【spring原始碼系列】之【FactoryBean型別的介面】
2022-06-13 06:02:26
1.概述
目前我們知道，spring建立bean有多種方式，比如xml方式建立，比如@Component，@Service，@Controler，@Repository註解建立，比如@Autowired依賴注入建立，後續還有通過springboot方式的設定註解@Configuration與@Bean方式結合建立，這裡不一一介紹，等分析spring boot原始碼的時候再做總結。
就spring本身，提供了一種介面方式建立bean，就是本節要討論的通過FactoryBean介面方式建立。
2.範例
FactoryBean介面的實現類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;
}
}
通過FactoryBean實現類，完成自定義類FactoryB的範例化，FactoryB：
package com.wzj.FactoryBean;
import lombok.Data;
@Data
public class FactoryB {
private String name = "wzj";
}
測試類：
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);
}
測試結果：
可以看出，當獲取名稱為factoryBeanDemo的範例時，得到的是getObject()方法裡建立的FactoryB型別的物件，而獲取加字首&的factoryBeanDemo的範例時，得到的是FactoryBeanDemo本身的範例。
3.原始碼
step1: FactoryBean 介面的呼叫入口在範例化和 IOC/DI 做完後，就會呼叫 FactoryBean 型別的介面如下圖所示
// Create bean instance.
// 建立bean範例
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的呼叫入口
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
step2: 如果要獲取到 FactoryBean 類本身，就必須加上&符號，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.
// 如果為name不為空，且以字首&打頭，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")只能獲取到 getObject()方法返回的範例。getObject 方法返回的範例會有單獨的快取儲存，跟其他範例不是同一個快取，對應的快取是：factoryBeanObjectCache
// 如果是不是以字首&打頭，並且是FactoryBean型別的
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
}
else {
// 從快取裡拿FactoryBean範例
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());
// 快取沒有的話，
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// 先從快取factoryBeanObjectCache取
Object object = this.factoryBeanObjectCache.get(beanName);
// 如果快取為空，
if (object == null) {
// 呼叫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)) {
// 最後放到快取中
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
......
小結：具體程式碼參考 getSingleton 方法之後 getObjectForBeanInstance
- 如果bean範例不是 FactoryBean 型別的或者 name 以&開始的則直接返回範例。
- 如果bean是 FacotyBean 並且不是以&開頭， 會通過方法doGetObjectFromFactoryBean 呼叫FactoryBean 內部繼承實現的 getObject 方法，並且判斷一級快取中如果存在該 bean 範例把範例快取到factoryBeanObjectCache 對應的 map 中，這個是單獨快取 FactoryBean 型別範例的 map。
4.總結
靈活建立所需範例物件的時候，通過實現FactoryBean介面的getObject方法定義範例化過程。
比如MyBatis提供mybatis-spring專案中的 org.mybatis.spring.SqlSessionFactoryBean
public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
// ...省略其他程式碼
public SqlSessionFactory getObject() throws Exception {
if (this.sqlSessionFactory == null) {
afterPropertiesSet();
}
return this.sqlSessionFactory;
}
}
sqlSessionFactory是SqlSessionFactoryBean的一個屬性，它的賦值是在通過回撥afterPropertiesSet()方法進行的。 因為SqlSessionFactoryBean實現了InitializingBean介面，所以在Spring初始化Bean的時候，能回撥afterPropertiesSet()方法。
public void afterPropertiesSet() throws Exception {
// buildSqlSessionFactory()方法會根據mybatis的設定進行初始化。
this.sqlSessionFactory = buildSqlSessionFactory();
}
在上面的afterPropertiesSet()方法中，buildSqlSessionFactory()方法會根據mybatis的設定，完成客戶所需要的的sessionFactory的初始化。