ReentrantLock的解读
Lock 加锁—公平所，FireSync
先看代码
class ReentrantLockExample{
int a = 0;
ReentrantLock lock = new ReentrantLock();
public void writer(){
lock.lock(); // 获取锁
try{
a ++; // 写
}finally{
lock.unlock(); // 释放锁
}
}
public void read(){
lock.lock(); // 获取锁
try{
int i = a; // 读
}finally{
lock.unlock(); // 释放锁
}
}
}
ReentrantLock依赖于Java的同步器框架AbstractQueuedSynchronizer（大名鼎鼎的AQS），AQS使用一个volatile int state来维护同步状态。这个state是ReentrantLock实现的关键。
贴一下ReentrantLock的部分源码：
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below. Uses AQS state to
* represent the number of holds on the lock.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
abstract void lock();
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*
* 非公平--尝试获取锁
* exclusive 独有的，排外的
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) { // 目前没有锁
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current); // 设置当前线程持有锁
return true;
}
} // 目前有锁，并且是当前线程，则当前线程继续持有锁，并且更新state的值
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
// 有锁且不为当前线程，返回false，取不到锁
return false;
}
// 释放锁
protected final boolean tryRelease(int releases) {
int c = getState() - releases; // 新的state值
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c); // 设置state值，释放时需要把最新值设置到state中
return free;
}
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
final ConditionObject newCondition() {
return new ConditionObject();
}
// Methods relayed from outer class
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
}
}
/**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1)) // expect 0，update to 1
setExclusiveOwnerThread(Thread.currentThread());//设置当前线程为hold锁线程
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
/**
* Sync object for fair locks
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive（递归的，循环的） call or no waiters（没有在等待的） or is first.
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState(); // 获取锁的开始，首先读AQS中volatile变量state
if (c == 0) {
// 关键是这个，这才是公平锁和非公平锁的区别，判断是否有别的线程比当前线程等的时间久
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) { //compareAndSetState
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;
}
}
......
/**
* Queries whether any threads have been waiting to acquire longer
* than the current thread.
*
* <p>An invocation of this method is equivalent to (but may be
* more efficient than):
* <pre> {@code
* getFirstQueuedThread() != Thread.currentThread() &&
* hasQueuedThreads()}</pre>
*
* <p>Note that because cancellations due to interrupts and
* timeouts may occur at any time, a {@code true} return does not
* guarantee that some other thread will acquire before the current
* thread. Likewise, it is possible for another thread to win a
* race to enqueue after this method has returned {@code false},
* due to the queue being empty.
*
* <p>This method is designed to be used by a fair synchronizer to
* avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
* Such a synchronizer's {@link #tryAcquire} method should return
* {@code false}, and its {@link #tryAcquireShared} method should
* return a negative value, if this method returns {@code true}
* (unless this is a reentrant acquire). For example, the {@code
* tryAcquire} method for a fair, reentrant, exclusive mode
* synchronizer might look like this:
*
* <pre> {@code
* protected boolean tryAcquire(int arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}</pre>
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*/
public final boolean hasQueuedPredecessors() {
// The correctness of this depends on head being initialized
// before tail and on head.next being accurate if the current
// thread is first in queue.
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());
}
......
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync(); // 默认为非公平锁
}
/**
* Creates an instance of {@code ReentrantLock} with the
* given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
/**
* Acquires the lock.
*
* <p>Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
* <p>If the current thread already holds the lock then the hold
* count is incremented by one and the method returns immediately.
*
* <p>If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until the lock has been acquired,
* at which time the lock hold count is set to one.
*/
public void lock() {
sync.lock();
}
/**
* Acquires the lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
* <p>If the current thread already holds this lock then the hold count
* is incremented by one and the method returns immediately.
*
* <p>If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of two things happens:
*
* <ul>
*
* <li>The lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
* current thread.
*
* </ul>
*
* <p>If the lock is acquired by the current thread then the lock hold
* count is set to one.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method; or
*
* <li>is {@linkplain Thread#interrupt interrupted} while acquiring
* the lock,
*
* </ul>
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to the
* interrupt over normal or reentrant acquisition of the lock.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
/**
* Acquires the lock only if it is not held by another thread at the time
* of invocation.
*
* <p>Acquires the lock if it is not held by another thread and
* returns immediately with the value {@code true}, setting the
* lock hold count to one. Even when this lock has been set to use a
* fair ordering policy, a call to {@code tryLock()} <em>will</em>
* immediately acquire the lock if it is available, whether or not
* other threads are currently waiting for the lock.
* This "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting for this lock, then use
* {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* <p>If the current thread already holds this lock then the hold
* count is incremented by one and the method returns {@code true}.
*
* <p>If the lock is held by another thread then this method will return
* immediately with the value {@code false}.
*
* @return {@code true} if the lock was free and was acquired by the
* current thread, or the lock was already held by the current
* thread; and {@code false} otherwise
*/
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
/**
* Acquires the lock if it is not held by another thread within the given
* waiting time and the current thread has not been
* {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the lock if it is not held by another thread and returns
* immediately with the value {@code true}, setting the lock hold count
* to one. If this lock has been set to use a fair ordering policy then
* an available lock <em>will not</em> be acquired if any other threads
* are waiting for the lock. This is in contrast to the {@link #tryLock()}
* method. If you want a timed {@code tryLock} that does permit barging on
* a fair lock then combine the timed and un-timed forms together:
*
* <pre> {@code
* if (lock.tryLock() ||
* lock.tryLock(timeout, unit)) {
* ...
* }}</pre>
*
* <p>If the current thread
* already holds this lock then the hold count is incremented by one and
* the method returns {@code true}.
*
* <p>If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
* <ul>
*
* <li>The lock is acquired by the current thread; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
* <li>The specified waiting time elapses
*
* </ul>
*
* <p>If the lock is acquired then the value {@code true} is returned and
* the lock hold count is set to one.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method; or
*
* <li>is {@linkplain Thread#interrupt interrupted} while
* acquiring the lock,
*
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* <p>In this implementation, as this method is an explicit
* interruption point, preference is given to responding to the
* interrupt over normal or reentrant acquisition of the lock, and
* over reporting the elapse of the waiting time.
*
* @param timeout the time to wait for the lock
* @param unit the time unit of the timeout argument
* @return {@code true} if the lock was free and was acquired by the
* current thread, or the lock was already held by the current
* thread; and {@code false} if the waiting time elapsed before
* the lock could be acquired
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if the time unit is null
*/
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
/**
* Attempts to release this lock.
*
* <p>If the current thread is the holder of this lock then the hold
* count is decremented. If the hold count is now zero then the lock
* is released. If the current thread is not the holder of this
* lock then {@link IllegalMonitorStateException} is thrown.
*
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock
*/
public void unlock() {
sync.release(1);
}
......
/**
* Returns {@code true} if this lock has fairness set true.
*
* @return {@code true} if this lock has fairness set true
*/
public final boolean isFair() {
return sync instanceof FairSync;
}
......
}
AbstractQueuedSynchronizer的部分源码（只是其中的state，其余的部分还需要找时间仔细瞅瞅）：
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
private static final long serialVersionUID = 7373984972572414691L;
......
/**
* The synchronization state.
*/
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);
}
......
/**
* 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();
}
......
}
所以对于公平锁的获取，lock的调用轨迹如下：
ReentrantLock : lock()
—> sync = FairSync : lock() // 公平锁，所以走FairSync内部静态类
—> AbstarctQueuedSynchronizer : acquire(int arg)
—> 调用ReentrantLock中的tryAcquire实现，AQS中的tryAquire并没有实现。
所以真正的lock实现，是在ReentrantLock中的tryAquire方法，首先读volatile变量state，根据state来判断是否可加锁。
Lock释放锁
FairSync和NonfairSync 在释放锁的场景实现一致。
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unlock调用的轨迹如下：
ReentrantLock : unlock()
—> sync = FairSync 调用父类 AbstractQueuedSynchronizer : release(int arg)
—> 调用ReentrantLock中的内部抽象类Sync ：tryRelease实现，
Lock加锁 — 非公平锁
非公平锁的lock()调用轨迹如下：
ReentrantLock ：lock()
—> NonfairSync : lock()
—> AbstractQueuedSynchronizer : compareAndSetState(int expect, int update)
可以根据上面的代码看到，AQS中的compareAndSetState方法直接调用的unsafe.compareAndSwapInt(this, stateOffset, expect, update)方法。unsafe'是调用本地方法，在openjdk中依次调用的cpp代码为：
unsafe.cpp —> atomic.cpp —> atomic_windows_x86.inline.hpp.
简单贴下这块代码在win平台x86上的实现
// Adding a lock prefix to an instruction on MP machine
// VC++ doesn't like the lock prefix to be on a single line.
// so we can't insert a label after the lock prefix.
// By emitting a lock prefix, we can define a label after it.
#define LOCK_IF_MP(mp) __asm cmp mp,0 \
__asm je L0 \
__asm _emit 0XF0 \
__asm L0:
inline jint Atomic::cmpxchg (jint exchange_value,
volatile jint* dest,
jint compare_value){
// alternative for InterlockedCompareExchange
int mp = os::is_MP();
__asm{
mov edx,dest
mov ecx,exchange_value
mov eax,compare_value
LOCK_IF_MP(mp)
cmpxchg dword ptr[edx],exx
}
}
这里MP—猜测表示multi-processor，也就是多处理器。如果程序是在多处理器上运行，就在cmpxchg指令前面加上lock指令。反之，如果是在单处理器上运行，就省略。因为单处理器会维护自身的顺序一致性，不需要lock前缀提供的内存屏障效果。
intel的手册对lock指令的说明如下：
- 带lock前缀的指令，其对内存的 读-改-写都是原子执行
- 禁止该指令与之前、之后的读/写指令重排
- 把写缓冲区中的所有数据刷新到主内存中。
其实看起来，除了原子性之外，跟volatile的特性基本一致。
对公平锁及非公平锁做个总结
- 公平锁和非公平锁，在释放时都需要写volatile state
- 公平锁在获取时，首先会去读volatile state
- 非公平锁在获取时，首先会用CAS更新volatile state (cas同时具有volatile 读和volatile写的内存语义，不会重排序)
从ReentrantLock的实现可以看出，锁的释放—获取，可以通过两种方式来实现：
- 利用volatile变量的写/读具有的内存语义来实现——公平锁
- 利用cas锁附带的等同于volatie读和volatile写的内存语义来实现 — 非公平锁