struts1.x的action是单例模式

经过对struts1.x源码的研读发现：
struts1.2获取action的方式是单例的，所有的action都被维护在一个hashMap里，当有请求到达时，先根据action的名称去hashMap里查找要请求的Action是否已经存在，如果存在，则直接返回hashMap里的action。如果不存在，则创建一个新的Action实例。


下面我们来分析一下源码：

请求到达ActionServlet时，首先会到达doGet()或doPost()方法，而ActionServlet转给了process()方法进行统一处理

public void doPost(HttpServletRequest request,
           HttpServletResponse response)
    throws IOException, ServletException {

    process(request, response);

}

    public void doPost(HttpServletRequest request,
               HttpServletResponse response)
        throws IOException, ServletException {

        process(request, response);

    }

而process()方法又会转给processor的process()方法进行处理

protected void process(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException {

    ...
    processor.process(request, response);

}

    protected void process(HttpServletRequest request, HttpServletResponse response)
        throws IOException, ServletException {

        ...
        processor.process(request, response);

    }

processor的process()方法里经过一系列处理后，最后通过processActionCreate方法来返回一个具体的action实例

public void process(HttpServletRequest request,
                        HttpServletResponse response)
        throws IOException, ServletException {

        ...

        // Create or acquire the Action instance to process this request
        Action action = processActionCreate(request, response, mapping);
        if (action == null) {
            return;
        }

     ...

    }

public void process(HttpServletRequest request,
                        HttpServletResponse response)
        throws IOException, ServletException {

        ...

        // Create or acquire the Action instance to process this request
        Action action = processActionCreate(request, response, mapping);
        if (action == null) {
            return;
        }

     ...

    }

那我们就到processActionCreate这个方法里去一窥究竟吧：
１、先获取类名
２、根据类名去一个名为actions的map里查寻实例是否已经存在
３、如果存在，则直接返回
４、如果不存在，则创建一个新实例
５、把创建好的action放到map里备用

protected Action processActionCreate(HttpServletRequest request,
                                         HttpServletResponse response,
                                         ActionMapping mapping)
        throws IOException {

        // Acquire the Action instance we will be using (if there is one)
        String className = mapping.getType();//１、先获取类名
        ...
        Action instance = null;
        synchronized (actions) {

            // Return any existing Action instance of this class
            instance = (Action) actions.get(className);//２、根据类名去map里查寻实例是否已经存在
            if (instance != null) {
                return (instance); //３、如果存在，则直接返回
            }

            // Create and return a new Action instance
            //４、如果不存在，则创建一个新实例
            instance = (Action) RequestUtils.applicationInstance(className)

            instance.setServlet(this.servlet);
            actions.put(className, instance);//５、把创建好的action放到map里
        }
        ...
        return (instance);

    }

  
protected Action processActionCreate(HttpServletRequest request,
                                         HttpServletResponse response,
                                         ActionMapping mapping)
        throws IOException {

        // Acquire the Action instance we will be using (if there is one)
        String className = mapping.getType();//１、先获取类名 
        ...
        Action instance = null;
        synchronized (actions) {

            // Return any existing Action instance of this class
            instance = (Action) actions.get(className);//２、根据类名去map里查寻实例是否已经存在
            if (instance != null) {
                return (instance); //３、如果存在，则直接返回
            }

            // Create and return a new Action instance
            //４、如果不存在，则创建一个新实例
            instance = (Action) RequestUtils.applicationInstance(className)

            instance.setServlet(this.servlet);
            actions.put(className, instance);//５、把创建好的action放到map里
        }
        ...
        return (instance);

    }

我们再来看看actions的定义：

/**
 * The set of Action instances that have been created and
 * initialized, keyed by the fully qualified Java class name of the
 * Action class.
 */
protected HashMap actions = new HashMap();

    /**
     * The set of Action instances that have been created and
     * initialized, keyed by the fully qualified Java class name of the
     * Action class.
     */
    protected HashMap actions = new HashMap();

结论：
struts1.x里的action是单例模式的，供多个请求共享。所以不是线程安全的。
其数据都通过actionForm进行传递。
因此， action实例里面不应该声明带有状态的实例变量