EL expressions [ new ] Why can salvage value in the value stack

 This post last edited by the iwieijakdf on 2014-01-07 08:16:12

I have a list value of a collection of objects into a stack Map stack in the background

  public String getLastVersionPD(){
  ActionContext.getContext().put("pdList", this.pdManager.getLastVersonPD());
  return "listAction";
  }

then iteration OGNL expressions in the foreground

  <s:iterator value="#pdList">
    <s:property value="name"/>
    <s:property value="version"/>
    <s:a action="pdManagerAction_delete?key=%{key}">删除</s:a>
    <a href="javascript: showProcessImage('${requestScope.deploymentId}')">查看流程图</a>
  </s:iterator>

name version key deploymentId is a property of an object , the value is placed on top of the stack when the stack object iteration , in front of the name version key expression can use OGNL value stack objects in the stack take the value , why use EL expressions can also be obtained at the request scoped attribute deploymentId it
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landlord this issue , why take the request scope with el things will also take it to the value of the contents of the stack . problem it is necessary to combine struts2 source , we know there is HttpServletRequest request in javaee it is all packaged inside an interface is implemented by javax.servlet.http.HttpServletRequestWrapper class . struts2 own javax.servlet.http . HttpServletRequestWrapper achieve , that is org.apache.struts2.dispatcher.StrutsRequestWrapper in struts2-core packet re getAttribute method was rewritten.
We know the value of request scope el expressions take is to call request.getAttribute (String s);
what this is struts2 getAttribute method of org.apache.struts2.dispatcher.StrutsRequestWrapper rewriting the source code for part

  public Object getAttribute(String s) {
          if (s != null && s.startsWith("javax.servlet")) {
              //如果是${requestScope.javax.servlet.属性名的}就直接去request里面取值
              return super.getAttribute(s);
          }
  //关键在下面,先从request里面取值取不到时就从值栈里面取值,所以就可以解释楼主这个为啥也能取到值了
          ActionContext ctx = ActionContext.getContext();
          Object attribute = super.getAttribute(s);
          if (ctx != null) {
              if (attribute == null) {
                  boolean alreadyIn = false;
                  Boolean b = (Boolean) ctx.get("__requestWrapper.getAttribute");
                  if (b != null) {
                      alreadyIn = b.booleanValue();
                  }
                  if (!alreadyIn && s.indexOf("#") == -1) {
                      try {
                          // If not found, then try the ValueStack
                          ctx.put("__requestWrapper.getAttribute", Boolean.TRUE);
                          ValueStack stack = ctx.getValueStack();
                          if (stack != null) {
                              attribute = stack.findValue(s);
                          }
                      } finally {
                          ctx.put("__requestWrapper.getAttribute", Boolean.FALSE);
                      }
                  }
              }
          }
          return attribute;
      }
  

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# non- root object is accessed , it is non- value data stack , such as ognl context and action context. # Can be understood as ActionContext.getContext ().
EL expressions, such as $ {username}, equivalent to request.getParameter (username), and then you go to understand what is meant by request it.
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<s:iterator value="#pdList">

this sentence is to traverse the collection every single element in turn placed on the stack , el expression values ​​also directly from the value stack , so we get to the amount
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understand Thank