有一個筆試題卵酪，A幌蚊、B、C三個線程溃卡，怎么保證A執(zhí)行完后再執(zhí)行B溢豆，B執(zhí)行完后再執(zhí)行C。
最簡單的就是join 了吧瘸羡。直接上代碼：
public static void main(String[] args) throws InterruptedException {
PrintThread threadA = new PrintThread("Thread A");
PrintThread threadB = new PrintThread("Thread B");
PrintThread threadC = new PrintThread("Thread C");
threadA.start();
threadA.join();
threadB.start();
threadB.join();
threadC.start();
}
static class PrintThread extends Thread {
PrintThread(String name) {
super(name);
}
@Override
public void run() {
System.out.println(getName() + " start");
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(getName() + " end ...");
}
}
執(zhí)行結(jié)果：
Thread A start
Thread A end ...
Thread B start
Thread B end ...
Thread C start
Thread C end ...
那么join是怎么實(shí)現(xiàn)的呢漩仙？看下源碼：
/**
* 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);
}
注意下注釋：等待此線程執(zhí)行完。
join()方法實(shí)際上調(diào)用了join(long millis),參數(shù)是0，再往下看：
/**
* 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;
}
}
}
翻譯一下注釋：等待millis 毫秒終止線程队他，假如這段時間內(nèi)該線程還沒執(zhí)行完卷仑，那么結(jié)束等待。如果是0麸折，意味著一直等待锡凝，知道線程終止。
很明顯垢啼，有一個while循環(huán)使該線程一直停留在阻塞狀態(tài)窜锯，再看一下isAlive()方法：
/**
* 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();
是一個native方法，但是看一下注釋：判斷該線程是否是正在執(zhí)行狀態(tài)芭析，正在執(zhí)行狀態(tài)是指已經(jīng)start了锚扎，但是還沒結(jié)束。
通過不停判斷該線程的狀態(tài)馁启，來決定是不是要wait驾孔，線程結(jié)束，或者達(dá)到指定的延遲時間后进统，終止代碼助币。
