ThreadLocal分析详解编程语言

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:29 
Thread-1 After Consume left:29 
Thread-3 After Consume left:29 
Thread-2 After Consume left:28 
Thread-1 After Consume left:28 
Thread-3 After Consume left:28 
Thread-2 After Consume left:27 
Thread-1 After Consume left:27 
Thread-2 After Consume left:26 
Thread-3 After Consume left:27 
Thread-1 After Consume left:25 
Thread-2 After Consume left:25 
Thread-3 After Consume left:25 
Thread-1 After Consume left:24 
Thread-2 After Consume left:24 
Thread-3 After Consume left:24 
Thread-1 After Consume left:23 
Thread-2 After Consume left:23 
Thread-3 After Consume left:23 
Thread-1 After Consume left:22 
Thread-2 After Consume left:22 
Thread-3 After Consume left:22 
Thread-1 After Consume left:21 
Thread-2 After Consume left:21 
Thread-3 After Consume left:21 
Thread-1 After Consume left:20 
Thread-2 After Consume left:20 
Thread-3 After Consume left:20 
Thread-1 After Consume left:19 
Thread-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:29 
Thread-3 After Consume left:29 
Thread-2 After Consume left:29 
Thread-1 After Consume left:28 
Thread-3 After Consume left:28 
Thread-2 After Consume left:28 
Thread-1 After Consume left:27 
Thread-3 After Consume left:27 
Thread-2 After Consume left:27 
Thread-1 After Consume left:26 
Thread-3 After Consume left:26 
Thread-2 After Consume left:26 
Thread-1 After Consume left:25 
Thread-3 After Consume left:25 
Thread-2 After Consume left:25 
Thread-1 After Consume left:24 
Thread-3 After Consume left:24 
Thread-2 After Consume left:24 
Thread-1 After Consume left:23 
Thread-3 After Consume left:23 
Thread-2 After Consume left:23 
Thread-1 After Consume left:22 
Thread-3 After Consume left:22 
Thread-2 After Consume left:22 
Thread-1 After Consume left:21 
Thread-3 After Consume left:21 
Thread-2 After Consume left:21 
Thread-1 After Consume left:20 
Thread-3 After Consume left:20 
Thread-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。