问题现象

如果我们通过如下的方式来注入Spring Bean对象，那么在启动的时候就会报如下错误

@Servicepublic class MyServiceImpl {    private String name;    public MyServiceImpl(String name) { this.name = name;    }}

警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'myServiceImpl': Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.lang.String' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'myServiceImpl': Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.lang.String' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798)at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1358)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517)at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202)at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:879)at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878)at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550)at com.wyl.learn.TestSpring.main(TestSpring.java:30)Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.lang.String' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1695)at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1253)at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1207)at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885)at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789)... 13 more

解决方式

解决方式很简单，只需要向Spring容器中注入一个名为name的bean对象即可

@Configurationpublic class BeanConfig {    @Bean    public String name() { return "小明";    }}

问题分析

首先通过有参的构造方法注入，会调用AbstractAutowireCapableBeanFactory的

protected BeanWrapper autowireConstructor(String beanName, RootBeanDefinition mbd, @Nullable Constructor<?>[] ctors, @Nullable Object[] explicitArgs)

具体在autowireConstructor方法中，会调用createArgumentArray方法，这个方法的目的就是对参数对象进行构建

argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames,getUserDeclaredConstructor(candidate), autowiring, candidates.length == 1);

然后调用DefaultListableBeanFactory中的doResolveDependency真正的解析构造方法中的参数对象

result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);

具体方法findAutowireCandidates提供，根据指定类型找到所需的bean实例

protected Map<String, Object> findAutowireCandidates(@Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor)

最后，通过DefaultListableBeanFactory的doGetBeanNamesForType，改方法负责最后在beanDefinitionNames中找目标对象的任务

String[] result = lbf.getBeanNamesForType(type, includeNonSingletons, allowEagerInit);resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);

总体思路

当Spring发现需要通过有参构造方法的方式构建对象实例时，就会挨个处理构造方法中的参数，且必须先对这些参数进行构建，因此会按照参数类型从beanDefinitionNames集合中寻找，看看容器中有没有相关的bean存在，如果找不到就会报错。

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