[Java] Understand ThreadLocal

Someone in the group said yesterday that Struts2.x was used in other layers of non-Action.
ServletActionContext. getRequest ()
Q: Why can I get it successfully?

Why do you think it cannot be obtained?
In fact, it is easier to prove that there are rabbits in the forest than to prove that there are no rabbits in the forest-we just need to catch rabbits.

The source code is clear.
First, let's take a look at what ServletActionContext. getRequest () has done:

/**     * Gets the HTTP servlet request object.     *     * @return the HTTP servlet request object.     */    public static HttpServletRequest getRequest() {        return (HttpServletRequest) ActionContext.getContext().get(HTTP_REQUEST);    }

ActionContext. getContext () returns an ActionContext. The get (String key) of ActionContext is only get in Map:

/**    * Returns a value that is stored in the current ActionContext by doing a lookup using the value's key.    *    * @param key the key used to find the value.    * @return the value that was found using the key or <tt>null</tt> if the key was not found.    */   public Object get(String key) {       return context.get(key);   }

In fact, the question is not get (), but ActionContext. getContext (). Read the comment that says Returns the ActionContext specific to the current thread:

/**    * Returns the ActionContext specific to the current thread.    *    * @return the ActionContext for the current thread, is never <tt>null</tt>.    */   public static ActionContext getContext() {       return actionContext.get();   }

The actionContext here is a ThreadLocal type:

static ThreadLocal<ActionContext> actionContext = new ThreadLocal<ActionContext>();

It may be because you do not understand ThreadLocal--|

It's not LocalThread, it's ThreadLocal, What Is ThreadLocal?
Oh, you may remember.
It is the local variable of the thread.
We once declared a static ThreadLocal in a class <...> or something.
We once override his initialValue (), called his get () set () remove ().

ThreadLocal contains a static class ThreadLocalMap.
Taking set () as an example, when we call set, we actually store this ThreadLocal as the Key in Map.

public void set(T value) {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);    }

You should be aware of how to obtain the Map, getMap (Thread. currentThread ()):

ThreadLocalMap getMap(Thread t) {    return t.threadLocals;}

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All our ThreadLocal objects are in this map:

/** * Construct a new map initially containing (firstKey, firstValue). * ThreadLocalMaps are constructed lazily, so we only create * one when we have at least one entry to put in it. */ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {    table = new Entry[INITIAL_CAPACITY];    int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);    table[i] = new Entry(firstKey, firstValue);    size = 1;    setThreshold(INITIAL_CAPACITY);}

/*** Set the value associated with key. ** @ param key the thread local object * @ param value the value to be set */private void set (ThreadLocal key, Object value) {// the source code is omitted}

To sum up, that's all:
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