Struts1.x单例模式分析

偏执的太偏执、 2022-05-15 03:54 178阅读 0赞

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。

**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

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

1.public void doPost(HttpServletRequest request,   
    2.           HttpServletResponse response)   
    3.    throws IOException, ServletException {   
    4.  
    5.    process(request, response);   
    6.  
    7.}

1.public void doPost(HttpServletRequest request,  
    2.           HttpServletResponse response)  
    3.    throws IOException, ServletException {  
    4.  
    5.    process(request, response);  
    6.  
    7.}

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

1.protected void process(HttpServletRequest request, HttpServletResponse response)   
    2.    throws IOException, ServletException {   
    3.  
    4.    ...   
    5.    processor.process(request, response);   
    6.  
    7.}

1.protected void process(HttpServletRequest request, HttpServletResponse response)  
    2.    throws IOException, ServletException {  
    3.  
    4.    ...  
    5.    processor.process(request, response);  
    6.  
    7.}

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

1.public void process(HttpServletRequest request,   
    2.                        HttpServletResponse response)   
    3.        throws IOException, ServletException {   
    4.  
    5.        ...   
    6.  
    7.        // Create or acquire the Action instance to process this request   
    8.        Action action = processActionCreate(request, response,
    
     mapping);   
    9.        if (action == null) {   
    10.            return;   
    11.        }   
    12.  
    13.     ...   
    14.  
    15.    }

1.public void process(HttpServletRequest request,  
    2.                        HttpServletResponse response)  
    3.        throws IOException, ServletException {  
    4.  
    5.        ...  
    6.  
    7.        // Create or acquire the Action instance to process this request  
    8.        Action action = processActionCreate(request, response, mapping);  
    9.        if (action == null) {  
    10.            return;  
    11.        }  
    12.  
    13.     ...  
    14.  
    15.    }

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

2.protected Action processActionCreate(HttpServletRequest request,   
    3.                                         HttpServletResponse response,   
    4.                                         ActionMapping mapping)   
    5.        throws IOException {   
    6.  
    7.        // Acquire the Action instance we will be using (if there is one)   
    8.        String className = mapping.getType();//１、先获取类名    
    9.        ...   
    10.        Action instance = null;   
    11.        synchronized (actions) {   
    12.  
    13.            // Return any existing Action instance of this class   
    14.            instance = (Action) actions.get(className);//２、根据类名去map里查寻实例是否已经存在   
    15.            if (instance != null) {   
    16.                return (instance); //３、如果存在，则直接返回   
    17.            }   
    18.  
    19.            // Create and return a new Action instance   
    20.            //４、如果不存在，则创建一个新实例   
    21.            instance = (Action) RequestUtils.applicationInstance(className)   
    22.  
    23.            instance.setServlet(this.servlet);   
    24.            actions.put(className, instance);//５、把创建好的action放到map里   
    25.        }   
    26.        ...   
    27.        return (instance);   
    28.  
    29.    }

1.    
    2.protected Action processActionCreate(HttpServletRequest request,  
    3.                                         HttpServletResponse response,  
    4.                                         ActionMapping mapping)  
    5.        throws IOException {  
    6.  
    7.        // Acquire the Action instance we will be using (if there is one)  
    8.        String className = mapping.getType();//１、先获取类名   
    9.        ...  
    10.        Action instance = null;  
    11.        synchronized (actions) {  
    12.  
    13.            // Return any existing Action instance of this class  
    14.            instance = (Action) actions.get(className);//２、根据类名去map里查寻实例是否已经存在  
    15.            if (instance != null) {  
    16.                return (instance); //３、如果存在，则直接返回  
    17.            }  
    18.  
    19.            // Create and return a new Action instance  
    20.            //４、如果不存在，则创建一个新实例  
    21.            instance = (Action) RequestUtils.applicationInstance(className)  
    22.  
    23.            instance.setServlet(this.servlet);  
    24.            actions.put(className, instance);//５、把创建好的action放到map里  
    25.        }  
    26.        ...  
    27.        return (instance);  
    28.  
    29.    }

我们再来看看actions的定义：

1./**  
    2. * The set of Action instances that have been created and  
    3. * initialized, keyed by the fully qualified Java class name of the  
    4. * Action class.  
    5. */  
    6.protected HashMap actions = new HashMap();  ```

1./** 
    2. * The set of Action instances that have been created and 
    3. * initialized, keyed by the fully qualified Java class name of the 
    4. * Action class. 
    5. */  
    6.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==