SpringBoot 2.6. 整合springfox 3.0报错问题解决

一、问题现象

SpringBoot版本：2.6.4

springfox swagger版本：

<dependency><groupId>io.springfox</groupId><artifactId>springfox-boot-starter</artifactId><version>3.0.0</version>
</dependency>

启动之后报错：

org.springframework.context.ApplicationContextException: Failed to start bean 'documentationPluginsBootstrapper'; nested exception is java.lang.NullPointerExceptionat org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:181) ~[spring-context-5.3.16.jar:5.3.16]at org.springframework.context.support.DefaultLifecycleProcessor.access$200(DefaultLifecycleProcessor.java:54) ~[spring-context-5.3.16.jar:5.3.16]at org.springframework.context.support.DefaultLifecycleProcessor$LifecycleGroup.start(DefaultLifecycleProcessor.java:356) ~[spring-context-5.3.16.jar:5.3.16]at java.lang.Iterable.forEach(Iterable.java:75) ~[na:1.8.0_202]at org.springframework.context.support.DefaultLifecycleProcessor.startBeans(DefaultLifecycleProcessor.java:155) ~[spring-context-5.3.16.jar:5.3.16]at org.springframework.context.support.DefaultLifecycleProcessor.onRefresh(DefaultLifecycleProcessor.java:123) ~[spring-context-5.3.16.jar:5.3.16]at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:935) ~[spring-context-5.3.16.jar:5.3.16]at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:586) ~[spring-context-5.3.16.jar:5.3.16]at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:145) ~[spring-boot-2.6.4.jar:2.6.4]at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:740) [spring-boot-2.6.4.jar:2.6.4]at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:415) [spring-boot-2.6.4.jar:2.6.4]at org.springframework.boot.SpringApplication.run(SpringApplication.java:303) [spring-boot-2.6.4.jar:2.6.4]at org.springframework.boot.SpringApplication.run(SpringApplication.java:1312) [spring-boot-2.6.4.jar:2.6.4]at org.springframework.boot.SpringApplication.run(SpringApplication.java:1301) [spring-boot-2.6.4.jar:2.6.4]at com.mycan.cplatform.CplatformApplication.main(CplatformApplication.java:12) [classes/:na]at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_202]at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_202]at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_202]at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_202]at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.6.4.jar:2.6.4]
Caused by: java.lang.NullPointerException: nullat springfox.documentation.spring.web.WebMvcPatternsRequestConditionWrapper.getPatterns(WebMvcPatternsRequestConditionWrapper.java:56) ~[springfox-spring-webmvc-3.0.0.jar:3.0.0]at springfox.documentation.RequestHandler.sortedPaths(RequestHandler.java:113) ~[springfox-core-3.0.0.jar:3.0.0]at springfox.documentation.spi.service.contexts.Orderings.lambda$byPatternsCondition$3(Orderings.java:89) ~[springfox-spi-3.0.0.jar:3.0.0]at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469) ~[na:1.8.0_202]at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355) ~[na:1.8.0_202]at java.util.TimSort.sort(TimSort.java:220) ~[na:1.8.0_202]at java.util.Arrays.sort(Arrays.java:1512) ~[na:1.8.0_202]at java.util.ArrayList.sort(ArrayList.java:1462) ~[na:1.8.0_202]at java.util.stream.SortedOps$RefSortingSink.end(SortedOps.java:387) ~[na:1.8.0_202]at java.util.stream.Sink$ChainedReference.end(Sink.java:258) ~[na:1.8.0_202]at java.util.stream.Sink$ChainedReference.end(Sink.java:258) ~[na:1.8.0_202]at java.util.stream.Sink$ChainedReference.end(Sink.java:258) ~[na:1.8.0_202]at java.util.stream.Sink$ChainedReference.end(Sink.java:258) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_202]at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_202]at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[na:1.8.0_202]at springfox.documentation.spring.web.plugins.WebMvcRequestHandlerProvider.requestHandlers(WebMvcRequestHandlerProvider.java:81) ~[springfox-spring-webmvc-3.0.0.jar:3.0.0]at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[na:1.8.0_202]at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_202]at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_202]at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_202]at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[na:1.8.0_202]at springfox.documentation.spring.web.plugins.AbstractDocumentationPluginsBootstrapper.withDefaults(AbstractDocumentationPluginsBootstrapper.java:107) ~[springfox-spring-web-3.0.0.jar:3.0.0]at springfox.documentation.spring.web.plugins.AbstractDocumentationPluginsBootstrapper.buildContext(AbstractDocumentationPluginsBootstrapper.java:91) ~[springfox-spring-web-3.0.0.jar:3.0.0]at springfox.documentation.spring.web.plugins.AbstractDocumentationPluginsBootstrapper.bootstrapDocumentationPlugins(AbstractDocumentationPluginsBootstrapper.java:82) ~[springfox-spring-web-3.0.0.jar:3.0.0]at springfox.documentation.spring.web.plugins.DocumentationPluginsBootstrapper.start(DocumentationPluginsBootstrapper.java:100) ~[springfox-spring-web-3.0.0.jar:3.0.0]at org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:178) ~[spring-context-5.3.16.jar:5.3.16]... 19 common frames omitted

二、问题排查

查看异常堆栈的顶部，发现是此处报空指针异常：springfox.documentation.spring.web.WebMvcPatternsRequestConditionWrapper#getPatterns，

跟踪condition变量，发现是从springfox.documentation.spring.web.WebMvcRequestHandler#getPatternsCondition方法返回的WebMvcPatternsRequestConditionWrapper类的对象返回的。

找到类org.springframework.web.servlet.mvc.method.RequestMappingInfo，发现有两个很相近的方法，查看方法注释可以知道当其中一个起作用时，另外一个则返回null。所以，springfox的代码似乎不应该这样写，而是应该加上一个判断。

实际上，SpringBoot 2.6.0开始，请求路径与Spring MVC处理映射匹配的默认策略已从AntPathMatcher更改为PathPatternParser。但是可以通过设置spring.mvc.pathmatch.matching-strategy为ant-path-matcher来改变。

所以我们在application.properties文件中加上一行配置：

spring.mvc.pathmatch.matching-strategy=ant_path_matcher

启动项目，发现还是报错。继续看，发现还是有HandlerMethod的patternsCondition为空。

那么为了不报空指针异常，只能把这些HandlerMethod过滤掉，这些HandlerMethod是从RequestMappingInfoHandlerMapping获取的，那么通过过滤RequestMappingInfoHandlerMapping就可以达到目标，通过判断条件getPatternParser() == null进行判断，这样过滤之后，所有的HandlerMethod的getPatternsCondition()都不是为空的了。

于是，我们对WebMvcRequestHandlerProvider类型的Bean进行改造，添加代码：

@Bean
public BeanPostProcessor springfoxHandlerProviderBeanPostProcessor() {return new BeanPostProcessor() {@Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {if (bean instanceof WebMvcRequestHandlerProvider || bean instanceof WebFluxRequestHandlerProvider) {customizeSpringfoxHandlerMappings(getHandlerMappings(bean));}return bean;}private <T extends RequestMappingInfoHandlerMapping> void customizeSpringfoxHandlerMappings(List<T> mappings) {List<T> copy = mappings.stream().filter(mapping -> mapping.getPatternParser() == null).collect(Collectors.toList());mappings.clear();mappings.addAll(copy);}@SuppressWarnings("unchecked")private List<RequestMappingInfoHandlerMapping> getHandlerMappings(Object bean) {try {Field field = ReflectionUtils.findField(bean.getClass(), "handlerMappings");field.setAccessible(true);return (List<RequestMappingInfoHandlerMapping>) field.get(bean);} catch (IllegalArgumentException | IllegalAccessException e) {throw new IllegalStateException(e);}}};}

三、解决方案：

1. 添加配置文件：

spring.mvc.pathmatch.matching-strategy=ant_path_matcher

2. 添加代码配置：

@Bean
public BeanPostProcessor springfoxHandlerProviderBeanPostProcessor() {return new BeanPostProcessor() {@Overridepublic Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {if (bean instanceof WebMvcRequestHandlerProvider || bean instanceof WebFluxRequestHandlerProvider) {customizeSpringfoxHandlerMappings(getHandlerMappings(bean));}return bean;}private <T extends RequestMappingInfoHandlerMapping> void customizeSpringfoxHandlerMappings(List<T> mappings) {List<T> copy = mappings.stream().filter(mapping -> mapping.getPatternParser() == null).collect(Collectors.toList());mappings.clear();mappings.addAll(copy);}@SuppressWarnings("unchecked")private List<RequestMappingInfoHandlerMapping> getHandlerMappings(Object bean) {try {Field field = ReflectionUtils.findField(bean.getClass(), "handlerMappings");field.setAccessible(true);return (List<RequestMappingInfoHandlerMapping>) field.get(bean);} catch (IllegalArgumentException | IllegalAccessException e) {throw new IllegalStateException(e);}}};}

四、其他

Spring fox 已经大半年没有更新了，为了使用swagger折腾的费劲，其实现在也有其他的api文档方案，比如Spring doc和Apifox，可以考虑更换。

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