深入剖析 Spring 框架的 BeanFactory

说到Spring框架，人们往往大谈特谈一些似乎高逼格的东西，比如依赖注入，控制反转，面向切面等等。但是却忘记了最基本的一点，Spring的本质是一个bean工厂(beanFactory)或者说bean容器，它按照我们的要求，生产我们需要的各个各样的bean，提供给我们使用。只是在生产bean的过程中，需要解决bean之间的依赖问题，才引入了依赖注入(DI)这种技术。也就是说依赖注入是beanFactory生产bean时为了解决bean之间的依赖的一种技术而已。

那么我们为什么需要Spring框架来给我们提供这个beanFactory的功能呢？原因是一般我们认为是，可以将原来硬编码的依赖，通过Spring这个beanFactory这个工程来注入依赖，也就是说原来只有依赖方和被依赖方，现在我们引入了第三方——spring这个beanFactory，由它来解决bean之间的依赖问题，达到了松耦合的效果；这个只是原因之一，还有一个更加重要的原因：在没有spring这个beanFactory之前，我们都是直接通过new来实例化各种对象，现在各种对象bean的生产都是通过beanFactory来实例化的，这样的话，spring这个beanFactory就可以在实例化bean的过程中，做一些小动作——在实例化bean的各个阶段进行一些额外的处理，也就是说beanFactory会在bean的生命周期的各个阶段中对bean进行各种管理，并且spring将这些阶段通过各种接口暴露给我们，让我们可以对bean进行各种处理，我们只要让bean实现对应的接口，那么spring就会在bean的生命周期调用我们实现的接口来处理该bean。下面我们看是如何实现这一点的。

1. bean容器的启动

bean在实例化之前，必须是在bean容器启动之后。所以就有了两个阶段：

1）bean容器的启动阶段；

2）容器中bean的实例化阶段；

在启动阶段，

1> 首先是读取bean的

public abstract class AbstractBeanDefinition extends BeanMetadataAttributeAccessor    implements BeanDefinition, Cloneable { private volatile Object beanClass;  private String scope = SCOPE_DEFAULT;  private boolean abstractFlag = false;  private boolean lazyInit = false;  private int autowireMode = AUTOWIRE_NO;  private int dependencyCheck = DEPENDENCY_CHECK_NONE;  private String[] dependsOn;private ConstructorArgumentValues constructorArgumentValues;  private MutablePropertyValues propertyValues;private String factoryBeanName;  private String factoryMethodName;  private String initMethodName;  private String destroyMethodName;

beanClass保存bean的class属性，scop保存bean是否单例，abstractFlag保存该bean是否抽象，lazyInit保存是否延迟初始化，autowireMode保存是否自动装配，dependencyCheck保存是否坚持依赖，dependsOn保存该bean依赖于哪些bean(这些bean必须提取初始化)，constructorArgumentValues保存通过构造函数注入的依赖，propertyValues保存通过setter方法注入的依赖，factoryBeanName和factoryMethodName用于factorybean，也就是工厂类型的bean，initMethodName和destroyMethodName分别对应bean的init-method和destory-method属性，比如：

<bean name="dataSource" class="com.alibaba.druid.pool.DruidDataSource" init-method="init" destroy-method="close">

读完配置文件之后，得到了很多的BeanDefinition对象，

2> 然后通过BeanDefinitionRegistry将这些bean注册到beanFactory中：

public interface BeanDefinitionRegistry extends AliasRegistry {  void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)throws BeanDefinitionStoreException;  void removeBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;  BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;  boolean containsBeanDefinition(String beanName);  String[] getBeanDefinitionNames();  int getBeanDefinitionCount();  boolean isBeanNameInUse(String beanName);}

BeanFactory的实现类，需要实现BeanDefinitionRegistry 接口：

@SuppressWarnings("serial")public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory    implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {  /** Map of bean definition objects, keyed by bean name */  private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(64);  
  @Override  public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)      throws BeanDefinitionStoreException {    // ... ...

    this.beanDefinitionMap.put(beanName, beanDefinition);    // ... ...  }

我们看到BeanDefinition被注册到了 DefaultListableBeanFactory，保存在它的一个ConcurrentHashMap中。

将BeanDefinition注册到了beanFactory之后，在这里Spring为我们提供了一个扩展的切口，允许我们通过实现接口BeanFactoryPostProcessor 在此处来插入我们定义的代码：

public interface BeanFactoryPostProcessor {  /**   * Modify the application context's internal bean factory after its standard   * initialization. All bean definitions will have been loaded, but no beans   * will have been instantiated yet. This allows for overriding or adding   * properties even to eager-initializing beans.   * @param beanFactory the bean factory used by the application context   * @throws org.springframework.beans.BeansException in case of errors   */  void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;}

典型的例子就是：PropertyPlaceholderConfigurer，我们一般在配置数据库的dataSource时使用到的占位符的值，就是它注入进去的：

public abstract class PropertyResourceConfigurer extends PropertiesLoaderSupport    implements BeanFactoryPostProcessor, PriorityOrdered {  @Override  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {    try {      Properties mergedProps = mergeProperties();      // Convert the merged properties, if necessary.      convertProperties(mergedProps);      // Let the subclass process the properties.      processProperties(beanFactory, mergedProps);    }    catch (IOException ex) {      throw new BeanInitializationException("Could not load properties", ex);    }  }

processProperties(beanFactory, mergedProps);在子类中实现的，功能就是将

  <bean name="dataSource" class="com.alibaba.druid.pool.DruidDataSource" init-method="init" destroy-method="close">    <property name="url" value="${jdbc_url}" />    <property name="username" value="${jdbc_username}" />    <property name="password" value="${jdbc_password}" />

${jdbc_username}等等这些替换成实际值。

bean的实例化阶段

实例化阶段主要是通过反射或者CGLIB对bean进行实例化，在这个阶段Spring又给我们暴露了很多的扩展点：

1> 各种的Aware接口，比如 BeanFactoryAware，MessageSourceAware，ApplicationContextAware

对于实现了这些Aware接口的bean，在实例化bean时Spring会帮我们注入对于的：BeanFactory， MessageSource，ApplicationContext的实例：

public interface BeanFactoryAware extends Aware {  /**   * Callback that supplies the owning factory to a bean instance.   * <p>Invoked after the population of normal bean properties   * but before an initialization callback such as   * {@link InitializingBean#afterPropertiesSet()} or a custom init-method.   * @param beanFactory owning BeanFactory (never {@code null}).   * The bean can immediately call methods on the factory.   * @throws BeansException in case of initialization errors   * @see BeanInitializationException   */  void setBeanFactory(BeanFactory beanFactory) throws BeansException;}

public interface ApplicationContextAware extends Aware {  /**   * Set the ApplicationContext that this object runs in.   * Normally this call will be used to initialize the object.   * <p>Invoked after population of normal bean properties but before an init callback such   * as {@link org.springframework.beans.factory.InitializingBean#afterPropertiesSet()}   * or a custom init-method. Invoked after {@link ResourceLoaderAware#setResourceLoader},   * {@link ApplicationEventPublisherAware#setApplicationEventPublisher} and   * {@link MessageSourceAware}, if applicable.   * @param applicationContext the ApplicationContext object to be used by this object   * @throws ApplicationContextException in case of context initialization errors   * @throws BeansException if thrown by application context methods   * @see org.springframework.beans.factory.BeanInitializationException   */  void setApplicationContext(ApplicationContext applicationContext) throws BeansException;}

public interface MessageSourceAware extends Aware {  /**   * Set the MessageSource that this object runs in.   * <p>Invoked after population of normal bean properties but before an init   * callback like InitializingBean's afterPropertiesSet or a custom init-method.   * Invoked before ApplicationContextAware's setApplicationContext.   * @param messageSource message sourceto be used by this object   */  void setMessageSource(MessageSource messageSource);}

2> BeanPostProcessor接口

实现了BeanPostProcessor接口的bean，在实例化bean时Spring会帮我们调用接口中的方法：

public interface BeanPostProcessor {  /**   * Apply this BeanPostProcessor to the given new bean instance <i>before</i> any bean   * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}   * or a custom init-method). The bean will already be populated with property values.   * The returned bean instance may be a wrapper around the original.*/  Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;  /**   * Apply this BeanPostProcessor to the given new bean instance <i>after</i> any bean   * initialization callbacks (like InitializingBean's {@code afterPropertiesSet}   * or a custom init-method). The bean will already be populated with property values.   * The returned bean instance may be a wrapper around the original.*/  Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;}

从注释中可以知道 postProcessBeforeInitialization方法在 InitializingBean接口的 afterPropertiesSet方法之前执行，而postProcessAfterInitialization方法在 InitializingBean接口的afterPropertiesSet方法之后执行。

3> InitializingBean接口

实现了BeanPostProcessor接口的bean，在实例化bean时Spring会帮我们调用接口中的方法：

public interface InitializingBean {  /**   * Invoked by a BeanFactory after it has set all bean properties supplied   * (and satisfied BeanFactoryAware and ApplicationContextAware).   * <p>This method allows the bean instance to perform initialization only   * possible when all bean properties have been set and to throw an   * exception in the event of misconfiguration.   * @throws Exception in the event of misconfiguration (such   * as failure to set an essential property) or if initialization fails.   */  void afterPropertiesSet() throws Exception;}

4> DisposableBean接口

实现了BeanPostProcessor接口的bean，在该bean死亡时Spring会帮我们调用接口中的方法：

public interface DisposableBean {  /**   * Invoked by a BeanFactory on destruction of a singleton.   * @throws Exception in case of shutdown errors.   * Exceptions will get logged but not rethrown to allow   * other beans to release their resources too.   */  void destroy() throws Exception;}

InitializingBean接口和 DisposableBean接口对应于 <bean /> 的 init-method 和 destory-method 属性，其经典的例子就是dataSource:

所以在Spring初始化 dataSource 这个bean之后会调用 DruidDataSource.init 方法:

  public void init() throws SQLException {    // ... ...try {      lock.lockInterruptibly();    } catch (InterruptedException e) {      throw new SQLException("interrupt", e);    }    boolean init = false;    try {       connections = new DruidConnectionHolder[maxActive];      SQLException connectError = null;      try {                for (int i = 0, size = getInitialSize(); i < size; ++i) {          Connection conn = createPhysicalConnection();          DruidConnectionHolder holder = new DruidConnectionHolder(this, conn);          connections[poolingCount++] = holder;        }        if (poolingCount > 0) {          poolingPeak = poolingCount;          poolingPeakTime = System.currentTimeMillis();        }      } catch (SQLException ex) {        LOG.error("init datasource error", ex);        connectError = ex;      }         } catch (SQLException e) {      LOG.error("dataSource init error", e);      throw e;    } catch (InterruptedException e) {      throw new SQLException(e.getMessage(), e);    } finally {      inited = true;      lock.unlock();    }  }

基本就是初始化数据库连接池。

在dataSource 这个bean死亡时会调用 DruidDataSource.close()方法:

  public void close() {    lock.lock();    try {     for (int i = 0; i < poolingCount; ++i) {        try {          DruidConnectionHolder connHolder = connections[i];          for (PreparedStatementHolder stmtHolder : connHolder.getStatementPool().getMap().values()) {            connHolder.getStatementPool().closeRemovedStatement(stmtHolder);          }          connHolder.getStatementPool().getMap().clear();          Connection physicalConnection = connHolder.getConnection();          physicalConnection.close();          connections[i] = null;          destroyCount.incrementAndGet();        } catch (Exception ex) {          LOG.warn("close connection error", ex);        }      }         } finally {      lock.unlock();    }  }

基本就是关闭连接池中的连接。

另外注解 @PostConstruct 和 @PreDestroy 也能达到 InitializingBean接口和 DisposableBean接口的效果。

2. 总结

spring容器接管了bean的实例化，不仅仅是通过依赖注入达到了松耦合的效果，同时给我们提供了各种的扩展接口，来在bean的生命周期的各个时期插入我们自己的代码：

0）BeanFactoryPostProcessor接口(在容器启动阶段)

1）各种的Aware接口

2）BeanPostProcessor接口

3）InitializingBean接口(@PostConstruct， init-method)

4）DisposableBean接口(@PreDestroy, destory-method)

原标题：深入剖析 Spring 框架的 BeanFactory

关键词：Spring