我试图理解Spring framework中@component和@Bean之间的区别，并提到了这个问题。
基于答案，我得到了一个公平的理解，并试图通过实现一个简单的例子来加强这个概念(以检查我是否真的理解了这个概念(。
下面是我写的代码。我想查看"第三方"库的@Bean(以便可以使用@Bean注释对其进行实例化(
package com.example.thirdparty.code;
public class ThirdPartyUtil {
public ThirdPartyUtil() {
}
public void print(String str) {
System.out.println("String passed is --> " + str);
}
}
在上面，我试图模拟一个"第三方库/类 - 非弹簧"，它不是基于弹簧的(因此该类中不会有任何@component。
现在，以下是我创建的主要应用程序：
package com.example.spring;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;
import com.example.thirdparty.code.ThirdPartyUtil;
@Configuration
@ComponentScan(basePackages = { "com.example.spring" })
@Component
public class SpringBasedMainApp {
@Autowired
private ApplicationContext ctx;
@Autowired
private static ThirdPartyUtil thirdPartyUtil;
@Bean
public ThirdPartyUtil beanThirdPartyUtil() {
try {
Class c = getClass().getClassLoader().loadClass("com.example.thirdparty.code.ThirdPartyUtil");
return (ThirdPartyUtil) ctx.getBean(c);
} catch (ClassNotFoundException e) {
throw new IllegalArgumentException("Exception");
}
}
public static void main(String[] args) {
ApplicationContext ctx = new AnnotationConfigApplicationContext(SpringBasedMainApp.class);
System.out.println("thirdPartyUtil is " + thirdPartyUtil);
}
}
我希望使用@Bean注释，它会创建ThirdPartyUtil作为Spring bean，然后将使用@Autowired注入到字段thirdPartyUtil中。
但是，当我运行此程序时，我得到了一个令人惊讶的异常，如下所示：
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'beanThirdPartyUtil' defined in com.example.spring.SpringBasedMainApp: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.thirdparty.code.ThirdPartyUtil]: Factory method 'beanThirdPartyUtil' threw exception; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'beanThirdPartyUtil': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:627)
at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:456)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1288)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1127)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:538)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:498)
at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:320)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:846)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:863)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:546)
at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:88)
at com.example.spring.SpringBasedMainApp.main(SpringBasedMainApp.java:41)
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.thirdparty.code.ThirdPartyUtil]: Factory method 'beanThirdPartyUtil' threw exception; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'beanThirdPartyUtil': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:185)
at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:622)
... 14 more
Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'beanThirdPartyUtil': Requested bean is currently in creation: Is there an unresolvable circular reference?
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:339)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:215)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:318)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:224)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveNamedBean(DefaultListableBeanFactory.java:1112)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveBean(DefaultListableBeanFactory.java:407)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:341)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:335)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1101)
at com.example.spring.SpringBasedMainApp.beanThirdPartyUtil(SpringBasedMainApp.java:33)
at com.example.spring.SpringBasedMainApp$$EnhancerBySpringCGLIB$$d93c67f4.CGLIB$beanThirdPartyUtil$0(<generated>)
at com.example.spring.SpringBasedMainApp$$EnhancerBySpringCGLIB$$d93c67f4$$FastClassBySpringCGLIB$$9349d69b.invoke(<generated>)
at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244)
at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:363)
at com.example.spring.SpringBasedMainApp$$EnhancerBySpringCGLIB$$d93c67f4.beanThirdPartyUtil(<generated>)
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)
... 15 more
我不明白为什么它会给出这个例外。我正在使用AnnotationConfigApplicationContext创建一个应用程序上下文，并将必要的详细信息传递给它。
我希望它能工作，但是得到了上述异常(我已经看到了处女时间(。
为什么抛出这个异常，我对@Bean的理解不正确吗？
@Bean注解的目标是注释一个方法，该方法的职责是创建一个对象，然后由 Spring 将其注册为 bean。
但是在你的方法中，你要求Spring(使用ctx.getBean()(给你你当前正在创建的豆子。所以你会得到这个例外。
以下是你的冰毒应该是什么样子的：
@Bean
public ThirdPartyUtil beanThirdPartyUtil() {
return new ThirdPartyUtil();
}
要扩展 Nizet 答案@JB请在 spring 构造函数中实例化您的第三方类，如下所示。
@Autowired
private static ThirdPartyUtil thirdPartyUtil;
@Autowired
public SpringBean() {
this.thirdPartyUtil = new ThirdPartyUtil();
}