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深入理解 ThreadLocal

ThreadLocal ，顾名思义就是用来提供线程（Thread）内部的局部（Local）变量的，主要应用场景为在同一个线程内方便地共享变量。

例如：一次用户请求，服务器会为其开一个线程，我们在线程中创建一个 ThreadLocal，里面存请求上下文信息，这样在之后获取这些信息时就可以很方便地拿到，而不用层层显式传参。

2.实现原理

ThreadLocal 的实现原理并不复杂，核心就是在 Thread 类里维护了一个 Map：

/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;

然后通过 ThreadLocal 的接口读写这个变量：

/**
* 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);
}

至于 ThreadLocalMap，是一个专门为 ThreadLocal 应用场景定制的一个 HashMap，实现细节可以先不去管，Key 为 ThreadLocal 的实例，Value 为线程局部变量。所以说，ThreadLocal 本身并不存储值，它只是作为一个 key 来让线程从 ThreadLocalMap 获取 value。

准备工作做好了，接下来看下 ThreadLocal 是怎么实现线程局部变量的读写的，核心代码也都很简单。

get：其实就是去读取 map 里的值。

/**
* 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();
}

当 map 为空或者没有读到值时，会触发初始化逻辑：

/**
* 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();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

initialValue() 是一个可重写的接口，用于让开发者自定义想要的初始值。

/**
* Returns the current thread's "initial value" for this
* thread-local variable.  This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
* method, in which case the {@code initialValue} method will not
* be invoked for the thread.  Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
* <p>This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
* value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden.  Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
    return null;
}

set：其实就是往 map 里面放值。

/**
* 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);
}

remove：用于使用完毕时清除数据，务必调用，不然会出现内存泄漏问题，见下文

/**
* Remove the entry for key.
*/
private void remove(ThreadLocal<?> key) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        if (e.get() == key) {
            e.clear();
            expungeStaleEntry(i);
            return;
        }
    }
}