Java ThreadLocal，javathreadlocal

Java ThreadLocal，javathreadlocal

ThreadLocal類，代表一個線程局部變量，通過把數據放在ThreadLocal中，可以讓每個線程創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，通過為每個線程創建一個變量的線程本地副本，從而避免並發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();
    }

}

運行結果：

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

由運行結果可知，各線程都用於各自的Local變量，並各自讀寫互不干擾。

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));
    }
    
    。。。
}

當某個線程需要自己的Looper及消息隊列時，就調用Looper.prepare()，它會為線程創建屬於線程的Looper對象及MessageQueue，並將Looper對象保存在ThreadLocal中。