9. 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：

1. NEW - 新创建的Thread，还没有开始执行

2. RUNNABLE - 可运行状态的Thread，包括准备运行和正在运行的。

3. BLOCKED - 正在等待资源锁的线程

4. WAITING - 正在无限期等待其他线程来执行某个特定操作

5. TIMED_WAITING - 在一定的时间内等待其他线程来执行某个特定操作

6. 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());
    }

    @Override
    public 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 {
    @Override
    public 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 {
    @Override
    public 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：

1. object.wait()

2. thread.join()

3. 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;
    @Override
    public 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：

1. thread.sleep(long millis)

2. wait(int timeout) 或者 wait(int timeout, int nanos)

3. thread.join(long millis)

4. LockSupport.parkNanos

5. LockSupport.parkUntil

我们举个例子：

public class TimedWaitThread implements  Runnable{
    @Override
    public 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 t1
        Thread.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{
    @Override
    public 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 complete
        Thread.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

更多教程请参考 flydean的博客