1.概述
目前我们知道，spring创建bean有多种方式，比如xml方式创建，比如@Component，@Service，@Controler，@Repository注解创建，比如@Autowired依赖注入创建，后续还有通过springboot方式的配置注解@Configuration与@Bean方式结合创建，这里不一一介绍，等分析spring boot源码的时候再做总结。
就spring本身，提供了一种接口方式创建bean，就是本节要讨论的通过FactoryBean接口方式创建。
2.实例
FactoryBean接口的实现类FactoryBeanDemo:
package com.wzj.FactoryBean;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Service;
@Service
public class FactoryBeanDemo implements FactoryBean {
@Override
public Object getObject() throws Exception {
return new FactoryB();
}
@Override
public Class<?> getObjectType() {
return FactoryB.class;
}
}
通过FactoryBean实现类，完成自定义类FactoryB的实例化，FactoryB：
package com.wzj.FactoryBean;
import lombok.Data;
@Data
public class FactoryB {
private String name = "wzj";
}
测试类：
public class TestSpring {
@Autowired
private ApplicationContext applicationContext;
@Test
public void testFactoryBean() {
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml");
FactoryB factoryB = (FactoryB)applicationContext.getBean("factoryBeanDemo");
System.out.println(factoryB);
FactoryBeanDemo factoryBeanDemo = (FactoryBeanDemo)applicationContext.getBean("&factoryBeanDemo");
System.out.println(factoryBeanDemo);
}
测试结果：
可以看出，当获取名称为factoryBeanDemo的实例时，得到的是getObject()方法里创建的FactoryB类型的对象，而获取加前缀&的factoryBeanDemo的实例时，得到的是FactoryBeanDemo本身的实例。
3.源码
step1: FactoryBean 接口的调用入口在实例化和 IOC/DI 做完后，就会调用 FactoryBean 类型的接口如下图所示
// Create bean instance.
// 创建bean实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
// FactoryBean的调用入口
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
step2: 如果要获取到 FactoryBean 类本身，就必须加上&符号，比如 beanFactory.getBean("&beanName") ，如下:
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
// 如果为name不为空，且以前缀&打头，直接返回bean本身
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
public static boolean isFactoryDereference(@Nullable String name) {
return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
}
String FACTORY_BEAN_PREFIX = "&";
stet3: BeanFactory.getBean("beanName")只能获取到 getObject()方法返回的实例。getObject 方法返回的实例会有单独的缓存存储，跟其他实例不是同一个缓存，对应的缓存是：factoryBeanObjectCache
// 如果是不是以前缀&打头，并且是FactoryBean类型的
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
// 从缓存里拿FactoryBean实例
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 缓存没有的话，
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// 先从缓存factoryBeanObjectCache取
Object object = this.factoryBeanObjectCache.get(beanName);
// 如果缓存为空，
if (object == null) {
// 调用getObject方法
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
if (containsSingleton(beanName)) {
// 最后放到缓存中
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
......
小结：具体代码参考 getSingleton 方法之后 getObjectForBeanInstance
- 如果bean实例不是 FactoryBean 类型的或者 name 以&开始的则直接返回实例。
- 如果bean是 FacotyBean 并且不是以&开头， 会通过方法doGetObjectFromFactoryBean 调用FactoryBean 内部继承实现的 getObject 方法，并且判断一级缓存中如果存在该 bean 实例把实例缓存到factoryBeanObjectCache 对应的 map 中，这个是单独缓存 FactoryBean 类型实例的 map。
4.总结
灵活创建所需实例对象的时候，通过实现FactoryBean接口的getObject方法定义实例化过程。
比如MyBatis提供mybatis-spring项目中的 org.mybatis.spring.SqlSessionFactoryBean
public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
// ...省略其他代码
public SqlSessionFactory getObject() throws Exception {
if (this.sqlSessionFactory == null) {
afterPropertiesSet();
}
return this.sqlSessionFactory;
}
}
sqlSessionFactory是SqlSessionFactoryBean的一个属性，它的赋值是在通过回调afterPropertiesSet()方法进行的。 因为SqlSessionFactoryBean实现了InitializingBean接口，所以在Spring初始化Bean的时候，能回调afterPropertiesSet()方法。
public void afterPropertiesSet() throws Exception {
// buildSqlSessionFactory()方法会根据mybatis的配置进行初始化。
this.sqlSessionFactory = buildSqlSessionFactory();
}
在上面的afterPropertiesSet()方法中，buildSqlSessionFactory()方法会根据mybatis的配置，完成客户所需要的的sessionFactory的初始化。