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深入 Spring Boot：怎样排查 expected single matching bean but found 2 的异常

写在前面

这个demo来说明怎么排查一个常见的spring expected single matching bean but found 2的异常。

https://github.com/hengyunabc/spring-boot-inside/tree/master/demo-expected-single

调试排查 expected single matching bean but found 2 的错误

把工程导入IDE里，直接启动应用，抛出来的异常信息是：

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'javax.sql.DataSource' available: expected single matching bean but found 2: h2DataSource1,h2DataSource2
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1041) ~[spring-beans-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:345) ~[spring-beans-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:340) ~[spring-beans-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1090) ~[spring-context-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    at org.springframework.boot.autoconfigure.jdbc.DataSourceInitializer.init(DataSourceInitializer.java:71) ~[spring-boot-autoconfigure-1.4.7.RELEASE.jar:1.4.7.RELEASE]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_112]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_112]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_112]
    at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_112]
    at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleElement.invoke(InitDestroyAnnotationBeanPostProcessor.java:366) ~[spring-beans-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleMetadata.invokeInitMethods(InitDestroyAnnotationBeanPostProcessor.java:311) ~[spring-beans-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor.postProcessBeforeInitialization(InitDestroyAnnotationBeanPostProcessor.java:134) ~[spring-beans-4.3.9.RELEASE.jar:4.3.9.RELEASE]
    ... 30 common frames omitted

很多人碰到这种错误时，就乱配置一通，找不到下手的办法。其实耐心排查下，是很简单的。

抛出异常的原因

异常信息写得很清楚了，在spring context里需要注入/获取到一个DataSource bean，但是现在spring context里出现了两个，它们的名字是：h2DataSource1,h2DataSource2

那么有两个问题：

应用是在哪里要注入/获取到一个DataSource bean？
h2DataSource1,h2DataSource2 是在哪里定义的？

使用 Java Exception Breakpoint

在IDE里，新建一个断点，类型是Java Exception Breakpoint（如果不清楚怎么添加，可以搜索对应IDE的使用文档），异常类是上面抛出来的NoUniqueBeanDefinitionException。

当断点停住时，查看栈，可以很清楚地找到是在DataSourceInitializer.init() line: 71这里要获取DataSource：

Thread [main] (Suspended (exception NoUniqueBeanDefinitionException))
    owns: ConcurrentHashMap<K,V>  (id=49)
    owns: Object  (id=50)
    DefaultListableBeanFactory.resolveNamedBean(Class<T>, Object...) line: 1041
    DefaultListableBeanFactory.getBean(Class<T>, Object...) line: 345
    DefaultListableBeanFactory.getBean(Class<T>) line: 340
    AnnotationConfigEmbeddedWebApplicationContext(AbstractApplicationContext).getBean(Class<T>) line: 1090
    DataSourceInitializer.init() line: 71
    NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method]
    NativeMethodAccessorImpl.invoke(Object, Object[]) line: 62
    DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 43
    Method.invoke(Object, Object...) line: 498
    InitDestroyAnnotationBeanPostProcessor$LifecycleElement.invoke(Object) line: 366
    InitDestroyAnnotationBeanPostProcessor$LifecycleMetadata.invokeInitMethods(Object, String) line: 311
    CommonAnnotationBeanPostProcessor(InitDestroyAnnotationBeanPostProcessor).postProcessBeforeInitialization(Object, String) line: 134
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).applyBeanPostProcessorsBeforeInitialization(Object, String) line: 409
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).initializeBean(String, Object, RootBeanDefinition) line: 1620
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).doCreateBean(String, RootBeanDefinition, Object[]) line: 555
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).createBean(String, RootBeanDefinition, Object[]) line: 483
    AbstractBeanFactory$1.getObject() line: 306
    DefaultListableBeanFactory(DefaultSingletonBeanRegistry).getSingleton(String, ObjectFactory<?>) line: 230
    DefaultListableBeanFactory(AbstractBeanFactory).doGetBean(String, Class<T>, Object[], boolean) line: 302
    DefaultListableBeanFactory(AbstractBeanFactory).getBean(String, Class<T>, Object...) line: 220
    DefaultListableBeanFactory.resolveNamedBean(Class<T>, Object...) line: 1018
    DefaultListableBeanFactory.getBean(Class<T>, Object...) line: 345
    DefaultListableBeanFactory.getBean(Class<T>) line: 340
    DataSourceInitializerPostProcessor.postProcessAfterInitialization(Object, String) line: 62
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).applyBeanPostProcessorsAfterInitialization(Object, String) line: 423
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).initializeBean(String, Object, RootBeanDefinition) line: 1633
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).doCreateBean(String, RootBeanDefinition, Object[]) line: 555
    DefaultListableBeanFactory(AbstractAutowireCapableBeanFactory).createBean(String, RootBeanDefinition, Object[]) line: 483
    AbstractBeanFactory$1.getObject() line: 306
    DefaultListableBeanFactory(DefaultSingletonBeanRegistry).getSingleton(String, ObjectFactory<?>) line: 230
    DefaultListableBeanFactory(AbstractBeanFactory).doGetBean(String, Class<T>, Object[], boolean) line: 302
    DefaultListableBeanFactory(AbstractBeanFactory).getBean(String) line: 197
    DefaultListableBeanFactory.preInstantiateSingletons() line: 761
    AnnotationConfigEmbeddedWebApplicationContext(AbstractApplicationContext).finishBeanFactoryInitialization(ConfigurableListableBeanFactory) line: 867
    AnnotationConfigEmbeddedWebApplicationContext(AbstractApplicationContext).refresh() line: 543
    AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).refresh() line: 122
    SpringApplication.refresh(ApplicationContext) line: 762
    SpringApplication.refreshContext(ConfigurableApplicationContext) line: 372
    SpringApplication.run(String...) line: 316
    SpringApplication.run(Object[], String[]) line: 1187
    SpringApplication.run(Object, String...) line: 1176
    DemoExpectedSingleApplication.main(String[]) line: 17

定位哪里要注入/使用DataSource

要获取DataSource具体的代码是：

//org.springframework.boot.autoconfigure.jdbc.DataSourceInitializer.init()
    @PostConstruct
    public void init() {
        if (!this.properties.isInitialize()) {
            logger.debug("Initialization disabled (not running DDL scripts)");
            return;
        }
        if (this.applicationContext.getBeanNamesForType(DataSource.class, false,
                false).length > 0) {
            this.dataSource = this.applicationContext.getBean(DataSource.class);
        }
        if (this.dataSource == null) {
            logger.debug("No DataSource found so not initializing");
            return;
        }
        runSchemaScripts();
    }

this.applicationContext.getBean(DataSource.class); 要求spring context里只有一个DataSource的bean，但是应用里有两个，所以抛出了NoUniqueBeanDefinitionException。

从BeanDefinition获取bean具体定义的代码

我们再来看 h2DataSource1,h2DataSource2 是在哪里定义的？

上面进程断在了DefaultListableBeanFactory.resolveNamedBean(Class<T>, Object…) 函数里的 throw new NoUniqueBeanDefinitionException(requiredType, candidates.keySet()); 这一行。

那么我们在这里执行一下（如果不清楚，先搜索下IDE怎么在断点情况下执行代码）：

this.getBeanDefinition("h2DataSource1")

返回的信息是：

Root bean: class [null]; scope=; abstract=false; lazyInit=false; autowireMode=3; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=demoExpectedSingleApplication; factoryMethodName=h2DataSource1; initMethodName=null; destroyMethodName=(inferred);
defined in com.example.demo.expected.single.DemoExpectedSingleApplication

可以很清楚地定位到h2DataSource1这个bean是在 com.example.demo.expected.single.DemoExpectedSingleApplication里定义的。

所以上面两个问题的答案是：

是spring boot代码里的DataSourceInitializer.init() line: 71这里要获取DataSource，并且只允许有一个DataSource实例
h2DataSource1,h2DataSource2 是在com.example.demo.expected.single.DemoExpectedSingleApplication里定义的

解决问题

上面排查到的原因是：应用定义了两个DataSource实例，但是spring boot却要求只有一个。那么有两种办法来解决：

使用@Primary来指定一个优先使用的DataSource，这样子spring boot里自动初始的代码会获取到@Primary的bean
把spring boot自动初始化DataSource相关的代码禁止掉，应用自己来控制所有的DataSource相关的bean

禁止的办法有两种：

在main函数上配置exclude

@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class, DataSourceTransactionManagerAutoConfiguration.class })

在application.properties里配置：

spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration