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[Java教程]Java ThreadLocal


ThreadLocal类,代表一个线程局部变量,通过把数据放在ThreadLocal中,可以让每个线程创建一个该变量的副本。也可以看成是线程同步的另一种方式吧,通过为每个线程创建一个变量的线程本地副本,从而避免并发线程同时读写同一个变量资源时的冲突。

示例如下:

import java.util.Random;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;import com.sun.javafx.webkit.Accessor;public class ThreadLocalTest {  static class ThreadLocalVariableHolder {    private static ThreadLocal<Integer> value = new ThreadLocal<Integer>() {      private Random random = new Random();            protected synchronized Integer initialValue() {        return random.nextInt(10000);      }    };        public static void increment() {      value.set(value.get() + 1);    }        public static int get() {      return value.get();    }  }    static class Accessor implements Runnable{    private final int id;        public Accessor(int id) {      this.id = id;    }        @Override    public void run() {      while (!Thread.currentThread().isInterrupted()) {        ThreadLocalVariableHolder.increment();        System.out.println(this);        Thread.yield();      }    }        @Override    public String toString() {      return "#" + id + ": " + ThreadLocalVariableHolder.get();    }      }    public static void main(String[] args) {    ExecutorService executorService = Executors.newCachedThreadPool();    for (int i = 0; i < 5; i++) {      executorService.execute(new Accessor(i));    }    try {      TimeUnit.MICROSECONDS.sleep(1);    } catch (InterruptedException e) {      e.printStackTrace();    }    executorService.shutdownNow();  }}

运行结果:

#1: 9685#1: 9686#2: 138#2: 139#2: 140#2: 141#0: 5255。。。

由运行结果可知,各线程都用于各自的Local变量,并各自读写互不干扰。

ThreadLocal共提供了三个方法来操作,set,get和remove。

  • 在Android 中的Looper,即使用了ThreadLocal来为每个线程都创建各自独立的Looper对象。
public final class Looper {  private static final String TAG = "Looper";  // sThreadLocal.get() will return null unless you've called prepare().  static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();  private static void prepare(boolean quitAllowed) {    if (sThreadLocal.get() != null) {      throw new RuntimeException("Only one Looper may be created per thread");    }    sThreadLocal.set(new Looper(quitAllowed));  }    。。。}

当某个线程需要自己的Looper及消息队列时,就调用Looper.prepare(),它会为线程创建属于线程的Looper对象及MessageQueue,并将Looper对象保存在ThreadLocal中。