Java并发编程实战（5）- 线程生命周期

在这篇文章中，我们来聊一下线程的生命周期。[toc]# 概述线程是操作系统中的一个概念，在Java中，它是实现并发程序的主要手段。Java中的线程，本质上就是操作系统中的线程。操作系统中的线程有“生老病死”，专业说法就是生命周期，虽然不同的开发语言对于操作系统的线程做了不同的封装，但是对于线程生命周期来说，基本上是大同小异的。**我们在学习线程的生命周期时，只要能理解生命周期中各个节点的状态转换机制就可以了。**# 操作系统中的线程生命周期操作系统中的线程有5种状态，可以用下面的图进行描述，该图也被称为五态模型。线程的五种状态描述如下：* **初始状态**。这是指线程已经被创建，但是还不允许分配CPU执行。这个状态属于编程语言特有的，也就是说，这里所谓的被创建，仅仅是在编程语言层面上被创建，在操作系统层面上，线程是还没有被创建的。* **可运行状态**。这是指线程可以分配CPU执行。在这个状态下，操作系统已经创建了线程，正在等待分配CPU。* **运行状态**。这是指CPU有空闲，操作系统将其分配给一个处于可运行状态的线程，被分配到CPU的线程，就可以正常执行线程中的逻辑了。* **休眠状态**。 这是指处于运行状态的线程调用了一个阻塞API或者等待某个事件。在休眠状态下，线程会释放CPU使用权，*处于休眠状态的线程，是永远没有机会获得CPU使用权的。* 当等待的事件出现了，线程就会从休眠状态转换到可运行状态，等待CPU重新分配。* **终止状态**。这是指线程执行完成或者抛出异常。终止状态的线程不会切换到其他任何状态，这也意味着进行终止状态的线程的生命周期结束了。# Java中的线程生命周期Java中的线程生命周期，基于操作系统的线程生命周期进行了定制，它包括六种状态：* NEW（初始化状态）* RUNNABLE（可运行/运行状态）* BLOCKED（阻塞状态）* WAITING（无时限等待）* TIMED_WAITING（有时限等待）* TERMINATED（终止状态）Java中的线程生命周期可以用下图来描述。和操作系统线程生命周期相比，Java中的线程生命周期主要有以下2个改动：* Java线程中对可运行状态和运行状态进行了合并。* Java线程中的休眠状态被细化为：阻塞状态、无时限等待和有时限等待。# Java线程状态转换Java线程状态中的阻塞、无时限等待和有时限等待可以理解为线程导致休眠状态的三种原因，我们来看一下这些状态之间是怎么转换的。## 运行状态和阻塞状态之间的转换在Java中，只有一种情况会出现这种状态转换：**线程等待synchronized隐式锁**。synchronized修饰的方法、代码块同一时刻只允许一个线程执行，其他线程只能等待，在这种情况下，等待的线程会从运行状态转换到阻塞状态，而当等待的线程获得synchronized锁后，状态会从阻塞状态转换为运行状态。线程调用阻塞式API时，会切换到阻塞状态吗？> 在操作系统层面，线程是会切换到休眠状态的，但是在JVM层面，Java线程的状态不会切换，也就说Java线程依然是运行状态。**JVM不关心操作系统调度相关的状态**。在JVM看来，等待CPU使用权和等待I/O没有区别，都是在等待某个资源，所以都属于可运行/运行状态。我们平时说的Java调用阻塞式API时，线程会被阻塞，我们指的是操作系统线程状态，而不是Java线程状态，这一点需要分清楚。## 运行状态和无时限等待状态的切换以下三种情况会触发运行状态和无时限等待状态的切换。* 获得synchronized锁的线程，调用了无参数的Object.wait()方法。* 调用无参数的Thread.join()方法。* 调用LockSupport.park()方法。## 运行状态和有时限等待状态的切换有时限等待和无时限等待的主要区别，在于触发条件中添加了超时参数。以下五种情况会触发运行状态和有时限等待状态的切换。* 调用带超时参数的Thread.sleep(long millis)方法。* 获得synchronized锁的线程，调用带超时参数的Object.wait(long timeout)方法。* 调用带超时参数的Thread.join(long millis)方法。* 调用带超时参数的LocakSupport.parkNanos(Object blocker, long deadline)方法。* 调用带超时参数的LockSupport.parkUntil(long deadlinie)方法。## 初始化状态和运行状态的切换Java刚创建出来的Thread对象就是初始化状态，有两种可以创建线程的方法：* 继承Thread类* 实现Runnable接口初始化状态的线程，并不会被操作系统调度，因此不会被执行。在调用线程对象的start()方法后，线程就会从初始化状态切换到运行状态。## 运行状态和终止状态的切换线程在以下两种情况时会自动切换到终止状态：* 正常执行完run()方法* run()方法中抛出异常### 手动终止线程我们有2种方法终止线程：* 调用stop()方法* 调用interrupt()方法我们不推荐使用stop()方法，在JDK中，它已经被标记为Deprecated。我们推荐使用interrupt()方法来终止线程。stop()方法和interrupt()方法的区别：* stop()方法会直接杀死线程，不给线程喘息的机会，如果此时线程持有锁，那么这个锁不会被释放，其他线程也没有办法获取这个锁。* interrupt()方法只是通知该线程，线程有机会执行一些后续操作，同时也可以无视这个通知。被调用了interrupt()方法的线程，有以下2种方式接收通知：* 异常，处于有时限等待或者无时限等待状态的线程， 在被调用interrupt()方法后，线程会返回运行状态，但同时会抛出InterruptedException。* 主动监测，线程可以调用isInterrupted()方法，来判断自己是不是被中断了。# 使用jstack查看多线程状态在查看了Java线程生命周期中的状态以及状态之间的切换后，我们来使用jstack来查看一下真实运行的线程的状态。我们以一个死锁的程序为例，来说明如何使用jstack。我们在解释互斥锁和死锁的时候，写了一些死锁示例，代码如下。“`public class BankTransferDemo {public void transfer(BankAccount sourceAccount, BankAccount targetAccount, double amount) {synchronized(sourceAccount) {synchronized(targetAccount) {if (sourceAccount.getBalance() > amount) {System.out.println(\”Start transfer.\”);System.out.println(String.format(\”Before transfer, source balance:%s, target balance:%s\”, sourceAccount.getBalance(), targetAccount.getBalance()));sourceAccount.setBalance(sourceAccount.getBalance() – amount);targetAccount.setBalance(targetAccount.getBalance() + amount);System.out.println(String.format(\”After transfer, source balance:%s, target balance:%s\”, sourceAccount.getBalance(), targetAccount.getBalance()));}}}}}“““public static void main(String[] args) throws InterruptedException {BankAccount sourceAccount = new BankAccount();sourceAccount.setId(1);sourceAccount.setBalance(50000);BankAccount targetAccount = new BankAccount();targetAccount.setId(2);targetAccount.setBalance(20000);BankTransferDemo obj = new BankTransferDemo();Thread t1 = new Thread(() ->{for (int i = 0; i {for (int i = 0; i < 10000; i++) {obj.transfer(targetAccount, sourceAccount, 1);}});t1.start();t2.start();t1.join();t2.join();System.out.println(\"Finished.\");}“`上述代码在运行过程中，因为资源争抢的原因，最后会进入死锁状态，下面我们来看一下如何使用jstack来获取具体信息。“`(base) ➜ ~ jstack -l 63044“`请注意上述的`63044`是运行的pid，运行程序多次产生的pid是不一样的。jstack的返回结果如下。“`2021-01-15 19:56:28Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.251-b08 mixed mode):\"RMI TCP Accept-0\" #14 daemon prio=9 os_prio=31 tid=0x00007fb1d80b6000 nid=0x5803 runnable [0x00007000059d8000]java.lang.Thread.State: RUNNABLEat java.net.PlainSocketImpl.socketAccept(Native Method)at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:409)at java.net.ServerSocket.implAccept(ServerSocket.java:545)at java.net.ServerSocket.accept(ServerSocket.java:513)at sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:52)at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:405)at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:377)at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:- None\"Attach Listener\" #12 daemon prio=9 os_prio=31 tid=0x00007fb1db03d800 nid=0x3617 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"Thread-1\" #11 prio=5 os_prio=31 tid=0x00007fb1db04e800 nid=0xa603 waiting for monitor entry [0x00007000057d2000]java.lang.Thread.State: BLOCKED (on object monitor)at com.concurrency.demo.BankTransferDemo.transfer(BankTransferDemo.java:8)- waiting to lock (a com.concurrency.demo.BankAccount)- locked (a com.concurrency.demo.BankAccount)at com.concurrency.demo.BankTransferDemo.lambda$1(BankTransferDemo.java:38)at com.concurrency.demo.BankTransferDemo$$Lambda$2/1044036744.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:- None\"Thread-0\" #10 prio=5 os_prio=31 tid=0x00007fb1d896e000 nid=0xa703 waiting for monitor entry [0x00007000056cf000]java.lang.Thread.State: BLOCKED (on object monitor)at com.concurrency.demo.BankTransferDemo.transfer(BankTransferDemo.java:8)- waiting to lock (a com.concurrency.demo.BankAccount)- locked (a com.concurrency.demo.BankAccount)at com.concurrency.demo.BankTransferDemo.lambda$0(BankTransferDemo.java:32)at com.concurrency.demo.BankTransferDemo$$Lambda$1/135721597.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:- None\"Service Thread\" #9 daemon prio=9 os_prio=31 tid=0x00007fb1de809000 nid=0x5503 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"C1 CompilerThread3\" #8 daemon prio=9 os_prio=31 tid=0x00007fb1df80a800 nid=0x3b03 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"C2 CompilerThread2\" #7 daemon prio=9 os_prio=31 tid=0x00007fb1df80a000 nid=0x3a03 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"C2 CompilerThread1\" #6 daemon prio=9 os_prio=31 tid=0x00007fb1df809000 nid=0x3e03 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"C2 CompilerThread0\" #5 daemon prio=9 os_prio=31 tid=0x00007fb1df008800 nid=0x3803 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"Signal Dispatcher\" #4 daemon prio=9 os_prio=31 tid=0x00007fb1de808800 nid=0x4103 runnable [0x0000000000000000]java.lang.Thread.State: RUNNABLELocked ownable synchronizers:- None\"Finalizer\" #3 daemon prio=8 os_prio=31 tid=0x00007fb1d8810800 nid=0x3203 in Object.wait() [0x0000700004db1000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)- locked (a java.lang.ref.ReferenceQueue$Lock)at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)Locked ownable synchronizers:- None\"Reference Handler\" #2 daemon prio=10 os_prio=31 tid=0x00007fb1d900b000 nid=0x3103 in Object.wait() [0x0000700004cae000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on (a java.lang.ref.Reference$Lock)at java.lang.Object.wait(Object.java:502)at java.lang.ref.Reference.tryHandlePending(Reference.java:191)- locked (a java.lang.ref.Reference$Lock)at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)Locked ownable synchronizers:- None\"main\" #1 prio=5 os_prio=31 tid=0x00007fb1db809000 nid=0x1003 in Object.wait() [0x000070000408a000]java.lang.Thread.State: WAITING (on object monitor)at java.lang.Object.wait(Native Method)- waiting on (a java.lang.Thread)at java.lang.Thread.join(Thread.java:1252)- locked (a java.lang.Thread)at java.lang.Thread.join(Thread.java:1326)at com.concurrency.demo.BankTransferDemo.main(BankTransferDemo.java:45)Locked ownable synchronizers:- None\"VM Thread\" os_prio=31 tid=0x00007fb1db821000 nid=0x4c03 runnable\"GC task thread#0 (ParallelGC)\" os_prio=31 tid=0x00007fb1db809800 nid=0x1f07 runnable\"GC task thread#1 (ParallelGC)\" os_prio=31 tid=0x00007fb1d8008800 nid=0x1b03 runnable\"GC task thread#2 (ParallelGC)\" os_prio=31 tid=0x00007fb1db009000 nid=0x1d03 runnable\"GC task thread#3 (ParallelGC)\" os_prio=31 tid=0x00007fb1db009800 nid=0x2a03 runnable\"GC task thread#4 (ParallelGC)\" os_prio=31 tid=0x00007fb1db00a000 nid=0x2c03 runnable\"GC task thread#5 (ParallelGC)\" os_prio=31 tid=0x00007fb1db00a800 nid=0x2d03 runnable\"GC task thread#6 (ParallelGC)\" os_prio=31 tid=0x00007fb1db80a000 nid=0x5203 runnable\"GC task thread#7 (ParallelGC)\" os_prio=31 tid=0x00007fb1db00b800 nid=0x5003 runnable\"GC task thread#8 (ParallelGC)\" os_prio=31 tid=0x00007fb1db00c000 nid=0x4f03 runnable\"GC task thread#9 (ParallelGC)\" os_prio=31 tid=0x00007fb1d900a800 nid=0x4d03 runnable\"VM Periodic Task Thread\" os_prio=31 tid=0x00007fb1d8028800 nid=0xa803 waiting on conditionJNI global references: 333Found one Java-level deadlock:=============================\"Thread-1\":waiting to lock monitor 0x00007fb1db8270a8 (object 0x000000076ab76ef0, a com.concurrency.demo.BankAccount),which is held by \"Thread-0\"\"Thread-0\":waiting to lock monitor 0x00007fb1db827158 (object 0x000000076ab76f10, a com.concurrency.demo.BankAccount),which is held by \"Thread-1\"Java stack information for the threads listed above:===================================================\"Thread-1\":at com.concurrency.demo.BankTransferDemo.transfer(BankTransferDemo.java:8)- waiting to lock (a com.concurrency.demo.BankAccount)- locked (a com.concurrency.demo.BankAccount)at com.concurrency.demo.BankTransferDemo.lambda$1(BankTransferDemo.java:38)at com.concurrency.demo.BankTransferDemo$$Lambda$2/1044036744.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)\"Thread-0\":at com.concurrency.demo.BankTransferDemo.transfer(BankTransferDemo.java:8)- waiting to lock (a com.concurrency.demo.BankAccount)- locked (a com.concurrency.demo.BankAccount)at com.concurrency.demo.BankTransferDemo.lambda$0(BankTransferDemo.java:32)at com.concurrency.demo.BankTransferDemo$$Lambda$1/135721597.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)Found 1 deadlock.“`我们从中可以看到线程的状态有RUNNABLE，WAITING，BLOCKED，例如：“`\"Thread-0\" #10 prio=5 os_prio=31 tid=0x00007fb1d896e000 nid=0xa703 waiting for monitor entry [0x00007000056cf000]java.lang.Thread.State: BLOCKED (on object monitor)at com.concurrency.demo.BankTransferDemo.transfer(BankTransferDemo.java:8)- waiting to lock (a com.concurrency.demo.BankAccount)- locked (a com.concurrency.demo.BankAccount)at com.concurrency.demo.BankTransferDemo.lambda$0(BankTransferDemo.java:32)at com.concurrency.demo.BankTransferDemo$$Lambda$1/135721597.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)Locked ownable synchronizers:- None“`下面是死锁的相关信息：“`Found one Java-level deadlock:=============================\"Thread-1\":waiting to lock monitor 0x00007fb1db8270a8 (object 0x000000076ab76ef0, a com.concurrency.demo.BankAccount),which is held by \"Thread-0\"\"Thread-0\":waiting to lock monitor 0x00007fb1db827158 (object 0x000000076ab76f10, a com.concurrency.demo.BankAccount),which is held by \"Thread-1\"“`从上面的描述中，我们可以清楚的看到2个线程在互相等待对方持有的锁对象。jstack是一个非常实用的工具，我会在后面找机会详细的说明如何使用它和其他相关工具。