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。