文章目錄

• • java中Thread的狀態
  • NEW
  • Runnable
  • BLOCKED
  • WAITING
  • TIMED_WAITING
  • TERMINATED

java中線程的生命周期

線程是java中繞不過去的一個話題， 今天本文將會詳細講解java中線程的生命周期，希望可以給大家一些啟發。

java中Thread的狀態

java中Thread有6種狀態，分別是：

NEW - 新創建的Thread，還沒有開始執行

RUNNABLE - 可運行狀態的Thread，包括準備運行和正在運行的。

BLOCKED - 正在等待資源鎖的線程

WAITING - 正在無限期等待其他線程來執行某個特定操作

TIMED_WAITING - 在一定的時間內等待其他線程來執行某個特定操作

TERMINATED - 線程執行完畢

我們可以用一個圖來直觀的表示：

JDK代碼中的定義如下：

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 表示線程創建了，但是還沒有開始執行。我們看一個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() {} }

上面的代碼將會輸出：

NEW

Runnable

Runnable表示線程正在可執行狀態。包括正在運行和準備運行兩種。

為什么這兩種都叫做Runnable呢？我們知道在多任務環境中，CPU的個數是有限的，所以任務都是輪循占有CPU來處理的，JVM中的線程調度器會為每個線程分配特定的執行時間，當執行時間結束后，線程調度器將會釋放CPU，以供其他的Runnable線程執行。

我們看一個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());} }

上面的代碼將會輸出：

RUNNABLE

BLOCKED

BLOCKED表示線程正在等待資源鎖，而目前該資源正在被其他線程占有。

我們舉個例子：

public class BlockThread implements Runnable {@Overridepublic void run() {loopResource();}public static synchronized void loopResource() {while(true) {//無限循環}}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是無限循環，將會一直占有資源鎖，導致t2無法獲取資源鎖，從而位于BLOCKED狀態。

我們會得到如下結果：

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

WAITING

WAITING 狀態表示線程正在等待其他的線程執行特定的操作。有三種方法可以導致線程處于WAITTING狀態：

object.wait()

thread.join()

LockSupport.park()

其中1，2方法不需要傳入時間參數。

我們看下使用的例子：

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();} }

在這個例子中，我們調用的t2.join()，這會使調用它的t1線程處于WAITTING狀態。

我們看下輸出結果：

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狀態表示在一個有限的時間內等待其他線程執行特定的某些操作。

java中有5中方式來達到這種狀態：

thread.sleep(long millis)

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

thread.join(long millis)

LockSupport.parkNanos

LockSupport.parkUntil

我們舉個例子：

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());} }

上面的例子中我們調用了Thread.sleep(5000)來讓線程處于TIMED_WAITING狀態。

看下輸出：

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

那么問題來了，TIMED_WAITING和WAITTING有什么區別呢？

TIMED_WAITING如果在給定的時間內沒有等到其他線程的特定操作，則會被喚醒，從而進入爭奪資源鎖的隊列，如果能夠獲取到鎖，則會變成Runnable狀態，如果獲取不到鎖，則會變成BLOCKED狀態。

TERMINATED

TERMINATED表示線程已經執行完畢。我們看下例子：

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());} }

輸出結果：

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

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

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更多教程請參考 flydean的博客

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