springboot自定义消息转换器HttpMessageConverter

https://segmentfault.com/a/1190000015975405

 

记一次踩坑:springboot2.0.2配置fastjson不生效

spring fastjson springboot

更新于 2018-08-13  约 18 分钟

最近在尝试搭建springboot+dubbo+shiro基于注解的一个项目,突发奇想想把消息转换器从jackson换成fastjson,于是就开始了折腾之路.

轻车熟路的去自定了一个SpringMvcConfigure去继承WebMvcConfigurerAdapter,然后就发现这个WebMvcConfigurerAdapter竟然过时了?what?点进去看源码:

/** * An implementation of {@link WebMvcConfigurer} with empty methods allowing * subclasses to override only the methods they're interested in. * * @author Rossen Stoyanchev * @since 3.1 * @deprecated as of 5.0 {@link WebMvcConfigurer} has default methods (made * possible by a Java 8 baseline) and can be implemented directly without the * need for this adapter */ @Deprecated public abstract class WebMvcConfigurerAdapter implements WebMvcConfigurer {}

可以看到从spring5.0开始就被@Deprecated,原来是java8中支持接口中有默认方法,所以我们现在可以直接实现WebMvcConfigurer,然后选择性的去重写某个方法,而不用实现它的所有方法.

于是就实现了WebMvcConfigurer:

@Configuration public class SpringMvcConfigure implements WebMvcConfigurer { /** * 配置消息转换器 * @param converters */ @Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter(); //自定义配置... FastJsonConfig config = new FastJsonConfig(); config.setSerializerFeatures(SerializerFeature.QuoteFieldNames, SerializerFeature.WriteEnumUsingToString, /*SerializerFeature.WriteMapNullValue,*/ SerializerFeature.WriteDateUseDateFormat, SerializerFeature.DisableCircularReferenceDetect); fastJsonHttpMessageConverter.setFastJsonConfig(config); converters.add(fastJsonHttpMessageConverter); } }

本以为这样子配置就可以完事儿,但是诡异的事情发生了,我明明注释了SerializerFeature.WriteMapNullValue,可是返回的json中仍然有为null的字段,然后我就去com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter中的write和writeInternal打了断点,再次执行,竟然什么都没有发生,根本没有走这两个方法,于是在自定义的SpringMvcConfigure中configureMessageConverters方法内打了断点,想看看这个方法参数converters里边到底有什么:

看到这里就想到,肯定是我自己添加的fastjson在后边,所以没有生效,所以就加了以下代码:

@Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { converters = converters.stream() .filter((converter)-> !(converter instanceof MappingJackson2HttpMessageConverter)) .collect(Collectors.toList()); FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter(); //自定义配置... FastJsonConfig config = new FastJsonConfig(); config.setSerializerFeatures(SerializerFeature.QuoteFieldNames, SerializerFeature.WriteEnumUsingToString, /*SerializerFeature.WriteMapNullValue,*/ SerializerFeature.WriteDateUseDateFormat, SerializerFeature.DisableCircularReferenceDetect); fastJsonHttpMessageConverter.setFastJsonConfig(config); converters.add(fastJsonHttpMessageConverter); }

竟然还没有生效,后来开始追踪,开始方法是从org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport类中的一个bean配置:

@Bean public RequestMappingHandlerAdapter requestMappingHandlerAdapter() { RequestMappingHandlerAdapter adapter = createRequestMappingHandlerAdapter(); adapter.setContentNegotiationManager(mvcContentNegotiationManager()); //就是从这里开始设置converters的,然后从这里一路追踪下去. 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; }

getMessageConverters()方法:

protected final List<HttpMessageConverter<?>> getMessageConverters() { if (this.messageConverters == null) { this.messageConverters = new ArrayList<>(); configureMessageConverters(this.messageConverters);//进入这一步 if (this.messageConverters.isEmpty()) { addDefaultHttpMessageConverters(this.messageConverters); } extendMessageConverters(this.messageConverters); } return this.messageConverters; }

org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration:

@Override protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) { this.configurers.configureMessageConverters(converters); } @Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { for (WebMvcConfigurer delegate : this.delegates) { delegate.configureMessageConverters(converters); } }

this.delegates包含了springboot的一个默认配置类类:org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration里边有一个参数

private final HttpMessageConverters messageConverters;

for循环里的delegate.configureMessageConverters(converters)调用了WebMvcAutoConfiguration中的configureMessageConverters方法:

@Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { converters.addAll(this.messageConverters.getConverters()); }

执行完这个后,就给converters中添加了10个转换器了,就是上图中的10个.
this.delegates中还有一个就是我们自定义的那个,执行完后,在我们自定义的那个SpringMvcConfigure发现我添加的fastjson添加进去了,但是org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport.getMessageConverters(),发现converters并没有发现我们添加进去的FastJsonHttpMessageConverter,这时突然又想起来:java8的stream api每次都是生成一个新的对象,所以导致converters已经不是传递过来的那个converters的引用了(这里也证明了java是值传递,不是引用传递).

于是再次改变那个lambda表达式为普通的增强for循环:

@Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { /*converters = converters.stream(). filter((converter)-> !(converter instanceof MappingJackson2HttpMessageConverter)) .collect(Collectors.toList());*/ for (HttpMessageConverter<?> converter : converters) { if (converter instanceof MappingJackson2HttpMessageConverter){ converters.remove(converter); } } FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter(); //自定义配置... FastJsonConfig config = new FastJsonConfig(); config.setSerializerFeatures(SerializerFeature.QuoteFieldNames, SerializerFeature.WriteEnumUsingToString, /*SerializerFeature.WriteMapNullValue,*/ SerializerFeature.WriteDateUseDateFormat, SerializerFeature.DisableCircularReferenceDetect); fastJsonHttpMessageConverter.setFastJsonConfig(config); converters.add(fastJsonHttpMessageConverter); }

再次运行,wtf?报错了:ConcurrentModificationException,原来使用for循环遍历过程中不能进行remove操作,于是换成Iterator:

@Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { /*converters = converters.stream() .filter((converter)-> !(converter instanceof MappingJackson2HttpMessageConverter)) .collect(Collectors.toList()); for (HttpMessageConverter<?> converter : converters) { if (converter instanceof MappingJackson2HttpMessageConverter){ converters.remove(converter); } }*/ Iterator<HttpMessageConverter<?>> iterator = converters.iterator(); while(iterator.hasNext()){ HttpMessageConverter<?> converter = iterator.next(); if(converter instanceof MappingJackson2HttpMessageConverter){ iterator.remove(); } } FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter(); //自定义配置... FastJsonConfig config = new FastJsonConfig(); config.setSerializerFeatures(SerializerFeature.QuoteFieldNames, SerializerFeature.WriteEnumUsingToString, /*SerializerFeature.WriteMapNullValue,*/ SerializerFeature.WriteDateUseDateFormat, SerializerFeature.DisableCircularReferenceDetect); fastJsonHttpMessageConverter.setFastJsonConfig(config); converters.add(fastJsonHttpMessageConverter); }

再次运行,我去,终于解决了,先是删除MappingJackson2HttpMessageConverter,然后添加FastJsonHttpMessageConverter,但是不是到为什么进过一系列操作后,MappingJackson2HttpMessageConverter还是添加进去了,但是由于FastJsonHttpMessageConverter在MappingJackson2HttpMessageConverter之前添加,所以对结果不影响.至此,解决了这个问题.

总结

1. 最重要的还是解决了springboot2.0.2配置fastjson不生效的问题
2. 更加明白stream api返回的都是全新的对象
3. 更理解java是值传递而不是引用传递
4. 了解到想要在迭代过程中对集合进行操作要用Iterator,而不是直接简单的for循环或者增强for循环

如有不正确的地方还请指出,谢谢.