ThreadLocal之印象修正

ThreadLocal之印象更正

? ? 今天熟悉公司TMS系统的时候发现自己之前对ThreadLocal的理解是错误的。之前以为ThreadLocal就是一个有自己变量副本的Thread。今天查看文档和源代码的时候才发现根本不是那回事，ThreadLocal根本不是Thread，而是保持Thread变量副本的“容器”，那ThreadLocal是怎么保存这个变量副本的呢？而这个变量副本又怎么能区分是来至那个Thread的呢？

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查看ThreadLocal的源代码，存储变量副本的是一个定制的Map，ThreadLocal里面的一个内部类。下面以ThreadLocal的set方法说明下


 /**     * 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) {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);    }    /**     * Create the map associated with a ThreadLocal. Overridden in     * InheritableThreadLocal.     *     * @param t the current thread     * @param firstValue value for the initial entry of the map     * @param map the map to store.     */    void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }         /**     * 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 {         ......  //Map的一些基础操作，详细请查看ThreadLocal源代码    }

?每一个ThreadLocalMap都保存在当前Thread里，所以当前Thread 通过ThreadLocal.get()拿出来的就是之前保存的变量副本


public class Thread implements Runnable {    /* ThreadLocal values pertaining to this thread. This map is maintained     * by the ThreadLocal class. */     ......    ThreadLocal.ThreadLocalMap threadLocals = null;    .......}

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public class ThreadLocal<T>{.....    /**     * 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() {        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();    }       /**     * Get the map associated with a ThreadLocal. Overridden in     * InheritableThreadLocal.     *     * @param  t the current thread     * @return the map     */    ThreadLocalMap getMap(Thread t) {        return t.threadLocals;    }.....}

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