Details of the problem are as follows:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'webMvcObjectMapperConfigurer' defined in class path resource [springfox/documentation/spring/web/SpringfoxWebMvcConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [springfox.documentation.spring.web.WebMvcObjectMapperConfigurer] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:597) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:213) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.context.support.PostProcessorRegistrationDelegate.registerBeanPostProcessors(PostProcessorRegistrationDelegate.java:270) ~[spring-context-5.3.13.jar:5.3.13]
	at org.springframework.context.support.AbstractApplicationContext.registerBeanPostProcessors(AbstractApplicationContext.java:762) ~[spring-context-5.3.13.jar:5.3.13]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:567) ~[spring-context-5.3.13.jar:5.3.13]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:145) ~[spring-boot-2.5.7.jar:2.5.7]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:765) [spring-boot-2.5.7.jar:2.5.7]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:445) [spring-boot-2.5.7.jar:2.5.7]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:338) [spring-boot-2.5.7.jar:2.5.7]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1354) [spring-boot-2.5.7.jar:2.5.7]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1343) [spring-boot-2.5.7.jar:2.5.7]
	at com.xx.xxApplication.main(xxApplication.java:12) [classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_121]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_121]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_121]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_121]
	at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) [spring-boot-devtools-2.5.7.jar:2.5.7]
Caused by: java.lang.IllegalStateException: Failed to introspect Class [springfox.documentation.spring.web.WebMvcObjectMapperConfigurer] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:481) ~[spring-core-5.3.13.jar:5.3.13]
	at org.springframework.util.ReflectionUtils.doWithLocalMethods(ReflectionUtils.java:321) ~[spring-core-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.buildLifecycleMetadata(InitDestroyAnnotationBeanPostProcessor.java:232) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.findLifecycleMetadata(InitDestroyAnnotationBeanPostProcessor.java:210) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(InitDestroyAnnotationBeanPostProcessor.java:149) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(CommonAnnotationBeanPostProcessor.java:305) ~[spring-context-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyMergedBeanDefinitionPostProcessors(AbstractAutowireCapableBeanFactory.java:1116) ~[spring-beans-5.3.13.jar:5.3.13]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:594) ~[spring-beans-5.3.13.jar:5.3.13]
	... 20 common frames omitted
Caused by: java.lang.NoClassDefFoundError: com/fasterxml/jackson/databind/ObjectMapper
	at java.lang.Class.getDeclaredMethods0(Native Method) ~[na:1.8.0_121]
	at java.lang.Class.privateGetDeclaredMethods(Class.java:2701) ~[na:1.8.0_121]
	at java.lang.Class.getDeclaredMethods(Class.java:1975) ~[na:1.8.0_121]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:463) ~[spring-core-5.3.13.jar:5.3.13]
	... 27 common frames omitted
Caused by: java.lang.ClassNotFoundException: com.fasterxml.jackson.databind.ObjectMapper
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_121]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_121]
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331) ~[na:1.8.0_121]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_121]
	... 31 common frames omitted


Process finished with exit code 0

The pom.XML file is as follows:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.7</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.xx</groupId>
    <artifactId>xx</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>xx</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <!--Web-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>com.fasterxml.jackson.core</groupId>
                    <artifactId>jackson-databind</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <!--jdbc-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-jdbc</artifactId>
            <version>2.5.7</version>
        </dependency>
        <!--Mybatis-->
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>2.2.0</version>
        </dependency>
        <!--devtools-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
            <optional>true</optional>
        </dependency>
        <!--postgresql-->
        <dependency>
            <groupId>org.postgresql</groupId>
            <artifactId>postgresql</artifactId>
            <scope>runtime</scope>
        </dependency>
        <!--spring-boot-starter-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <!--restdocs-->
        <dependency>
            <groupId>org.springframework.restdocs</groupId>
            <artifactId>spring-restdocs-mockmvc</artifactId>
            <scope>test</scope>
        </dependency>
        <!--shiro-->
        <dependency>
            <groupId>org.apache.shiro</groupId>
            <artifactId>shiro-web</artifactId>
            <version>1.5.3</version>
        </dependency>
        <dependency>
            <groupId>org.apache.shiro</groupId>
            <artifactId>shiro-spring</artifactId>
            <version>1.5.3</version>
        </dependency>
        <!--mysql-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.25</version>
        </dependency>
        <!--gson-->
        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
        </dependency>
        <!--lombok-->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <scope>provided</scope>
        </dependency>
        <!--swagger-->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-boot-starter</artifactId>
            <version>3.0.0</version>
        </dependency>
        <!--junit-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.asciidoctor</groupId>
                <artifactId>asciidoctor-maven-plugin</artifactId>
                <version>1.5.8</version>
                <executions>
                    <execution>
                        <id>generate-docs</id>
                        <phase>prepare-package</phase>
                        <goals>
                            <goal>process-asciidoc</goal>
                        </goals>
                        <configuration>
                            <backend>html</backend>
                            <doctype>book</doctype>
                        </configuration>
                    </execution>
                </executions>
                <dependencies>
                    <dependency>
                        <groupId>org.springframework.restdocs</groupId>
                        <artifactId>spring-restdocs-asciidoctor</artifactId>
                        <version>${spring-restdocs.version}</version>
                    </dependency>
                </dependencies>
            </plugin>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <version>2.19.1</version>
            </plugin>
        </plugins>
    </build>

</project>

Solution:

First, Check pom.xml file that we have excluded the jackjson , which comes with spring-boot-starter-web:

            <exclusions>
                <exclusion>
                    <groupId>com.fasterxml.jackson.core</groupId>
                    <artifactId>jackson-databind</artifactId>
                </exclusion>
            </exclusions>

Gson is used instead of Jackson :

        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
        </dependency>

We also introduced the swagger dependency:

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

This is the problem. No matter which version of swaggerwe introduce, the above problems will occur, because swagger uses Jackson to convert JSON, but gson is incompatible with swagger.

Solution:

Use the default Jackson

Customize the configuration of Gson