泛型的问题

package com.mmall.common;

import org.codehaus.jackson.annotate.JsonIgnore;
import org.codehaus.jackson.map.annotate.JsonSerialize;

import java.io.Serializable;

/**
 * Created by geely
 */
public class ServerResponse<T> implements Serializable {

    private int status;
    private String msg;
    private T data;

    private ServerResponse(int status){
        this.status = status;
    }
    public ServerResponse(int status,T data){
        this.status = status;
        this.data = data;
    }

    private ServerResponse(int status,String msg,T data){
        this.status = status;
        this.msg = msg;
        this.data = data;
    }

    public ServerResponse(int status,String msg){
        this.status = status;
        this.msg = msg;
    }

    public static void main(String[] args) {
        ServerResponse serverResponse1 = new ServerResponse(1,new Object());
        ServerResponse serverResponse2 = new ServerResponse(1,"msg");
        System.out.println("finis");
    }
}

运行结果：

图片.png

分析：
msg是成功或者失败返回时的附加信息。
data是成功返回时的泛型数据。
构造函数第二个参数是非字符串，则调用ServerResponse(int status,String msg)；
构造函数第二个参数是字符串，则调用ServerResponse(int status,T data)
那存在这么一种情景，成功返回的泛型数据是String类型的怎么办？如果调用ServerResponse(int,String),那这个data数据就会被附到msg上，这不是我们想要。

解决办法：
把所有构造函数都设为private，开放其他函数：如下。当调用createBySuccess(String)时，由于参数类型是T，那么调用的肯定是ServerResponse(int status,T data)这个方法。

...
public static <T> ServerResponse<T> createBySuccessMessage(String msg){
        return new ServerResponse<T>(ResponseCode.SUCCESS.getCode(),msg);
    }

    public static <T> ServerResponse<T> createBySuccess(T data){
        return new ServerResponse<T>(ResponseCode.SUCCESS.getCode(),data);
    }
...

下面把这个代码挂上。

package com.mmall.common;

import org.codehaus.jackson.annotate.JsonIgnore;
import org.codehaus.jackson.map.annotate.JsonSerialize;

import java.io.Serializable;

/**
 * Created by geely
 */
@JsonSerialize(include =  JsonSerialize.Inclusion.NON_NULL)
//保证序列化json的时候,如果是null的对象,key也会消失
public class ServerResponse<T> implements Serializable {

    private int status;
    private String msg;
    private T data;

    private ServerResponse(int status){
        this.status = status;
    }
    private ServerResponse(int status,T data){
        this.status = status;
        this.data = data;
    }

    private ServerResponse(int status,String msg,T data){
        this.status = status;
        this.msg = msg;
        this.data = data;
    }

    private ServerResponse(int status,String msg){
        this.status = status;
        this.msg = msg;
    }

    @JsonIgnore
    //使之不在json序列化结果当中
    public boolean isSuccess(){
        return this.status == ResponseCode.SUCCESS.getCode();
    }

    public int getStatus(){
        return status;
    }
    public T getData(){
        return data;
    }
    public String getMsg(){
        return msg;
    }


    public static <T> ServerResponse<T> createBySuccess(){
        return new ServerResponse<T>(ResponseCode.SUCCESS.getCode());
    }

    public static <T> ServerResponse<T> createBySuccessMessage(String msg){
        return new ServerResponse<T>(ResponseCode.SUCCESS.getCode(),msg);
    }

    public static <T> ServerResponse<T> createBySuccess(T data){
        return new ServerResponse<T>(ResponseCode.SUCCESS.getCode(),data);
    }

    public static <T> ServerResponse<T> createBySuccess(String msg,T data){
        return new ServerResponse<T>(ResponseCode.SUCCESS.getCode(),msg,data);
    }


    public static <T> ServerResponse<T> createByError(){
        return new ServerResponse<T>(ResponseCode.ERROR.getCode(),ResponseCode.ERROR.getDesc());
    }


    public static <T> ServerResponse<T> createByErrorMessage(String errorMessage){
        return new ServerResponse<T>(ResponseCode.ERROR.getCode(),errorMessage);
    }

    public static <T> ServerResponse<T> createByErrorCodeMessage(int errorCode,String errorMessage){
        return new ServerResponse<T>(errorCode,errorMessage);
    }
}