SpringBoot中注入RedisTemplate实例异常解决

小渣渣

最近，在项目开发过程中使用了RedisTemplate，进行单元测试时提示“Field redisTemplate in com.example.demo1.dao.RedisDao required a bean of type ‘org.springframework.data.redis.core.RedisTemplate’ that could not be found”，翻译过来就是“找不到类型为RedisTemplate的bean”。当然，仅看这句话我们无法确定为什么会出现这个问题。现在，贴出详细的错误日志。

2018-08-10 14:53:49.761 - Caught exception while allowing TestExecutionListener [org.springframework.test.context.web.ServletTestExecutionListener@5af3afd9] to prepare test instance [com.example.demo1.Demo1ApplicationTests@2361365c]
java.lang.IllegalStateException: Failed to load ApplicationContext
    at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:124)
    at org.springframework.test.context.support.DefaultTestContext.getApplicationContext(DefaultTestContext.java:83)
    at org.springframework.test.context.web.ServletTestExecutionListener.setUpRequestContextIfNecessary(ServletTestExecutionListener.java:189)
    at org.springframework.test.context.web.ServletTestExecutionListener.prepareTestInstance(ServletTestExecutionListener.java:131)
    at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:230)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:228)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:287)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:289)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:247)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:94)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:191)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'redisDao': Unsatisfied dependency expressed through field 'redisTemplate'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.data.redis.core.RedisTemplate<java.lang.String, java.lang.Object>' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1264)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:693)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:360)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:303)
    at org.springframework.boot.test.context.SpringBootContextLoader.loadContext(SpringBootContextLoader.java:120)
    at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContextInternal(DefaultCacheAwareContextLoaderDelegate.java:98)
    at org.springframework.test.context.cache.DefaultCacheAwareContextLoaderDelegate.loadContext(DefaultCacheAwareContextLoaderDelegate.java:116)
    ... 25 common frames omitted
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.data.redis.core.RedisTemplate<java.lang.String, java.lang.Object>' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1493)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1104)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1066)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585)
    ... 43 common frames omitted

看到这一大段错误日志是不是有点晕，其实我们只需要看关键的错误日志就可以定位到问题。错误日志中的第二个Caused by打印了关键的错误日志：No qualifying bean of type ‘org.springframework.data.redis.core.RedisTemplate< java.lang.String, java.lang.Object>。

笔主回看写的代码，在注入RedisTemplate< K, V>时指定了具体的类型。

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根据错误日志查看DefaultListableBeanFactory这个类的第1493行的代码，源码如下：

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这两个方法说明了没有找到RedisTemplate< String, Object>可以匹配的Bean，那么这个问题该如何解决呢？不妨在使用RedisTemplate< K, V>时不指定具体的类型，修改代码如下：

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重新启动服务，启动日志没有报错，RedisTemplate注入Bean成功了。为什么RedisTemplate< String, Object>注入Bean会失败呢，很是纳闷。思考很久，想到RedisTemplate在SpringBoot框架中是自动配置的，容器中默认的就是RedisTemplate的实例。想到这里，就需要翻下官网的文档，看看官网文档有没有什么说明。果然，官方文档第30.1.1章节还是针对RedisTemplate做了说明。

官方文档地址：https://docs.spring.io/spring-bo ... ference/htmlsingle/

If you add your own @Bean of any of the auto-configured types, it replaces the default (except in the case of RedisTemplate, when the exclusion is based on the bean name, redisTemplate, not its type).

如果针对自动配置类型添加自己的Bean，它将取代默认的。我的代码好像写的没问题啊，等等…括号中这句话才是重点啊。翻译一波，在RedisTemplate的情况下除外，当排除基于Bean的名称，而不是它的类型。英文不太好，怎么翻译好像都不大通顺。

现在好像明白了，刚才的RedisTemplate< String, Object>注入时用到了@Autowired，@Autowired默认按照类型装配的。也就是说，想要获取RedisTemplate< String, Object>的Bean，要根据名字装配。那么自然想到使用@Resource，它默认按照名字装配。再次修改代码如下：

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注解由@Autowired改成了@Resource 项目启动不报错了，完美解决！

再次重新启动服务，启动日志没有报错，RedisTemplate< String, Object>注入Bean成功了。通过这个错误，学习到SpringBoot框架中自动配置类的一个特性。同时，也反映出学习一门技术要从官网开始，避免踩坑。
在实际开发过程中，使用RedisTemplate< K, V>不是必须指定K、V的类型，使用默认的Bean就能满足需求。

最后想再验证一个小的问题，再次修改代码如下：

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上边这段代码在注入RedisTemplate时指定K、V都为String类型，在compare()方法中比较redisTemplate和stringRedisTemplate是否是同一个对象。运行下单元测试类，测试结果如下：

2018-08-10 18:30:57.075 - Started Demo1ApplicationTests in 15.58 seconds (JVM running for 16.974)
2018-08-10 18:30:57.360 - redisTemplate的hashcode：1996087296
2018-08-10 18:30:57.360 - stringRedisTemplate的hashcode：1996087296
2018-08-10 18:30:57.363 - equal()的结果：true
2018-08-10 18:30:57.364 - redisTemplate和stringRedisTemplate的比较结果：true

测试结果表明redisTemplate和stringRedisTemplate是同一个对象。等等…这次注入RedisTemplate时指定K、V都为String为什么没有错呢？而且，两个对象竟然是同一个对象。还是来看下StringRedisTemplate的源码吧。

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看了源码是不是理解了，因为StringRedisTemplate类的父类就是RedisTemplate< String, String>，而Bean默认是单例的，两个是自然是同一个对象了。