Struts2中的ModelDriven机制及其运用(2013-04-23 16:32:31)转载▼标签: it分类: web随记
所谓ModelDriven,意思是直接把实体类当成页面数据的收集对象。比如,有实体类User如下:
package cn.com.leadfar.struts2.actions;
publicclass User {
privateintid;
private String username;
private String password;
privateintage;
private String address;
public String getUsername() {
returnusername;
}
publicvoid setUsername(String username) {
this.username = username;
}
public String getPassword() {
returnpassword;
}
publicvoid setPassword(String password) {
this.password = password;
}
publicint getAge() {
returnage;
}
publicvoid setAge(int age) {
this.age = age;
}
public String getAddress() {
returnaddress;
}
publicvoid setAddress(String address) {
this.address = address;
}
publicint getId() {
returnid;
}
publicvoid setId(int id) {
this.id = id;
}
}
假如要写一个Action,用来添加User。
第一种做法是直接在Action中定义所有需要的属性,然后在JSP中直接用属性名称来提交数据:
UserAction:
publicclass UserAction {
privateintid;
private String username;
private String password;
privateintage;
private String address;
public String add(){
User user = new User();
user.setId(id);
user.setUsername(username);
user.setPassword(password);
user.setAge(age);
user.setAddress(address);
new UserManager().addUser(user);
return"success";
}
publicint getId() {
returnid;
}
publicvoid setId(int id) {
this.id = id;
}
public String getUsername() {
returnusername;
}
publicvoid setUsername(String username) {
this.username = username;
}
public String getPassword() {
returnpassword;
}
publicvoid setPassword(String password) {
this.password = password;
}
publicint getAge() {
returnage;
}
publicvoid setAge(int age) {
this.age = age;
}
public String getAddress() {
returnaddress;
}
publicvoid setAddress(String address) {
this.address = address;
}
}
add_input.jsp:
<</span>form action="test/user.action" method="post">
<</span>input type="hidden" name="method:add">
username:<</span>input type="text" name="username"><</span>br/>
password:<</span>input type="text" name="password"><</span>br/>
age:<</span>input type="text" name="age"><</span>br/>
address:<</span>input type="text" name="address"><</span>br/>
<</span>input type="submit" name="submit" value="添加用户">
</</span>form><</span>br/>
上述做法不好之处是:如果实体类的属性非常多,那么Action中也要定义相同的属性。
第二种做法是将User对象定义到UserAction中,然后在JSP中通过user属性来给user赋值:
UserAction:
publicclass UserAction {
private User user;
public String add(){
new UserManager().addUser(user);
return"success";
}
public User getUser() {
returnuser;
}
publicvoid setUser(User user) {
this.user = user;
}
}
add_input.jsp:
<</span>form action="test/user.action" method="post">
<</span>input type="hidden" name="method:add">
username:<</span>input type="text" name="user.username"><</span>br/>
password:<</span>input type="text" name="user.password"><</span>br/>
age:<</span>input type="text" name="user.age"><</span>br/>
address:<</span>input type="text" name="user.address"><</span>br/>
<</span>input type="submit" name="submit" value="添加用户">
</</span>form><</span>br/>
这种做法不好的地方是:JSP页面上表单域中的命名变得太长
第三种做法是利用ModelDriven机制,让UserAction实现一个ModelDriven接口,同时实现接口中的方法:getModel()。如下所示:
publicclass UserAction implements ModelDriven{
private User user;
@Override
public Object getModel() {
if(user == null){
user = new User();
}
returnuser;
}
public String add(){
new UserManager().addUser(user);
return"success";
}
public User getUser() {
returnuser;
}
publicvoid setUser(User user) {
this.user = user;
}
}
JSP的代码如下:
<</span>form action="test/user.action" method="post">
<</span>input type="hidden" name="method:add">
username:<</span>input type="text" name="username"><</span>br/>
password:<</span>input type="text" name="password"><</span>br/>
age:<</span>input type="text" name="age"><</span>br/>
<</span>input type="submit" name="submit" value="添加用户">
</</span>form><</span>br/>
可见,第三种做法是比较好的,Action和JSP写起来都比较简单。
ModelDriven背后的机制?
ModelDriven背后的机制就是ValueStack。界面通过:username/age/address这样的名称,就能够被直接赋值给user对象,这证明user对象正是ValueStack中的一个root对象!
那么,为什么user对象会在ValueStack中呢?它是什么时候被压入ValueStack的呢?答案是:ModelDrivenInterceptor(关于Interceptor的概念,请参考后续章节的说明)。ModelDrivenInterceptor是缺省的拦截器链的一部分,当一个请求经过ModelDrivenInterceptor的时候,在这个拦截器中,会判断当前要调用的Action对象是否实现了ModelDriven接口,如果实现了这个接口,则调用getModel()方法,并把返回值(本例是返回user对象)压入ValueStack。
请看ModelDrivenInterceptor的代码:
publicclass ModelDrivenInterceptor extends AbstractInterceptor {
protectedbooleanrefreshModelBeforeResult = false;
publicvoid setRefreshModelBeforeResult(boolean val) {
this.refreshModelBeforeResult = val;
}
@Override
public String intercept(ActionInvocation invocation) throws Exception {
Object action = invocation.getAction();
if (action instanceof ModelDriven) {
ModelDriven modelDriven = (ModelDriven) action;
ValueStack stack = invocation.getStack();
Object model = modelDriven.getModel();
if (model != null) {
stack.push(model);
}
if (refreshModelBeforeResult) {
invocation.addPreResultListener(new RefreshModelBeforeResult(modelDriven, model));
}
}
return invocation.invoke();
}
从ModelDrivenInterceptor中,即可以看到model对象被压入ValueStack中!
其中的refreshModelBeforeResult是为了接下来描述的一个问题而提供的解决方法。
理解常见的陷阱及其解决方法
假设我们要更新一个实体对象,那么第一步首先是打开更新界面,请看下述模拟打开更新界面的代码:
publicclass UserAction implements ModelDriven{
private User user;
@Override
public Object getModel() {
if(user == null){
user = new User();
//user.setUsername("这是原来的User对象");
}
returnuser;
}
public String updateInput(){
//根据ID,查询数据库,得到User对象
user = new UserManager().findUserById(user.getId());
return"update_input";
}
上述代码中,new UserManager().findUserById(user.getId());这一行,将从数据库中查询相应的记录,同时转换为User对象返回。而return “update_input”;将转向更新显示页面。
更新页面如下:
<</span>form action="test/user.action" method="post">
<</span>input type="hidden" name="method:update">
id:<</span>input type="text" name="id" value="<</span>s:propertyvalue="id"/>"><</span>br/>
username:<</span>input type="text" name="username"value="<</span>s:propertyvalue="username"/>"><</span>br/>
password:<</span>input type="text" name="password"value="<</span>s:propertyvalue="password"/>"><</span>br/>
age:<</span>input type="text" name="age" value="<</span>s:propertyvalue="age"/>"><</span>br/>
address:<</span>input type="text" name="address"value="<</span>s:propertyvalue="address"/>"><</span>br/>
<</span>input type="submit" name="submit" value="更新用户">
</</span>form><</span>br/>
上述代码运行起来之后,你在更新界面上将看不到数据(id属性有值,其它属性无显示)。关键的原因是在执行到updateInput之前,user对象(在getMode()方法中创建的对象)被压到ValueStack中,这时候,UserAction和ValueStack都指向同一个user对象;但紧接着,UserAction中的user被一个新的user对象覆盖,这时候,UserAction和ValueStack不再指向同一个user对象!ValueStack中是旧的user对象,而UserAction中是新的user对象!我们在JSP中,直接通过username/address等直接访问,当然是要访问ValueStack中的旧user对象,所以它们的属性都是空的(id属性除外)!
理解上述问题很重要,当你理解了问题,那么问题的解决方法就可以有很多了:
比如,你可以把新对象的属性拷贝到旧对象上;比如,你可以先把旧对象从ValueStack中移除,然后再把新对象压入ValueStack等……
在最新的struts2版本中,ModelDrivenInterceptor提供了一个配置参数:refreshModelBeforeResult,只要将它定义为true,上述问题就被解决了!struts2的解决方案就是:先把旧的model对象从ValueStack中移除,然后再把新的model对象压入ValueStack!
当返回result前对业务对象修改了,但是值栈中对应的对象依然是之前的就值,若想在返回result之前刷新值栈中业务对象各个属性的根对象,需在该action中配置如下参数:[html] view plaincopyprint?01.<action name="studentAction" 02. class="com.amouse.student.action.StudentAction"> 03. <interceptor-ref name="defaultStack"> 04. <!-- 渲染页面前刷新model在ValueStack的root的引用 --> 05. <param name="modelDriven.refreshModelBeforeResult"> 06. true 07. </param> 08. </interceptor-ref> 09. <result name="main">/pages/student/student.jsp</result> 10. <result name="login">/pages/student/login.jsp</result> 11. <result name="success">/pages/student/studentInfo.jsp</result> 12.</action>