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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