ThreadLocal類，代表一個線程局部變量，通過把數(shù)據(jù)放在ThreadLocal中，可以讓每個線程創(chuàng)建一個該變量的副本。也可以看成是線程同步的另一種方式吧，通過為每個線程創(chuàng)建一個變量的線程本地副本，從而避免并發(fā)線程同時讀寫同一個變量資源時的沖突。

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();
 }

}

運行結(jié)果：

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

由運行結(jié)果可知，各線程都用于各自的Local變量，并各自讀寫互不干擾。

ThreadLocal共提供了三個方法來操作，set,get和remove。

在Android 中的Looper，即使用了ThreadLocal來為每個線程都創(chuàng)建各自獨立的Looper對象。

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));
 }
 
 。。。
}

當(dāng)某個線程需要自己的Looper及消息隊列時，就調(diào)用Looper.prepare()，它會為線程創(chuàng)建屬于線程的Looper對象及MessageQueue，并將Looper對象保存在ThreadLocal中。

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