详解SpringBoot定制@ResponseBody注解返回的Json格式

 1、引言

在SpringMVC的使用中，后端与前端的交互一般是使用Json格式进行数据传输，SpringMVC的@ResponseBody注解可以很好的帮助我们进行转换，但是后端返回数据给前端往往都有约定固定的格式，这时候我们在后端返回的时候都要组拼成固定的格式，每次重复的操作非常麻烦。

2、SpringMVC对@ResponseBody的处理

SpringMVC处理@ResponseBody注解声明的Controller是使用默认的.RequestResponseBodyMethodProcessor类来实现，RequestResponseBodyMethodProcessor类实现了HandlerMethodReturnValueHandler接口并实现了接口中的supportsReturnType()和handleReturnValue()方法。

/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.method.support;

import org.springframework.core.MethodParameter;
import org.springframework.lang.Nullable;
import org.springframework.web.context.request.NativeWebRequest;

/**
 * Strategy interface to handle the value returned from the invocation of a
 * handler method .
 *
 * @author Arjen Poutsma
 * @since 3.1
 * @see HandlerMethodArgumentResolver
 */
public interface HandlerMethodReturnValueHandler {

 /**
 * Whether the given {@linkplain MethodParameter method return type} is
 * supported by this handler.
 * @param returnType the method return type to check
 * @return {@code true} if this handler supports the supplied return type;
 * {@code false} otherwise
 */
 boolean supportsReturnType(MethodParameter returnType);

 /**
 * Handle the given return value by adding attributes to the model and
 * setting a view or setting the
 * {@link ModelAndViewContainer#setRequestHandled} flag to {@code true}
 * to indicate the response has been handled directly.
 * @param returnValue the value returned from the handler method
 * @param returnType the type of the return value. This type must have
 * previously been passed to {@link #supportsReturnType} which must
 * have returned {@code true}.
 * @param mavContainer the ModelAndViewContainer for the current request
 * @param webRequest the current request
 * @throws Exception if the return value handling results in an error
 */
 void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
  ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;

}

3、实现思路

知道@ResponseBody是由RequestResponseBodyMethodProcessor进行处理的，这时候我们可以自己定义一个处理返回数据的Handler来实现我们的定制化Json格式数据返回，但是如果直接把我们定制的Handler加入到SpringMVC的ReturnValueHandlers中，因为我们定制的Handler在RequestResponseBodyMethodProcessor之后，所以我们定制的Handler还是不会生效，这时候我们可以想办法把RequestResponseBodyMethodProcessor替换成我们定制的Handler。

4、代码实现

4.1、定制Json返回格式实体

package com.autumn.template;

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.experimental.Accessors;

/**
 * JSON信息交互对象模板
 * @Author Autumn、
 * @Date 2019/4/8 23:46
 * @Description
 */
@Setter
@Getter
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Result implements BaseBean {

  ......(这里只展示一些必要字段)
  /** 响应码 */
  private Integer code;
  /** 响应信息 */
  private String message;
  /** 数据 */
  private Object data;
  /** 请求地址 */
  private String url;

  ......
}

4.2、定义定制Json返回格式Handler

package com.autumn.component.handler;

import com.autumn.template.Result;

import org.springframework.core.MethodParameter;
import org.springframework.lang.Nullable;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
import org.springframework.web.method.support.ModelAndViewContainer;

/**
 * 统一处理ResponseBody数据格式
 * @Author: Autumn、
 * @Date: 2019/4/24 23:59
 * @Description:
 **/
public class ResultWarpReturnValueHandler implements HandlerMethodReturnValueHandler {

  private final HandlerMethodReturnValueHandler delegate;

  /** 委托 */
  public ResultWarpReturnValueHandler(HandlerMethodReturnValueHandler delegate) {
    this.delegate = delegate;
  }

  /**
   * 判断返回类型是否需要转成字符串返回
   * @param returnType 方法返回类型
   * @return 需要转换返回true，否则返回false
   */
  @Override
  public boolean supportsReturnType(MethodParameter returnType) {
    return delegate.supportsReturnType(returnType);
  }

  /**
   * 返回值转换
   */
  @Override
  public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
   // 委托SpringMVC默认的RequestResponseBodyMethodProcessor进行序列化
    delegate.handleReturnValue(returnValue instanceof Result ? returnValue : Result.succeed(returnValue), returnType, mavContainer, webRequest);
  }
}

4.3、替换默认的RequestResponseBodyMethodProcessor

package com.autumn.config;

import com.autumn.component.handler.ResultWarpReturnValueHandler;

import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.cache.annotation.EnableCaching;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;
import org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor;

import java.util.ArrayList;
import java.util.List;

import lombok.extern.slf4j.Slf4j;

/**
 * 替换默认的RequestResponseBodyMethodProcessor
 * @Author Autumn、
 * @Date 2019/4/8 23:46
 * @Description
 */
@Slf4j
@Configuration
@EnableCaching
public class ApplicationContext implements WebMvcConfigurer, InitializingBean {

  @Autowired(required = false)
  private RequestMappingHandlerAdapter adapter;

  @Override
  public void afterPropertiesSet() throws Exception {
    // 获取SpringMvc的ReturnValueHandlers
    List<HandlerMethodReturnValueHandler> returnValueHandlers = adapter.getReturnValueHandlers();
    // 新建一个List来保存替换后的Handler的List
    List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(returnValueHandlers);
    // 循环遍历找出RequestResponseBodyMethodProcessor
    for (HandlerMethodReturnValueHandler handler : handlers) {
      if (handler instanceof RequestResponseBodyMethodProcessor) {
       // 创建定制的Json格式处理Handler
        ResultWarpReturnValueHandler decorator = new ResultWarpReturnValueHandler(handler);
        // 使用定制的Json格式处理Handler替换原有的RequestResponseBodyMethodProcessor
        int index = handlers.indexOf(handler);
        handlers.set(index, decorator);
        break;
      }
    }
    // 重新设置SpringMVC的ReturnValueHandlers
    adapter.setReturnValueHandlers(handlers);
  }
}

5、总结

至此完成了定制@ResponseBody注解返回的Json格式，在Controller中返回任何的字符串都可以定制成为我们想要的Json格式。此外SpringMVC还提供了非常多的Handler接口来进行Controller的增强，可以使用此思路对参数等进行定制化。

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