菜鸟学Struts2——Struts工作原理

Posted 2020-08-20

　　在完成Struts2的HelloWorld后，对Struts2的工作原理进行学习。Struts2框架可以按照模块来划分为Servlet Filters，Struts核心模块，拦截器和用户实现部分，其中需要用户实现的部分只有三个，那就是struts.xml，Action，Template（JSP），如下图：

 

2.3.31中的org.apache.struts2.dispatcher.ActionContextCleanUp已经被标记为@Deprecated Since Struts 2.1.3，2.1.3之后FilterDispatcher已经改为org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter，2.5.0之后又被移到了父包中，即不在ng包下面，直接在父包中org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter

 

（1） 在struts2.3.31中FilterDispatcher就是StrutsPrepareAndExecuteFilter，Struts的过滤器根据<url-pattern>/*</url-pattern>拦截请求，doFilter(org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter)核心代码如下：

 1 public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
 2     HttpServletRequest request = (HttpServletRequest) req;
 3     HttpServletResponse response = (HttpServletResponse) res;
 4     try {
 5         if (excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
 6             // 如果常量struts.action.excludePattern指定了不被拦截的Url这里直接放行
 7             chain.doFilter(request, response);
 8         } else {
 9             prepare.setEncodingAndLocale(request, response);
10             prepare.createActionContext(request, response);
11             prepare.assignDispatcherToThread();
12             request = prepare.wrapRequest(request);
13             // 通过ActionMapper获取ActionMapping
14             ActionMapping mapping = prepare.findActionMapping(request, response, true);
15             if (mapping == null) {
16                 boolean handled = execute.executeStaticResourceRequest(request, response);
17                 if (!handled) {
18                     chain.doFilter(request, response);
19                 }
20             } else {
21                 // 调用dispatcher.serviceAction()执行Action
22                 execute.executeAction(request, response, mapping);
23             }
24         }
25     } finally {
26         prepare.cleanupRequest(request);
27     }
28 }

 

 （2）StrutsPrepareAndExecuteFilter通过Dispatcher（org.apache.struts2.dispatcher.Dispatcher）的serviceAction()方法获取ActionProxy去执行Action，核心代码如下：

 1 public void serviceAction(HttpServletRequest request, HttpServletResponse response, ActionMapping mapping)
 2         throws ServletException {
 3     Map<String, Object> extraContext = createContextMap(request, response, mapping);
 4     // 获取值栈OgnlValueStack implements ValueStack
 5     // If there was a previous value stack, then create a new copy and pass it in to be used by the new Action
 6     ValueStack stack = (ValueStack) request.getAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY);
 7     boolean nullStack = stack == null;
 8     if (nullStack) {
 9         ActionContext ctx = ActionContext.getContext();
10         if (ctx != null) {
11             stack = ctx.getValueStack();
12         }
13     }
14     if (stack != null) {
15         extraContext.put(ActionContext.VALUE_STACK, valueStackFactory.createValueStack(stack));
16     }
17 
18     String timerKey = "Handling request from Dispatcher";
19     try {
20         UtilTimerStack.push(timerKey);
21         String namespace = mapping.getNamespace();
22         String name = mapping.getName();
23         String method = mapping.getMethod();
24         // 获取Action代理执行Action
25         ActionProxy proxy = getContainer().getInstance(ActionProxyFactory.class).createActionProxy(
26             namespace, name, method, extraContext, true, false);
27         
28         request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, proxy.getInvocation().getStack());
29 
30         // if the ActionMapping says to go straight to a result, do it!
31         if (mapping.getResult() != null) {
32             Result result = mapping.getResult();
33             result.execute(proxy.getInvocation());
34         } else {
35             // Action代理执行Action
36             proxy.execute();
37         }
38 
39         // If there was a previous value stack then set it back onto the request
40         if (!nullStack) {
41             request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);
42         }
43     } catch (ConfigurationException e) {
44         logConfigurationException(request, e);
45         sendError(request, response, HttpServletResponse.SC_NOT_FOUND, e);
46     } catch (Exception e) {
47         if (handleException || devMode) {
48             sendError(request, response, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e);
49         } else {
50             throw new ServletException(e);
51         }
52     } finally {
53         UtilTimerStack.pop(timerKey);
54     }
55 }

（3）Dispatcher通过ActionProxy的execute()方法执行Action，ActionProxy默认的实现是com.opensymphony.xwork2.DefaultActionProxy，ActionProxy执行ActionInvocation.invoke()代理执行Action，核心代码如下：

 1 public String execute() throws Exception {
 2     ActionContext nestedContext = ActionContext.getContext();
 3     ActionContext.setContext(invocation.getInvocationContext());
 4 
 5     String retCode = null;
 6 
 7     String profileKey = "execute: ";
 8     try {
 9         UtilTimerStack.push(profileKey);
10         // invoke代理执行Action
11         retCode = invocation.invoke();
12     } finally {
13         if (cleanupContext) {
14             ActionContext.setContext(nestedContext);
15         }
16         UtilTimerStack.pop(profileKey);
17     }
18     return retCode;
19 }

 

（4）ActionInvocation会执行一系列的Struts的拦截器如上图，而拦截器就只有一个参数，那就是ActionInvocation，在拦截器里面加入一些其他逻辑然后又调用ActionInvocation.invoke()又回到ActionInvocation.invoke()，这样循环直到interceptors.hasNext()没有拦截器为止才执行Action，即调用invokeActionOnly()执行Action，Action执行之后会接着执行executeResult()，上图的Result在Action执行之后，并将executed标记为true，这样result就生成了，当执行拦截器调用ActionInvocation.invoke()之后的代码完成就不会再对result造成任何影响了，核心代码如下：

 1 public String invoke() throws Exception {
 2     String profileKey = "invoke: ";
 3     try {
 4         UtilTimerStack.push(profileKey);
 5 
 6         if (executed) {
 7             throw new IllegalStateException("Action has already executed");
 8         }
 9 
10         if (interceptors.hasNext()) {
11             final InterceptorMapping interceptor = interceptors.next();
12             String interceptorMsg = "interceptor: " + interceptor.getName();
13             UtilTimerStack.push(interceptorMsg);
14             try {
15                 // 执行拦截器，拦截器中执行invocation.invoke()回到这里调用下一个拦截器
16                 resultCode = interceptor.getInterceptor().intercept(DefaultActionInvocation.this);
17                 // 所有拦截的执行invocation.invoke()之后的代码完成，此时下面的executed已经被标记成true不会再次执行
18             }
19             finally {
20                 UtilTimerStack.pop(interceptorMsg);
21             }
22         } else {
23             // 所有拦截器invocation.invoke()之前的代码执行完后执行Action
24             resultCode = invokeActionOnly();
25         }
26 
27         // this is needed because the result will be executed, then control will return to the Interceptor, which will
28         // return above and flow through again
29         if (!executed) {
30             if (preResultListeners != null) {
31                 LOG.trace("Executing PreResultListeners for result [#0]", result);
32         
33                 for (Object preResultListener : preResultListeners) {
34                     PreResultListener listener = (PreResultListener) preResultListener;
35         
36                     String _profileKey = "preResultListener: ";
37                     try {
38                         UtilTimerStack.push(_profileKey);
39                         listener.beforeResult(this, resultCode);
40                     }
41                     finally {
42                         UtilTimerStack.pop(_profileKey);
43                     }
44                 }
45             }
46             // Action执行完成，执行Result
47             // now execute the result, if we‘re supposed to
48             if (proxy.getExecuteResult()) {
49                 executeResult();
50             }
51             // 并标记executed为true
52             executed = true;
53         }
54         // 返回ResultCode
55         return resultCode;
56     }
57     finally {
58         UtilTimerStack.pop(profileKey);
59     }
60 }

 下面是Struts2配置的默认拦截器：

 

至此，Struts2一次请求就完成了。

未完，待续。