JAVA并发学习之ThreadLocal

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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<ThreadResource> resoursePackage=new ThreadLocal<ThreadResource>(){        @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<ThreadLocal<?>> {        /** 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。