ThreadLocal<T>

图解：

说明：在我们Thread 的类里面，存在一个属性，这个属性是：ThreadLocal类实现的内部类（ThreadLocalMap），所以对于每一个线程来说，他都具有一个本地的map，保存属于自己的参数，

对于回收，ThreadLocalMap的实现使用了弱引用，

    static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

　　

另外注意使用规范：

    private static final ThreadLocal<List<Runnable>> RUNNABLES = new ThreadLocal<List<Runnable>>();
    private ExecutorService threadPool = Executors.newFixedThreadPool(20);

    @Override
    public void execute(Runnable runnable) {
        List<Runnable> threadRunnables = RUNNABLES.get();
        if (threadRunnables == null) {
            threadRunnables = new ArrayList<Runnable>();
            RUNNABLES.set(threadRunnables);
            TransactionSynchronizationManager.registerSynchronization(this);
        }
        threadRunnables.add(runnable);
    }

　　TheadLocal的添加，是通过用户自己先判断，get(),是否为null，如果为空，set一个new值

 

java.lang.ThreadLocal<T>实现

 public void set(T value) {  
       Thread t = Thread.currentThread();  
       ThreadLocalMap map = getMap(t);  
       if (map != null)  
           map.set(this, value);  
       else  
           createMap(t, value);  
   } 

　　

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();  
} 

　　

 private T setInitialValue() {  
       T value = initialValue();  
       Thread t = Thread.currentThread();  
       ThreadLocalMap map = getMap(t);  
       if (map != null)  
           map.set(this, value);  
       else  
           createMap(t, value);  
       return value;  
   } 

与java.lang.Thread  implements Runnable （）{。。。}的关联

此类的变量中存在

ThreadLocal.ThreadLocalMap threadLocals = null;