彻底解决springboot使用Swagger报Failed to start bean ‘documentationPluginsBootstrapper异常的问题

问题背景

springboot项目 2.6.x 版本与swagger-ui 2.9.x 版本进行整合实现接口文档功能

在项目启动时出现
Failed to start bean ‘documentationPluginsBootstrapper’; nested exception is java.lang.NullPointerException

org.springframework.context.ApplicationContextException: Failed to start bean 'documentationPluginsBootstrapper'; nested exception is java.lang.NullPointerException
	at org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:181)
	at org.springframework.context.support.DefaultLifecycleProcessor.access$200(DefaultLifecycleProcessor.java:54)
	at org.springframework.context.support.DefaultLifecycleProcessor$LifecycleGroup.start(DefaultLifecycleProcessor.java:356)
	at java.lang.Iterable.forEach(Iterable.java:75)
	at org.springframework.context.support.DefaultLifecycleProcessor.startBeans(DefaultLifecycleProcessor.java:155)
	at org.springframework.context.support.DefaultLifecycleProcessor.onRefresh(DefaultLifecycleProcessor.java:123)
	at org.springframework.context.support.AbstractApplicationContext.finishRefresh(AbstractApplicationContext.java:935)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:586)
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:145)
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:745)
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:420)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:307)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1317)
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1306)
	at com.ihome.business.SpringBusinessApplication.main(SpringBusinessApplication.java:40)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49)
Caused by: java.lang.NullPointerException: null
	at springfox.documentation.spi.service.contexts.Orderings$8.compare(Orderings.java:113)
	at springfox.documentation.spi.service.contexts.Orderings$8.compare(Orderings.java:110)
	at com.google.common.collect.ComparatorOrdering.compare(ComparatorOrdering.java:37)
	at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
	at java.util.TimSort.sort(TimSort.java:234)
	at java.util.Arrays.sort(Arrays.java:1438)
	at com.google.common.collect.Ordering.sortedCopy(Ordering.java:852)
	at springfox.documentation.spring.web.plugins.WebMvcRequestHandlerProvider.requestHandlers(WebMvcRequestHandlerProvider.java:57)
	at springfox.documentation.spring.web.plugins.DocumentationPluginsBootstrapper$2.apply(DocumentationPluginsBootstrapper.java:138)
	at springfox.documentation.spring.web.plugins.DocumentationPluginsBootstrapper$2.apply(DocumentationPluginsBootstrapper.java:135)
	at com.google.common.collect.Iterators$6.transform(Iterators.java:785)
	at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
	at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:47)
	at com.google.common.collect.Iterators$ConcatenatedIterator.hasNext(Iterators.java:1332)
	at com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:270)
	at com.google.common.collect.ImmutableList.copyOf(ImmutableList.java:234)
	at com.google.common.collect.FluentIterable.toList(FluentIterable.java:617)
	at springfox.documentation.spring.web.plugins.DocumentationPluginsBootstrapper.defaultContextBuilder(DocumentationPluginsBootstrapper.java:109)
	at springfox.documentation.spring.web.plugins.DocumentationPluginsBootstrapper.buildContext(DocumentationPluginsBootstrapper.java:96)
	at springfox.documentation.spring.web.plugins.DocumentationPluginsBootstrapper.start(DocumentationPluginsBootstrapper.java:167)
	at org.springframework.context.support.DefaultLifecycleProcessor.doStart(DefaultLifecycleProcessor.java:178)
	... 19 common frames omitted

先综合一下网上其他的解决方案
1.给springboot降版本
此方案在企业或生产环境中需要全面评估项目大版本更换的影响，所以属于成本较大的改动。
2.排查异常原因，是由swagger-ui和项目内其他依赖冲突导致的。
主要由于项目中使用了spring-boot-starter-actuator或者其他依赖中引入了spring-boot-starter-actuator导致swagger识别web入口出现异常
举例：本项目中引入了redisson-spring-boot-starter 而其中依赖包含了spring-boot-starter-actuator 也会导致此异常。

解决方案2
修改handlerMapping处理兼容依赖场景，以及通过enable参数控制在非必要时不加载swagger

import io.swagger.annotations.ApiOperation;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.actuate.autoconfigure.endpoint.web.CorsEndpointProperties;
import org.springframework.boot.actuate.autoconfigure.endpoint.web.WebEndpointProperties;
import org.springframework.boot.actuate.autoconfigure.web.server.ManagementPortType;
import org.springframework.boot.actuate.endpoint.ExposableEndpoint;
import org.springframework.boot.actuate.endpoint.web.EndpointLinksResolver;
import org.springframework.boot.actuate.endpoint.web.EndpointMapping;
import org.springframework.boot.actuate.endpoint.web.EndpointMediaTypes;
import org.springframework.boot.actuate.endpoint.web.ExposableWebEndpoint;
import org.springframework.boot.actuate.endpoint.web.WebEndpointsSupplier;
import org.springframework.boot.actuate.endpoint.web.annotation.ControllerEndpointsSupplier;
import org.springframework.boot.actuate.endpoint.web.annotation.ServletEndpointsSupplier;
import org.springframework.boot.actuate.endpoint.web.servlet.WebMvcEndpointHandlerMapping;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;
import org.springframework.util.StringUtils;
import springfox.documentation.builders.RequestHandlerSelectors;
import springfox.documentation.spi.DocumentationType;
import springfox.documentation.spring.web.plugins.Docket;
import springfox.documentation.swagger2.annotations.EnableSwagger2;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

@Configuration
@EnableSwagger2
public class SwaggerConfig {

    @Value("${swagger.enable:true}")
    private boolean enableSwagger;

    @Bean
    public Docket swaggerSpringMvcPlugin() {
        return new Docket(DocumentationType.SWAGGER_2).enable(enableSwagger)
                .select()
                .apis(RequestHandlerSelectors.withMethodAnnotation(ApiOperation.class))
                .build();
    }

    @Bean
    public WebMvcEndpointHandlerMapping webEndpointServletHandlerMapping(
            WebEndpointsSupplier webEndpointsSupplier, ServletEndpointsSupplier servletEndpointsSupplier,
            ControllerEndpointsSupplier controllerEndpointsSupplier, EndpointMediaTypes endpointMediaTypes,
            CorsEndpointProperties corsProperties, WebEndpointProperties webEndpointProperties,
            Environment environment) {
        List<ExposableEndpoint<?>> allEndpoints = new ArrayList<>();
        Collection<ExposableWebEndpoint> webEndpoints = webEndpointsSupplier.getEndpoints();
        allEndpoints.addAll(webEndpoints);
        allEndpoints.addAll(servletEndpointsSupplier.getEndpoints());
        allEndpoints.addAll(controllerEndpointsSupplier.getEndpoints());
        String basePath = webEndpointProperties.getBasePath();
        EndpointMapping endpointMapping = new EndpointMapping(basePath);
        boolean shouldRegisterLinksMapping = shouldRegisterLinksMapping(webEndpointProperties,
                environment, basePath);
        return new WebMvcEndpointHandlerMapping(endpointMapping, webEndpoints, endpointMediaTypes,
                corsProperties.toCorsConfiguration(), new EndpointLinksResolver(allEndpoints, basePath),
                shouldRegisterLinksMapping, null);
    }

    /**
     * shouldRegisterLinksMapping
     * @param webEndpointProperties webEndpointProperties
     * @param environment environment
     * @param basePath /
     * @return boolean
     */
    private boolean shouldRegisterLinksMapping(WebEndpointProperties webEndpointProperties,
                                               Environment environment, String basePath) {
        return webEndpointProperties.getDiscovery().isEnabled() && (StringUtils.hasText(basePath)
                || ManagementPortType.get(environment).equals(ManagementPortType.DIFFERENT));
    }

    public boolean isEnableSwagger() {
        return enableSwagger;
    }

    public void setEnableSwagger(boolean enableSwagger) {
        this.enableSwagger = enableSwagger;
    }
}

配置application.yml

spring:
  mvc:
    pathmatch:
    #更改web路径匹配器
      matching-strategy: ant_path_matcher

注意 swagger-ui依赖guava
还需在maven引入相关依赖

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>27.0-jre</version>
</dependency>