记一次Java Bug的解决过程

Q:Bug描述

前端form表单数据提交时，后端出现Resolved [org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/x-www-form-urlencoded;charset=UTF-8' not supported]这样的提示，也没有触发Controller的响应方法。

A: 第一种解决办法

ajax请求的时候，把Content-Type，设置为application/json; charset=utf-8

A: 第二种解决办法

自定义参数解析器：

1. 首先定义注解

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER})
public @interface P {
    // ...
}

2. 自定义参数解析器的实现

public class Form2PojoArgumentResolver implements HandlerMethodArgumentResolver
{
	@Override
	public boolean supportsParameter(MethodParameter parameter)
	{
		return parameter.hasParameterAnnotation(P.class);
	}

	@Nullable
	@Override
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception
	{
        // 返回对应的参数类型的数据 
	}
}

3. 配置参数解析器

@Configuration
public class ArgumentResolversConfig implements WebMvcConfigurer
{
    @Override
	public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers)
	{
		resolvers.add(new Form2PojoArgumentResolver());
	}
}

4. 相应的控制器的方法中

public class TestController 
{
    @RequestMapping("xxx")
    public testArgumentResolver(@P TestEntity entity) 
    {
        // ... 
    }
}

通常情况下，前端form表单的数据到Controller中时就转换成的对象。但是我这里出现了特殊情形。 由于我的TestEntity实现了Map接口，而Spring Boot在启动时会自动加载一系列的解析器，而这些解析其中有一个叫MapMethodProcessor的处理器，它的supportsParameter方法是这样实现的：

@Override
public boolean supportsParameter(MethodParameter parameter) {
	return Map.class.isAssignableFrom(parameter.getParameterType());
}

这就导致了，在调用获取方法参数的方法时（即调用InvocableHandlerMethod.java中的getMethodArgumentValues时）直接使用了MapMethodProcessor参数解析器（处理器），而自定义的参数解析器并没有被调用。

翻了一下HandlerMethodArgumentResolverComposite.java的源码，参数解析器的选取是顺序遍历的，源码如下:

/** * Find a registered {@link HandlerMethodArgumentResolver} that supports the given method parameter. */
	@Nullable
	private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
		HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
		if (result == null) {
	        // 遍历参数解析器，寻找符合要求的
			for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
				if (logger.isTraceEnabled()) {
					logger.trace("Testing if argument resolver [" + methodArgumentResolver + "] supports [" +
							parameter.getGenericParameterType() + "]");
				}
				if (methodArgumentResolver.supportsParameter(parameter)) {
					result = methodArgumentResolver;
					this.argumentResolverCache.put(parameter, result);
					break;
				}
			}
		}
		return result;
	}

又翻了一下RequestMappingHandlerAdapter.java的源码，参数解析器的加载顺序是这个样子的：

/** * Return the list of argument resolvers to use including built-in resolvers * and custom resolvers provided via {@link #setCustomArgumentResolvers}. */
	private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
		List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>();

		// Annotation-based argument resolution
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
		resolvers.add(new RequestParamMapMethodArgumentResolver());
		resolvers.add(new PathVariableMethodArgumentResolver());
		resolvers.add(new PathVariableMapMethodArgumentResolver());
		resolvers.add(new MatrixVariableMethodArgumentResolver());
		resolvers.add(new MatrixVariableMapMethodArgumentResolver());
		resolvers.add(new ServletModelAttributeMethodProcessor(false));
		resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new RequestHeaderMapMethodArgumentResolver());
		resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new SessionAttributeMethodArgumentResolver());
		resolvers.add(new RequestAttributeMethodArgumentResolver());

		// Type-based argument resolution
		resolvers.add(new ServletRequestMethodArgumentResolver());
		resolvers.add(new ServletResponseMethodArgumentResolver());
		resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RedirectAttributesMethodArgumentResolver());
		resolvers.add(new ModelMethodProcessor());
		// 卧槽，你在这儿啊
		resolvers.add(new MapMethodProcessor());
		resolvers.add(new ErrorsMethodArgumentResolver());
		resolvers.add(new SessionStatusMethodArgumentResolver());
		resolvers.add(new UriComponentsBuilderMethodArgumentResolver());

		// Custom arguments
		// 妈蛋，我在这儿
		if (getCustomArgumentResolvers() != null) {
			resolvers.addAll(getCustomArgumentResolvers());
		}

		// Catch-all
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
		resolvers.add(new ServletModelAttributeMethodProcessor(true));

		return resolvers;
	}

好吧，怪不得轮不到自定义的参数解析器来处理。既然如此，那我改变一下自定义参数解析器的加载顺序吧————渣渣我要骑到你们头上去！

@Configuration
public class ArgumentResolversConfig
{
	@Autowired
	private RequestMappingHandlerAdapter adapter;

	@PostConstruct
	public void injectSelfMethodArgumentResolver() 
	{
		List<HandlerMethodArgumentResolver> argumentResolvers = new ArrayList<>();
		argumentResolvers.add(new Form2PojoArgumentResolver());
		argumentResolvers.addAll(adapter.getArgumentResolvers());
		adapter.setArgumentResolvers(argumentResolvers);
	}
}

好了，如此，完美的解决了上述问题。

这个Bug就聊到这儿吧。