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