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 value = new ThreadLocal() {
            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 sThreadLocal = new ThreadLocal();

    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。