自定义统一api返回json格式（app后台框架搭建三）

Posted 2020-09-26 北冥大帝

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在统一json自定义格式的方式有多种：1，直接重写@reposeBody的实现，2，自定义一个注解，自己去解析对象成为json字符串进行返回

第一种方式，我就不推荐，想弄得的话，可以自己去研究一下源码

第二种方式，主要通过定义注解，通过 HandlerMethodReturnValueHandler 对返回值的处理，而不让他进去viewResolver处理

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1，讲解HandlerMethodReturnValueHandler 处理原理（简单介绍，网上资料很多）

2，如何实现自定义统一的json返回格式

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1，HandlerMethodReturnValueHandler

HandlerMethodReturnValueHandler是RequestMappingHandlerAdapter用来处理完映射控制类方法返回的值处理，RequestMappingHandlerAdapter

类包含默认的值处理器链的所有处理引擎，默认是会加入handlers.add(new ModelAttributeMethodProcessor(true)); getDefaultReturnValueHandlers()方法里可以查看到所有默认的处理引擎。

对处理后的对象，会调用RequestMappingHandlerAdapter里invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod)

方法：

protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
            HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

        ServletWebRequest webRequest = new ServletWebRequest(request, response);
        try {
            WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
            ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

            ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
            invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
            invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
            invocableMethod.setDataBinderFactory(binderFactory);
            invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

            ModelAndViewContainer mavContainer = new ModelAndViewContainer();
            mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
            modelFactory.initModel(webRequest, mavContainer, invocableMethod);
            mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

            AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
            asyncWebRequest.setTimeout(this.asyncRequestTimeout);

            WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
            asyncManager.setTaskExecutor(this.taskExecutor);
            asyncManager.setAsyncWebRequest(asyncWebRequest);
            asyncManager.registerCallableInterceptors(this.callableInterceptors);
            asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

            if (asyncManager.hasConcurrentResult()) {
                Object result = asyncManager.getConcurrentResult();
                mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
                asyncManager.clearConcurrentResult();
                if (logger.isDebugEnabled()) {
                    logger.debug("Found concurrent result value [" + result + "]");
                }
                invocableMethod = invocableMethod.wrapConcurrentResult(result);
            }

            invocableMethod.invokeAndHandle(webRequest, mavContainer);
            if (asyncManager.isConcurrentHandlingStarted()) {
                return null;
            }

            return getModelAndView(mavContainer, modelFactory, webRequest);
        }
        finally {
            webRequest.requestCompleted();
        }
    }

关键是调用 invocableMethod.invokeAndHandle(webRequest, mavContainer);

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
        setResponseStatus(webRequest);

        if (returnValue == null) {
            if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
                mavContainer.setRequestHandled(true);
                return;
            }
        }
        else if (StringUtils.hasText(getResponseStatusReason())) {
            mavContainer.setRequestHandled(true);
            return;
        }

        mavContainer.setRequestHandled(false);
        try {
            this.returnValueHandlers.handleReturnValue(
                    returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
        }
        catch (Exception ex) {
            if (logger.isTraceEnabled()) {
                logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
            }
            throw ex;
        }
    }

首先设置 mavContainer.setRequestHandled(false);说明是会按处理链进行处理，如果设置为true就是处理完就结束了。

 this.returnValueHandlers.handleReturnValue(
                    returnValue, getReturnValueType(returnValue), mavContainer, webRequest); 是真正获取处理链，然后从处理链中选择合适的引擎并依次处理。

HandlerMethodReturnValueHandler 只提供两个接口，

public interface HandlerMethodReturnValueHandler {
   boolean supportsReturnType(MethodParameter returnType);
   void handleReturnValue(Object returnValue, MethodParameter returnType,
         ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
}

第一方法是判断是否支持值处理引擎，第二方法是对值进行处理，并确定是否继续进行下一个处理引擎执行。

所以，第一个supportsReturnType（MethodParameter returnType）的判断一定要正确，如果不正确，就永远不会被执行，这是关键。

具体的执行，就可以按这几个关键的类去debug一下就可以了。

-----------------------------------------------------------------------------------------------------------------

2，具体的实现

2.1 定义注解：用于识别json转换处理的

import java.lang.annotation.*;

/**
 * Created by ThinkPad on 2017/6/22.
 */
@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface AppResponsBody {
    String type() default "json";
}

2.2 处理引擎的：需要实现 HandlerMethodReturnValueHandler

/**
 * Created by ThinkPad on 2017/6/22.
 */
public class FormatJsonReturnValueHandler implements HandlerMethodReturnValueHandler{
    @Override
    public boolean supportsReturnType(MethodParameter returnType) {
        System.out.println("===========sdfdsf==============="+ returnType.getMethodAnnotation(AppResponsBody.class));
        boolean hasJSONAnno = returnType.getMethodAnnotation(AppResponsBody.class) != null || returnType.getMethodAnnotation(AppResponsBody.class) != null;
        return hasJSONAnno;
    }

    @Override
    public void handleReturnValue(Object obj, MethodParameter methodParameter, ModelAndViewContainer modelAndViewContainer,
                                  NativeWebRequest nativeWebRequest) throws Exception {
           modelAndViewContainer.setRequestHandled(true);
          AppResponsBody res=methodParameter.getMethodAnnotation(AppResponsBody.class);
          String type = res.type();
        HttpServletResponse response=nativeWebRequest.getNativeResponse(HttpServletResponse.class);
        response.setContentType(MediaType.APPLICATION_JSON_UTF8_VALUE);
        PrintWriter writer = null;
        Gson gson=new Gson();
        ResultInfo info=new ResultInfo();
         info.setData(obj);
        try {
            writer = response.getWriter();
            writer.write(gson.toJson(info));
            writer.flush();
        } catch (IOException ex) {
            ex.printStackTrace();
        } finally {
            if (writer != null)
                writer.close();
        }
      }

}

2.3 注册你编写的HandlerMethodReturnValueHandler引擎到webConfig配置文件里

@Configuration
@EnableWebMvc
@ComponentScan(basePackages = {"com.ouyang.teson"},useDefaultFilters = true)
public class WebConfig extends WebMvcConfigurerAdapter{

配置文件里增加以下：

@Bean
public FormatJsonReturnValueHandler JsonReturnHandler(){
    FormatJsonReturnValueHandler formatJsonReturnValueHandler=new FormatJsonReturnValueHandler();
    return formatJsonReturnValueHandler;
}
@Override
public void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> returnValueHandlers) {
    returnValueHandlers.add(JsonReturnHandler());
}

另外，还需要定义一个响应类处理统一格式的：ResultInfo.java

public class ResultInfo {

    public long code;
    public String message;
    public Object data;

    public long getCode() {
        if(code==0l){
            code=200l;
        }
        return code;
    }

    public void setCode(long code) {
        this.code = code;
    }

    public String getMessage() {
        if(message==null){
            message="处理成功！";
        }
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }
}