目录
本篇博客基于jdk1.8
Thread类概述
Thread的一个实例,即是一个程序中执行的一个线程,java虚拟机允许一个应用程序同时有多个并发的线程运行。
每一个线程都有一个优先级。优先级高的线程比优先级低的线程优先执行。每一个线程也都可能被标记为一个守护线程。当代码执行到某一个线程去创建另一个线程时,新创建的线程的优先级会初始化成和创建它的线程一样。并且当且仅当创建线程为守护线程时,被创建线程才会被设为守护线程。
当Java虚拟机启动时,通常会有一个单独的非守护进程线程(通常这个非守护线程会去调用一个指定的类的main方法)。Java虚拟机器继续执行线程,直到以下任一一种情况发生:
Runtime类的exit方法被调用并且security manager(安全管理器)允许退出操作操作发生。
所有的非守护线程全部都已经终结,要么通过从调用返回到运行方法或由抛出超出运行的异常方法。
创建一个新的执行线程有两种方法:
声明一个Thread类的子类。这个子类应当去重写Thread类里的run方法。然后子类的实例就可以分配和启动。例如,计算大于指定值的素数的线程可以写成如下:
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
...
}
}
下面的代码就可以创建一个线程并启动它:
PrimeThread p = new PrimeThread(143);
p.start();
创建线程的另一种方法是声明一个类实现Runable接口,这个类需要实现run方法,然后这个类的实例就可以在创建线程时被当做参数传递进去,并且启动,同一个例子的另一种样式如下:
class PrimeRun implements Runnable {
long minPrime;
PrimeRun(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
...
}
}
然后,下面的代码就可以创建一个线程并启动它:
PrimeRun p = new PrimeRun(143);
new Thread(p).start();
每个线程都有用于标识目的的名称。多个线程可能具有相同的名称。如果创建一个线程时未指定名称,则会为其生成一个新名称。
除非另有说明,否则,向Thread类中的构造函数或方法传递null参数将导致抛出NullPointerException。
另外一点特别有意思的是,Thread类本身就是实现了Runnable接口的一个类。
成员变量
private volatile String name; //线程的名字
private int priority; //线程优先级
/* Whether or not to single_step this thread. */
private boolean single_step;
/* Whether or not the thread is a daemon thread. */
private boolean daemon = false; //是否是守护线程
/* What will be run. */
private Runnable target; //将会被执行的Runnable.
/* The group of this thread */
private ThreadGroup group; //这个线程的组
/* The context ClassLoader for this thread */
private ClassLoader contextClassLoader; //这个线程的上下文
/* The inherited AccessControlContext of this thread */
private AccessControlContext inheritedAccessControlContext; //继承的请求控制
/* For autonumbering anonymous threads. */
private static int threadInitNumber; //默认线程的自动编号
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null; //当前线程附属的ThreadLocal,而ThreadLocalMap会被ThreadLocal维护)
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
// 主要作用:为子线程提供从父线程那里继承的值
//在创建子线程时,子线程会接收所有可继承的线程局部变量的初始值,以获得父线程所具有的值
// 创建一个线程时如果保存了所有 InheritableThreadLocal 对象的值,那么这些值也将自动传递给子线程
//如果一个子线程调用 InheritableThreadLocal 的 get() ,那么它将与它的父线程看到同一个对象
/*
* The requested stack size for this thread, or 0 if the creator did
* not specify a stack size. It is up to the VM to do whatever it
* likes with this number; some VMs will ignore it.
*/
private long stackSize; //该线程请求的堆栈大小 默认一般都是忽略
/*
* Thread ID
*/
private long tid; // 每个线程都有专属ID,但名字可能重复
/* For generating thread ID */
private static long threadSeqNumber; //用来生成thread ID
/* Java thread status for tools,
* initialized to indicate thread 'not yet started'
*/
private volatile int threadStatus = 0; //标识线程状态,默认是线程未启动
/* The object in which this thread is blocked in an interruptible I/O
* operation, if any. The blocker's interrupt method should be invoked
* after setting this thread's interrupt status.
*/
private volatile Interruptible blocker; //阻塞器锁,主要用于处理阻塞情况
/**
* The minimum priority that a thread can have.
*/
public final static int MIN_PRIORITY = 1;//线程的优先级中最小的
/**
* The default priority that is assigned to a thread.
*/
public final static int NORM_PRIORITY = 5; //线程的优先级中第二的同时也是默认的优先级
/**
* The maximum priority that a thread can have.
*/
public final static int MAX_PRIORITY = 10; //最高的优先级
构造方法
据完全统计,Thread类里一共提供了九个构造方法...其中有一个是非public的方法。
但是这九个构造方法事实上最后都是调用了一个叫做init的方法,该方法定义如下:
/**
* Initializes a Thread.
*
* @param g the Thread group
* @param target the object whose run() method gets called
* @param name the name of the new Thread
* @param stackSize the desired stack size for the new thread, or
* zero to indicate that this parameter is to be ignored.
* @param acc the AccessControlContext to inherit, or
* AccessController.getContext() if null
* @param inheritThreadLocals if {@code true}, inherit initial values for
* inheritable thread-locals from the constructing thread
*/
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null) {
g = security.getThreadGroup();
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup();
}
}
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
该方法一共六个参数:
1.ThreadGroup 线程组
2.Runnable target 一个Runnable对象,即调用run方法的对象
3.String name 新线程的名字
4.long stackSize 新线程分配所需堆栈的大小
5.AccessControlContext acc 权限控制
6.boolean inheritThreadLocals 是否继承ThreadLocals, 这个参数总是true。
方法的实现大致总结起来就是,给变量赋初值,大部分的初值都是通过该线程的父线程赋值,获取父线程对象的代码为:
Thread parent = currentThread();
而currentThread方法的实现是一个本地方法。
start方法
start方法的作用就是让该线程开始执行,java虚拟机会调用这个线程的run方法。
执行start方法的结果是两个线程会并发的运行,当前线程即执行start方法的线程,以及另一个调用run方法的线程。
启动一个线程两次永远是非法的,会抛出IllegalThreadStateException。代码如下:
public synchronized void start() {
/**
* This method is not invoked for the main method thread or "system"
* group threads created/set up by the VM. Any new functionality added
* to this method in the future may have to also be added to the VM.
*
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented. */
group.add(this);
boolean started = false;
try {
start0();
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
可以看到上来会验证线程的状态,如果线程已启动则抛异常。核心实现代码为方法start0,是一个本地方法,在这个本地方法的实现当中,虚拟机会去调用该线程的run方法,在java代码中并看不出这一点。
这就是我们可以通过start方法启动线程,并且run方法会执行的原因所在。
run方法
Thread类实现了Runable接口,自然也就实现了run方法,代码如下
@Override
public void run() {
if (target != null) {
target.run();
}
}
target定义如下:
/* What will be run. */
private Runnable target;
target就是Runnable的一个实例,是Thread类中执行run的对象。注意:这里的代码同时说明了我们new一个Thread的时候问什么要传入一个Runable的对象,因为事实上后来这个Thread启动调用run方法的时候,就是调用的这个Runable的run方法。
问题:Thread直接调用run()和start()方法的区别?
run()方法: 在本线程内调用该Runnable对象的run()方法,可以重复多次调用;
start()方法:启动一个线程,然后Java虚拟机会调用该Runnable对象,即new一个Thread的时候传入的Runable对象,的run()方法,不能多次启动一个线程。
你调用run()方法的时候,只会是在原来的线程中调用,没有新的线程启动,start()方法才会启动新线程。
sleep方法
有两个重载的sleep方法。其中一个代码如下:
public static void sleep(long millis, int nanos)
throws InterruptedException {
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
}
if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
}
sleep(millis);
}
在调用另一个sleep方法之前,就是做了一些range check,即范围检查,然后就直接调用了另一个native的sleep方法。
yield方法
代码如下
public static native void yield();
作用是使当前线程从执行状态(运行状态)变为可执行态(就绪状态)。也是一个native方法。
interrupt方法
public void interrupt() {
if (this != Thread.currentThread())
checkAccess();
synchronized (blockerLock) {
Interruptible b = blocker;
if (b != null) {
interrupt0(); // Just to set the interrupt flag
b.interrupt(this);
return;
}
}
interrupt0();
}
实现也是调用了一个interrupt0的本地方法,有一句关键的注释:只是去set了interrupt flag,即不会真正的去停止线程,关于这一部分的内容,可以参考我的另一篇博客:停止线程。
join方法
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
join的作用就是使得当前线程阻塞,一直到调用join方法的线程对象执行完run方法中的任务且销毁之后才继续执行后面的代码。
从上面的代码实现来看,join方法是通过和wait方法配合实现的。
getState方法
该方法是返回线程的状态,它的实现很有意思,如下:
public State getState() {
// get current thread state
return sun.misc.VM.toThreadState(threadStatus);
}
public static State toThreadState(int var0) {
if ((var0 & 4) != 0) {
return State.RUNNABLE;
} else if ((var0 & 1024) != 0) {
return State.BLOCKED;
} else if ((var0 & 16) != 0) {
return State.WAITING;
} else if ((var0 & 32) != 0) {
return State.TIMED_WAITING;
} else if ((var0 & 2) != 0) {
return State.TERMINATED;
} else {
return (var0 & 1) == 0 ? State.NEW : State.RUNNABLE;
}
}
toThreadState方法的实现就是根据线程状态的数字,返回对应的状态而已,它们只是有一个mapping的关系。State的定义则是一个枚举,如下:
public enum State {
/**
* Thread state for a thread which has not yet started.
*/
NEW,
/**
* Thread state for a runnable thread. A thread in the runnable
* state is executing in the Java virtual machine but it may
* be waiting for other resources from the operating system
* such as processor.
*/
RUNNABLE,
/**
* Thread state for a thread blocked waiting for a monitor lock.
* A thread in the blocked state is waiting for a monitor lock
* to enter a synchronized block/method or
* reenter a synchronized block/method after calling
* {@link Object#wait() Object.wait}.
*/
BLOCKED,
/**
* Thread state for a waiting thread.
* A thread is in the waiting state due to calling one of the
* following methods:
* <ul>
* <li>{@link Object#wait() Object.wait} with no timeout</li>
* <li>{@link #join() Thread.join} with no timeout</li>
* <li>{@link LockSupport#park() LockSupport.park}</li>
* </ul>
*
* <p>A thread in the waiting state is waiting for another thread to
* perform a particular action.
*
* For example, a thread that has called <tt>Object.wait()</tt>
* on an object is waiting for another thread to call
* <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
* that object. A thread that has called <tt>Thread.join()</tt>
* is waiting for a specified thread to terminate.
*/
WAITING,
/**
* Thread state for a waiting thread with a specified waiting time.
* A thread is in the timed waiting state due to calling one of
* the following methods with a specified positive waiting time:
* <ul>
* <li>{@link #sleep Thread.sleep}</li>
* <li>{@link Object#wait(long) Object.wait} with timeout</li>
* <li>{@link #join(long) Thread.join} with timeout</li>
* <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
* <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
* </ul>
*/
TIMED_WAITING,
/**
* Thread state for a terminated thread.
* The thread has completed execution.
*/
TERMINATED;
}
可以看到java的线程一共是有六种状态的。
holdsLock方法判断线程是否持有锁对象
/**
* Returns <tt>true</tt> if and only if the current thread holds the
* monitor lock on the specified object.
*
* <p>This method is designed to allow a program to assert that
* the current thread already holds a specified lock:
* <pre>
* assert Thread.holdsLock(obj);
* </pre>
*
* @param obj the object on which to test lock ownership
* @throws NullPointerException if obj is <tt>null</tt>
* @return <tt>true</tt> if the current thread holds the monitor lock on
* the specified object.
* @since 1.4
*/
public static native boolean holdsLock(Object obj);
为什么wait和notify方法不在Thread类里定义
需要说明为什么把这些方法放在Object类里是有意义的,还有不把它放在Thread类里的原因。一个很明显的原因是JAVA提供的锁是对象级的而不是线程级的,每个对象都有锁,通过线程获得。如果线程需要等待某些锁那么调用对象中的wait()方法就有意义了。如果wait()方法定义在Thread类中,线程正在等待的是哪个锁就不明显了。简单的说,由于wait,notify和notifyAll都是锁级别的操作,所以把他们定义在Object类中因为锁属于对象
总结
经过上面的分析,可以发现事实上Thread类因为设计到线程的操作,因此大量的还是依靠了native方法实现。java核心逻辑并不多。