记一次WebMVC配置问题以及源码阅读
version：
spring-boot 2.5.x
背景
后端Long类型在前端会出现精度丢失问题，可以通过Jackson配置序列化为String（其实已经过时了）
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spring:
jackson:
generator:
write_numbers_as_strings: true
但在某次代码更新后发现失效了。
解决
通过debug跟踪，发现通过ObjectMapper生成的UTF8JsonGenerator并不符合预期
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public class UTF8JsonGenerator extends JsonGeneratorImpl {
.......
@Override
public void writeNumber(long l) throws IOException
{
_verifyValueWrite(WRITE_NUMBER);
if (_cfgNumbersAsStrings) { //_cfgNumbersAsStrings为false，导致序列化出现异常
_writeQuotedLong(l);
return;
}
if ((_outputTail + 21) >= _outputEnd) {
// up to 20 digits, minus sign
_flushBuffer();
}
_outputTail = NumberOutput.outputLong(l, _outputBuffer, _outputTail);
}
......
}
跟踪后发现MappingJackson2HttpMessageConverter并不是通过Jackson自动装配注入而来，而是通过WebMvcConfigurationSupport#addDefaultHttpMessageConverters而来，并没有读取使用配置。最后查看代码提交和控制变量法，发现是由于为了自定义枚举的转换，但却添加了@EnableWebMvc导致的。
去除@EnableWebMvc其实就已经可以了，但是发现上述配置已经过时了，于是采用了WebMvcConfigurer重新配置。
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//@EnableWebMvc
@Configuration
public class CustomizeWebConfig implements WebMvcConfigurer {
@Override
public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
// 修改默认MappingJackson2HttpMessageConverter 配置jackson 序列化 long to string
MappingJackson2HttpMessageConverter jackson2HttpMessageConverter = null;
for (HttpMessageConverter<?> converter : converters) {
if (converter instanceof MappingJackson2HttpMessageConverter) {
jackson2HttpMessageConverter = (MappingJackson2HttpMessageConverter) converter;
break;
}
}
if (Objects.isNull(jackson2HttpMessageConverter)) {
jackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();
}
ObjectMapper objectMapper = jackson2HttpMessageConverter.getObjectMapper();
SimpleModule module = new SimpleModule();
module.addSerializer(Long.class, ToStringSerializer.instance);
objectMapper.registerModule(module);
converters.add(jackson2HttpMessageConverter);
}
...
}
源码分析
代码较多，截取部分重要代码，同时以HttpMessageConverter配置为例。
@EnableWebMvc
可以看到EnableWebMvc通过@Import注入了DelegatingWebMvcConfiguration。
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@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}
所以接下来看DelegatingWebMvcConfiguration的父类WebMvcConfigurationSupport :D
WebMvcConfigurationSupport
可以看到WebMvcConfigurationSupport中定义了很多通过@Bean注入的Spring Web MVC的Bean，同时暴露了一些方法去扩展Bean的行为（典型的模板方法）。可以通过@EnableWebMvc来注入，或者继承该类使用@Configuration来注入，同时可以通过覆盖该类的@Bean方法（记得添加@Bean），或者重写扩展方法来自定义MVC的配置。
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/**
* This is the main class providing the configuration behind the MVC Java config.
* It is typically imported by adding {@link EnableWebMvc @EnableWebMvc} to an
* application {@link Configuration @Configuration} class. An alternative more
* advanced option is to extend directly from this class and override methods as
* necessary, remembering to add {@link Configuration @Configuration} to the
* subclass and {@link Bean @Bean} to overridden {@link Bean @Bean} methods.
* For more details see the javadoc of {@link EnableWebMvc @EnableWebMvc}.
* ......
*/
public class WebMvcConfigurationSupport implements ApplicationContextAware, ServletContextAware {
......
@Bean
public RouterFunctionMapping routerFunctionMapping(
@Qualifier("mvcConversionService") FormattingConversionService conversionService,
@Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {
RouterFunctionMapping mapping = new RouterFunctionMapping();
mapping.setOrder(3);
mapping.setInterceptors(getInterceptors(conversionService, resourceUrlProvider));
mapping.setCorsConfigurations(getCorsConfigurations());
// 调用了getMessageConverters()方法获取MessageConverters
mapping.setMessageConverters(getMessageConverters());
PathPatternParser patternParser = getPathMatchConfigurer().getPatternParser();
if (patternParser != null) {
mapping.setPatternParser(patternParser);
}
return mapping;
}
/**
* 首先调用configureMessageConverters配置HttpMessageConverter
* 若没有配置则获取默认的HttpMessageConverter
* 最后通过extendMessageConverters扩展HttpMessageConverter
*/
protected final List<HttpMessageConverter<?>> getMessageConverters() {
if (this.messageConverters == null) {
this.messageConverters = new ArrayList<>();
configureMessageConverters(this.messageConverters);
if (this.messageConverters.isEmpty()) {
addDefaultHttpMessageConverters(this.messageConverters);
}
extendMessageConverters(this.messageConverters);
}
return this.messageConverters;
}
protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
}
protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
}
......
}
DelegatingWebMvcConfiguration
@EnableWebMvc实际导入的类，通过继承WebMvcConfigurationSupport导入一堆Bean，同时自定义了部分MVC的配置。这里出现了两个新的类WebMvcConfigurer和WebMvcConfigurerComposite，而对于MVC的配置都是通过WebMvcConfigurer和WebMvcConfigurerComposite完成的，可以看到注入了所有的WebMvcConfigurer，同时委托了WebMvcConfigurerComposite代为执行。
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/**
* A subclass of {@code WebMvcConfigurationSupport} that detects and delegates
* to all beans of type {@link WebMvcConfigurer} allowing them to customize the
* configuration provided by {@code WebMvcConfigurationSupport}. This is the
* class actually imported by {@link EnableWebMvc @EnableWebMvc}.
*
* @author Rossen Stoyanchev
* @since 3.1
*/
@Configuration(proxyBeanMethods = false)
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 configureMessageConverters(List<HttpMessageConverter<?>> converters) {
this.configurers.configureMessageConverters(converters);
}
@Override
protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
this.configurers.extendMessageConverters(converters);
}
......
}
WebMvcConfigurer
定义了很多用于配置MVC的方法，结合@EnableWebMvc可以注入所有的WebMvcConfigurer，同时MVC组件会调用所有的配置方法进行自定义的配置。
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/**
* Defines callback methods to customize the Java-based configuration for
* Spring MVC enabled via {@code @EnableWebMvc}.
*
* <p>{@code @EnableWebMvc}-annotated configuration classes may implement
* this interface to be called back and given a chance to customize the
* default configuration.
*
* @author Rossen Stoyanchev
* @author Keith Donald
* @author David Syer
* @since 3.1
*/
public interface WebMvcConfigurer {
......
/**
* Configure the {@link HttpMessageConverter HttpMessageConverter}s for
* reading from the request body and for writing to the response body.
* <p>By default, all built-in converters are configured as long as the
* corresponding 3rd party libraries such Jackson JSON, JAXB2, and others
* are present on the classpath.
* <p><strong>Note</strong> use of this method turns off default converter
* registration. Alternatively, use
* {@link #extendMessageConverters(java.util.List)} to modify that default
* list of converters.
* @param converters initially an empty list of converters
*/
default void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
}
/**
* Extend or modify the list of converters after it has been, either
* {@link #configureMessageConverters(List) configured} or initialized with
* a default list.
* <p>Note that the order of converter registration is important. Especially
* in cases where clients accept {@link org.springframework.http.MediaType#ALL}
* the converters configured earlier will be preferred.
* @param converters the list of configured converters to be extended
* @since 4.1.3
*/
default void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
}
......
}
WebMvcConfigurerComposite
代理（组合）了所有的WebMvcConfigurer，通过循环执行所有的自定义配置。
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/**
* A {@link WebMvcConfigurer} that delegates to one or more others.
*
* @author Rossen Stoyanchev
* @since 3.1
*/
class WebMvcConfigurerComposite implements WebMvcConfigurer {
private final List<WebMvcConfigurer> delegates = new ArrayList<>();
public void addWebMvcConfigurers(List<WebMvcConfigurer> configurers) {
if (!CollectionUtils.isEmpty(configurers)) {
this.delegates.addAll(configurers);
}
}
......
@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
for (WebMvcConfigurer delegate : this.delegates) {
delegate.configureMessageConverters(converters);
}
}
@Override
public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
for (WebMvcConfigurer delegate : this.delegates) {
delegate.extendMessageConverters(converters);
}
}
......
}
WebMvcAutoConfiguration
再来看看Spring Boot 的自动装配是如何实现MVC配置的。
从@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)可以知道@EnableWebMvc为什么会导致自动装配失效。
再看WebMvcAutoConfiguration#EnableWebMvcConfiguration，继承了DelegatingWebMvcConfiguration，引入了很多的配置，使我们能通过文件配置Spring MVC。（注入了EnableWebMvcConfiguration就相当于@EnableWebMvc注入了DelegatingWebMvcConfiguration）
最后再看WebMvcAutoConfiguration#WebMvcAutoConfigurationAdapter，实现了WebMvcConfigurer，通过它Spring Boot完成了对MVC组件的配置。
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@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class, WebMvcConfigurer.class })
//可以看到自动装配的条件之一就是Bean中没有WebMvcConfigurationSupport，所以当我们使用了@EnableWebMvc之后自动装配便会失效
@ConditionalOnMissingBean(WebMvcConfigurationSupport.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
@AutoConfigureAfter({ DispatcherServletAutoConfiguration.class, TaskExecutionAutoConfiguration.class,
ValidationAutoConfiguration.class })
public class WebMvcAutoConfiguration {
......
@SuppressWarnings("deprecation")
@Configuration(proxyBeanMethods = false)
@Import(EnableWebMvcConfiguration.class)
@EnableConfigurationProperties({ WebMvcProperties.class, WebProperties.class })
@Order(0)
public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer, ServletContextAware {
......
private final ObjectProvider<HttpMessageConverters> messageConvertersProvider;
public WebMvcAutoConfigurationAdapter(WebProperties webProperties, WebMvcProperties mvcProperties,
ListableBeanFactory beanFactory, ObjectProvider<HttpMessageConverters> messageConvertersProvider,
ObjectProvider<ResourceHandlerRegistrationCustomizer> resourceHandlerRegistrationCustomizerProvider,
ObjectProvider<DispatcherServletPath> dispatcherServletPath,
ObjectProvider<ServletRegistrationBean<?>> servletRegistrations) {
......
this.messageConvertersProvider = messageConvertersProvider;
......
}
......
@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
/**
* 通过ObjectProvider获取了HttpMessageConverters，
* 而HttpMessageConverters包含了所有自动装配的HttpMessageConverter（例如MappingJackson2HttpMessageConverter），
* 具体可以看HttpMessageConvertersAutoConfiguration
*/
this.messageConvertersProvider
.ifAvailable((customConverters) -> converters.addAll(customConverters.getConverters()));
}
......
}
/**
* Configuration equivalent to {@code @EnableWebMvc}.
*/
@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(WebProperties.class)
public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {
......
public EnableWebMvcConfiguration(WebMvcProperties mvcProperties, WebProperties webProperties,
ObjectProvider<WebMvcRegistrations> mvcRegistrationsProvider,
ObjectProvider<ResourceHandlerRegistrationCustomizer> resourceHandlerRegistrationCustomizerProvider,
ListableBeanFactory beanFactory) {
this.resourceProperties = webProperties.getResources();
this.mvcProperties = mvcProperties;
this.webProperties = webProperties;
this.mvcRegistrations = mvcRegistrationsProvider.getIfUnique();
this.beanFactory = beanFactory;
}
......
}
}
