No qualifying bean of type ‘java.lang.String‘ available: expected at least 1 bean which qualifies-CFANZ编程社区

一、问题复现和详细异常

今天一个同事在开发时给一个类（AServiceImpl）加了@AllArgsConstructor，希望通过private final 的方式将XxClass 和 XxxClass注入到当前类：

@Service
@AllArgsConstructor
public class AServiceImpl implements AService{

    private final XxClass xxClass;

    private final XxxClass xxxClass;

    @Value("${my.value:myValue}")
    private String value;

    .....
}

运行时报错如下：

No qualifying bean of type ‘java.lang.String‘ available: expected at least 1 bean which qualifies_java

org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.lang.String' available: expected at least 1 bean which qualifies as autowire candidate. annotations: {}
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1777) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1333) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1287) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:887) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:229) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1356) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1206) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:571) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:531) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944) ~[spring-beans-5.3.2.jar:5.3.2]
  at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:923) ~[spring-context-5.3.2.jar:5.3.2]
  at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:588) ~[spring-context-5.3.2.jar:5.3.2]
  at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:144) ~[spring-boot-2.4.1.jar:2.4.1]
  at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:767) [spring-boot-2.4.1.jar:2.4.1]
  at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:759) [spring-boot-2.4.1.jar:2.4.1]
  at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:426) [spring-boot-2.4.1.jar:2.4.1]
  at org.springframework.boot.SpringApplication.run(SpringApplication.java:326) [spring-boot-2.4.1.jar:2.4.1]
  at org.springframework.boot.SpringApplication.run(SpringApplication.java:1309) [spring-boot-2.4.1.jar:2.4.1]
  at org.springframework.boot.SpringApplication.run(SpringApplication.java:1298) [spring-boot-2.4.1.jar:2.4.1]
  at com.saint.DemoTestApplication.main(DemoTestApplication.java:10) [classes/:na]
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. annotations: {}
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1777)
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1333)
  at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1287)
  at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:887)
  at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:791)
  ... 28

二、原因分析

从报错提示可以看出是没有找到NoSuchBeanDefinition；根本原因是因为springioc容器加载bean默认使用无参构造进行初始化，就示例而言，需要用到只有一个入参的构造函数（AClassImpl(XsClass)）；

由于此处使用的是@AllArgsConstructor注解，所以对AClassImpl而言，只有一个构造函数（AClassImpl(XxClass, XxxClass, String)），而构造函数的第三个参数是用@Value注入导入的一个属性值，其并不会存在于Spring容器，所以报错。

源码分析

从报错信息找到出错的入口：org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1356)

No qualifying bean of type ‘java.lang.String‘ available: expected at least 1 bean which qualifies_jar_02

这里可以看到获取到的AServiceImpl的构造函数中有三个入参，最后一个入参是String类型；然后顺着报错日志的执行链路，找到

No qualifying bean of type ‘java.lang.String‘ available: expected at least 1 bean which qualifies_jar_03

这里ConstructorResolver#resolveAutowiredArgument()方法会判断用于解析指定参数的模板方法是否支持被自动注入（autowired）；非Class类型均不行。

抛出异常的源码位置如下：

No qualifying bean of type ‘java.lang.String‘ available: expected at least 1 bean which qualifies_spring cloud_04

因为采用构造函数注入属性的方式，属性是必须要存在于Spring容器的，所以这里检查参数必须性时会校验不通过（即不会提前return），进而报错；

三、解决方案

一般可以给AServiceImpl提供一个默认的构造方法来解决，但此处需要注入XxClass 和 XxxClass，所以仅需生成一个只包含XxClass 和 XxxClass两个参数的构造方法即可。
因此，我们选择使用@RequiredArgsConstructor注解来替换@AllArgsConstructor注解；

No qualifying bean of type ‘java.lang.String‘ available: expected at least 1 bean which qualifies_java_05

@RequiredArgsConstructor 和 @AllArgsConstructor的区别？

@RequiredArgsConstructor会将类的每一个final字段或者non-null字段生成一个构造方法
@AllArgsConstructor 生成一个包含所有字段的构造方法；
@NoArgsConstructor 生成无参的构造方法；

使用@RequiredArgsConstructor可以代替@Autowrited注解；而@RequiredArgsConstructor是根据构造器注入的，所以会有循坏依赖的问题。

@RequiredArgsConstructor循环依赖问题解决措施

有三种方式可以解决：

改为使用@Autowired注解做属性方法注入Bean；
不过，Spring 从 4.0 开始，就不推荐使用属性注入模式了，原因在于它可以让我们忽略掉一些代码可能变坏的隐患。
使用@RequiredArgsConstructor(onConstructor =@_(@Autowired) )，这样默认都是通过@Autowired注入Bean的；
使用@RequiredArgsConstructor(onConstructor_={@Lazy} )，做懒加载处理。