Table of contents


Introduction to Struts 1 framework

Struts is an application development framework that is designed for and used with the popular J2EE platform. It cuts time out of the development process and makes developers more productive by providing them a series of tools and components to build applications. Struts is maintained as a part of Apache Jakarta project and is open source.

Struts 1 was first release of MVC based framework by the Apache Software foundation in May 2000.


How Struts 1 works

To describe the way Struts 1 work, we will seee the below image:

How Struts 1 work

  • At deployment time or at the first user request, container will create ActionServlet instance, and it will call init method. (In web.xml file, we have to configure it with *.do URL pattern).

  • init method read struts-config.xml and it will store that information into ModuleConfig config reference and when user sends a request, protected void initModuleFormBeans(ModuleConfig config) will create form bean instance and it will keep it in scope (request or session).

  • When user sends a request, container will send that request to void doGet(HttpServletRequest request, HttpServletResponse response) and void doPost(HttpServletRequest request, HttpServletResponse response) methods. After that, doGet() and doPost() methods will delegate to void process(HttpServletRequest request, HttpServletResponse response) method of ActionServlet.

  • process() method of ActionServlet will create one Singleton instance of a RequestProcessor and call void init(ActionServlet servlet, ModuleConfig moduleConfig) method of RequestProcessor. Note that for each and every module ActionServlet will create one RequestProcessor object.

  • Then, process() method of ActionServlet will delegate request to process() method of RequestProcessor. process() method of RequestProcessor will call boolean processPreprocess(HttpServletRequest request, HttpServletResponse response) method of the same class. If processPreprocess() method returns true, then only process() method will do remain process. Otherwise, it will not process the request to next level. By default, processPreprocess() method’s implementation will return true.

  • process() method of RequestProcessor will populate read user input form data and populates that form data into ActionForm instance by calling setter methods.

  • Then, it will call the ActionErrors validate(ActionMapping mapping, javax.servlet.http.HttpServletRequest request) method for validation. It will return ActionErrors object which is the subclass of ActionMessage.

  • In ActionMessage class, isEmpty() method will check error messages what we added in our conditions:

      if (name.equals("")) {
      ae.add("name", new ActionMessage("mgs"));
  }

  • If count is zero, then process() method will redirect input page to the User for populating error messages.

  • If error count is not equals to zero, the process() method will create ActionClass object. In single factor (for first user request), it will call execute method of our Controller class.

  • For that execute() method, it pass (ActionMapping, ActionForm, HttpServletRequest, HttpServletResponse) as execute method parameters.

  • By using request or ActionForm parameter, we can read form data to process next Model layer classes.

  • From execute() method, process() method of RequestProcessor will expect one ActionForward that is return type. In that ActionForward, we can configure the required page redirection configuration.

  • Based on that configuration, process() method will forward required configured page what we have configured under <forward> tag in struts-config.xml file.


Core componenets in Struts 1 framework

Belows are some main components that we need to understand:

  • ActionServlet is a front controller Servlet in Struts, and the only Servlet exists in Struts framework. Every request comming to Struts must first come to ActionServlet clss instance. When ActionServlet class call init() method, ActionServlet reads struts-config.xml file to get data and stores it into one ModuleConfig object.

    ModuleConfig object contains all 8 struts-config.xml file elements. Once after moduleConfig object is created that object is passed into one new instance of RequestProcessor constructor.

      RequestProcessor reqProcessor = new RequestProcessor(this, moduleConfig);

    this is current ActionServlet class instance, and moduleConfig is ModuleConfig object.

ActionServlet works

In the process() method, ActionServlet invokes process() method of RequestProcessor.

  • ModuleConfig

    ModuleConfig contains the content of struts-config.xml file. It contains all ActionForm, Action, ActionForward, ActionMapping class names.

    Whenever client request comes, ActionServlet sends request to RequestProcessor, RequestProcessor uses ModuleConfig object to find out the ActionForm and Action class names of the requested action path uri.


Wrapping up

  • Understanding about the flows between components in Struts 1 helps us in coding, debugging.

Refer:

https://studylib.net/doc/9379291/struts-config.xml-file

http://tool.oschina.net/uploads/apidocs/struts-1.3.10/org/apache/struts/action/ActionServlet.html