定义返回 Controller 返回的对象字段和格式，用于统一数据格式，并让 SpringMVC 自动将对象序列化成 JSON 字符串后进行响应

消息结构固定

• 状态码

• 消息

• 额外数据（响应对象）

public class ApiCommonResponse<T> {
    private Integer status;
    private String msg;
    private T data;
    //常用的返回码和消息
    private static final int OK_CODE = 10000;
    private static final String OK_MSG = "SUCCESS";

    //单参构造函数：不传递参数即默认使用成功的code和msg
    public ApiCommonResponse() {
        this(OK_CODE,OK_MSG);
    }
    //三参构造函数
    public ApiCommonResponse(Integer status, String msg, T data) {
        this.status = status;
        this.msg = msg;
        this.data = data;
    }
    //双参构造函数
    public ApiCommonResponse(Integer status, String msg) {
        this.status = status;
        this.msg = msg;
    }

    //getter&setter

		//返回成功的方法
    public static <T> ApiCommonResponse<T> success(){
        return new ApiCommonResponse();
    }

    public static <T> ApiCommonResponse<T> success(T result){
        ApiCommonResponse<T> response = new ApiCommonResponse<>();
        response.setData(result);
        return response;
    }
		//返回失败的方法
    public static <T> ApiCommonResponse<T> error(ExceptionEnum ee){
        return new ApiCommonResponse<>(ee.getCode(),ee.getMsg());
    }
    public static <T> ApiCommonResponse<T> error(Integer status,String msg){
        return new ApiCommonResponse<>(status,msg);
    }
}

HashMap 扩展消息结构

public class R extends HashMap<String, Object> {

    public R() {
        put("code", HttpStatus.SC_OK);
        put("msg", "success");
    }

    @Override
    public R put(String key, Object value) {
        super.put(key, value);
        return this;
    }

    public static R ok() {
        return new R();
    }

    public static R ok(String msg) {
        R r = new R();
        r.put("msg", msg);
        return r;
    }

    public static R ok(Map<String, Object> map) {
        R r = new R();
        r.putAll(map);
        return r;
    }

    public static R error(int code, String msg) {
        R r = new R();
        r.put("code", code);
        r.put("msg", msg);
        return r;
    }

    public static R error(String msg) {
        return error(HttpStatus.SC_INTERNAL_SERVER_ERROR, msg);
    }

    public static R error() {
        return error(HttpStatus.SC_INTERNAL_SERVER_ERROR, "未知异常，请联系管理员");
    }

}