深入理解BeanFactory

1.BeanFactory概述

      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，生产我们需要的各种各样的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？第一个原因是，我们可以将原来硬编码的对象依赖通过Spring这个beanFactory这个工厂来注入依赖，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，达到了对象间松耦合的效果；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。

• 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：
• bean容器的启动阶段；
• 容器中bean的实例化阶段；

       在启动阶段，1> 首先是读取bean的xml配置文件，然后解析xml文件中的各种bean的定义，将xml文件中的每一个<bean />元素分别转换成一个BeanDefinition对象，其中保存了从配置文件中读取到的该bean的各种信息：

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会帮我们注入对应的：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InitializingBean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:<bean name="dataSource" class="com.alibaba.druid.pool.DruidDataSource" init-method="init" destroy-method="close">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：

• BeanFactoryPostProcessor接口(在容器启动阶段)
• 各种的Aware接口
• BeanPostProcessor接口
• InitializingBean接口(@PostConstruct， init-method)
• DisposableBean接口(@PreDestroy, destory-method)

3. FactoryBean接口

      实现了FactoryBean接口的bean是一类叫做factory的bean。其特点是，spring会在使用getBean()调用获得该bean时，会自动调用该bean的getObject()方法，所以返回的不是factory这个bean，而是这个bean.getOjbect()方法的返回值：

public interface FactoryBean<T> {
    T getObject() throws Exception;
    Class<?> getObjectType();
    boolean isSingleton();
}

      典型的例子有spring与mybatis的结合：

    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
      <property name="dataSource" ref="dataSource" />
      <property name="configLocation" value="classpath:config/mybatis-config-master.xml" />
      <property name="mapperLocations" value="classpath*:config/mappers/master/**/*.xml" />
    </bean>

        我们看上面该bean，因为实现了FactoryBean接口，所以返回的不是 SqlSessionFactoryBean 的实例，而是她的 SqlSessionFactoryBean.getObject() 的返回值：

public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {

  private static final Log logger = LogFactory.getLog(SqlSessionFactoryBean.class);

  private Resource configLocation;

  private Resource[] mapperLocations;

  private DataSource dataSource;
  
  public SqlSessionFactory getObject() throws Exception {
    if (this.sqlSessionFactory == null) {
      afterPropertiesSet();
    }

    return this.sqlSessionFactory;
  }

      其实他是一个专门生产 sqlSessionFactory 的工厂，所以才叫 SqlSessionFactoryBean。 而SqlSessionFactory又是生产SqlSession的工厂。还有spring与ibatis的结合：

    <!-- Spring提供的iBatis的SqlMap配置 -->
    <bean id="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">
        <property name="configLocation" value="classpath:sqlmap/sqlmap-config.xml" />
        <property name="dataSource" ref="dataSource" />
    </bean>

public class SqlMapClientFactoryBean implements FactoryBean<SqlMapClient>, InitializingBean {
    private Resource[] configLocations;
    private Resource[] mappingLocations;
    private Properties sqlMapClientProperties;
    private DataSource dataSource;
    private boolean useTransactionAwareDataSource = true;
    private Class transactionConfigClass = ExternalTransactionConfig.class;
    private Properties transactionConfigProperties;
    private LobHandler lobHandler;
    private SqlMapClient sqlMapClient;
    public SqlMapClient getObject() {
        return this.sqlMapClient;
    }

  SqlMapClientFactoryBean返回的是getObject()中返回的sqlMapClient,而不是SqlMapClientFactoryBean自己的实例。

4. 依赖注入(DI)

      1）依赖注入的方式分为接口注入、构造函数注入和setter方法注入：

<bean id="exampleBean" class="examples.ExampleBean">
    <constructor-arg index="0" value="7500000"/>
    <constructor-arg index="1" ref="bar"/>
</bean>
<bean id="bar" class="x.y.Bar"/>

      构造函数注入使用：<constructor-arg index="0" value="7500000"/>，<constructor-arg type="int" value="7500000"/>，对于非简单参数，需要使用ref <constructor-arg index="1" ref="bar"/>

    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
      <property name="dataSource" ref="dataSource" />
      <property name="configLocation" value="classpath:config/mybatis-config.xml" />
      <property name="mapperLocations" value="classpath*:config/mappers/**/*.xml" />
    </bean>

        setter方法注入使用 <property name="username" value="xxx"/>, 非简单类型属性使用ref <property name="xxbean" ref="xxx"/>

      2）集合等复杂类型的注入：

<bean id="moreComplexObject" class="example.ComplexObject">
    <!-- results in a setAdminEmails(java.util.Properties) call -->
    <property name="adminEmails">
        <props>
            <prop key="administrator">[email protected]</prop>
            <prop key="support">[email protected]</prop>
            <prop key="development">[email protected]</prop>
        </props>
    </property>
    <!-- results in a setSomeList(java.util.List) call -->
    <property name="someList">
        <list>
            <value>a list element followed by a reference</value>
            <ref bean="myDataSource" />
        </list>
    </property>
    <!-- results in a setSomeMap(java.util.Map) call -->
    <property name="someMap">
        <map>
            <entry key="an entry" value="just some string"/>
            <entry key ="a ref" value-ref="myDataSource"/>
        </map>
    </property>
    <!-- results in a setSomeSet(java.util.Set) call -->
    <property name="someSet">
        <set>
            <value>just some string</value>
            <ref bean="myDataSource" />
        </set>
    </property>
</bean>

也很简单，list属性就是 <list>里面包含<value>或者<ref>或者<bean>, set也类似。map是<map>里面包含<entry>这个也好理解，因为map的实现就是使用内部类Entry来存储key和value. Properties是 <props>里面包含<prop>.

5. <bean> 元素可以配置的属性

      <bean> 除了 id 和 class 属性之外，还有一些可选的属性:

1) scope属性，默认<bean> 的 scope就是 singleton="true", springmvc和struts2的重要区别之一就是spring的controll是单例的，而struts2的action是：scope="prototype" ，还有 scope="request" ， scope="session"，scope="globalSession"(仅用于portlet)；

2）abstract属性，是否是抽象的bean：

    <bean id="baseDAO" abstract="true">
        <property name="dataSource" ref="dataSource" />
        <property name="sqlMapClient" ref="sqlMapClient" />
    </bean>    
    <bean id="collectionDAO" class="net.minisns.dal.dao.CollectionDAOImpl" parent="baseDAO" />
    <bean id="commentDAO" class="net.minisns.dal.dao.CommentDAOImpl" parent="baseDAO" />

3）depends-on 依赖于某个bean，其必须先初始化：<bean id="xxx" class="xxx" depends-on="refbean" />；

4）lazy-init="true" 是否延迟初始化，默认为 false；

5) dependency-check 是否对bean依赖的其它bean进行检查，默认值为 none，可取值有：none, simple, object, all等；

6）factory-method 和 factory-bean用于静态工厂和非静态工厂：

<bean id="bar" class="...StaticBarInterfaceFactory" factory-method="getInstance"/>
<bean id="barFactory" class="...NonStaticBarInterfaceFactory"/> 
<bean id="bar" factory-bean="barFactory" factory-method="getInstance"/>

7）init-method, destory-method 指定bean初始化和死亡时调用的方法，常用于 dataSource的连接池的配置；

8) lookup-method 方法注入：

<bean id="newsBean" class="..xxx" singleton="false"> 
<bean id="mockPersister" class="..impl.MockNewsPersister">
  <lookup-method name="getNewsBean" bean="newsBean"/> 
</bean>  

      表示 mockPersister 有一个依赖属性 newsBean，该属性的每次注入都是通过调用newsBean.getNewsBean() 方法获得的。

9) autowire 是否启用自动装配依赖，默认为 no, 其它取值还有：byName, byType, constructor。

原文地址：https://www.cnblogs.com/digdeep/p/4518571.html