java主线程结束和子线程结束之间的关系

情况1：正常情况下，主线程启动了子线程，主线程、子线程各自执行，彼此不受影响。

当你在run一个Java application的时候，这个时候系统会开一个进程。然后这个进程启动了Main线程。Java进程确定虚拟机中没有线程运行的时候，退出进程。或者也可以用System.exit(0);强制退出进程



代码示例如下：参考Thinkingin java代码

class LiftOff implements Runnable {

Logger logger = LoggerFactory.getLogger(LiftOff.class);

protected int countDown = 10; // Default
private static int taskCount = 0;
private final int id = taskCount++;
public LiftOff() {}
public LiftOff(int countDown) {
this.countDown = countDown;
}
public String status() {
return "#" + id + "(" +
(countDown > 0 ? countDown : "Liftoff!") + "), ";
}
@Override
public void run() {
while(countDown-- > 0) {
logger.info(status());
Thread.yield();
}
}
}

public class MainThread{

Logger logger = LoggerFactory.getLogger(MainClass.class);

@Test
public void test(){
Thread t = new Thread(new LiftOff());
t.start();
logger.info("waiting for liftoff");
}
}

显示结果：



情况2：需求是主线程执行结束，由主线程启动的子线程都结束



代码如下

class SimpleDaemons implements Runnable {
Logger logger = LoggerFactory.getLogger(SimpleDaemons.class);
public void run() {
try {
while (true) {
TimeUnit.MILLISECONDS.sleep(100);
logger.info("run..");
}
} catch (InterruptedException e) {
logger.info("sleep() interrupted");
}
}
}

public class MainThread {
Logger logger = LoggerFactory.getLogger(MainThread.class);
@Test
public void test() {
for(int i = 0; i < 5; i++) {
Thread daemon = new Thread(new SimpleDaemons());
daemon.setDaemon(true); // Must call before start()
daemon.start();
}
logger.info("All daemons started");
try {
TimeUnit.MILLISECONDS.sleep(175);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

运行结果：



情况3：需求是子线程执行结束，主线程等待启动的子线程都结束之后再结束



代码：

public class IsAliveTest {
public static void main(String args[]) throws Exception {

Thread t = new Thread(new ThreadDemo());
// this will call run() function
t.start();

// waits for this thread to die
t.join();

// tests if this thread is alive
System.out.println("thread t status = " + t.isAlive());
System.out.println("thread main status = " + Thread.currentThread().isAlive());
}
}

class ThreadDemo implements Runnable {

public void run() {

Thread t = Thread.currentThread();
// tests if this thread is alive
System.out.println("status = " + t.isAlive());
try {
t.sleep(3000L);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

测试结果



什么情况下一个java thread reach the ‘Die’ state?

 

From the ThreadAPI, here is a complete list:

  1. If the run() method returns. 例如join()之后

  2. If an exception is thrown that propagates beyond the run method.

  3. If it is a daemon thread and all non-daemonthreads have 'died' 非后台线程都结束

  4. If the exit method of class Runtime has been called (even at another thread).

实际上，我们对Thread类没有什么控制权，我们几乎不能设置线程的任何状态，我们只能创建任务，并通过某种方式使用线程驱动这个任务

所以，在编写多线程代码的时候，遵循规则就变得非常重要