去实现加锁与解锁
*/
private ReentrantLock lock = new ReentrantLock(true);
/**
* 需要保证多个线程使用的是同一个ReentrantLock对象
* @return
*/
public void modifyResources(String threadName){
System.out.println("通知《管理员》线程:--->"+threadName+"准备打水");
//默认创建的是独占锁，排它锁；同一时刻读或者写只允许一个线程获取锁
lock.lock();
System.out.println("线程:--->"+threadName+"第一次加锁");
counter++;
System.out.println("线程:"+threadName+"打第"+counter+"桶水");
//重入该锁,我还有一件事情要做,没做完之前不能把锁资源让出去
lock.lock();
System.out.println("线程:--->"+threadName+"第二次加锁");
counter++;
System.out.println("线程:"+threadName+"打第"+counter+"桶水");
lock.unlock();
System.out.println("线程:"+threadName+"释放一个锁");
lock.unlock();
System.out.println("线程:"+threadName+"释放一个锁");
}
public static void main(String[] args){
LockTemplete tp = new LockTemplete();
new Thread(()->{
String threadName = Thread.currentThread().getName();
tp.modifyResources(threadName);
},"Thread1").start();
/*new Thread(()->{
String threadName = Thread.currentThread().getName();
tp.modifyResources(threadName);
},"Thread2").start();*/
}
}
Print the result:
the second bucket of water can be hit here, indicating that ReentranLock is a reentrant lock
Non-reentrant lock
If the lock has been acquired during the execution of the current thread, if the lock is acquired again, it will not be acquired and blocked
Read-write lock ReentrantReadWriteLock
- Write lock (exclusive lock, exclusive lock): It means that the lock can only be held by one thread. If thread T adds an exclusive lock to data A, other threads can no longer add any type of lock to A and obtain the write lock. The lock thread can both read data and change data.
- Read lock (shared lock): It means that the lock can be held by multiple threads. If thread T adds a shared lock to data A, other threads can only add a shared lock to A, and cannot add an exclusive lock. The thread of the lock can only read data but not write data
The state field in AQS (int type, 32 bits), where the state describes the number of read locks and write locks respectively, so the state variable is divided into two parts by bit cutting
- The upper 16 bits indicate the state of the read lock (the number of read locks)
- The lower 16 bits indicate the state of the write lock (the number of write locks)
ReentrantLock source code with Chinese annotations:
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
/**
* 内部调用AQS的动作，都基于该成员属性实现
*/
private final Sync sync;
/**
* ReentrantLock锁同步操作的基础类,继承自AQS框架.
* 该类有两个继承类，1、NonfairSync 非公平锁，2、FairSync公平锁
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* 加锁的具体行为由子类实现
*/
abstract void lock();
/**
* 尝试获取非公平锁
*/
final boolean nonfairTryAcquire(int acquires) {
//acquires = 1
final Thread current = Thread.currentThread();
int c = getState();
/**
* 不需要判断同步队列（CLH）中是否有排队等待线程
* 判断state状态是否为0，不为0可以加锁
*/
if (c == 0) {
//unsafe操作，cas修改state状态
if (compareAndSetState(0, acquires)) {
//独占状态锁持有者指向当前线程
setExclusiveOwnerThread(current);
return true;
}
}
/**
* state状态不为0，判断锁持有者是否是当前线程，
* 如果是当前线程持有 则state+1
*/
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
//加锁失败
return false;
}
/**
* 释放锁
*/
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
/**
* 判断持有独占锁的线程是否是当前线程
*/
protected final boolean isHeldExclusively() {
return getExclusiveOwnerThread() == Thread.currentThread();
}
//返回条件对象
final ConditionObject newCondition() {
return new ConditionObject();
}
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
final boolean isLocked() {
return getState() != 0;
}
/**
* Reconstitutes the instance from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
/**
* 非公平锁
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* 加锁行为
*/
final void lock() {
/**
* 第一步：直接尝试加锁
* 与公平锁实现的加锁行为一个最大的区别在于，此处不会去判断同步队列(CLH队列)中
* 是否有排队等待加锁的节点，上来直接加锁（判断state是否为0,CAS修改state为1）
* ，并将独占锁持有者 exclusiveOwnerThread 属性指向当前线程
* 如果当前有人占用锁，再尝试去加一次锁
*/
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
//AQS定义的方法,加锁
acquire(1);
}
/**
* 父类AbstractQueuedSynchronizer.acquire()中调用本方法
*/
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
/**
* 公平锁
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
/**
* 重写aqs中的方法逻辑
* 尝试加锁，被AQS的acquire()方法调用
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
/**
* 与非公平锁中的区别，需要先判断队列当中是否有等待的节点
* 如果没有则可以尝试CAS获取锁
*/
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
//独占线程指向当前线程
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
/**
* 默认构造函数，创建非公平锁对象
*/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* 根据要求创建公平锁或非公平锁
*/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
/**
* 加锁
*/
public void lock() {
sync.lock();
}
/**
* 尝试获去取锁，获取失败被阻塞，线程被中断直接抛出异常
*/
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
/**
* 尝试加锁
*/
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
/**
* 指定等待时间内尝试加锁
*/
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
/**
* 尝试去释放锁
*/
public void unlock() {
sync.release(1);
}
/**
* 返回条件对象
*/
public Condition newCondition() {
return sync.newCondition();
}
/**
* 返回当前线程持有的state状态数量
*/
public int getHoldCount() {
return sync.getHoldCount();
}
/**
* 查询当前线程是否持有锁
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldExclusively();
}
/**
* 状态表示是否被Thread加锁持有
*/
public boolean isLocked() {
return sync.isLocked();
}
/**
* 是否公平锁？是返回true 否则返回 false
*/
public final boolean isFair() {
return sync instanceof FairSync;
}
/**
* Returns the thread that currently owns this lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* best-effort approximation of current lock status. For example,
* the owner may be momentarily {@code null} even if there are
* threads trying to acquire the lock but have not yet done so.
* This method is designed to facilitate construction of
* subclasses that provide more extensive lock monitoring
* facilities.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getOwner();
}
/**
* 判断队列当中是否有在等待获取锁的Thread节点
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* 当前线程是否在同步队列中等待
*/
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
/**
* Returns an estimate of the number of threads waiting to
* acquire this lock. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring of the system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
/**
* 返回Thread集合，排队中的所有节点Thread会被返回
*/
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* 条件队列当中是否有正在等待的节点
*/
public boolean hasWaiters(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this lock. Note that because
* timeouts and interrupts may occur at any time, the estimate
* serves only as an upper bound on the actual number of waiters.
* This method is designed for use in monitoring of the system
* state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public int getWaitQueueLength(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this lock.
* Because the actual set of threads may change dynamically while
* constructing this result, the returned collection is only a
* best-effort estimate. The elements of the returned collection
* are in no particular order. This method is designed to
* facilitate construction of subclasses that provide more
* extensive condition monitoring facilities.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes either the String {@code "Unlocked"}
* or the String {@code "Locked by"} followed by the
* {@linkplain Thread#getName name} of the owning thread.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
Thread o = sync.getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
}
AQS source code with Chinese annotations:
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
/**
* Creates a new {@code AbstractQueuedSynchronizer} instance
* with initial synchronization state of zero.
*/
protected AbstractQueuedSynchronizer() {
}
/**
* Wait queue node class.
*
* <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
* Hagersten) lock queue. CLH locks are normally used for
* spinlocks. We instead use them for blocking synchronizers, but
* use the same basic tactic of holding some of the control
* information about a thread in the predecessor of its node. A
* "status" field in each node keeps track of whether a thread
* should block. A node is signalled when its predecessor
* releases. Each node of the queue otherwise serves as a
* specific-notification-style monitor holding a single waiting
* thread. The status field does NOT control whether threads are
* granted locks etc though. A thread may try to acquire if it is
* first in the queue. But being first does not guarantee success;
* it only gives the right to contend. So the currently released
* contender thread may need to rewait.
*
* <p>To enqueue into a CLH lock, you atomically splice it in as new
* tail. To dequeue, you just set the head field.
* <pre>
* +------+ prev +-----+ +-----+
* head | | <---- | | <---- | | tail
* +------+ +-----+ +-----+
* </pre>
*
* <p>Insertion into a CLH queue requires only a single atomic
* operation on "tail", so there is a simple atomic point of
* demarcation from unqueued to queued. Similarly, dequeuing
* involves only updating the "head". However, it takes a bit
* more work for nodes to determine who their successors are,
* in part to deal with possible cancellation due to timeouts
* and interrupts.
*
* <p>The "prev" links (not used in original CLH locks), are mainly
* needed to handle cancellation. If a node is cancelled, its
* successor is (normally) relinked to a non-cancelled
* predecessor. For explanation of similar mechanics in the case
* of spin locks, see the papers by Scott and Scherer at
* http://www.cs.rochester.edu/u/scott/synchronization/
*
* <p>We also use "next" links to implement blocking mechanics.
* The thread id for each node is kept in its own node, so a
* predecessor signals the next node to wake up by traversing
* next link to determine which thread it is. Determination of
* successor must avoid races with newly queued nodes to set
* the "next" fields of their predecessors. This is solved
* when necessary by checking backwards from the atomically
* updated "tail" when a node's successor appears to be null.
* (Or, said differently, the next-links are an optimization
* so that we don't usually need a backward scan.)
*
* <p>Cancellation introduces some conservatism to the basic
* algorithms. Since we must poll for cancellation of other
* nodes, we can miss noticing whether a cancelled node is
* ahead or behind us. This is dealt with by always unparking
* successors upon cancellation, allowing them to stabilize on
* a new predecessor, unless we can identify an uncancelled
* predecessor who will carry this responsibility.
*
* <p>CLH queues need a dummy header node to get started. But
* we don't create them on construction, because it would be wasted
* effort if there is never contention. Instead, the node
* is constructed and head and tail pointers are set upon first
* contention.
*
* <p>Threads waiting on Conditions use the same nodes, but
* use an additional link. Conditions only need to link nodes
* in simple (non-concurrent) linked queues because they are
* only accessed when exclusively held. Upon await, a node is
* inserted into a condition queue. Upon signal, the node is
* transferred to the main queue. A special value of status
* field is used to mark which queue a node is on.
*
* <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
* Scherer and Michael Scott, along with members of JSR-166
* expert group, for helpful ideas, discussions, and critiques
* on the design of this class.
*/
static final class Node {
/**
* 标记节点未共享模式
* */
static final Node SHARED = new Node();
/**
* 标记节点为独占模式
*/
static final Node EXCLUSIVE = null;
/**
* 在同步队列中等待的线程等待超时或者被中断，需要从同步队列中取消等待
* */
static final int CANCELLED = 1;
/**
* 后继节点的线程处于等待状态，而当前的节点如果释放了同步状态或者被取消，
* 将会通知后继节点，使后继节点的线程得以运行。
*/
static final int SIGNAL = -1;
/**
* 节点在等待队列中，节点的线程等待在Condition上，当其他线程对Condition调用了signal()方法后，
* 该节点会从等待队列中转移到同步队列中，加入到同步状态的获取中
*/
static final int CONDITION = -2;
/**
* 表示下一次共享式同步状态获取将会被无条件地传播下去
*/
static final int PROPAGATE = -3;
/**
* 标记当前节点的信号量状态 (1,0,-1,-2,-3)5种状态
* 使用CAS更改状态，volatile保证线程可见性，高并发场景下，
* 即被一个线程修改后，状态会立马让其他线程可见。
*/
volatile int waitStatus;
/**
* 前驱节点，当前节点加入到同步队列中被设置
*/
volatile Node prev;
/**
* 后继节点
*/
volatile Node next;
/**
* 节点同步状态的线程
*/
volatile Thread thread;
/**
* 等待队列中的后继节点，如果当前节点是共享的，那么这个字段是一个SHARED常量，
* 也就是说节点类型(独占和共享)和等待队列中的后继节点共用同一个字段。
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* 返回前驱节点
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() {
// Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) {
// Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) {
// Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
/**
* 指向同步等待队列的头节点
*/
private transient volatile Node head;
/**
* 指向同步等待队列的尾节点
*/
private transient volatile Node tail;
/**
* 同步资源状态
*/
private volatile int state;
/**
* Returns the current value of synchronization state.
* This operation has memory semantics of a {@code volatile} read.
* @return current state value
*/
protected final int getState() {
return state;
}
/**
* Sets the value of synchronization state.
* This operation has memory semantics of a {@code volatile} write.
* @param newState the new state value
*/
protected final void setState(int newState) {
state = newState;
}
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
*/
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
// Queuing utilities
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices
* to improve responsiveness with very short timeouts.
*/
static final long spinForTimeoutThreshold = 1000L;
/**
* 节点加入CLH同步队列
*/
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) {
// Must initialize
//队列为空需要初始化，创建空的头节点
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
//set尾部节点
if (compareAndSetTail(t, node)) {
//当前节点置为尾部
t.next = node; //前驱节点的next指针指向当前节点
return t;
}
}
}
}
/**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
// 1. 将当前线程构建成Node类型
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
// 2. 1当前尾节点是否为null？
if (pred != null) {
// 2.2 将当前节点尾插入的方式
node.prev = pred;
// 2.3 CAS将节点插入同步队列的尾部
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
/**
* Sets head of queue to be node, thus dequeuing. Called only by
* acquire methods. Also nulls out unused fields for sake of GC
* and to suppress unnecessary signals and traversals.
*
* @param node the node
*/
private void setHead(Node node) {
head = node;
node.thread = null;
node.prev = null;
}
/**
*
*/
private void unparkSuccessor(Node node) {
//获取wait状态
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);// 将等待状态waitStatus设置为初始值0
/**
* 若后继结点为空，或状态为CANCEL（已失效），则从后尾部往前遍历找到最前的一个处于正常阻塞状态的结点
* 进行唤醒
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);//唤醒线程
}
/**
* 把当前结点设置为SIGNAL或者PROPAGATE
* 唤醒head.next(B节点)，B节点唤醒后可以竞争锁，成功后head->B，然后又会唤醒B.next，一直重复直到共享节点都唤醒
* head节点状态为SIGNAL，重置head.waitStatus->0，唤醒head节点线程，唤醒后线程去竞争共享锁
* head节点状态为0，将head.waitStatus->Node.PROPAGATE传播状态，表示需要将状态向后继节点传播
*/
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
//head是SIGNAL状态
/* head状态是SIGNAL，重置head节点waitStatus为0，这里不直接设为Node.PROPAGATE,
* 是因为unparkSuccessor(h)中，如果ws < 0会设置为0，所以ws先设置为0，再设置为PROPAGATE
* 这里需要控制并发，因为入口有setHeadAndPropagate跟release两个，避免两次unpark
*/
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; //设置失败，重新循环
/* head状态为SIGNAL，且成功设置为0之后,唤醒head.next节点线程
* 此时head、head.next的线程都唤醒了，head.next会去竞争锁，成功后head会指向获取锁的节点，
* 也就是head发生了变化。看最底下一行代码可知，head发生变化后会重新循环，继续唤醒head的下一个节点
*/
unparkSuccessor(h);
/*
* 如果本身头节点的waitStatus是出于重置状态（waitStatus==0）的，将其设置为“传播”状态。
* 意味着需要将状态向后一个节点传播
*/
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) //如果head变了，重新循环
break;
}
}
/**
* 把node节点设置成head节点，且Node.waitStatus->Node.PROPAGATE
*/
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; //h用来保存旧的head节点
setHead(node);//head引用指向node节点
/* 这里意思有两种情况是需要执行唤醒操作
* 1.propagate > 0 表示调用方指明了后继节点需要被唤醒
* 2.头节点后面的节点需要被唤醒（waitStatus<0），不论是老的头结点还是新的头结点
*/
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())//node是最后一个节点或者 node的后继节点是共享节点
/* 如果head节点状态为SIGNAL，唤醒head节点线程，重置head.waitStatus->0
* head节点状态为0(第一次添加时是0)，设置head.waitStatus->Node.PROPAGATE表示状态需要向后继节点传播
*/
doReleaseShared();
}
}
// Utilities for various versions of acquire
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return;
node.thread = null;
// Skip cancelled predecessors
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
Node predNext = pred.next;
// Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* 若前驱结点的状态是SIGNAL，意味着当前结点可以被安全地park
*/
return true;
if (ws > 0) {
/*
* 前驱节点状态如果被取消状态，将被移除出队列
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* 当前驱节点waitStatus为 0 or PROPAGATE状态时
* 将其设置为SIGNAL状态，然后当前结点才可以可以被安全地park
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
/**
* Convenience method to interrupt current thread.
*/
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
/**
* 阻塞当前节点，返回当前Thread的中断状态
* LockSupport.park 底层实现逻辑调用系统内核功能 pthread_mutex_lock 阻塞线程
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);//阻塞
return Thread.interrupted();
}
/*
* Various flavors of acquire, varying in exclusive/shared and
* control modes. Each is mostly the same, but annoyingly
* different. Only a little bit of factoring is possible due to
* interactions of exception mechanics (including ensuring that we
* cancel if tryAcquire throws exception) and other control, at
* least not without hurting performance too much.
*/
/**
* 已经在队列当中的Thread节点，准备阻塞等待获取锁
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
//死循环
final Node p = node.predecessor();//找到当前结点的前驱结点
if (p == head && tryAcquire(arg)) {
//如果前驱结点是头结点，才tryAcquire，其他结点是没有机会tryAcquire的。
setHead(node);//获取同步状态成功，将当前结点设置为头结点。
p.next = null; // help GC
failed = false;
return interrupted;
}
/**
* 如果前驱节点不是Head，通过shouldParkAfterFailedAcquire判断是否应该阻塞
* 前驱节点信号量为-1，当前线程可以安全被parkAndCheckInterrupt用来阻塞线程
*/
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 与acquireQueued逻辑相似，唯一区别节点还不在队列当中需要先进行入队操作
*/
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.EXCLUSIVE);//以独占模式放入队列尾部
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 独占模式定时获取
*/
private boolean doAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.EXCLUSIVE);//加入队列
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return true;
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;//超时直接返回获取失败
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
//阻塞指定时长，超时则线程自动被唤醒
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())//当前线程中断状态
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* 尝试获取共享锁
*/
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED);//入队
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();//前驱节点
if (p == head) {
int r = tryAcquireShared(arg); //非公平锁实现，再尝试获取锁
//state==0时tryAcquireShared会返回>=0(CountDownLatch中返回的是1)。
// state为0说明共享次数已经到了，可以获取锁了
if (r >= 0) {
//r>0表示state==0,前继节点已经释放锁，锁的状态为可被获取
//这一步设置node为head节点设置node.waitStatus->Node.PROPAGATE，然后唤醒node.thread
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
//前继节点非head节点，将前继节点状态设置为SIGNAL，通过park挂起node节点的线程
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Acquires in shared timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
// Main exported methods
/**
* Attempts to acquire in exclusive mode. This method should query
* if the state of the object permits it to be acquired in the
* exclusive mode, and if so to acquire it.
*
* <p>This method is always invoked by the thread performing
* acquire. If this method reports failure, the acquire method
* may queue the thread, if it is not already queued, until it is
* signalled by a release from some other thread. This can be used
* to implement method {@link Lock#tryLock()}.
*
* <p>The default
* implementation throws {@link UnsupportedOperationException}.
*
* @param arg the acquire argument. This value is always the one
* passed to an acquire method, or is the value saved on entry
* to a condition wait. The value is otherwise uninterpreted
* and can represent anything you like.
* @return {@code true} if successful. Upon success, this object has
* been acquired.
* @throws IllegalMonitorStateException if acquiring would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in exclusive
* mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this object is now in a fully released
* state, so that any waiting threads may attempt to acquire;
* and {@code false} otherwise.
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if exclusive mode is not supported
*/
protected boolean tryRelease(int arg) {
throw new UnsupportedOperationException();
}
/**
* 共享式：共享式地获取同步状态。对于独占式同步组件来讲，同一时刻只有一个线程能获取到同步状态，
* 其他线程都得去排队等待，其待重写的尝试获取同步状态的方法tryAcquire返回值为boolean，这很容易理解；
* 对于共享式同步组件来讲，同一时刻可以有多个线程同时获取到同步状态，这也是“共享”的意义所在。
* 本方法待被之类覆盖实现具体逻辑
* 1.当返回值大于0时，表示获取同步状态成功，同时还有剩余同步状态可供其他线程获取；
*
* 2.当返回值等于0时，表示获取同步状态成功，但没有可用同步状态了；
* 3.当返回值小于0时，表示获取同步状态失败。
*/
protected int tryAcquireShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Attempts to set the state to reflect a release in shared mode.
*
* <p>This method is always invoked by the thread performing release.
*
* <p>The default implementation throws
* {@link UnsupportedOperationException}.
*
* @param arg the release argument. This value is always the one
* passed to a release method, or the current state value upon
* entry to a condition wait. The value is otherwise
* uninterpreted and can represent anything you like.
* @return {@code true} if this release of shared mode may permit a
* waiting acquire (shared or exclusive) to succeed; and
* {@code false} otherwise
* @throws IllegalMonitorStateException if releasing would place this
* synchronizer in an illegal state. This exception must be
* thrown in a consistent fashion for synchronization to work
* correctly.
* @throws UnsupportedOperationException if shared mode is not supported
*/
protected boolean tryReleaseShared(int arg) {
throw new UnsupportedOperationException();
}
/**
* Returns {@code true} if synchronization is held exclusively with
* respect to the current (calling) thread. This method is invoked
* upon each call to a non-waiting {@link ConditionObject} method.
* (Waiting methods instead invoke {@link #release}.)
*
* <p>The default implementation throws {@link
* UnsupportedOperationException}. This method is invoked
* internally only within {@link ConditionObject} methods, so need
* not be defined if conditions are not used.
*
* @return {@code true} if synchronization is held exclusively;
* {@code false} otherwise
* @throws UnsupportedOperationException if conditions are not supported
*/
protected boolean isHeldExclusively() {
throw new UnsupportedOperationException();
}
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (!tryAcquire(arg))
doAcquireInterruptibly(arg);
}
/**
* Attempts to acquire in exclusive mode, aborting if interrupted,
* and failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquire}, returning on success. Otherwise, the thread is
* queued, possibly repeatedly blocking and unblocking, invoking
* {@link #tryAcquire} until success or the thread is interrupted
* or the timeout elapses. This method can be used to implement
* method {@link Lock#tryLock(long, TimeUnit)}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquire(arg) ||
doAcquireNanos(arg, nanosTimeout);
}
/**
* 释放独占模式持有的锁
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
//释放一次锁
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);//唤醒后继结点
return true;
}
return false;
}
/**
* 请求获取共享锁
*/
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)//返回值小于0，获取同步状态失败，排队去；获取同步状态成功，直接返回去干自己的事儿。
doAcquireShared(arg);
}
/**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument.
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
/**
* Attempts to acquire in shared mode, aborting if interrupted, and
* failing if the given timeout elapses. Implemented by first
* checking interrupt status, then invoking at least once {@link
* #tryAcquireShared}, returning on success. Otherwise, the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted or the timeout elapses.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquireShared} but is otherwise uninterpreted
* and can represent anything you like.
* @param nanosTimeout the maximum number of nanoseconds to wait
* @return {@code true} if acquired; {@code false} if timed out
* @throws InterruptedException if the current thread is interrupted
*/
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
// Queue inspection methods
/**
* Queries whether any threads are waiting to acquire. Note that
* because cancellations due to interrupts and timeouts may occur
* at any time, a {@code true} return does not guarantee that any
* other thread will ever acquire.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there may be other threads waiting to acquire
*/
public final boolean hasQueuedThreads() {
return head != tail;
}
/**
* Queries whether any threads have ever contended to acquire this
* synchronizer; that is if an acquire method has ever blocked.
*
* <p>In this implementation, this operation returns in
* constant time.
*
* @return {@code true} if there has ever been contention
*/
public final boolean hasContended() {
return head != null;
}
/**
* Returns the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued.
*
* <p>In this implementation, this operation normally returns in
* constant time, but may iterate upon contention if other threads are
* concurrently modifying the queue.
*
* @return the first (longest-waiting) thread in the queue, or
* {@code null} if no threads are currently queued
*/
public final Thread getFirstQueuedThread() {
// handle only fast path, else relay
return (head == tail) ? null : fullGetFirstQueuedThread();
}
/**
* Version of getFirstQueuedThread called when fastpath fails
*/
private Thread fullGetFirstQueuedThread() {
/*
* The first node is normally head.next. Try to get its
* thread field, ensuring consistent reads: If thread
* field is nulled out or s.prev is no longer head, then
* some other thread(s) concurrently performed setHead in
* between some of our reads. We try this twice before
* resorting to traversal.
*/
Node h, s;
Thread st;
if (((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null) ||
((h = head) != null && (s = h.next) != null &&
s.prev == head && (st = s.thread) != null))
return st;
/*
* Head's next field might not have been set yet, or may have
* been unset after setHead. So we must check to see if tail
* is actually first node. If not, we continue on, safely
* traversing from tail back to head to find first,
* guaranteeing termination.
*/
Node t = tail;
Thread firstThread = null;
while (t != null && t != head) {
Thread tt = t.thread;
if (tt != null)
firstThread = tt;
t = t.prev;
}
return firstThread;
}
/**
* 判断当前线程是否在队列当中
*/
public final boolean isQueued(Thread thread) {
if (thread == null)
throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
return true;
return false;
}
/**
* Returns {@code true} if the apparent first queued thread, if one
* exists, is waiting in exclusive mode. If this method returns
* {@code true}, and the current thread is attempting to acquire in
* shared mode (that is, this method is invoked from {@link
* #tryAcquireShared}) then it is guaranteed that the current thread
* is not the first queued thread. Used only as a heuristic in
* ReentrantReadWriteLock.
*/
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
return (h = head) != null &&
(s = h.next) != null &&
!s.isShared() &&
s.thread != null;
}
/**
* 判断当前节点是否有前驱节点
*/
public final boolean hasQueuedPredecessors() {
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
}
// Instrumentation and monitoring methods
/**
* 同步队列长度
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.thread != null)
++n;
}
return n;
}
/**
* 获取队列等待thread集合
*/
public final Collection<Thread> getQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
return list;
}
/**
* 获取独占模式等待thread线程集合
*/
public final Collection<Thread> getExclusiveQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (!p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* 获取共享模式等待thread集合
*/
public final Collection<Thread> getSharedQueuedThreads() {
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node p = tail; p != null; p = p.prev) {
if (p.isShared()) {
Thread t = p.thread;
if (t != null)
list.add(t);
}
}
return list;
}
/**
* Returns a string identifying this synchronizer, as well as its state.
* The state, in brackets, includes the String {@code "State ="}
* followed by the current value of {@link #getState}, and either
* {@code "nonempty"} or {@code "empty"} depending on whether the
* queue is empty.
*
* @return a string identifying this synchronizer, as well as its state
*/
public String toString() {
int s = getState();
String q = hasQueuedThreads() ? "non" : "";
return super.toString() +
"[State = " + s + ", " + q + "empty queue]";
}
// Internal support methods for Conditions
/**
* 判断节点是否在同步队列中
*/
final boolean isOnSyncQueue(Node node) {
//快速判断1：节点状态或者节点没有前置节点
//注：同步队列是有头节点的，而条件队列没有
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
//快速判断2：next字段只有同步队列才会使用，条件队列中使用的是nextWaiter字段
if (node.next != null) // If has successor, it must be on queue
return true;
//上面如果无法判断则进入复杂判断
return findNodeFromTail(node);
}
/**
* Returns true if node is on sync queue by searching backwards from tail.
* Called only when needed by isOnSyncQueue.
* @return true if present
*/
private boolean findNodeFromTail(Node node) {
Node t = tail;
for (;;) {
if (t == node)
return true;
if (t == null)
return false;
t = t.prev;
}
}
/**
* 将节点从条件队列当中移动到同步队列当中，等待获取锁
*/
final boolean transferForSignal(Node node) {
/*
* 修改节点信号量状态为0，失败直接返回false
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* 加入同步队列尾部当中，返回前驱节点
*/
Node p = enq(node);
int ws = p.waitStatus;
//前驱节点不可用 或者 修改信号量状态失败
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread); //唤醒当前节点
return true;
}
/**
* Transfers node, if necessary, to sync queue after a cancelled wait.
* Returns true if thread was cancelled before being signalled.
*
* @param node the node
* @return true if cancelled before the node was signalled
*/
final boolean transferAfterCancelledWait(Node node) {
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
enq(node);
return true;
}
/*
* If we lost out to a signal(), then we can't proceed
* until it finishes its enq(). Cancelling during an
* incomplete transfer is both rare and transient, so just
* spin.
*/
while (!isOnSyncQueue(node))
Thread.yield();
return false;
}
/**
* 入参就是新创建的节点，即当前节点
*/
final int fullyRelease(Node node) {
boolean failed = true;
try {
//这里这个取值要注意，获取当前的state并释放，这从另一个角度说明必须是独占锁
//可以考虑下这个逻辑放在共享锁下面会发生什么？
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
//如果这里释放失败，则抛出异常
throw new IllegalMonitorStateException();
}
} finally {
/**
* 如果释放锁失败，则把节点取消，由这里就能看出来上面添加节点的逻辑中
* 只需要判断最后一个节点是否被取消就可以了
*/
if (failed)
node.waitStatus = Node.CANCELLED;
}
}
// Instrumentation methods for conditions
/**
* Queries whether the given ConditionObject
* uses this synchronizer as its lock.
*
* @param condition the condition
* @return {@code true} if owned
* @throws NullPointerException if the condition is null
*/
public final boolean owns(ConditionObject condition) {
return condition.isOwnedBy(this);
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this synchronizer. Note that because timeouts
* and interrupts may occur at any time, a {@code true} return
* does not guarantee that a future {@code signal} will awaken
* any threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if exclusive synchronization
* is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this synchronizer
* @throws NullPointerException if the condition is null
*/
public final boolean hasWaiters(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.hasWaiters();
}
/**
* 获取条件队列长度
*/
public final int getWaitQueueLength(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitQueueLength();
}
/**
* 获取条件队列当中所有等待的thread集合
*/
public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
if (!owns(condition))
throw new IllegalArgumentException("Not owner");
return condition.getWaitingThreads();
}
/**
* 条件对象，实现基于条件的具体行为
*/
public class ConditionObject implements Condition, java.io.Serializable {
private static final long serialVersionUID = 1173984872572414699L;
/** First node of condition queue. */
private transient Node firstWaiter;
/** Last node of condition queue. */
private transient Node lastWaiter;
/**
* Creates a new {@code ConditionObject} instance.
*/
public ConditionObject() {
}
// Internal methods
/**
* 1.与同步队列不同，条件队列头尾指针是firstWaiter跟lastWaiter
* 2.条件队列是在获取锁之后，也就是临界区进行操作，因此很多地方不用考虑并发
*/
private Node addConditionWaiter() {
Node t = lastWaiter;
//如果最后一个节点被取消，则删除队列中被取消的节点
//至于为啥是最后一个节点后面会分析
if (t != null && t.waitStatus != Node.CONDITION) {
//删除所有被取消的节点
unlinkCancelledWaiters();
t = lastWaiter;
}
//创建一个类型为CONDITION的节点并加入队列，由于在临界区，所以这里不用并发控制
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
/**
* 发信号，通知遍历条件队列当中的节点转移到同步队列当中，准备排队获取锁
*/
private void doSignal(Node first) {
do {
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) && //转移节点
(first = firstWaiter) != null);
}
/**
* 通知所有节点移动到同步队列当中，并将节点从条件队列删除
*/
private void doSignalAll(Node first) {
lastWaiter = firstWaiter = null;
do {
Node next = first.nextWaiter;
first.nextWaiter = null;
transferForSignal(first);
first = next;
} while (first != null);
}
/**
* 删除条件队列当中被取消的节点
*/
private void unlinkCancelledWaiters() {
Node t = firstWaiter;
Node trail = null;
while (t != null) {
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) {
t.nextWaiter = null;
if (trail == null)
firstWaiter = next;
else
trail.nextWaiter = next;
if (next == null)
lastWaiter = trail;
}
else
trail = t;
t = next;
}
}
// public methods
/**
* 发新号，通知条件队列当中节点到同步队列当中去排队
*
*/
public final void signal() {
if (!isHeldExclusively())//节点不能已经持有独占锁
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
/**
* 发信号通知条件队列的节点准备到同步队列当中去排队
*/
doSignal(first);
}
/**
* 唤醒所有条件队列的节点转移到同步队列当中
*/
public final void signalAll() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignalAll(first);
}
/**
* Implements uninterruptible condition wait.
* <ol>
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* </ol>
*/
public final void awaitUninterruptibly() {
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean interrupted = false;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if (Thread.interrupted())
interrupted = true;
}
if (acquireQueued(node, savedState) || interrupted)
selfInterrupt();
}
/*
* For interruptible waits, we need to track whether to throw
* InterruptedException, if interrupted while blocked on
* condition, versus reinterrupt current thread, if
* interrupted while blocked waiting to re-acquire.
*/
/** 该模式表示在退出等待时重新中断 */
private static final int REINTERRUPT = 1;
/** 异常中断 */
private static final int THROW_IE = -1;
/**
* 这里的判断逻辑是：
* 1.如果现在不是中断的，即正常被signal唤醒则返回0
* 2.如果节点由中断加入同步队列则返回THROW_IE，由signal加入同步队列则返回REINTERRUPT
*/
private int checkInterruptWhileWaiting(Node node) {
return Thread.interrupted() ?
(transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
0;
}
/**
* 根据中断时机选择抛出异常或者设置线程中断状态
*/
private void reportInterruptAfterWait(int interruptMode)
throws InterruptedException {
if (interruptMode == THROW_IE)
throw new InterruptedException();
else if (interruptMode == REINTERRUPT)
selfInterrupt();
}
/**
* 加入条件队列等待，条件队列入口
*/
public final void await() throws InterruptedException {
//如果当前线程被中断则直接抛出异常
if (Thread.interrupted())
throw new InterruptedException();
//把当前节点加入条件队列
Node node = addConditionWaiter();
//释放掉已经获取的独占锁资源
int savedState = fullyRelease(node);
int interruptMode = 0;
//如果不在同步队列中则不断挂起
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
//这里被唤醒可能是正常的signal操作也可能是中断
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
/**
* 走到这里说明节点已经条件满足被加入到了同步队列中或者中断了
* 这个方法很熟悉吧？就跟独占锁调用同样的获取锁方法，从这里可以看出条件队列只能用于独占锁
* 在处理中断之前首先要做的是从同步队列中成功获取锁资源
*/
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
//走到这里说明已经成功获取到了独占锁，接下来就做些收尾工作
//删除条件队列中被取消的节点
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
//根据不同模式处理中断
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* </ol>
*/
public final long awaitNanos(long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return deadline - System.nanoTime();
}
/**
* Implements absolute timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean awaitUntil(Date deadline)
throws InterruptedException {
long abstime = deadline.getTime();
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (System.currentTimeMillis() > abstime) {
timedout = transferAfterCancelledWait(node);
break;
}
LockSupport.parkUntil(this, abstime);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
/**
* Implements timed condition wait.
* <ol>
* <li> If current thread is interrupted, throw InterruptedException.
* <li> Save lock state returned by {@link #getState}.
* <li> Invoke {@link #release} with saved state as argument,
* throwing IllegalMonitorStateException if it fails.
* <li> Block until signalled, interrupted, or timed out.
* <li> Reacquire by invoking specialized version of
* {@link #acquire} with saved state as argument.
* <li> If interrupted while blocked in step 4, throw InterruptedException.
* <li> If timed out while blocked in step 4, return false, else true.
* </ol>
*/
public final boolean await(long time, TimeUnit unit)
throws InterruptedException {
long nanosTimeout = unit.toNanos(time);
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
final long deadline = System.nanoTime() + nanosTimeout;
boolean timedout = false;
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
if (nanosTimeout <= 0L) {
timedout = transferAfterCancelledWait(node);
break;
}
if (nanosTimeout >= spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
nanosTimeout = deadline - System.nanoTime();
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null)
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
return !timedout;
}
// support for instrumentation
/**
* Returns true if this condition was created by the given
* synchronization object.
*
* @return {@code true} if owned
*/
final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
return sync == AbstractQueuedSynchronizer.this;
}
/**
* Queries whether any threads are waiting on this condition.
* Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
*
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final boolean hasWaiters() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
return true;
}
return false;
}
/**
* Returns an estimate of the number of threads waiting on
* this condition.
* Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
*
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if {@link #isHeldExclusively}
* returns {@code false}
*/
protected final int getWaitQueueLength() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int n = 0;
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION)
++n;
}
return n;
}
/**
* 得到同步队列当中所有在等待的Thread集合
*/
protected final Collection<Thread> getWaitingThreads() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
ArrayList<Thread> list = new ArrayList<Thread>();
for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
if (w.waitStatus == Node.CONDITION) {
Thread t = w.thread;
if (t != null)
list.add(t);
}
}
return list;
}
}
/**
* Setup to support compareAndSet. We need to natively implement
* this here: For the sake of permitting future enhancements, we
* cannot explicitly subclass AtomicInteger, which would be
* efficient and useful otherwise. So, as the lesser of evils, we
* natively implement using hotspot intrinsics API. And while we
* are at it, we do the same for other CASable fields (which could
* otherwise be done with atomic field updaters).
* unsafe魔法类，直接绕过虚拟机内存管理机制，修改内存
*/
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long stateOffset;
private static final long headOffset;
private static final long tailOffset;
private static final long waitStatusOffset;
private static final long nextOffset;
static {
try {
stateOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
headOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
tailOffset = unsafe.objectFieldOffset
(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
waitStatusOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("waitStatus"));
nextOffset = unsafe.objectFieldOffset
(Node.class.getDeclaredField("next"));
} catch (Exception ex) {
throw new Error(ex); }
}
/**
* CAS 修改头部节点指向. 并发入队时使用.
*/
private final boolean compareAndSetHead(Node update) {
return unsafe.compareAndSwapObject(this, headOffset, null, update);
}
/**
* CAS 修改尾部节点指向. 并发入队时使用.
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}
/**
* CAS 修改信号量状态.
*/
private static final boolean compareAndSetWaitStatus(Node node,
int expect,
int update) {
return unsafe.compareAndSwapInt(node, waitStatusOffset,
expect, update);
}
/**
* 修改节点的后继指针.
*/
private static final boolean compareAndSetNext(Node node,
Node expect,
Node update) {
return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
}
}