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。
时间： 2024-08-25 19:10:36