java 中ThreadLocal實(shí)例分析
從概念上理解，threadlocal使變量在多個(gè)線程中相互隔離實(shí)現(xiàn)線程安全，threadlocal包裝的變量最終都專(zhuān)屬于對(duì)應(yīng)的每個(gè)線程，線程之間相互獨(dú)立，用一個(gè)具體實(shí)現(xiàn)來(lái)說(shuō)明：
|
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
|
public interface Consumer { int consume();}public class ComsumeThread implements Runnable { private Consumer consumer; public ComsumeThread(Consumer consumer) { this.consumer = consumer; } @Override public void run() { for(int i=0;i<10;i++){ System.out.println(Thread.currentThread().getName()+" After Consume left:"+consumer.consume()); } }}public class ConsumeClientA implements Consumer { private static int leftNum = 30; @Override public int consume() { int orgLeftNum = leftNum; Random random = new Random(System.currentTimeMillis()); try { Thread.sleep(random.nextInt(3)); } catch (InterruptedException e) { e.printStackTrace(); } orgLeftNum = orgLeftNum -1; leftNum = orgLeftNum; return leftNum; } public static void main(String[] args){ Consumer consumer = new ConsumeClientA(); Thread thread1 = new Thread(new ComsumeThread(consumer)); Thread thread2 = new Thread(new ComsumeThread(consumer)); Thread thread3 = new Thread(new ComsumeThread(consumer)); thread1.start(); thread2.start(); thread3.start(); }} |
ConsumeClientA是在沒(méi)有做任何線程安全處理，結(jié)果如下：
|
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
|
Thread-2 After Consume left:29Thread-1 After Consume left:29Thread-3 After Consume left:29Thread-2 After Consume left:28Thread-1 After Consume left:28Thread-3 After Consume left:28Thread-2 After Consume left:27Thread-1 After Consume left:27Thread-2 After Consume left:26Thread-3 After Consume left:27Thread-1 After Consume left:25Thread-2 After Consume left:25Thread-3 After Consume left:25Thread-1 After Consume left:24Thread-2 After Consume left:24Thread-3 After Consume left:24Thread-1 After Consume left:23Thread-2 After Consume left:23Thread-3 After Consume left:23Thread-1 After Consume left:22Thread-2 After Consume left:22Thread-3 After Consume left:22Thread-1 After Consume left:21Thread-2 After Consume left:21Thread-3 After Consume left:21Thread-1 After Consume left:20Thread-2 After Consume left:20Thread-3 After Consume left:20Thread-1 After Consume left:19Thread-3 After Consume left:18 |
增加threadlocal處理，每個(gè)線程相互獨(dú)立，實(shí)現(xiàn)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
|
public class ConsumeClientB implements Consumer { private ThreadLocal<Integer> leftNumThreadLocal = new ThreadLocal<Integer>(){ @Override protected Integer initialValue() { return 30; } }; @Override public int consume() { int orgLeftNum = leftNumThreadLocal.get(); Random random = new Random(System.currentTimeMillis()); try { Thread.sleep(random.nextInt(3)); } catch (InterruptedException e) { e.printStackTrace(); } orgLeftNum = orgLeftNum -1; leftNumThreadLocal.set(orgLeftNum); return leftNumThreadLocal.get(); } public static void main(String[] args){ Consumer consumer = new ConsumeClientB(); Thread thread1 = new Thread(new ComsumeThread(consumer)); Thread thread2 = new Thread(new ComsumeThread(consumer)); Thread thread3 = new Thread(new ComsumeThread(consumer)); thread1.start(); thread2.start(); thread3.start(); }} |
運(yùn)行的結(jié)果如下：
|
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
|
Thread-1 After Consume left:29Thread-3 After Consume left:29Thread-2 After Consume left:29Thread-1 After Consume left:28Thread-3 After Consume left:28Thread-2 After Consume left:28Thread-1 After Consume left:27Thread-3 After Consume left:27Thread-2 After Consume left:27Thread-1 After Consume left:26Thread-3 After Consume left:26Thread-2 After Consume left:26Thread-1 After Consume left:25Thread-3 After Consume left:25Thread-2 After Consume left:25Thread-1 After Consume left:24Thread-3 After Consume left:24Thread-2 After Consume left:24Thread-1 After Consume left:23Thread-3 After Consume left:23Thread-2 After Consume left:23Thread-1 After Consume left:22Thread-3 After Consume left:22Thread-2 After Consume left:22Thread-1 After Consume left:21Thread-3 After Consume left:21Thread-2 After Consume left:21Thread-1 After Consume left:20Thread-3 After Consume left:20Thread-2 After Consume left:20 |
每個(gè)線程擁有自己的獨(dú)立變量，相互隔離實(shí)現(xiàn)線程安全。
那ThreadLocal是怎樣實(shí)現(xiàn)這種線程隔離的線程安全的呢？
從ThreadLocal源碼可以看到，真正實(shí)現(xiàn)線程隔離，與線程掛鉤的，其實(shí)是ThreadLocal.ThreadLocalMap這個(gè)實(shí)現(xiàn)類(lèi)，最明顯的體現(xiàn)就在于Thread類(lèi)源碼的這樣一個(gè)變量申明說(shuō)明了ThreadLocal.ThreadLocalMap與Thread的關(guān)系：
|
1
|
ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals; |
Thread類(lèi)是包含threadLocals對(duì)象的，ThreadLocal的具體實(shí)現(xiàn)就是根據(jù)提供的get,set等接口，對(duì)當(dāng)前thread的threadLocals變量進(jìn)行相關(guān)操作的，如get操作代碼如下：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) return (T)e.value; } return setInitialValue();}ThreadLocal.ThreadLocalMap getMap(Thread t) { return t.threadLocals;} |
可以看到，getMap()方法就是從當(dāng)前thread獲取對(duì)應(yīng)的threadLocals變量，然后從這個(gè)ThreadLocal.ThreadLocalMap類(lèi)型的threadLocals變量中獲取對(duì)應(yīng)線程中該ThreadLocal對(duì)象對(duì)應(yīng)的變量值。
set方法的操作也是一樣：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
public void set(T value) { Thread t = Thread.currentThread(); ThreadLocal.ThreadLocalMap map = getMap(t); if(map != null) { map.set(this, value); } else { this.createMap(t, value); }}void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue);} |
|
1
2
3
4
5
6
7
8
|
static class Entry extends WeakReference<ThreadLocal> { Object value; Entry(ThreadLocal var1, Object var2) { super(var1); this.value = var2; } } |
ThreadLocalMap中存的是內(nèi)部類(lèi)Entry的數(shù)組,Entry是繼承WeakReference實(shí)現(xiàn)，WeakReference的好處是保存對(duì)象引用，而又不干擾該對(duì)象被GC回收，線程執(zhí)行完回收threadLocals變量時(shí)不會(huì)受到Entry封裝的變量的干擾。
而且ThreadLocalMap中的key是ThreadLocal，所以一個(gè)ThreadLocal對(duì)象只能在一個(gè)Thread對(duì)象中保存一個(gè)ThreadLocal的value。
綜上，很多人說(shuō)ThreadLocal的實(shí)現(xiàn)是ThreadLocalMap中存Thread對(duì)象為key，變量為value的map結(jié)構(gòu)，其實(shí)是錯(cuò)誤的。
感謝閱讀，希望能幫助到大家，謝謝大家對(duì)本站的支持！
原文鏈接：http://blog.csdn.net/jamesjxin/article/details/45173201
