Every day SpringBoot, still do not understand RESTful API returns a unified data format is how to achieve?

Spring on the global process, I would say there are two aspects:

Unified data format to return
Unified exception handling
in order to clearly illustrate two issues, will be divided into two chapters illustrate, this chapter said first point

There are children's shoes, we have done a project of this treatment, that is, each individually API Tools will return the value package, but this is not elegant; I wanted to write a minimum of code to get this done, there may be children's shoes, he said, add a few notes to solve the problem, he said yes, but this article is mainly to explain why a few add annotations to solve the problem in the hope that you know why .

In order to better illustrate the problem, we first explain how to achieve, and then a detailed analysis of realization of the principle (which is crucial)

Why do unified data format to return

Separating the front and rear end of today's mainstream form of service, how to design a good API RESTful , and how to make the front end of a small partner can handle standard response JSON data structures are essential in order to allow a better front-end logic to interact with the page display processing , every RESTful requests should contain the following information:

name	description
status	Status code, identification request success, such as [1: success; -1: Failed]
errorCode	Error code, give a clear error code, better respond to business exceptions; the request is successful this value may be null
errorMsg	Error message, the error code corresponding to a more particular description of the abnormality information
resultBody	Return results typically Bean JSON data corresponding to the object, usually in order to deal with different types of return value, which is a generic type declaration

achieve

Return value generic class definition

According to the above description, with the Java Bean to reflect this structure is this:

@Data
public final class CommonResult<T> {

    private int status = 1;

    private String errorCode = "";

    private String errorMsg = "";

    private T resultBody;

    public CommonResult() {
    }

    public CommonResult(T resultBody) {
        this.resultBody = resultBody;
    }
}

Configuration

Yes, we need the help of a few key notes to complete what configuration:

@EnableWebMvc
@Configuration
public class UnifiedReturnConfig {

    @RestControllerAdvice("com.example.unifiedreturn.api")
    static class CommonResultResponseAdvice implements ResponseBodyAdvice<Object>{
        @Override
        public boolean supports(MethodParameter methodParameter, Class<? extends HttpMessageConverter<?>> aClass) {
            return true;
        }

        @Override
        public Object beforeBodyWrite(Object body, MethodParameter methodParameter, MediaType mediaType, Class<? extends HttpMessageConverter<?>> aClass, ServerHttpRequest serverHttpRequest, ServerHttpResponse serverHttpResponse) {
            if (body instanceof CommonResult){
                return body;
            }

            return new CommonResult<Object>(body);
        }
    }
}

To end here, we can indulge in any write RESTful API, all return values will have a unified structure JSON

test

The UserController new, add the appropriate RESTful API, written in relatively simple test, just to illustrate the process returns value

@RestController
@RequestMapping("/users")
public class UserController {


    @GetMapping("")
    public List<UserVo> getUserList(){
        List<UserVo> userVoList = Lists.newArrayListWithCapacity(2);
        userVoList.add(UserVo.builder().id(1L).name("日拱一兵").age(18).build());
        userVoList.add(UserVo.builder().id(2L).name("tan").age(19).build());
        return userVoList;
    }
}

打开浏览器输入地址测试: http://localhost:8080/users/ ，我们可以看到返回了 List JSON 数据

继续添加 RESTful API，根据用户 ID 查询用户信息

@GetMapping("/{id}")
public UserVo getUserByName(@PathVariable Long id){
    return UserVo.builder().id(1L).name("日拱一兵").age(18).build();
}

打开浏览器输入地址测试: http://localhost:8080/users/1 ，我们可以看到返回了单个 User JSON 数据

添加一个返回值类型为 ResponseEntity 的 API

@GetMapping("/testResponseEntity")
public ResponseEntity getUserByAge(){
    return new ResponseEntity(UserVo.builder().id(1L).name("日拱一兵").age(18).build(), HttpStatus.OK);
}

打开浏览器输入地址测试: http://localhost:8080/users/testResponseEntity ，我们可以看到同样返回了单个 User JSON 数据

解剖实现过程

我会将关键部分一一说明清楚，断案还需小伙伴自己去案发现场(打开自己的 IDE 查看)

故事要从 @EnableWebMvc 这个注解说起，打开该注解看:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}

通过 @Import 注解引入了 DelegatingWebMvcConfiguration.class，那来看这个类吧:

@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
    ...
}

有 @Configuration 注解，你应该很熟悉了，该类的父类 WebMvcConfigurationSupport 中却隐藏着一段关键代码:

@Bean
public RequestMappingHandlerAdapter requestMappingHandlerAdapter() {
    RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter();
    ...
    return adapter;
}

RequestMappingHandlerAdapter 是每一次请求处理的关键，来看该类的定义:

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
        implements BeanFactoryAware, InitializingBean {
    ...
}

该类实现了 InitializingBean 接口，我在 Spring Bean 生命周期之“我从哪里来”？这篇文章中明确说明了 Spring Bean 初始化的几个关键，其中 InitializingBean 接口的
afterPropertiesSet 方法就是关键之一，在 RequestMappingHandlerAdapter 类中同样重写了该方法:

@Override
public void afterPropertiesSet() {
    // Do this first, it may add ResponseBody advice beans
    initControllerAdviceCache();

    if (this.argumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.initBinderArgumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
        this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.returnValueHandlers == null) {
        List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
        this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
    }
}

该方法内容都非常关键，但我们先来看 initControllerAdviceCache 方法，其他内容后续再单独说明:

private void initControllerAdviceCache() {
        ...
    if (logger.isInfoEnabled()) {
        logger.info("Looking for @ControllerAdvice: " + getApplicationContext());
    }

    List<ControllerAdviceBean> beans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
    AnnotationAwareOrderComparator.sort(beans);

    List<Object> requestResponseBodyAdviceBeans = new ArrayList<Object>();

    for (ControllerAdviceBean bean : beans) {
        ...
        if (ResponseBodyAdvice.class.isAssignableFrom(bean.getBeanType())) {
            requestResponseBodyAdviceBeans.add(bean);
        }
    }
}

通过 ControllerAdviceBean 静态方法扫描 ControllerAdvice 注解，可是我们在实现上使用的是 @RestControllerAdvice 注解，打开看该注解:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@ControllerAdvice
@ResponseBody
public @interface RestControllerAdvice {

该注解由 @ControllerAdvice 和 @ResponseBody 标记，就好比你熟悉的 @RestController 注解由 @Controller 和 @ResponseBody 标记是一样的

到这里你已经知道我们用 @RestControllerAdvice 标记的 Bean 是如何被加载到 Spring 上下文的，接下来就要知道是 Spring 是如何使用我们的 bean 以及对返回 body 做处理的

其实在 HttpMessageConverter是如何转换数据的？这篇文章中已经说明了一部分，希望小伙伴先看这篇文章，下面的部分就会秒懂了，我们在这里做进一步的说明

在 AbstractMessageConverterMethodProcessor 的 writeWithMessageConverters 方法中，有一段核心代码:

if (messageConverter instanceof GenericHttpMessageConverter) {
    if (((GenericHttpMessageConverter) messageConverter).canWrite(
            declaredType, valueType, selectedMediaType)) {
        outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                inputMessage, outputMessage);
            ...
        return;
    }
}

可以看到通过 getAdvice() 调用了 beforeBodyWrite 方法，我们已经接近真相了

protected RequestResponseBodyAdviceChain getAdvice() {
    return this.advice;
}

RequestResponseBodyAdviceChain，看名字带有 Chain，很明显用到了「责任链设计模式」，这些内容在不得不知的责任链设计模式文章中明确说明过，只不过它传递责任链以循环的方式完成:

class RequestResponseBodyAdviceChain implements RequestBodyAdvice, ResponseBodyAdvice<Object> {

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType contentType,
            Class<? extends HttpMessageConverter<?>> converterType,
            ServerHttpRequest request, ServerHttpResponse response) {

        return processBody(body, returnType, contentType, converterType, request, response);
    }

    @SuppressWarnings("unchecked")
    private <T> Object processBody(Object body, MethodParameter returnType, MediaType contentType,
            Class<? extends HttpMessageConverter<?>> converterType,
            ServerHttpRequest request, ServerHttpResponse response) {

        for (ResponseBodyAdvice<?> advice : getMatchingAdvice(returnType, ResponseBodyAdvice.class)) {
            if (advice.supports(returnType, converterType)) {
                body = ((ResponseBodyAdvice<T>) advice).beforeBodyWrite((T) body, returnType,
                        contentType, converterType, request, response);
            }
        }
        return body;
    }
}

我们重写的 beforeBodyWrite 方法终究会被调用到，真相就是这样了!!!

其实还没完，你有没有想过，如果我们的 API 方法返回值是 org.springframework.http.ResponseEntity<T> 类型，我们可以指定 HTTP 返回状态码，但是这个返回值会直接放到我们的 beforeBodyWrite 方法的 body 参数中吗？如果这样做很明显是错误的，因为 ResponseEntity 包含很多我们非业务数据在里面，那 Spring 是怎么帮我们处理的呢？

在我们方法取得返回值并且在调用 beforeBodyWrite 方法之前，还要选择 HandlerMethodReturnValueHandler 用于处理不同的 Handler 来处理返回值

在类 HandlerMethodReturnValueHandlerComposite 中的 handleReturnValue 方法中

@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

    HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
    if (handler == null) {
        throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
    }
    handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}

通过调用 selectHandler 方法来选择合适的 handler，Spring 内置了很多个 Handler，我们来看类图:

HttpEntityMethodProcessor 就是其中之一，它重写了 supportsParameter 方法，支持 HttpEntity 类型，即支持 ResponseEntity 类型:

@Override
public boolean supportsParameter(MethodParameter parameter) {
    return (HttpEntity.class == parameter.getParameterType() ||
            RequestEntity.class == parameter.getParameterType());
}

所以当我们返回的类型为 ResponseEntity 时，就要通过 HttpEntityMethodProcessor 的 handleReturnValue 方法来处理我们的结果:

@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

    ...
    if (responseEntity instanceof ResponseEntity) {
        int returnStatus = ((ResponseEntity<?>) responseEntity).getStatusCodeValue();
        outputMessage.getServletResponse().setStatus(returnStatus);
        if (returnStatus == 200) {
            if (SAFE_METHODS.contains(inputMessage.getMethod())
                    && isResourceNotModified(inputMessage, outputMessage)) {
                // Ensure headers are flushed, no body should be written.
                outputMessage.flush();
                // Skip call to converters, as they may update the body.
                return;
            }
        }
    }

    // Try even with null body. ResponseBodyAdvice could get involved.
    writeWithMessageConverters(responseEntity.getBody(), returnType, inputMessage, outputMessage);

    // Ensure headers are flushed even if no body was written.
    outputMessage.flush();
}

该方法提取出 responseEntity.getBody()，并传递个 MessageConverter，然后再继续调用 beforeBodyWrite 方法，这才是真相!!!

这是 RESTful API 正常返回内容的情况，下一篇文章，让我们来侦查一下统一异常情况的处理以及实现原理

灵魂追问

返回值是非 ResponseEntity 类型时，用的是什么 handler？它支持的返回值类型是什么？看过你也许就知道为什么要用 @ResponseBody 注解了
你有追踪过 DispatchServlet 的整个请求过程吗？