java多线程2：Thread中的方法

静态方法：

Thread类中的静态方法表示操作的线程是"正在执行静态方法所在的代码块的线程"。

为什么Thread类中要有静态方法，这样就能对CPU当前正在运行的线程进行操作。下面来看一下Thread类中的静态方法：

1：currentThread

/**
     * Returns a reference to the currently executing thread object.
     *
     * @return  the currently executing thread.
     */
    public static native Thread currentThread();

　currentThread()方法返回的是对当前正在执行线程对象的引用。

/**
 * 测试Thread.currentThread()：返回代码段正在被哪个线程调用的信息
 * this 始终表示线程实例本身
 *
 * @author yuxiaoyu
 */
public class Mythread4 extends Thread {
　　static {
    　　System.out.println("Mythread4静态块的打印：Thread.currentThread().getName()=" + Thread.currentThread().getName());
　　}
    public Mythread4() {
        System.out.println("Mythread4构造方法：Thread.currentThread().getName()=" + Thread.currentThread().getName());
        System.out.println("Mythread4构造方法：this.getName()=" + this.getName());
    }

    public void run() {
        System.out.println("run 方法：Thread.currentThread().getName()=" + Thread.currentThread().getName());
        System.out.println("run 方法：this.getName()=" + this.getName());
    }

}

@Test
	public void test4() {
		Mythread4 a = new Mythread4();
		a.start();
	}

　　执行结果：

Mythread4静态块的打印：Thread.currentThread().getName()=main
Mythread4构造方法：Thread.currentThread().getName()=main
Mythread4构造方法：this.getName()=Thread-0
run 方法：Thread.currentThread().getName()=Thread-0
run 方法：this.getName()=Thread-0

　　这个结果说明，线程类的构造方法、静态块是被main线程调用的，而线程类的run()方法才是应用线程自己调用的，this 始终表示线程实例本身。

2：sleep

/**
     * Causes the currently executing thread to sleep (temporarily cease
     * execution) for the specified number of milliseconds, subject to
     * the precision and accuracy of system timers and schedulers. The thread
     * does not lose ownership of any monitors.
     *
     * @param  millis
     *         the length of time to sleep in milliseconds
     *
     * @throws  IllegalArgumentException
     *          if the value of {@code millis} is negative
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    public static native void sleep(long millis) throws InterruptedException;　

该方法是让正在执行的线路休眠指定毫秒，注意API的注释中“The thread does not lose ownership of any monitors.”表示线程不会失去任何监视器的所有权。

简单说就是sleep代码上下文如果被加锁了，那么在休眠的时间内，锁依然在，但是CPU资源会让出给其他线程。

3：yield

/**
     * A hint to the scheduler that the current thread is willing to yield
     * its current use of a processor. The scheduler is free to ignore this
     * hint.
     *
     * <p> Yield is a heuristic attempt to improve relative progression
     * between threads that would otherwise over-utilise a CPU. Its use
     * should be combined with detailed profiling and benchmarking to
     * ensure that it actually has the desired effect.
     *
     * <p> It is rarely appropriate to use this method. It may be useful
     * for debugging or testing purposes, where it may help to reproduce
     * bugs due to race conditions. It may also be useful when designing
     * concurrency control constructs such as the ones in the
     * {@link java.util.concurrent.locks} package.
     */
    public static native void yield();

　　该方法表示给调度程序的一个提示:当前线程愿意让出它当前对处理器的使用。调度程序可以自由地忽略这个提示。

public class Mythread5 extends Thread {

    public void run() {
        long beginTime = System.currentTimeMillis();
        int count = 0;
        for (int i = 0; i < 50000000; i++)
        {
            Thread.yield();
            count = count + i + 1;
        }
        long endTime = System.currentTimeMillis();
        System.out.println("用时：" + (endTime - beginTime) + "毫秒！");
    }
}

　　第一次运行结果：用时：34872毫秒！

　　第二次运行结果：用时：33738毫秒！

如果将 Thread.yield(); 注释掉的话， 运行结果：用时：48毫秒！。

看到，每次执行的用时都不一样，证明了yield()方法放弃CPU的时间并不确定，而且yield方法将CPU资源让给其他资源导致变慢。

4：interrupted()

/**
     * Tests whether the current thread has been interrupted.  The
     * <i>interrupted status</i> of the thread is cleared by this method.  In
     * other words, if this method were to be called twice in succession, the
     * second call would return false (unless the current thread were
     * interrupted again, after the first call had cleared its interrupted
     * status and before the second call had examined it).
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     * @return  <code>true</code> if the current thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see #isInterrupted()
     * @revised 6.0
     */
    public static boolean interrupted() {
        return currentThread().isInterrupted(true);
    }

/**
     * Tests if some Thread has been interrupted.  The interrupted state
     * is reset or not based on the value of ClearInterrupted that is
     * passed.
     */
    private native boolean isInterrupted(boolean ClearInterrupted);

　　

　　测试当前线程是否已经中断，并清除线程的中断状态。

实例方法

1：start

/**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>
     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * <code>start</code> method) and the other thread (which executes its
     * <code>run</code> method).
     * <p>
     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */
    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 */
            }
        }
    }

　　

此方法告诉线程执行器，这个线程可以执行了。

2：isAlive

/**
     * Tests if this thread is alive. A thread is alive if it has
     * been started and has not yet died.
     *
     * @return  <code>true</code> if this thread is alive;
     *          <code>false</code> otherwise.
     */
    public final native boolean isAlive();

　　判断当前的线程是否处于活动状态，活动状态就是线程已经启动且尚未终止。线程处于正在运行或准备开始运行的状态，就认为线程是“存活”的。

public class Mythread7_2 extends Thread {

    public Mythread7_2() {
        System.out.println("Mythread7_2---begin");

        System.out.println("Thread.currentThread().getName()=" + Thread.currentThread().getName());
        System.out.println("Thread.currentThread().isAlive()=" + Thread.currentThread().isAlive());

        System.out.println("this.getName()=" + this.getName());
        System.out.println("this.isAlive()=" + this.isAlive());

        System.out.println("Mythread7_2---end");
    }

    @Override
    public void run() {
        System.out.println("run---begin");

        System.out.println("Thread.currentThread().getName()=" + Thread.currentThread().getName());
        System.out.println("Thread.currentThread().isAlive()=" + Thread.currentThread().isAlive());

        System.out.println("this.getName()=" + this.getName());
        System.out.println("this.isAlive()=" + this.isAlive());

        System.out.println("run---end");
    }

}

@Test
	public void test7_2() {
		Mythread7_2 c = new Mythread7_2();
		System.out.println("main begin t1 isAlive=" + c.isAlive());
		c.setName("A");
		c.start();
		System.out.println("main end t1 isAlive=" + c.isAlive());
	}

　　执行结果：

Mythread7_2---begin
Thread.currentThread().getName()=main
Thread.currentThread().isAlive()=true
this.getName()=Thread-0
this.isAlive()=false
Mythread7_2---end
main begin t1 isAlive=false
run---begin
Thread.currentThread().getName()=A
Thread.currentThread().isAlive()=true
this.getName()=A
this.isAlive()=true
run---end
main end t1 isAlive=false

　　可以看出：只有在当前实例对象调用了start方法时，this.isAlive()才返回true。

3：interrupt

/**
     * Interrupts this thread.
     *
     * <p> Unless the current thread is interrupting itself, which is
     * always permitted, the {@link #checkAccess() checkAccess} method
     * of this thread is invoked, which may cause a {@link
     * SecurityException} to be thrown.
     *
     * <p> If this thread is blocked in an invocation of the {@link
     * Object#wait() wait()}, {@link Object#wait(long) wait(long)}, or {@link
     * Object#wait(long, int) wait(long, int)} methods of the {@link Object}
     * class, or of the {@link #join()}, {@link #join(long)}, {@link
     * #join(long, int)}, {@link #sleep(long)}, or {@link #sleep(long, int)},
     * methods of this class, then its interrupt status will be cleared and it
     * will receive an {@link InterruptedException}.
     *
     * <p> If this thread is blocked in an I/O operation upon an {@link
     * java.nio.channels.InterruptibleChannel InterruptibleChannel}
     * then the channel will be closed, the thread's interrupt
     * status will be set, and the thread will receive a {@link
     * java.nio.channels.ClosedByInterruptException}.
     *
     * <p> If this thread is blocked in a {@link java.nio.channels.Selector}
     * then the thread's interrupt status will be set and it will return
     * immediately from the selection operation, possibly with a non-zero
     * value, just as if the selector's {@link
     * java.nio.channels.Selector#wakeup wakeup} method were invoked.
     *
     * <p> If none of the previous conditions hold then this thread's interrupt
     * status will be set. </p>
     *
     * <p> Interrupting a thread that is not alive need not have any effect.
     *
     * @throws  SecurityException
     *          if the current thread cannot modify this thread
     *
     * @revised 6.0
     * @spec JSR-51
     */
    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();
    }

　　interrupt并不会立刻停止线程，interrupt0的作用仅仅是为当前线程打一个中断的标志。

4：isInterrupted

/**
     * Tests whether this thread has been interrupted.  The <i>interrupted
     * status</i> of the thread is unaffected by this method.
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     * @return  <code>true</code> if this thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see     #interrupted()
     * @revised 6.0
     */
    public boolean isInterrupted() {
        return isInterrupted(false);
    }

　　测试线程是否已经中断，但不清除状态标识。这个和interrupted()方法区别就是不清除状态标识。

参考文献

1：《Java并发编程的艺术》

2：《Java多线程编程核心技术》