java - 具有AnnotationConfigApplicationContext的Spring应用程序:获取运行时异常

这是我的一个SO问题的延续-> here

我扩展了相同的示例，并且希望能“很好”地工作，但是我遇到了NullPointerException。

这是完整的源代码：

FirstBean.java

package com.example;

import org.springframework.stereotype.Component;

@Component
public class FirstBean {

    public FirstBean() {

    }

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "FirstBean [name=" + name + "]";
    }
}

SomeBean.java

package com.example;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Component;

@Component
@Configuration
@ComponentScan(basePackages = { "com.example" })
public class SomeBean {

    @Autowired
    private FirstBean fb;

    @Bean
    FirstBean instantiateFirstBean() {
        return new FirstBean();
    }

    public SomeBean() {
        // this.fb.setName("Some Name"); -> this was causing problem as
        // bean isn't still created fully
    }

    public void print() {
        this.fb.toString();
    }

    @PostConstruct
    void post() {
        fb.setName("Some name");
    }
}

MainDriver.java

package com.example;

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.context.annotation.PropertySource;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;

@Configuration
@ComponentScan(basePackages = { "com.example" })
@PropertySource(ignoreResourceNotFound = true, value = "classpath:/application.props")
public class MainDriver {

    @Autowired
    private Environment env;

    @Autowired
    private ConfigurableEnvironment cenv;

    @Autowired
    ApplicationContext ctx;

    @Autowired
    private SomeBean sb;

    @Bean
    public SomeBean instantiateSomeBean() {
        return new SomeBean();
    }

    public static void main(String[] args) {

        ApplicationContext ctx = new AnnotationConfigApplicationContext(MainDriver.class);

        MainDriver l = ctx.getBean(MainDriver.class);
        System.out.println("In main() the ctx is " + ctx);
        l.test();
    }

    public void test() {
        System.out.println("hello");
        sb.print();
    }
}

我期望SomeBean得到Autowired（ctx，env，cenv的Autowired完全正确），但是却没有得到，并且我正在获得运行时异常。

Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'mainDriver': Unsatisfied dependency expressed through field 'sb'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'someBean' defined in file [/Users/vkoul/understand-code/platform/stream-base/trunk/target/classes/com/example/SomeBean.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.SomeBean$$EnhancerBySpringCGLIB$$fad3571b]: Constructor threw exception; nested exception is java.lang.NullPointerException
Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'mainDriver': Unsatisfied dependency expressed through field 'sb'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'someBean' defined in file [/Users/vkoul/understand-code/platform/stream-base/trunk/target/classes/com/example/SomeBean.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.SomeBean$$EnhancerBySpringCGLIB$$fad3571b]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:584)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:370)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1336)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:572)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:759)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:867)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:548)
    at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:88)
    at com.example.MainDriver.main(MainDriver.java:37)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'someBean' defined in file [/Users/vkoul/understand-code/platform/stream-base/trunk/target/classes/com/example/SomeBean.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.SomeBean$$EnhancerBySpringCGLIB$$fad3571b]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1228)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1127)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:535)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:495)
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:251)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1135)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1062)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:581)
    ... 14 more
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.example.SomeBean$$EnhancerBySpringCGLIB$$fad3571b]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:182)
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:87)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1220)
    ... 25 more
Caused by: java.lang.NullPointerException
    at com.example.SomeBean.<init>(SomeBean.java:23)
    at com.example.SomeBean$$EnhancerBySpringCGLIB$$fad3571b.<init>(<generated>)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:170)
    ... 27 more

我还提到了@Bean，我希望它能告诉您如何创建这些bean，但仍然会出现异常。

任何人都可以帮助我滞后于了解以及如何解决的地方。

最佳答案

我认为这是因为Spring首先必须实例化您的SomeBean类，然后它将尝试通过反射自动连接FirstBean，因为您将@Autowired注释放在了字段上。

您正在尝试访问FirstBean构造函数中的SomeBean依赖项，但是在调用SomeBean的构造函数时，在创建上下文时，FirstBean依赖项为null，因为Spring尚未自动关联该依赖项-它将尝试创建对象后通过反射对其进行自动布线。这就是为什么您在那里到达NullPointerException的原因。您的SomeBean类也会发生相同的情况-选择一种方法。

另外，创建同时用@Configuration和@Component注释的类很奇怪。请参阅this SO question以查看潜在的错误。

此外，您的配置中还存在一些问题。您用FirstBean注释对@Component进行注释，并且仍然创建一个@Bean带注释的方法，该方法返回此类的实例。您应该确定一个方法-使用@Bean注释为应用程序上下文实例化它，或者使用@Component注释此类，然后让组件扫描自动为您创建它。

编辑：

您应该决定是要使用@ComponentScan还是要通过@Bean注释为上下文创建bean-现在，您正在混合使用不同的类型，并且在尝试按类型自动装配时，对于重复的bean定义会出现错误，因为在您当前的实现中，在您的上下文中将有2个SomeBean bean和2个FirstBean bean-它们将只有不同的ID。请注意，我用NPE期望值指出的错误与它无关。我将提出使用自动包裹扫描的解决方案：

FirstBean类

@Component
public class FirstBean {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "FirstBean [name=" + name + "]";
    }
}

SomeBean类

@Component
public class SomeBean {

    private FirstBean fb;

    @Autowired
    public SomeBean(FirstBean firstBean) {
        this.fb = firstBean;
        this.fb.setName("Some Name");
    }

    public void print() {
        this.fb.toString();
    }
}

MyConfig类

@Configuration
@ComponentScan(basePackages = { "com.example" })
public class MyConfig {

}

MainDriver类

@Component
public class MainDriver {

    @Autowired
    private Environment env;

    @Autowired
    protected ConfigurableEnvironment cenv;

    @Autowired
    ApplicationContext ctx;

    @Autowired
    private SomeBean sb;


    public static void main(String[] args) {

        ApplicationContext ctx = new AnnotationConfigApplicationContext(MyConfig.class);

        MainDriver l = ctx.getBean(MainDriver.class);
        System.out.println("In main() the ctx is " + ctx);
        l.test();
    }

    public void test() {
        System.out.println("hello");
        sb.print();
    }
}