Chapter 5: Struts Work Flow
Struts principle and practice
上圖是Struts的工作流程，前邊我們提到，所有的請求都提交給ActionServlet來處理。
ActionServlet是一個FrontController，它是一個標準的Servlet，它將request轉發給RequestProcessor來處理，
ActionMapping是ActionConfig的子類，實質上是對struts-config.xml的一個映射，從中可以取得所有的配置信息
RequestProcessor根據提交過來的url，如*.do，從ActionMapping 中得到相應的ActionForn和Action。然後將request的參數對應到ActionForm中，進行form驗證。如果驗證通過則調用Action的execute()方法來執行Action，最終返回ActionFoward。
ActionFoward是對mapping中一個foward的包裝，對應於一個url
ActionForm使用了ViewHelper模式，是對HTML中form的一個封裝。其中包含有validate方法，用於驗證form數據的有效性。ActionForm是一個符合JavaBean規範的類，所有的屬性都應滿足get和set對應。對於一些複雜的系統，還可以採用DynaActionForm來構造動態的Form，即通過預製參數來生成Form。這樣可以更靈活的擴展程序。
ActionErrors是對錯誤信息的包裝，一旦在執行action或者form.validate中出現異常，即可產生一個ActionError並最終加入到ActionErrors。在Form驗證的過程中，如果有Error發生，則會將頁面重新導向至輸入頁，並提示錯誤。
Action是用於執行業務邏輯的RequsestHandler。每個Action都只建立一個instance。Action不是線程安全的，所以不應該在Action中訪問特定資源。一般來說，應改使用 Business Delegate 模式來對Business tier進行訪問以解除耦合。
Struts提供了多種Action供選擇使用。普通的Action只能通過調用execute執行一項任務，而DispatchAction可以根據配置參數執行，而不是僅進入execute()函數，這樣可以執行多種任務。如insert,update等。LookupDispatchAction可以根據提交表單按鈕的名稱來執行函數。
我們可以先回到剛纔的例子，理解一下Struts的流程。
下面我們看Struts自帶的example實例：
Chapter 6: Example 2: Login Application
Struts principle and practice
說明：實例二是Struts自帶的example程序, 實現了登錄，註冊，修改功能
代碼中大量應用了struts taglib，並且採用validator插件進行form的驗證
但是代碼樹立了一個不好的榜樣，即把大量的業務邏輯寫在了action中。
部分代碼如下：
登錄：logon.jsp
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struts-config.xml配置
<form-beans> <!-- Logon form bean --> <form-bean name="logonForm" type="org.apache.struts.validator.DynaValidatorForm"> <form-property name="username" type="java.lang.String"/> <form-property name="password" type="java.lang.String"/> </form-bean> <!-- Subscription form bean --> <form-bean name="subscriptionForm" type="org.apache.struts.webapp.example.SubscriptionForm"/> </form-beans> <action-mappings> <!-- Edit mail subscription --> <action path="/editSubscription" type="org.apache.struts.webapp.example.EditSubscriptionAction" attribute="subscriptionForm" scope="request" validate="false"> <forward name="failure" path="/mainMenu.jsp"/> <forward name="success" path="/subscription.jsp"/> </action> ...
subscriptionForm 是一個標準的ActionForm,其中reset方法用於清除form的值,validate方法用於驗證
public final class SubscriptionForm extends ActionForm { // The maintenance action we are performing (Create or Edit). private String action = "Create"; // Should we auto-connect at startup time? private boolean autoConnect = false; // The host name. private String host = null; private String password = null; private String type = null; private String username = null; public String getAction() { return (this.action); } public void setAction(String action) { this.action = action; } public boolean getAutoConnect() { return (this.autoConnect); } public void setAutoConnect(boolean autoConnect) { this.autoConnect = autoConnect; } public String getHost() { return (this.host); } public void setHost(String host) { this.host = host; } public String getPassword() { return (this.password); } public void setPassword(String password) { this.password = password; } public String getType() { return (this.type); } public void setType(String type) { this.type = type; } public String getUsername() { return (this.username); } public void setUsername(String username) { this.username = username; } /** * Reset all properties to their default values. * * @param mapping The mapping used to select this instance * @param request The servlet request we are processing */ public void reset(ActionMapping mapping, HttpServletRequest request) { this.action = "Create"; this.autoConnect = false; this.host = null; this.password = null; this.type = null; this.username = null; } /** * Validate the properties that have been set from this HTTP request, * and return an <code>ActionErrors</code> object that encapsulates any * validation errors that have been found. If no errors are found, return * <code>null</code> or an <code>ActionErrors</code> object with no * recorded error messages. * * @param mapping The mapping used to select this instance * @param request The servlet request we are processing */ public ActionErrors validate(ActionMapping mapping, HttpServletRequest request) { ActionErrors errors = new ActionErrors(); if ((host == null) || (host.length() < 1)) errors.add("host", new ActionError("error.host.required")); if ((username == null) || (username.length() < 1)) errors.add("username", new ActionError("error.username.required")); if ((password == null) || (password.length() < 1)) errors.add("password", new ActionError("error.password.required")); if ((type == null) || (type.length() < 1)) errors.add("type", new ActionError("error.type.required")); else if (!"imap".equals(type) && !"pop3".equals(type)) errors.add("type", new ActionError("error.type.invalid", type)); return (errors); }}
logonAction
public final class LogonAction extends Action { /** * Process the specified HTTP request, and create the corresponding HTTP * response (or forward to another web component that will create it). * Return an <code>ActionForward</code> instance describing where and how * control should be forwarded, or <code>null</code> if the response has * already been completed. * * @param mapping The ActionMapping used to select this instance * @param form The optional ActionForm bean for this request (if any) * @param request The HTTP request we are processing * @param response The HTTP response we are creating * * @exception Exception if business logic throws an exception */ public ActionForward execute(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception { // Extract attributes we will need Locale locale = getLocale(request); MessageResources messages = getResources(request); User user = null; // Validate the request parameters specified by the user ActionErrors errors = new ActionErrors(); String username = (String) PropertyUtils.getSimpleProperty(form, "username"); String password = (String) PropertyUtils.getSimpleProperty(form, "password"); UserDatabase database = (UserDatabase) servlet.getServletContext().getAttribute(Constants.DATABASE_KEY); if (database == null) errors.add(ActionErrors.GLOBAL_ERROR, new ActionError("error.database.missing")); else { user = getUser(database, username); if ((user != null) && !user.getPassword().equals(password)) user = null; if (user == null) errors.add(ActionErrors.GLOBAL_ERROR, new ActionError("error.password.mismatch")); } // Report any errors we have discovered back to the original form if (!errors.isEmpty()) { saveErrors(request, errors); return (mapping.getInputForward()); } // Save our logged-in user in the session HttpSession session = request.getSession(); session.setAttribute(Constants.USER_KEY, user); if (log.isDebugEnabled()) { log.debug("LogonAction: User '" + user.getUsername() + "' logged on in session " + session.getId()); } // Remove the obsolete form bean if (mapping.getAttribute() != null) { if ("request".equals(mapping.getScope())) request.removeAttribute(mapping.getAttribute()); else session.removeAttribute(mapping.getAttribute()); } // Forward control to the specified success URI return (mapping.findForward("success")); } /** * Look up the user, throwing an exception to simulate business logic * rule exceptions. * * @param database Database in which to look up the user * @param username Username specified on the logon form * * @exception ModuleException if a business logic rule is violated */ public User getUser(UserDatabase database, String username) throws ModuleException { // Force an ArithmeticException which can be handled explicitly if ("arithmetic".equals(username)) { throw new ArithmeticException(); } // Force an application-specific exception which can be handled if ("expired".equals(username)) { throw new ExpiredPasswordException(username); } // Look up and return the specified user return ((User) database.findUser(username)); }}