java中线程的生命周期

文章目录

java中Thread的状态
NEW
Runnable
BLOCKED
WAITING
TIMED_WAITING
TERMINATED

java中线程的生命周期

线程是java中绕不过去的一个话题，今天本文将会详细讲解java中线程的生命周期，希望可以给大家一些启发。

java中Thread的状态

java中Thread有6种状态，分别是：

NEW - 新创建的Thread，还没有开始执行
RUNNABLE - 可运行状态的Thread，包括准备运行和正在运行的。
BLOCKED - 正在等待资源锁的线程
WAITING - 正在无限期等待其他线程来执行某个特定操作
TIMED_WAITING - 在一定的时间内等待其他线程来执行某个特定操作
TERMINATED - 线程执行完毕

我们可以用一个图来直观的表示：

JDK代码中的定义如下：

public enum State {/*** Thread state for a thread which has not yet started.*/NEW,/*** Thread state for a runnable thread.  A thread in the runnable* state is executing in the Java virtual machine but it may* be waiting for other resources from the operating system* such as processor.*/RUNNABLE,/*** Thread state for a thread blocked waiting for a monitor lock.* A thread in the blocked state is waiting for a monitor lock* to enter a synchronized block/method or* reenter a synchronized block/method after calling* {@link Object#wait() Object.wait}.*/BLOCKED,/*** Thread state for a waiting thread.* A thread is in the waiting state due to calling one of the* following methods:* <ul>*   <li>{@link Object#wait() Object.wait} with no timeout</li>*   <li>{@link #join() Thread.join} with no timeout</li>*   <li>{@link LockSupport#park() LockSupport.park}</li>* </ul>** <p>A thread in the waiting state is waiting for another thread to* perform a particular action.** For example, a thread that has called <tt>Object.wait()</tt>* on an object is waiting for another thread to call* <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on* that object. A thread that has called <tt>Thread.join()</tt>* is waiting for a specified thread to terminate.*/WAITING,/*** Thread state for a waiting thread with a specified waiting time.* A thread is in the timed waiting state due to calling one of* the following methods with a specified positive waiting time:* <ul>*   <li>{@link #sleep Thread.sleep}</li>*   <li>{@link Object#wait(long) Object.wait} with timeout</li>*   <li>{@link #join(long) Thread.join} with timeout</li>*   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>*   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>* </ul>*/TIMED_WAITING,/*** Thread state for a terminated thread.* The thread has completed execution.*/TERMINATED;}

NEW

NEW 表示线程创建了，但是还没有开始执行。我们看一个NEW的例子：

public class NewThread implements Runnable{public static void main(String[] args) {Runnable runnable = new NewThread();Thread t = new Thread(runnable);log.info(t.getState().toString());}@Overridepublic void run() {}
}

上面的代码将会输出：

NEW

Runnable

Runnable表示线程正在可执行状态。包括正在运行和准备运行两种。

为什么这两种都叫做Runnable呢？我们知道在多任务环境中，CPU的个数是有限的，所以任务都是轮循占有CPU来处理的，JVM中的线程调度器会为每个线程分配特定的执行时间，当执行时间结束后，线程调度器将会释放CPU，以供其他的Runnable线程执行。

我们看一个Runnable的例子：

public class RunnableThread implements Runnable {@Overridepublic void run() {}public static void main(String[] args) {Runnable runnable = new RunnableThread();Thread t = new Thread(runnable);t.start();log.info(t.getState().toString());}
}

上面的代码将会输出：

RUNNABLE

BLOCKED

BLOCKED表示线程正在等待资源锁，而目前该资源正在被其他线程占有。

我们举个例子：

public class BlockThread implements Runnable {@Overridepublic void run() {loopResource();}public static synchronized void loopResource() {while(true) {//无限循环}}public static void main(String[] args) throws InterruptedException {Thread t1 = new Thread(new BlockThread());Thread t2 = new Thread(new BlockThread());t1.start();t2.start();Thread.sleep(1000);log.info(t1.getState().toString());log.info(t2.getState().toString());System.exit(0);}
}

上面的例子中，由于t1是无限循环，将会一直占有资源锁，导致t2无法获取资源锁，从而位于BLOCKED状态。

我们会得到如下结果：

12:40:11.710 [main] INFO com.flydean.BlockThread - RUNNABLE
12:40:11.713 [main] INFO com.flydean.BlockThread - BLOCKED

WAITING

WAITING 状态表示线程正在等待其他的线程执行特定的操作。有三种方法可以导致线程处于WAITTING状态：

object.wait()
thread.join()
LockSupport.park()

其中1，2方法不需要传入时间参数。

我们看下使用的例子：

public class WaitThread implements  Runnable{public static Thread t1;@Overridepublic void run() {Thread t2 = new Thread(()->{try {Thread.sleep(10000);} catch (InterruptedException e) {Thread.currentThread().interrupt();log.error("Thread interrupted", e);}log.info("t1"+t1.getState().toString());});t2.start();try {t2.join();} catch (InterruptedException e) {Thread.currentThread().interrupt();log.error("Thread interrupted", e);}log.info("t2"+t2.getState().toString());}public static void main(String[] args) {t1 = new Thread(new WaitThread());t1.start();}
}

在这个例子中，我们调用的t2.join()，这会使调用它的t1线程处于WAITTING状态。

我们看下输出结果：

12:44:12.958 [Thread-1] INFO com.flydean.WaitThread - t1 WAITING
12:44:12.964 [Thread-0] INFO com.flydean.WaitThread - t2 TERMINATED

TIMED_WAITING

TIMED_WAITING状态表示在一个有限的时间内等待其他线程执行特定的某些操作。

java中有5中方式来达到这种状态：

thread.sleep(long millis)
wait(int timeout) 或者 wait(int timeout, int nanos)
thread.join(long millis)
LockSupport.parkNanos
LockSupport.parkUntil

我们举个例子：

public class TimedWaitThread implements  Runnable{@Overridepublic void run() {try {Thread.sleep(5000);} catch (InterruptedException e) {Thread.currentThread().interrupt();log.error("Thread interrupted", e);}}public static void main(String[] args) throws InterruptedException {TimedWaitThread obj1 = new TimedWaitThread();Thread t1 = new Thread(obj1);t1.start();// The following sleep will give enough time for ThreadScheduler// to start processing of thread t1Thread.sleep(1000);log.info(t1.getState().toString());}
}

上面的例子中我们调用了Thread.sleep(5000)来让线程处于TIMED_WAITING状态。

看下输出：

12:58:02.706 [main] INFO com.flydean.TimedWaitThread - TIMED_WAITING

那么问题来了，TIMED_WAITING和WAITTING有什么区别呢？

TIMED_WAITING如果在给定的时间内没有等到其他线程的特定操作，则会被唤醒，从而进入争夺资源锁的队列，如果能够获取到锁，则会变成Runnable状态，如果获取不到锁，则会变成BLOCKED状态。

TERMINATED

TERMINATED表示线程已经执行完毕。我们看下例子：

public class TerminatedThread implements Runnable{@Overridepublic void run() {}public static void main(String[] args) throws InterruptedException {Thread t1 = new Thread(new TerminatedThread());t1.start();// The following sleep method will give enough time for// thread t1 to completeThread.sleep(1000);log.info(t1.getState().toString());}
}

输出结果：

13:02:38.868 [main] INFO com.flydean.TerminatedThread - TERMINATED

本文的例子可以参考https://github.com/ddean2009/learn-java-concurrency/tree/master/thread-lifecycle

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