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？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这个工长来注入依赖，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：
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)
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">administrator@example.org</prop> <prop key="support">support@example.org</prop> <prop key="development">development@example.org</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
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