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。 | 
