【Java】ThreadLocal 可以在指定线程内存储数据，只有指定线程可以得到存储数据

一般事务会用到 ThreadLocal 可以保障同一个线程用同一个 Connection

可以参考

ThreadLocal 是线程的内部存储类，可以在指定线程内存储数据。只有指定线程可以得到存储数据。

返回当前线程的这个副本的值
线程局部变量。如果变量没有值
当前线程，它首先被初始化为返回的值
通过调用{@link #initialValue}方法。

 @返回当前线程的线程本地值

/**
     * 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();
    }

获取与ThreadLocal关联的映射。覆盖在
 InheritableThreadLocal。

 @param t当前线程
 @返回地图

/**
     * 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;
    }

每个线程都有一个ThreadLocalMap的实例对象，并且通过ThreadLocal管理ThreadLocalMap。

/**
     * 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);
    }