Spring中Bean的作用域Scope你知道多少？如何自定义作用域？

1 Scope作用

通过@Scope注解可以指定Bean的作用域，默认情况都是单例的（ConfigurableBeanFactory.SCOPE_SINGLETON=singleton）

在创建bean实例时就是根据当前定义BeanDefinition中的Scope来做不同的创建，源码如下：

protected  T doGetBean(
            String name, @Nullable Class requiredType, @Nullable Object[] args, boolean typeCheckOnly)
            throws BeansException {
  String beanName = transformedBeanName(name);
  Object bean;
  // Eagerly check singleton cache for manually registered singletons.
  Object sharedInstance = getSingleton(beanName);
  if (sharedInstance != null && args == null) {
    // other code
  } else {
    // other code
    try {
      RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      checkMergedBeanDefinition(mbd, beanName, args);


      // Guarantee initialization of beans that the current bean depends on.
      // other code
      // Create bean instance.
      // 根据BeanDefinition中定义的Scope创建实例
      // 判断如果是单例
      if (mbd.isSingleton()) {
        // 如果是单例Bean会将Bean保存到缓存中singletonObjects  
        sharedInstance = getSingleton(beanName, () -> {
          try {
            return createBean(beanName, mbd, args);
          } catch (BeansException ex) {
            destroySingleton(beanName);
            throw ex;
          }
        });
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
      }
      // 判断如果是原型（多例）
      else if (mbd.isPrototype()) {
        // It's a prototype -> create a new instance.
        Object prototypeInstance = null;
        try {
          beforePrototypeCreation(beanName);
          prototypeInstance = createBean(beanName, mbd, args);
        } finally {
          afterPrototypeCreation(beanName);
        }
        bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      } 
      else {
        String scopeName = mbd.getScope();
        if (!StringUtils.hasLength(scopeName)) {
          throw new IllegalStateException("No scope name defined for bean 麓" + beanName + "'");
        }
        Scope scope = this.scopes.get(scopeName);
        // 当集合中也不存在时抛出异常  
        if (scope == null) {
          throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
        }
        try {
          Object scopedInstance = scope.get(beanName, () -> {
            beforePrototypeCreation(beanName);
            try {
              return createBean(beanName, mbd, args);
            } finally {
              afterPrototypeCreation(beanName);
            }
          });
          bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
        } catch (IllegalStateException ex) {
          throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider " + "defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex);
        }
      }
    } catch (BeansException ex) {
      cleanupAfterBeanCreationFailure(beanName);
      throw ex;
    }
  }
  // other code
  return (T) bean;
}

从上面源码看到分别判断是了 是否是 Singleton及Proptotype，如果都不是则会从Map scopes中获取。如果当前你配置的@Scope不是singleton及prototype那么从scopes集合中取（这个集合是通过AbstractBeanFactory#registerScope方法进行注册的，一般我们可以通过BeanDefinitionRegistryPostProcessor进行注册），如果集合中也不存在那么就会抛出异常。如果存在就会执行Scope#get方法。

Scope scope = this.scopes.get(scopeName);
Object scopedInstance = scope.get(beanName, () -> {
  beforePrototypeCreation(beanName);
  try {
    return createBean(beanName, mbd, args);
  } finally {
    afterPrototypeCreation(beanName);
  }
});

2 自定义Scope

自定义Scope

public class CustomScope implements Scope {
    
  private Object target ;


  @Override
  public Object get(String name, ObjectFactory objectFactory) {
    return target != null ? target : objectFactory.getObject() ;
  }
  // 如果调用了这个方法，那么下次在注入有@Scope("custom")的bean时 将会重写调用objectFactory.getObject()方法。
  @Override
  public Object remove(String name) {
    target = null ;
    return "success" ;
  }


  @Override
  public void registerDestructionCallback(String name, Runnable callback) {
  }


  @Override
  public Object resolveContextualObject(String key) {
    return null;
  }


  @Override
  public String getConversationId() {
    return null;
  }


}

注册Scope

@Component
public class CustomScopeRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor {
  @Override
  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    beanFactory.registerScope("custom", new CustomScope()) ;
  }
  @Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
  }
}

使用Scope

@Component
@Scope("custom")
public class ApplyScopeBean {
}

示例

@RestController
@RequestMapping("/refresh")
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE)
public class RefreshController implements ApplicationContextAware{
  @Resource
  private ApplyScopeBean scopeBean ;
  @Resource
  private CustomScope customScope ;
  @GetMapping("/custom")
  public String custom() {
    return scopeBean.getCustom() ;
  }
  @GetMapping("/remove") 
  public Object remove() {
    return customScope.remove("applyScopeBean") ;
  }  
}

这里将Controller设置为多例，以便查看效果。交替执行上面的接口，只要删除了就会创建新的实例。

3 多例注入

如果一个Bean 设置了@Scope(value =ConfigurableBeanFactory.SCOPE_PROTOTYPE) 当这个Bean需要在一个单例Bean中被注入时，需要如下配置才可

@Component
@Scope(value = ConfigurableBeanFactory.SCOPE_PROTOTYPE, proxyMode = ScopedProxyMode.TARGET_CLASS)
public class ApplyScopeBean {
}

这样才能正确地注入Bean，否则因为本身使用者是单例的，属性只会被初始化一次。也可以在每次使用前调用BeanFactory#getBean()。