1、现在有T1、T2、T3三个线程，你怎样保证T2在T1执行完后执行，T3在T2执行完后执行？
package com.mianshi.easy;
class JoinDemo implements Runnable{
public void run() {
for(int i=0;i<100;i++){
System.out.println(Thread.currentThread().getName()+"……"+ i);
}
}
} public class JoinMethod { public static void main(String[] args) throws InterruptedException {
JoinDemo jd = new JoinDemo();
Thread T1 = new Thread(jd);
Thread T2 = new Thread(jd);
Thread T3 = new Thread(jd); T1.start();
//当t1拿着执行权把这些数据都打印完了，主线程才恢复到运行中来
T1.join(); T2.start();
T2.join(); T3.start();
T3.join(); }
}
结果：
Thread-0……0
Thread-0……1
Thread-0……2
Thread-0……3
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2、练习demo
（1）主线程向下转时，碰到了t1.join(),t1要申请加入到运行中来，就是要CPU执行权。这时CPU执行权在主线程手里，主线程就把CPU执行权给放开，陷入冻结状态。活着的只有t1了，只有当t1拿着执行权把这些数据都打印完了，主线程才恢复到运行中来。
package com.mianshi.easy;
class JoinDemo implements Runnable{
public void run() {
for(int i=0;i<100;i++){
System.out.println(Thread.currentThread().getName()+"……"+ i);
}
}
}
/**
* 主线程向下转时，碰到了t1.join(),t1要申请加入到运行中来，就是要CPU执行权。
* 这时候CPU执行权在主线程手里，主线程就把CPU执行权给放开，陷入冻结状态。活着的
* 只有t1了，只有当t1拿着执行权把这些数据都打印完了，主线程才恢复到运行中来。
* */
public class JoinMethod {
public static void main(String[] args) throws InterruptedException {
JoinDemo jd = new JoinDemo();
Thread t1 = new Thread(jd);
Thread t2 = new Thread(jd);
t1.start();
t1.join();
t2.start();
for(int j =0; j<100; j++){
System.out.println(Thread.currentThread().getName()+"……"+j);
}
}
}
结果：
Thread-0……0
Thread-0……1
Thread-0……2
Thread-0……3
Thread-0……4
Thread-0……5
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（2）改变顺序，主线程启动了两个子线程A、B时，其中一个子线程A调用了join()方法后，主线程将释放执行权，进入冻结状态。子线程A并不会影响子线程B，两个子线程将会交替执行，直到A线程run()方法执行完，主线程将会重新获得执行权。
package com.mianshi.easy;
class JoinDemo implements Runnable{
public void run() {
for(int i=0;i<200;i++){
System.out.println(Thread.currentThread().getName()+"……"+ i);
}
}
}
/**
* 将t1.join()和t2.start()位置互换。主线程开启了t1、t2，这时候CPU执行权还在主线程手里。
* 当主线程碰到了t1.join(),释放执行权，处于冻结状态。活着的t1、t2都具备执行资格，这时候CPU
* 便对t1和t2交替执行。主线程要等到t1结束才能活，至于t2结不结束，与主线程没有丝毫关系。如果t1
* 结束了，t2还没结束，主线程就会和t2抢执行权执行。主线程碰到谁的join，它就等谁。也就是说，谁
* 让它把执行权放出来，它就等谁死。至于谁去抢，它不管。
*
* */
public class JoinMethod {
public static void main(String[] args) throws InterruptedException {
JoinDemo jd = new JoinDemo();
Thread t1 = new Thread(jd);
Thread t2 = new Thread(jd);
t1.start();
t2.start();
t1.join();
for(int j =0; j<100; j++){
System.out.println(Thread.currentThread().getName()+"……"+j);
}
}
}
结果：
Thread-1……0
Thread-0……0
Thread-0……1
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顺便贴下java.lang.Thread中关于join()方法的实现代码：
/**
* 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;
}
}
} /**
* Waits at most {@code millis} milliseconds plus
* {@code nanos} nanoseconds for this thread to die.
*
* <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
*
* @param nanos
* {@code 0-999999} additional nanoseconds to wait
*
* @throws IllegalArgumentException
* if the value of {@code millis} is negative, or the value
* of {@code nanos} is not in the range {@code 0-999999}
*
* @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, int nanos)
throws InterruptedException { if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
} if (nanos < 0 || nanos > 999999) {
throw new IllegalArgumentException(
"nanosecond timeout value out of range");
} if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
millis++;
} join(millis);
} /**
* 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);
}