一，wait

是object類的方法，並不是執行緒的方法，使用此方法會使當前執行緒進入阻塞狀態，過程中執行緒會釋放cpu資源和鎖資源,獻策韓國你進入等待池，可以通過notify或者notifyall將執行緒喚醒，是執行緒重新進入就緒狀態，在wait，notify和notifyall方法使用過程中一定要注意在同步方法或同步程式碼塊中使用，因為在同步中才會獲得鎖，才能釋放鎖，java API強制規定，否則將報出java.lang.IllegalMonitorStateException異常。

二，sleep

sleep使Thread的方法，使強制使執行緒進入睡眠狀態Thread.sleep(time)。在此過程中該執行緒只會釋放掉cpu資源並不會釋放鎖資源，也就意味著目前物件例項的搶佔到剛釋放的cpu資源的執行緒不能獲得同步資源鎖，但是可以去搶佔沒有被佔用的同步資源鎖。

三，join

join方法的原始碼

    /**
     * Waits for this thread to die.
     *
     * <p> An invocation of this method behaves in exactly the same
     * way as the invocation
     *
     * <blockquote>
     * {@linkplain #join(long) join}{@code (0)}
     * </blockquote>
     *
     * @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 void join() throws InterruptedException {
        join(0);            //join()等同於join(0)
    }
    /**
     * 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);           //join(0)等同於wait(0)，即wait無限時間直到被notify
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }
 

可以看出join是一個同步方法，當主執行緒呼叫t的jion方法時會像去獲得t的鎖，然後主執行緒呼叫wait方法進入等待池，直到t執行緒執行結束main執行緒才能繼續執行。

四，yield

執行緒讓步，是將該執行緒，讓出cpu資源進入可執行狀態，然後與同優先順序的執行緒再一併競爭搶佔資源，也就是說並不意味著該執行緒一定會處於可執行狀態，當其再一次搶到資源，會立即又轉變成執行狀態。