package cn.com.leadfar.struts2.actions;public class User {
private int id ;
private String username ;
private String password ;
private int age ;
private String address ;
public String getUsername() {
return username ;
}
public void setUsername(String username) {
this . username = username;
}
public String getPassword() {
return password ;
}
public void setPassword(String password) {
this . password = password;
}
public int getAge() {
return age ;
}
public void setAge( int age) {
this . age = age;
}
public String getAddress() {
return address ;
}
public void setAddress(String address) {
this . address = address;
}
public int getId() {
return id ;
}
public void setId( int id) {
this . id = id;
}}

UserAction:public class UserAction {
private int id ;
private String username ;
private String password ;
private int age ;
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” ;
}public int getId() {
return id ;
}
public void setId( int id) {
this . id = id;
}
public String getUsername() {
return username ;
}
public void setUsername(String username) {
this . username = username;
}
public String getPassword() {
return password ;
}
public void setPassword(String password) {
this . password = password;
}
public int getAge() {
return age ;
}
public void setAge( int age) {
this . age = age;
}
public String getAddress() {
return address ;
}
public void setAddress(String address) {
this . address = address;
}}add_input.jsp:username:
password:
age:
address: 

UserAction:public class UserAction {private User user ;public String add(){new UserManager().addUser( user );return “success” ;
}public User getUser() {
return user ;
}public void setUser(User user) {
this . user = user;
}}add_input.jsp:username:
password:
age:
address: 

public class UserAction implements ModelDriven{private User user ;@Override
public Object getModel() {
if ( user == null ){
user = new User();
}
return user ;
}public String add(){new UserManager().addUser( user );return “success” ;
}public User getUser() {
return user ;
}public void setUser(User user) {
this . user = user;
}
}JSP 的代码如下：username:
password:
age: 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：public class ModelDrivenInterceptor extends AbstractInterceptor {protected boolean refreshModelBeforeResult = false ;public void 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();
}

public class 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 “);
}
return user ;
}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” ;
}