1、源码分析

此处以JDK1.8版本分析

1.1 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);    }

此处的ThreadLocalMap为ThreadLocal的一个内部类，用Entry来存储要设置的值。如下

　　　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
     
      > {            /** The value associated with this ThreadLocal. */            Object value;            Entry(ThreadLocal
       k, Object v) {                super(k);                value = v;            }        } 　　}
     

看下getMap()方法，用于返回当前ThreadLocal维护的ThreadLocalMap对象，如下

/**     * 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;    }

此处的t.threadLocals为当前Thread维护的ThreadLocalMap对象

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

再看下createMap()方法，用于为当前Thread没有ThreadLocalMap时新建一个ThreadLocalMap，如下

　　/**     * 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);    }

从源码中可以看出set方法过程：获取当前线程，并作为句柄去获取ThreadLocalMap，如果ThreadLocalMap存在，则以当前ThreadLocal对象为key，传入的对象为value进行存储。否则新建一个和当前线程相关联的ThreadLocalMap来存储。

1.2 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();    }

从源码可以看出也是以当前线程为句柄获取ThreadLocalMap对象，如果存在则返回当前线程的ThreadLocal为key对应的值，否则设置初始值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();        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);        return value;    }

其中initialValue返回null

/**     * 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}.     *     * 
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;    }

1.3 remove()方法

/**     * Removes the current thread's value for this thread-local     * variable.  If this thread-local variable is subsequently     * {
    @linkplain #get read} by the current thread, its value will be     * reinitialized by invoking its {
    @link #initialValue} method,     * unless its value is {
    @linkplain #set set} by the current thread     * in the interim.  This may result in multiple invocations of the     * {
    @code initialValue} method in the current thread.     *     * @since 1.5     */     public void remove() {         ThreadLocalMap m = getMap(Thread.currentThread());         if (m != null)             m.remove(this);     }

从源码可以很容易看出remove的作用过程，不做详细分析。为了避免内存泄漏问题，尽量在当前线程处理完之后手动进行remove操作。

2、总结

从上面的源码可以看出ThreadLocal设置的值和线程相关，可以理解为是线程的私有的访问区域，其他线程无法访问，实现了“数据隔离”效果。和解决并发问题和共享变量无关

每个Thread都维护者一个ThreadLocalMap的引用。