java 中ThreadLocal实例分析

从概念上理解，threadlocal使变量在多个线程中相互隔离实现线程安全，threadlocal包装的变量最终都专属于对应的每个线程，线程之间相互独立，用一个具体实现来说明：

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是在没有做任何线程安全处理，结果如下：

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处理，每个线程相互独立，实现如下：

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(); }}

运行的结果如下：

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

每个线程拥有自己的独立变量，相互隔离实现线程安全。

那ThreadLocal是怎样实现这种线程隔离的线程安全的呢？

从ThreadLocal源码可以看到，真正实现线程隔离，与线程挂钩的，其实是ThreadLocal.ThreadLocalMap这个实现类，最明显的体现就在于Thread类源码的这样一个变量申明说明了ThreadLocal.ThreadLocalMap与Thread的关系：

ThreadLocal.ThreadLocalMap threadLocals, inheritableThreadLocals;

Thread类是包含threadLocals对象的，ThreadLocal的具体实现就是根据提供的get,set等接口，对当前thread的threadLocals变量进行相关操作的，如get操作代码如下：

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()方法就是从当前thread获取对应的threadLocals变量，然后从这个ThreadLocal.ThreadLocalMap类型的threadLocals变量中获取对应线程中该ThreadLocal对象对应的变量值。

set方法的操作也是一样：

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); }static class Entry extends WeakReference<ThreadLocal> { Object value; Entry(ThreadLocal var1, Object var2) { super(var1); this.value = var2; } }

ThreadLocalMap中存的是内部类Entry的数组,Entry是继承WeakReference实现，WeakReference的好处是保存对象引用，而又不干扰该对象被GC回收，线程执行完回收threadLocals变量时不会受到Entry封装的变量的干扰。

而且ThreadLocalMap中的key是ThreadLocal，所以一个ThreadLocal对象只能在一个Thread对象中保存一个ThreadLocal的value。

综上，很多人说ThreadLocal的实现是ThreadLocalMap中存Thread对象为key，变量为value的map结构，其实是错误的。

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