wait/notify

释放锁吗？
释放。
因为wait一定是在出现同步代码块里，所以，wait的时候，必须释放锁，其他线程才能获取锁执行，然后其他线程执行完成之后再notify这个线程。

sleep

释放锁吗？
不释放。
它的作用只是，让当前线程暂停几秒，即暂时睡眠几秒。

看官方文档API

/**
     * 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;

如果时间是0呢？

yield

是否锁吗？
不释放。

作用
是让线程放弃cpu。

官方文档API

/**
     * 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();

什么时候恢复呢？

共同点

1.作用
让当前线程暂时停止执行。

2.针对的对象是谁？
都是针对线程，即当前线程。

总结

参考