JAVA并发学习之ThreadLocal

关于java并发编程的相关文章都是阅读了《java并发编程实战》之后的读书笔记总结

概述

ThreadLocal其实是线程封闭的一种规范化的实现，它通过提供一组get和set的接口为每个使用该变量的线程保存一份独立的副本。对于那种按线程多实例（每个线程对应一个实例）的对象的访问，并且这个对象很多地方都要用到的情况（例如数据库连接管理、会话session管理以及线程私有的消息队列等），ThreadLocal就会展现出它的魅力。

下面的这个小例子展示了ThreadLocal的常规使用：

/**
* 线程资源类
* 这里只有简单的两个标志线程属性的变量,我们还可以往里面增添其他线程的属性(一般是我们对应于那个线程都要使用并且各不相同的属性)
*/
public class ThreadResource {

    private String threadName;
    private int threadId;

    public ThreadResource(int threadId,String threadName){
        this.threadId=threadId;
        this.threadName=threadName;
    }

    public String getThreadName() {
        return threadName;
    }

    public void setThreadName(String threadName) {
        this.threadName = threadName;
    }

    public int getThreadId() {
        return threadId;
    }

    public void setThreadId(int threadId) {
        this.threadId = threadId;
    }
}

public class Test {
    //以一个静态实例的方式持有一个ThreadLocal对象,它里面以map的形式存储了线程的局部变量
    static ThreadLocal resoursePackage=new ThreadLocal(){
        @Override
        protected ThreadResource initialValue() {
            return new ThreadResource(0,"initialThread");
        }
    };

    private static class TestThread extends Thread {
        @Override
        public void run(){
            resoursePackage.set(new ThreadResource(1,"testThread"));
            System.out.println(resoursePackage.get().getThreadName()+ resoursePackage.get().getThreadId());
        }
    }

    public static void main (String[] args){
        System.out.println(resoursePackage.get().getThreadName()+ resoursePackage.get().getThreadId());
        new TestThread().start();
    }
}

源码解析ThreadLocal的实现

• get()方法的实现

  /**
  * Returns the value in the current thread's copy of this
  * thread-local variable.  If the variable has no value for the
  * current thread, it is first initialized to the value returned
  * by an invocation of the {@link #initialValue} method.
  * 
  * @return the current thread's value of this thread-local
  */
  public T get() {        
      //获取当前threadlocal变量所属的线程
      Thread t = Thread.currentThread();
      //根据线程获取到一个ThreadLocalMap的对象
      ThreadLocalMap map = getMap(t);
      //如果线程已经绑定了一个ThreadLocalMap对象的话，则从中获取到里面所保存的值，否则使用初始化的值
      if (map != null) {
          ThreadLocalMap.Entry e = map.getEntry(this);
          if (e != null) {
              @SuppressWarnings("unchecked")
              T result = (T)e.value;
              return result;
          }
      }
      return setInitialValue();
  }
  

  我们看一下上面getMap()的方法的实现

  /**
  * Get the map associated with a ThreadLocal. Overridden in
  * InheritableThreadLocal.
  *
  * @param  t the current thread
  * @return the map
  */
  ThreadLocalMap getMap(Thread t) {
      //返回线程的一个ThreadLocalMap的成员变量,下面是该成员变量在threa类中的声明
      //ThreadLocal values pertaining to this thread. This map is maintained by the ThreadLocal class.
      //ThreadLocal.ThreadLocalMap threadLocals = ull;
      return t.threadLocals;
  }
  

  再看一下线程尚未绑定ThreadLocalMap对象的时候，调用的setInitialValue（）的方法的实现

  /**
  * Variant of set() to establish initialValue. Used instead
  * of set() in case user has overridden the set() method.
  *
  * @return the initial value
  */
  private T setInitialValue() {
      //initialValue（）就是我们在新建一个ThreadLocal变量的时候，实现的protected的那个方法。
      T value = initialValue();
      Thread t = Thread.currentThread();
      ThreadLocalMap map = getMap(t);
      if (map != null)
          map.set(this, value);
      else
          //由此我们可知，线程尚未绑定到ThreadLocalMap对象的时候
          //ThreadLocal为我们使用在initialValue设置的值初始化了一个对象值，并绑定到该线程上。
          createMap(t, value);
      return value;
  }
  

  在上面的代码中，我们一直提及到了一个ThreadLocalMap的类，它其实是在ThreadLocal的一个静态内部类，它是一个ThreadLocal自定义的hash map对象，用于保存线程的局部变量。在它里面，又包含了一个Entry的静态内部类，它里面就是对应的所要存储的值。我们看一下源码的实现

  /**
  * ThreadLocalMap is a customized hash map suitable only for
  * maintaining thread local values. No operations are exported
  * outside of the ThreadLocal class. The class is package private to
  * allow declaration of fields in class Thread.  To help deal with
  * very large and long-lived usages, the hash table entries use
  * WeakReferences for keys. However, since reference queues are not
  * used, stale entries are guaranteed to be removed only when
  * the table starts running out of space.
  */
  static class ThreadLocalMap {
  
      /**
      * The entries in this hash map extend WeakReference, using
      * its main ref field as the key (which is always a
      * ThreadLocal object).  Note that null keys (i.e. entry.get()
      * == null) mean that the key is no longer referenced, so the
      * entry can be expunged from table.  Such entries are referred to
      * as "stale entries" in the code that follows.
      */
      static class Entry extends WeakReference> {
          /** The value associated with this ThreadLocal. */
          Object value;
          //以ThreadLocal对象作为键值，保存threadlocal变量所包含的值
          //我们在ThreadLocal的get方法当中也是根据threadlocal变量取出所存储的值
          Entry(ThreadLocal k, Object v) {
              super(k);
              value = v;
          }
      }
  ｝
  

• set()方法的实现

  /**
  * Sets the current thread's copy of this thread-local variable
  * to the specified value.  Most subclasses will have no need to
  * override this method, relying solely on the {@link #initialValue}
  * method to set the values of thread-locals.
  *
  * @param value the value to be stored in the current thread's copy of
  *        this thread-local.
  */
  public void set(T value) {
      //获取threadlocal对象所属的线程
      Thread t = Thread.currentThread();
      //获取线程所绑定的ThreadLocalMap对象
      ThreadLocalMap map = getMap(t);
      if (map != null)
          map.set(this, value);
      else
          //在线程尚未初始化并绑定ThreadLocalMap对象的时候，使用给定的value值新建一个，并将线程与该对对象关联起来
          createMap(t, value);
  }
  

在阅读了上面的源码之后，我们大概已经明白了ThreadLocal是怎么做到为每个线程保存一份引用对象的拷贝的值的。每个thread对象都会持有一个ThreadLocal.ThreadLocalMap的对象的引用，而我们通过ThreadLocal对象找到了线程所持有的这个ThreadLocalMap对象，并往其中添加、移除或获得我们所要保存的引用对象的值。

关于ThreadLocalMap里面实现的自定义的hash map我们可以在ThreadLocal的源码中深入了解，这里不做进一步的深入。