使用 @Bean Spring 注释创建 bean 时出现异常

Question

我试图了解Spring framework @component和@Bean之间的@component ，并参考了这个 SO question 。

基于这个答案，我有了一个大致的了解，并试图通过实现一个简单的例子来强化这个概念（检查我是否真的理解了这个概念）。

下面是我写的代码。 我想看到“第三方”库的@Bean （以便它可以使用@Bean注释实例化）

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@component中不会有任何@component 。

现在，下面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，它将创建ThirdPartyUtil作为 Spring bean，然后使用@Autowired注入到字段thirdPartyUtil 。

然而，当我运行这个程序时，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 applicationContext 并将必要的详细信息传递给它。

我希望它可以工作，但是遇到了上述异常（我见过处女时间）。

为什么会抛出这个异常，我对@Bean理解不正确吗？

Answer 1

@Bean注解的目标是注解一个方法，该方法的职责是创建一个对象，然后由 Spring 将其注册为 bean。

但是在您的方法中，您要求 Spring（使用ctx.getBean() ）为您提供您当前正在创建的 bean。 所以你得到了那个例外。

您的方法应该如下所示：

@Bean
public ThirdPartyUtil beanThirdPartyUtil() {
    return new ThirdPartyUtil();
}

Answer 2

要扩展@JB Nizet 答案，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();
}