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，從而避免并發線程同時讀寫同一個變量資源時的沖突。
示例如下：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
|
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
2
3
4
5
6
7
8
|
#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來為每個線程都創建各自獨立的Looper對象。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
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()，它會為線程創建屬于線程的Looper對象及MessageQueue，并將Looper對象保存在ThreadLocal中。
