线程本地变量 ThreadLocal

不能解决共享变量的并发问题，只能维护本线程变量

原理
各线程持有一个用于存放变量的map（ThreadLocalMap），以变量对应的ThreadLocal自身为键

应用场景
数据库连接
Session管理

//获取线程中本地变量map的值（键为当前ThreadLocal）
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();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

//设置线程中本地变量map的值（键为当前ThreadLocal）
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

//移除线程中本地变量map的值（键为当前ThreadLocal）
public void remove() {
    ThreadLocalMap m = getMap(Thread.currentThread());
    if (m != null)
        m.remove(this);
}

//获取线程的本地变量map
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

//为线程初始化其本地变量map
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}