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();
- }
- }
- }
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