通过对源码解读, 来了解 Spring 异常处理机制
分析一下常见的 starter 配置原理
众所周知, 如果我们想自定义一个 springboot 的 starter, 那么需要在编码完成之后, 在 META-INF 文件夹中编写个 spring.factories 文件, 其内需要 指定改 starter 的入口配置在哪:
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org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gomyck.jdbc.converter.GomyckConverterConfiguration
那其实仔细看这个配置, 不难发现, EnableAutoConfiguration其实是一个类, 其是 springboot autoconfigure 包下的一个注解, 而且应用到了 importSelector
其 import 的值就是
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@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
那么 spring.factories 是如何被 spring 加载并执行的呢?
其实 spring framework 是提供一种类似 java SPI 的一种动态加载机制的: SpringFactoriesLoader, 该类可以加载.factories 文件, 并且收集相关配置信息
通过代码跟踪, 我们能看到, AutoConfigurationImportSelector这个类的
selectImports -> getAutoConfigurationEntry -> fireAutoConfigurationImportEvents -> getAutoConfigurationImportListeners
通过上述方法, 把所有的 starter 入口类加载实例化
而AutoConfigurationImportSelector这个类是如何被加载实例化的呢? 看注解: @EnableAutoConfiguration, 这个注解被包含在: @SpringBootApplication 之中, 被显式声明到服务函数入口类中
了解 spring 异常处理机制
spring 中如果想拦截并处理指定异常, 需要声明类, 并使用注解: @RestControllerAdvice 或 @ControllerAdvice
通过追踪两个注解, 最终可得到一个该注解的包装类: ControllerAdviceBean, 该类描述了一个 controller 增强类的一些详细信息, 包括名称, 包名等信息
追踪该类可得到, 该类是由 ExceptionHandlerExceptionResolver 初始化, 追踪该类, 可看到, 该类实现了 InitializingBean, 在初始化之后, 扫描了上下文, 过滤出有ControllerAdvice注解的 bean
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private void initExceptionHandlerAdviceCache() {
if (getApplicationContext() == null) {
return;
}
List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
for (ControllerAdviceBean adviceBean : adviceBeans) {
Class<?> beanType = adviceBean.getBeanType();
if (beanType == null) {
throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
}
ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
if (resolver.hasExceptionMappings()) {
this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
}
if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
this.responseBodyAdvice.add(adviceBean);
}
}
if (logger.isDebugEnabled()) {
int handlerSize = this.exceptionHandlerAdviceCache.size();
int adviceSize = this.responseBodyAdvice.size();
if (handlerSize == 0 && adviceSize == 0) {
logger.debug("ControllerAdvice beans: none");
}
else {
logger.debug("ControllerAdvice beans: " +
handlerSize + " @ExceptionHandler, " + adviceSize + " ResponseBodyAdvice");
}
}
}
并且, 初始化了内部的参数解析器(argumentResolver) 和 返回值处理器(returnValueHandler), 感兴趣的可以看看这两个处理器的作用
而具体的处理异常的方法块, 可以去看 mvc 的核心类: dispatchServlet
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// line: 1125
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
其他的异常处理器
DefaultErrorAttributes 这个类, 在 ErrorMvcAutoConfiguration 中初始化了实例, 其主要作用为配合 BasicErrorController 组合使用(我们平时看见的 spring 错误白页)
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@Override
public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object handler,
Exception ex) {
storeErrorAttributes(request, ex);
return null;
}
private void storeErrorAttributes(HttpServletRequest request, Exception ex) {
request.setAttribute(ERROR_ATTRIBUTE, ex);
}
@Override
public Map<String, Object> getErrorAttributes(WebRequest webRequest, ErrorAttributeOptions options) {
Map<String, Object> errorAttributes = getErrorAttributes(webRequest, options.isIncluded(Include.STACK_TRACE));
if (Boolean.TRUE.equals(this.includeException)) {
options = options.including(Include.EXCEPTION);
}
if (!options.isIncluded(Include.EXCEPTION)) {
errorAttributes.remove("exception");
}
if (!options.isIncluded(Include.STACK_TRACE)) {
errorAttributes.remove("trace");
}
if (!options.isIncluded(Include.MESSAGE) && errorAttributes.get("message") != null) {
errorAttributes.put("message", "");
}
if (!options.isIncluded(Include.BINDING_ERRORS)) {
errorAttributes.remove("errors");
}
return errorAttributes;
}
DefaultHandlerExceptionResolver这个类, 可以为异步请求返回各种不支持的错误:
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@Override
@Nullable
protected ModelAndView doResolveException(
HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {
try {
if (ex instanceof HttpRequestMethodNotSupportedException) {
return handleHttpRequestMethodNotSupported(
(HttpRequestMethodNotSupportedException) ex, request, response, handler);
}
else if (ex instanceof HttpMediaTypeNotSupportedException) {
return handleHttpMediaTypeNotSupported(
(HttpMediaTypeNotSupportedException) ex, request, response, handler);
}
else if (ex instanceof HttpMediaTypeNotAcceptableException) {
return handleHttpMediaTypeNotAcceptable(
(HttpMediaTypeNotAcceptableException) ex, request, response, handler);
}
else if (ex instanceof MissingPathVariableException) {
return handleMissingPathVariable(
(MissingPathVariableException) ex, request, response, handler);
}
else if (ex instanceof MissingServletRequestParameterException) {
return handleMissingServletRequestParameter(
(MissingServletRequestParameterException) ex, request, response, handler);
}
else if (ex instanceof ServletRequestBindingException) {
return handleServletRequestBindingException(
(ServletRequestBindingException) ex, request, response, handler);
}
else if (ex instanceof ConversionNotSupportedException) {
return handleConversionNotSupported(
(ConversionNotSupportedException) ex, request, response, handler);
}
else if (ex instanceof TypeMismatchException) {
return handleTypeMismatch(
(TypeMismatchException) ex, request, response, handler);
}
else if (ex instanceof HttpMessageNotReadableException) {
return handleHttpMessageNotReadable(
(HttpMessageNotReadableException) ex, request, response, handler);
}
else if (ex instanceof HttpMessageNotWritableException) {
return handleHttpMessageNotWritable(
(HttpMessageNotWritableException) ex, request, response, handler);
}
else if (ex instanceof MethodArgumentNotValidException) {
return handleMethodArgumentNotValidException(
(MethodArgumentNotValidException) ex, request, response, handler);
}
else if (ex instanceof MissingServletRequestPartException) {
return handleMissingServletRequestPartException(
(MissingServletRequestPartException) ex, request, response, handler);
}
else if (ex instanceof BindException) {
return handleBindException((BindException) ex, request, response, handler);
}
else if (ex instanceof NoHandlerFoundException) {
return handleNoHandlerFoundException(
(NoHandlerFoundException) ex, request, response, handler);
}
else if (ex instanceof AsyncRequestTimeoutException) {
return handleAsyncRequestTimeoutException(
(AsyncRequestTimeoutException) ex, request, response, handler);
}
}
catch (Exception handlerEx) {
if (logger.isWarnEnabled()) {
logger.warn("Failure while trying to resolve exception [" + ex.getClass().getName() + "]", handlerEx);
}
}
return null;
}
springboot 中使用 @ControllerAdvice 来处理异常
如果使用 @ControllerAdvice + @ExceptionHandler + @ResponseBody + @ResponseStatus(HttpStatus.OK) 来处理跨域请求异常, HttpStatus 必须是 200, 否则浏览器则认为跨域请求被拒绝, 导致浏览器 302 错误