首先给出结论：t.join()方法只会使主线程(或者说调用t.join()的线程进入等待池并等待t线程执行完毕后才会被唤醒。并不影响同一时刻处在运行中的其他线程)
下面则是分析过程：
之前对于join方法只是了解它能够使得t.join()中的t优先执行，当t执行完才会执行其他线程。能够使得线程之间的并行执行变成串行执行。
public class TestJoin{
public static void main(String[] args) throws InterruptedException{
Thread t1 = new ThreadTest("A");
Thread t2 = new ThreadTest("B");
t1.start();
t2.start();
}
}
class ThreadTest extends Thread{
private String name;
public ThreadTest(String name){
this.name = name;
}
public void run(){
for(int i=1;i<=5;i++){
System.out.println(name+"-"+i);
}
}
}
运行结果：
A-1
B-1
B-2
B-3
A-2
B-4
A-3
B-5
A-4
A-5
可以看出A线程和B线程是交替执行的
而在其中加入join()方法后(后面的代码都省略了ThreadTest类的定义)
public class TestJoin {
public static void main(String[] args) throws InterruptedException {
// TODO Auto-generated method stub
ThreadTest t1=new ThreadTest("A");
ThreadTest t2=new ThreadTest("B");
t1.start();
t1.join();
t2.start();
}
}
运行结果：
A-1
A-2
A-3
A-4
A-5
B-1
B-2
B-3
B-4
B-5
显然,使用t1.join()之后，B线程需要等A线程执行完毕之后才能执行。需要注意的是t1.join()需要等t1.start()执行之后执行才有效果，此外如果t1.join()放在t2.start()之后的话，仍然会交替执行，然而并不是没有效果。
为了深入理解，我们先看一看join()的源码。
/**
* 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()等同于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)等同于wait(0)，即wait无限时间直到被notify
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
可以看出,join()方法的底层是利用了wait()方法实现。由此，join方法是一个同步方法，当主线程调用t1.join方法时,主线程先获得了t1对象的锁，随后进入方法，调用了t1对象的wait()方法，使主线程进入了t1对象的等待池，此时A线程则还在执行，并且随后的t2.start()还没被执行，因此B线程还没开始。等到A线程执行完毕之后，主线程继续执行，走到了t2.start(),B线程才会开始执行。
此外,对于join()的位置和作用的关系，我们可以用下面的例子来分析：
public class TestJoin {
public static void main(String[] args) throws InterruptedException {
// TODO Auto-generated method stub
System.out.println(Thread.currentThread().getName()+" start");
ThreadTest t1=new ThreadTest("A");
ThreadTest t2=new ThreadTest("B");
ThreadTest t3=new ThreadTest("C");
System.out.println("t1start");
t1.start();
System.out.println("t2start");
t2.start();
System.out.println("t3start");
t3.start();
System.out.println(Thread.currentThread().getName()+" end");
}
}
运行结果为：
main start
t1start
t1end
t2start
t2end
t3start
t3end
A-1
A-2
main end
C-1
C-2
C-3
C-4
C-5
A-3
B-1
B-2
B-3
B-4
B-5
A-4
A-5
A、B、C和主线程交替运行。加入join()方法之后
public class TestJoin {
public static void main(String[] args) throws InterruptedException {
// TODO Auto-generated method stub
System.out.println(Thread.currentThread().getName()+" start");
ThreadTest t1=new ThreadTest("A");
ThreadTest t2=new ThreadTest("B");
ThreadTest t3=new ThreadTest("C");
System.out.println("t1start");
t1.start();
System.out.println("t1end");
System.out.println("t2start");
t2.start();
System.out.println("t2end");
t1.join();
System.out.println("t3start");
t3.start();
System.out.println("t3end");
System.out.println(Thread.currentThread().getName()+" end");
}
}
运行记结果为
main start
t1start
t1end
t2start
t2end
t3start
t3end
A-1
A-2
main end
C-1
C-2
C-3
C-4
C-5
A-3
B-1
B-2
B-3
B-4
B-5
A-4
A-5
