Thread Join 簡單理解就是等待被調(diào)用join的線程先執(zhí)行完，再自己執(zhí)行；或者并發(fā)線程串行化。

不要輕易的使用join否則嚴(yán)重降低線程并發(fā)性能；jdk 并不建議使用join。

jdk 解釋join原文：

public final void?join()?throws InterruptedException Waits for this thread to die. Throws: InterruptedException? - if any thread has interrupted the current thread. The?interrupted status?of the current thread is cleared when this exception is thrown.

demo:

package test;public class ThreadA extends Thread{@Overridepublic void run() {// TODO Auto-generated method stubfor(int i = 0;i < 5; i++){System.out.printf("ThreadA " + "run i=" + i + "\n");try {Thread.sleep(2000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}System.out.printf("ThreadA " + "END..." + "\n");} } package test;public class ThreadB extends Thread{private ThreadA at;public ThreadB(ThreadA at){this.at = at;}@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.printf("ThreadB " + "START..." + "\n");System.out.printf("ThreadB " + "END..." + "\n");try {at.join();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}} package test;public class threadMain {public static void main(String[] args) {// TODO Auto-generated method stubThreadA at = new ThreadA();ThreadB bt = new ThreadB(at);at.start();try {Thread.sleep(2000);System.out.println("main " + "run..." + "\n");System.out.println("main " + "end..." + "\n");bt.join();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}bt.start();}}

日志打印：

ThreadA run i=0
main run...

main end...

ThreadB START...
ThreadB END...
ThreadA run i=1
ThreadA run i=2
ThreadA run i=3
ThreadA run i=4
ThreadA END...

可以看出使用join之前的代碼是可以先被執(zhí)行的，所以join使用位置很重要。

代碼調(diào)整如下：

package test;public class ThreadA extends Thread{@Overridepublic void run() {// TODO Auto-generated method stubfor(int i = 0;i < 5; i++){System.out.printf("ThreadA " + "run i=" + i + "\n");try {Thread.sleep(2000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}System.out.printf("ThreadA " + "END..." + "\n");} } package test;public class ThreadB extends Thread{private ThreadA at;public ThreadB(ThreadA at){this.at = at;}@Overridepublic void run() {try {at.join();} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}// TODO Auto-generated method stubSystem.out.printf("ThreadB " + "START..." + "\n");System.out.printf("ThreadB " + "END..." + "\n");}} package test;public class threadMain {public static void main(String[] args) {// TODO Auto-generated method stubThreadA at = new ThreadA();ThreadB bt = new ThreadB(at);at.start();try {//Thread.sleep(2000);bt.join();System.out.println("main " + "run..." + "\n");System.out.println("main " + "end..." + "\n");} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}bt.start();}}

日志打印：

ThreadA run i=0
main run...

main end...

ThreadA run i=1
ThreadA run i=2
ThreadA run i=3
ThreadA run i=4
ThreadA END...
ThreadB START...
ThreadB END...

為什么main還是早于bt執(zhí)行，在于main中先調(diào)用join然后bt.start;如果調(diào)用join 的時(shí)候線程沒有運(yùn)行join是不生效的。

main做修改：

package test;public class threadMain {public static void main(String[] args) {// TODO Auto-generated method stubThreadA at = new ThreadA();ThreadB bt = new ThreadB(at);at.start();bt.start();try {//Thread.sleep(2000);System.out.println("main sleep over, and bt join.");bt.join();System.out.println("main " + "run..." + "\n");System.out.println("main " + "end..." + "\n");} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}

日志打印：

ThreadA run i=0
main sleep over, and bt join.
ThreadA run i=1
ThreadA run i=2
ThreadA run i=3
ThreadA run i=4
ThreadA END...
ThreadB START...
ThreadB END...
main run...

main end...

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