1. 复现问题

今天在启动spring boot项目时，出现如下错误：

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'mappingJackson2HttpMessageConverter' defined in class path resource [org/springframework/boot/autoconfigure/http/JacksonHttpMessageConvertersConfiguration$MappingJackson2HttpMessageConverterConfiguration.class]: Unsatisfied dependency expressed through method 'mappingJackson2HttpMessageConverter' parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'jacksonObjectMapper' defined in class path resource [org/springframework/boot/autoconfigure/jackson/JacksonAutoConfiguration$JacksonObjectMapperConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [com.fasterxml.jackson.databind.ObjectMapper] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:800)
	at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:541)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1334)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:564)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.addCandidateEntry(DefaultListableBeanFactory.java:1605)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:1562)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveMultipleBeans(DefaultListableBeanFactory.java:1406)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1338)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory$DependencyObjectProvider.resolveStream(DefaultListableBeanFactory.java:2108)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory$DependencyObjectProvider.orderedStream(DefaultListableBeanFactory.java:2102)
	at org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration.messageConverters(HttpMessageConvertersAutoConfiguration.java:70)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154)
	... 45 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'jacksonObjectMapper' defined in class path resource [org/springframework/boot/autoconfigure/jackson/JacksonAutoConfiguration$JacksonObjectMapperConfiguration.class]: Post-processing of merged bean definition failed; nested exception is java.lang.IllegalStateException: Failed to introspect Class [com.fasterxml.jackson.databind.ObjectMapper] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:579)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208)
	at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1380)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300)
	at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:887)
	at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791)
	... 67 common frames omitted
Caused by: java.lang.IllegalStateException: Failed to introspect Class [com.fasterxml.jackson.databind.ObjectMapper] from ClassLoader [sun.misc.Launcher$AppClassLoader@18b4aac2]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:481)
	at org.springframework.util.ReflectionUtils.doWithLocalMethods(ReflectionUtils.java:321)
	at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.buildLifecycleMetadata(InitDestroyAnnotationBeanPostProcessor.java:232)
	at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.findLifecycleMetadata(InitDestroyAnnotationBeanPostProcessor.java:210)
	at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(InitDestroyAnnotationBeanPostProcessor.java:149)
	at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessMergedBeanDefinition(CommonAnnotationBeanPostProcessor.java:294)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyMergedBeanDefinitionPostProcessors(AbstractAutowireCapableBeanFactory.java:1098)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:576)
	... 77 common frames omitted
Caused by: java.lang.NoClassDefFoundError: com/fasterxml/jackson/core/exc/StreamWriteException
	at java.lang.Class.getDeclaredMethods0(Native Method)
	at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
	at java.lang.Class.getDeclaredMethods(Class.java:1975)
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:463)
	... 84 common frames omitted
Caused by: java.lang.ClassNotFoundException: com.fasterxml.jackson.core.exc.StreamWriteException
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	... 88 common frames omitted

即Error creating bean with name 'mappingJackson2HttpMessageConverter' defined in class path resource 这个问题，但从这个错误信息，根本看不到具体的错误。

因而，继续往下看错误日志，看到最后发现报了这个错误，如下图所示：

2. 分析问题

java.lang.ClassNotFoundException: com.fasterxml.jackson.core.exc.StreamWriteException。出现这个错误，一般会有如下三种情况：

1. 版本不兼容

spring的版本和jackson的版本不兼容导致的。

2. jackson包冲突导致的

• 如何查看maven的包冲突？

可以使用快捷键Ctrl+Shift+Alt+U，或者是打开pom.xml,右键选择maven，然后选择Show Dependencies。

• 进入依赖包结构之后，可以用走上方的加减图标进行放大和缩小

• 红线的就是有冲突的包，可以选择右键—>exclude，把冲突的包排除掉

• 剔除冲突的包，回到pom.xml进行刷新后，冲突的包就被排除了

3. 缺少相应的依赖

我查看我的pom文件，果然没有相应的依赖。

3. 解决问题

既然缺少相应的依赖，加上如下依赖即可：

<!-- properties中增加jackson.version，便于dependncy调用 -->
<properties>
    <java.version>1.8</java.version>
    <jackson.version>2.10.1</jackson.version>
</properties>

<!-- jackson -->
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>${jackson.verson}</version>
</dependency>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-core</artifactId>
  <version>${jackson.verson}</version>
</dependency>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-annotations</artifactId>
  <version>${jackson.verson}</version>
</dependency>