struts.xml
<package name="user_json" extends="json-default">
        <action name="findByName" class="org.mln.bbs.action.FindNameAction">
            <result type="json">
                <!-- 这里指定将被Struts2序列化的属性，该属性在action中必须有对应的getter方法 -->
                 <param name="root">jsonResult</param> 
            </result>
        </action>
    </package>

action代码
@Component("user1")
@Scope("prototype")
public class FindNameAction extends ActionSupport {
    private UserRegisterInfo userRegisterInfo;
    private UserService userService;
    private Map<String,Object> jsonResult;
    
    @Override
    public String execute() {
        jsonResult = new HashMap<String, Object>();
        int userNum = userService.userNumByName(userRegisterInfo.getUsername());
        if(userNum > 0) {
            jsonResult.put("valid", false);
        } else {
            jsonResult.put("valid", true);
        }
        return "success";
    }

    public Map<String, Object> getJsonResult() {
        return jsonResult;
    }
    public void setJsonResult(Map<String, Object> jsonResult) {
        this.jsonResult = jsonResult;
    }
    
    public UserService getUserService() {
        return userService;
    }
    @Resource(name="userService")
    public void setUserService(UserService userService) {
        this.userService = userService;
    }

    public UserRegisterInfo getUserRegisterInfo() {
        return userRegisterInfo;
    }

    public void setUserRegisterInfo(UserRegisterInfo userRegisterInfo) {
        this.userRegisterInfo = userRegisterInfo;
    }
}