Java ThreadLocal，javathreadlocal

Java ThreadLocal，javathreadlocal

ThreadLocal類，代表一個線程局部變數，通過把資料放在ThreadLocal中，可以讓每個線程建立一個該變數的副本。也可以看成是線程同步的另一種方式吧，通過為每個線程建立一個變數的執行緒區域副本，從而避免並發線程同時讀寫同一個變數資源時的衝突。

樣本如下：

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變數，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來為每個線程都建立各自獨立的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及訊息佇列時，就調用Looper.prepare()，它會為線程建立屬於線程的Looper對象及MessageQueue，並將Looper對象儲存在ThreadLocal中。