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java 中ThreadLocal实例分析

java 中ThreadLocal实例分析

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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 leftNumThreadLocal = new ThreadLocal(){
    @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 {
      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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