ThreadLocal源码分析

一 TheadLocal是什么？

ThreadLocal，很多地方叫做线程本地变量，也有些地方叫做线程本地存储，其实意思差不多。ThreadLocal为变量在每个线程中都创建了一个副本，那么每个线程可以访问自己内部的副本变量。

二 TheadLocal实现原理

以前的理解是：ThreadLocal类里面有一个ThreadLocalMap类，这个map以线程为key，存储所需的变量。大错

正确的理解：Thread实例里面有一个ThreadLocalMap变量，这个map以TheadLocal为key，存储所需的变量。

详细的表达：ThreadLocalMap是ThreadLocal类的一个静态内部类，它实现了键值对的设置和获取（对比Map对象来理解），每个线程中都有一个独立的ThreadLocalMap副本，它所存储的值，只能被当前线程读取和修改。ThreadLocal类通过操作每一个线程特有的ThreadLocalMap副本，从而实现了变量访问在不同线程中的隔离。因为每个线程的变量都是自己特有的，完全不会有并发错误。还有一点就是，ThreadLocalMap存储的键值对中的键是this对象指向的ThreadLocal对象，而值就是你所设置的对象了。

三 源码分析

先看看set（）方法：

    /**     * 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);    }

在看看最主要的 getMap（）以及createMap（）

    /**     * 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;    }    /**     * 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     */    void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }

在看看get（）方法

    /**     * 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) {                @SuppressWarnings("unchecked")                T result = (T)e.value;                return result;            }        }        return setInitialValue();    }    /**     * 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() {        T value = initialValue(); // 方法内直接return null        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);        return value;    }

然后就是最重要的ThreadLocalMap（看注释就理解了）

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

四 总结

线程私有方式提供了解决并发问题的思路