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。
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