【Android筆記】Thread類中關於join()方法的源碼分析

一、join()方法的做用：

例若有一個線程對象爲Thread1，在main()方法中調用Thread1.join()方法可以使得當前線程(即主線程)阻塞，而執行Thread1線程。

二、源碼分析(以上面的例子爲例)

/**

 * 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  (等待時間爲毫秒)
 *         the time to wait in milliseconds
 *
 * @throws  IllegalArgumentException (當 millis 爲負數時拋出此異常)

 * if the value of {@code millis} is negative
 *
 * @throws InterruptedException (若是有任何線程中斷了當前線程（當前線程爲main線程），拋出此異常，當前線程的中斷狀態將被清除。該異常由源碼中的wait()拋出)

 * if any thread has interrupted the current thread. The
 * <i>interrupted status</i> of the current thread is
 * cleared when this exception is thrown.
 */

//這段代碼是由main線程執行的
public final void join(long millis) throws InterruptedException { synchronized(lock) { //這裏的lock是在Thread1中建立的，源碼爲 private final Object lock=new Object();
//main線程獲取對象lock的鎖 long base = System.currentTimeMillis();//獲取系統當前時間毫秒值 long now = 0; //表示等待了多少時間 if (millis < 0) { //拋出 millis 爲負數的異常 throw new IllegalArgumentException("timeout value is negative"); } if (millis == 0) { //毫秒值爲零，意味着等待到Thread1結束 while (isAlive()) { //當Thread1線程還活着時 lock.wait(0); //使得擁有lock對象的鎖的線程(main線程)中止，並使得當前線程釋放鎖，那何時調用notifyAll呢？這個答案在 jvm源碼中，在此不細述 } //中斷異常也是由wait()方法拋出 } else { //不然，等待millis while (isAlive()) { long delay = millis - now; if (delay <= 0) { break; // 等待時間結束，跳出了join()方法，main線程繼續執行 } lock.wait(delay); //延時等待 now = System.currentTimeMillis() - base; //更新now } } } }