关于 Spring 的全局处理，有两方面要说:
- 统一数据返回格式
- 统一异常处理
通用返回值类定义：
public class GlobalResponse<T> implements POJO {
/**
*
*/
private static final long serialVersionUID = 1L;
@ApiModelProperty(notes = "数据")
private T data;
@ApiModelProperty(notes = "不为空。等于200时表示业务成功，其他表示业务失败")
private int code = 200;
@ApiModelProperty(notes = "错误信息，如果不为空，展示给用户")
private String msg;
public GlobalResponse() {
}
}配置
没错，我们需要借助几个关键注解来完成一下相关配置:
@EnableWebMvc
public class UnifiedResponseHandler {
@RestControllerAdvice
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 returnType, MediaType selectedContentType,
Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request,
ServerHttpResponse response) {
if (body instanceof GlobalResponse) {
// 兼容旧版本的数据，已经用GlobalResponse封装好了的，就不用再进行处理了
return body;
} else if (body instanceof POJO) {
// 目前只针对POJO的返回对象进行封装
return new GlobalResponse<Object>(body);
} else {
return body;
}
}
}
}到这里就结束了，我们就可以纵情的写任何 RESTful API 了，所有的返回值都会有统一的 JSON 结构
解剖实现过程
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 {
private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();
@Autowired(required = false)
public void setConfigurers(List<WebMvcConfigurer> configurers) {
if (!CollectionUtils.isEmpty(configurers)) {
this.configurers.addWebMvcConfigurers(configurers);
}
}
@Override
protected void configurePathMatch(PathMatchConfigurer configurer) {
this.configurers.configurePathMatch(configurer);
}
@Override
protected void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
this.configurers.configureContentNegotiation(configurer);
}
@Override
protected void configureAsyncSupport(AsyncSupportConfigurer configurer) {
this.configurers.configureAsyncSupport(configurer);
}
@Override
protected void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
this.configurers.configureDefaultServletHandling(configurer);
}
@Override
protected void addFormatters(FormatterRegistry registry) {
this.configurers.addFormatters(registry);
}
@Override
protected void addInterceptors(InterceptorRegistry registry) {
this.configurers.addInterceptors(registry);
}
@Override
protected void addResourceHandlers(ResourceHandlerRegistry registry) {
this.configurers.addResourceHandlers(registry);
}
@Override
protected void addCorsMappings(CorsRegistry registry) {
this.configurers.addCorsMappings(registry);
}
@Override
protected void addViewControllers(ViewControllerRegistry registry) {
this.configurers.addViewControllers(registry);
}
@Override
protected void configureViewResolvers(ViewResolverRegistry registry) {
this.configurers.configureViewResolvers(registry);
}
@Override
protected void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers) {
this.configurers.addArgumentResolvers(argumentResolvers);
}
@Override
protected void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> returnValueHandlers) {
this.configurers.addReturnValueHandlers(returnValueHandlers);
}
@Override
protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
this.configurers.configureMessageConverters(converters);
}
@Override
protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
this.configurers.extendMessageConverters(converters);
}
@Override
protected void configureHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {
this.configurers.configureHandlerExceptionResolvers(exceptionResolvers);
}
@Override
protected void extendHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {
this.configurers.extendHandlerExceptionResolvers(exceptionResolvers);
}
@Override
@Nullable
protected Validator getValidator() {
return this.configurers.getValidator();
}
@Override
@Nullable
protected MessageCodesResolver getMessageCodesResolver() {
return this.configurers.getMessageCodesResolver();
}
}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();
adapter.setContentNegotiationManager(mvcContentNegotiationManager());
adapter.setMessageConverters(getMessageConverters());
adapter.setWebBindingInitializer(getConfigurableWebBindingInitializer());
adapter.setCustomArgumentResolvers(getArgumentResolvers());
adapter.setCustomReturnValueHandlers(getReturnValueHandlers());
if (jackson2Present) {
adapter.setRequestBodyAdvice(Collections.singletonList(new JsonViewRequestBodyAdvice()));
adapter.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));
}
AsyncSupportConfigurer configurer = new AsyncSupportConfigurer();
configureAsyncSupport(configurer);
if (configurer.getTaskExecutor() != null) {
adapter.setTaskExecutor(configurer.getTaskExecutor());
}
if (configurer.getTimeout() != null) {
adapter.setAsyncRequestTimeout(configurer.getTimeout());
}
adapter.setCallableInterceptors(configurer.getCallableInterceptors());
adapter.setDeferredResultInterceptors(configurer.getDeferredResultInterceptors());
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，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()) {
("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UnifiedResponseHandler中实际实现上使用的是 @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是这样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」，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 来处理不同类型的返回值
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!!!
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