在被注入的原型 bean 中获取父 bean答案

【问题标题】：Get parent bean in prototype bean that gets injected在被注入的原型 bean 中获取父 bean
【发布时间】：2010-10-20 23:55:11
【问题描述】：

我想要一个像这样的 Bean 和一个 SubBean：

@Scope(BeanDefinition.SCOPE_PROTOTYPE)
@Component
public class SubBean implements ApplicationContextAware{
  private Object parent;
  public void setApplicationContext(ApplicationContext ctx){
    this.parent = doSomeMagicToGetMyParent(ctx);
  }
  public Object getParent(){ 
    return parent; 
  }
}

@Component
public class SomeBean implements InitializingBean{
  @Resource
  private SubBean sub;

  public void afterPropertiesSet(){
    Assert.isTrue(this == sub.getParent());
  }
}

我想要实现的技巧是，SubBean 自动获取对它注入的 Bean 的引用。因为子 bean 的范围是原型，所以它将作为一个新实例注入到每个希望它被注入的父级中。

我的大想法是利用这种模式来编写一个可以注入到普通 bean 中的 LoggerBean。子 bean 应该像 SLF4J 记录器一样工作。

那么有谁知道使这项工作发挥作用的魔力吗？ :)

编辑：我找到了使用自定义 BeanPostProcessor 的解决方案：

@Component
public class DependencyInjectionAwareBeanPostProcessor implements BeanPostProcessor {
  @Override
  public Object postProcessBeforeInitialization(Object bean, String beanName) {
    return bean;
  }
  @Override
  public Object postProcessAfterInitialization(Object bean, String beanName) {
    for (Field f : bean.getClass().getFields()) {
        if (f.getType().isInstance(IDependencyInjectionAware.class)) {
            ReflectionUtils.makeAccessible(f);
            try {
                IDependencyInjectionAware diAware = (IDependencyInjectionAware) f.get(bean);
                diAware.injectedInto(bean);
            } catch (IllegalArgumentException e) {
                ReflectionUtils.handleReflectionException(e);
            } catch (IllegalAccessException e) {
                ReflectionUtils.handleReflectionException(e);
            }
        }
    }
    return bean;
  }
}

界面如下：

public interface IDependencyInjectionAware {
  void injectedInto(Object parent);
}

这里有一个 Bean 使用它：

@Scope(BeanDefinition.SCOPE_PROTOTYPE)
@Component
public class SomeAwareBean implements IDependencyInjectionAware {
  private Object parent;
  public void injectedInto(Object parent){
    this.parent = parent;
  }
  public Object getParent(){
    return parent;
  }
}

这里用正常的 Bean 进行测试，效果很好：

@Component 
public class UsingBean implements InitializingBean {
  @Resource
  private SomeAwareBean b;
  public void afterPropertiesSet(){
    Assert.notNull(b); //works
    Assert.isTrue(b.getParent() == this); //works
  }
}

但是，当将其与通过@Configurable 注入依赖项的普通类一起使用时，测试会失败：

@Configurable
public class UsingPlainClass implements InitializingBean {
  @Resource
  private SomeAwareBean b;
  public void afterPropertiesSet(){
    Assert.notNull(b); //works
    Assert.isTrue(b.getParent() == this); //fails because null is returned
  }
}

所以这似乎让我想到了另一个问题：为什么我的自定义 BeanPostProcessor 不能在 @Configurable 类上运行？也许我不得不求助于 AspectJ...

编辑：只是为了更新状态。毕竟我没有实现这个，因为这是过度工程......

【问题讨论】：

标签： spring dependency-injection design-patterns slf4j javabeans

【解决方案1】：

我觉得这更简单：

@Scope(BeanDefinition.SCOPE_PROTOTYPE)
@Component
public class SubBean implements ApplicationContextAware{
  private Object parent;
  public void setApplicationContext(ApplicationContext ctx){
    ...
  }
  public Object getParent(){ 
    return parent; 
  }
  //ADDED CODE
  public void setParent(Object parent) {
    this.parent = parent;
  }
  //END ADDED CODE
}

@Component
public class SomeBean implements InitializingBean{

  private SubBean sub;
  //ADDED CODE
  @Resource
  public void setSub(SubBean sub) {
      this.sub = sub;
      sub.setParent(this);
  }
  //END ADDED CODE

  public void afterPropertiesSet(){
    Assert.isTrue(this == sub.getParent());
  }
}

【讨论】：

SomeBean 范围是什么？在您的应用程序中 SomeBean 是一个实例？

【解决方案2】：

用原帖者给出的解决方案修复了几个错误：

import java.lang.reflect.Field;

import org.apache.log4j.Logger;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.util.ReflectionUtils;

public interface DependencyInjectionAware {

void injectedInto(final Object bean, final String beanName);

public static class DependencyInjectionAwareBeanPostProcessor implements
        BeanPostProcessor {

    private static final Logger logger =   Logger.getLogger(DependencyInjectionAwareBeanPostProcessor.class);

    @Override
    public Object postProcessBeforeInitialization(final Object bean,
            final String beanName) {
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(final Object bean,
            final String beanName) {
        for (final Field f : bean.getClass().getDeclaredFields()) {
            logger.info("scanning field " + f.getName() + " of bean " + beanName + " (class= " + bean.getClass() + ")");

            if (DependencyInjectionAware.class.isAssignableFrom(f.getType())) {
                ReflectionUtils.makeAccessible(f);
                try {
                    final DependencyInjectionAware diAware = (DependencyInjectionAware) f.get(bean);
                    diAware.injectedInto(bean, beanName);
                } catch (final IllegalArgumentException e) {
                    ReflectionUtils.handleReflectionException(e);
                } catch (final IllegalAccessException e) {
                    ReflectionUtils.handleReflectionException(e);
                }                   
            }
        }
        return bean;
    }
}

}

【讨论】：