java多线程之yield,join,wait,sleep的区别

Java多线程之yield,join,wait,sleep的区别

Java多线程中，经常会遇到yield,join,wait和sleep方法。容易混淆他们的功能及作用。自己仔细研究了下，他们主要的区别是在cpu的占用和共享资源的锁上面。

wait:是继承自Object的方法，当前线程调用wait方法，是在告诉别的线程，我需要等待了，既会释放cpu，也会释放共享资源的锁，进入挂起状态。wait必须在synchronized代码块内部执行，因为wait需要获得共享资源的锁并且释放锁。需要notify/notifyAll来唤醒。wait主要和sleep进行区别，

join:Thread的非静态方法。底层用了wait方法。假如现在有两个线程，main线程和t线程.main线程里调用t.join.那么这时，main会取得线程对象t的锁，然后main线程 wait,释放cpu，t线程执行，

直到t线程执行完后，main线程继续执行。

jdk中join的源代码：

    /**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever.
     *
     * <p> This implementation uses a loop of {@code this.wait} calls
     * conditioned on {@code this.isAlive}. As a thread terminates the
     * {@code this.notifyAll} method is invoked. It is recommended that
     * applications not use {@code wait}, {@code notify}, or
     * {@code notifyAll} on {@code Thread} instances.
     *
     * @param  millis
     *         the time to wait 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 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;
            }
        }
    }

    /**
     * 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();

通过源代码，可以看出，如果t线程生成了，但是还未start，那么join方法是没有效果的。

sleep:是Thread的静态方法，会使当前线程释放cpu，但不会释放锁资源，可以理解为只和cpu有关，不涉及锁资源。涉及锁资源的，是wait,join方法。

yield:也是Thread的静态方法，和sleep方法类似，会使当前线程释放cpu，但不会释放锁资源。和sleep不同的是，sleep必须设置时间，但是yield不能设置时间，时间值是随机的