java 中ThreadLocal實例分析
從概念上理解，threadlocal使變量在多個線程中相互隔離實現(xiàn)線程安全，threadlocal包裝的變量最終都專屬于對應(yīng)的每個線程，線程之間相互獨立，用一個具體實現(xiàn)來說明：
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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是在沒有做任何線程安全處理，結(jié)果如下：
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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，每個線程相互獨立，實現(xiàn)如下：
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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(); }} |
運行的結(jié)果如下：
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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 |
每個線程擁有自己的獨立變量，相互隔離實現(xiàn)線程安全。
那ThreadLocal是怎樣實現(xiàn)這種線程隔離的線程安全的呢？
從ThreadLocal源碼可以看到，真正實現(xiàn)線程隔離，與線程掛鉤的，其實是ThreadLocal.ThreadLocalMap這個實現(xiàn)類，最明顯的體現(xiàn)就在于Thread類源碼的這樣一個變量申明說明了ThreadLocal.ThreadLocalMap與Thread的關(guān)系：
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ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals; |
Thread類是包含threadLocals對象的，ThreadLocal的具體實現(xiàn)就是根據(jù)提供的get,set等接口，對當前thread的threadLocals變量進行相關(guān)操作的，如get操作代碼如下：
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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()方法就是從當前thread獲取對應(yīng)的threadLocals變量，然后從這個ThreadLocal.ThreadLocalMap類型的threadLocals變量中獲取對應(yīng)線程中該ThreadLocal對象對應(yīng)的變量值。
set方法的操作也是一樣：
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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);} |
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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Entry的數(shù)組,Entry是繼承WeakReference實現(xiàn)，WeakReference的好處是保存對象引用，而又不干擾該對象被GC回收，線程執(zhí)行完回收threadLocals變量時不會受到Entry封裝的變量的干擾。
而且ThreadLocalMap中的key是ThreadLocal，所以一個ThreadLocal對象只能在一個Thread對象中保存一個ThreadLocal的value。
綜上，很多人說ThreadLocal的實現(xiàn)是ThreadLocalMap中存Thread對象為key，變量為value的map結(jié)構(gòu)，其實是錯誤的。
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原文鏈接：http://blog.csdn.net/jamesjxin/article/details/45173201
