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通常情况下，如果我们的API接口需要返回 JSON、XML 等格式化的数据时，只需要在控制器上注解@ResponseBody且指定produces = "application/json;charset=UTF-8"数据类型即可，参考代码如下：

/**
 * 登录提交
 */
@RequestMapping(value = "loginSubmit", produces = "application/json;charset=UTF-8")
@ResponseBody
public String loginSubmit(LoginRequest loginRequest) {
    // 登录的业务逻辑
    //......
    
    return "{"\code\": \"0\", \"msg\": \"登录成功\"}"
}

但是每个接口都加上这段代码：produces = "application/json;charset=UTF-8"，貌似不那么清爽，于是想到是否能在“拦截器”里做些手脚，让它自动输出这个JSON文档类型（即 Http Json头信息），但实践证明，针对 @ResponseBody，在“拦截器”中的设置是无效的，如：

package com.wenjianbao.interceptor;

import cn.hutool.core.util.StrUtil;
import org.springframework.lang.Nullable;
import org.springframework.web.servlet.handler.HandlerInterceptorAdapter;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * 系统拦截器
 */
public class SystemInterceptor extends HandlerInterceptorAdapter {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        // 针对 @ResponseBody 标注的控制器，这段代码是无效的！
        response.setHeader("Content-Type", "application/json;charset=UTF-8");
        return true;
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable Exception ex) throws Exception {
        // 针对 @ResponseBody 标注的控制器，这段代码是无效的！
        response.setHeader("Content-Type", "application/json;charset=UTF-8");
    }
    
}

通过这篇文章：SpringBoot ResponseBodyAdvice 接口实现自定义返回数据类型（响应头）学习到，

通过实现ResponseBodyAdvice接口，可对返回的字符串（json/xml等）对象，做一些强制篡改，包括 Http头信息的修改，参考代码如下：

package com.wanma.printer.interceptor;

import cn.hutool.core.util.StrUtil;
import com.wanma.printer.util.RequestUtil;
import org.springframework.core.MethodParameter;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.server.ServerHttpRequest;
import org.springframework.http.server.ServerHttpResponse;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;

@ControllerAdvice
public class ResponseBodyInterceptor implements ResponseBodyAdvice<Object> {
    /**
     *  判断哪些需要拦截
     */
    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return true;
    }

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
        if (body instanceof String) {
            String bodyStr = String.valueOf(body);
            
            // 输出 XML 头信息
            String inAjax = RequestUtil.getRequest().getParameter("inAjax");
            if (inAjax != null && inAjax.equals("1")) {
                response.getHeaders().setContentType(MediaType.TEXT_XML);
            }

            // 输出 JSON 头信息
            if (StrUtil.startWith(bodyStr, "{") && StrUtil.endWith(bodyStr, "}")) {
                response.getHeaders().setContentType(MediaType.APPLICATION_JSON);
            }
        }
        
        return body;
    }
}

以后，Api控制器，就无需加上：produces = "application/json;charset=UTF-8"，简化如下：

/**
 * 登录提交
 */
@RequestMapping(value = "loginSubmit")
@ResponseBody
public String loginSubmit(LoginRequest loginRequest) {
    // 登录的业务逻辑
    //......
    
    return "{"\code\": \"0\", \"msg\": \"登录成功\"}"
}