java 中ThreadLocal实例分析

2024-07-13 10:08:59

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java/138964.html">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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