看一个例子
factorybeanDemo类
@Service
public class factorybeanDemo implements factorybean {
@Override
public Object getobject() throws Exception {
return new FactoryB();
}
@Override
public Class<?> getobjectType() {
return FactoryB.class;
}
}FactoryB类`
public class FactoryB {
}测试代码
public void test4() {
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext("cn.com.dq");
FactoryB factoryB = (FactoryB) applicationContext.getBean("factorybeanDemo");
System.out.println(factoryB);
factorybeanDemo factorybeanDemo = (factorybeanDemo) applicationContext.getBean("&factorybeanDemo");
System.out.println(factorybeanDemo);
}输出结果
从上面的代码可以启发2点思考
1.我们通过beanName=[factorybeanDemo]拿到的bean不是factorybeanDemo的实例，而是FactoryB 的实例
2.我们要想拿到factorybeanDemo的实例而是通过&factorybeanDemo拿到
下面我们从源码上分析上述情况
源码分析
在bean的实例化中，当bean实例化完成以后都会看到这样一行代码，如下：
Abstractbeanfactory类
doGetBean()方法
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
...
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;
}
});
//该方法是factorybean接口的调用入口
bean = getobjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
...
}`
factorybean接口的调用入口getobjectForBeanInstance();
getobjectForBeanInstance()
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.
if (beanfactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof factorybean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
}
// 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.
//如果实例不是factorybean类型的，或者name是以&号开头的，则直接返回实例
if (!(beanInstance instanceof factorybean) || beanfactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
//如果代码能走下来，则说明 beanName不是以&开头，并且beanInstance是factorybean类型的
Object object = null;
if (mbd == null) {
//从缓存里面拿factorybean类型的实例
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());
//关键方法
object = getobjectFromfactorybean(factory, beanName, !synthetic);
}
return object;
}如下代码
if (!(beanInstance instanceof factorybean) || beanfactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}beanfactoryUtils的isFactoryDereference方法如下
public static boolean isFactoryDereference(@Nullable String name) {
return (name != null && name.startsWith(beanfactory.FACTORY_BEAN_PREFIX));
}1.上述代码可以解释要拿到factorybeanDemo的实例则是通过&factorybeanDemo拿到
2.如果beanName不是以&开头，并且beanInstance是factorybean类型，会优先在factorybeanObjectCache缓存中拿。
3.第一次肯定是拿不到的，则会执行getobjectFromfactorybean()方法
factorybeanRegistrySupport类
getobjectFromfactorybean()方法
protected Object getobjectFromfactorybean(factorybean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factorybeanObjectCache.get(beanName);
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)) {
//把实例缓存到factorybeanObjectCache map中，这个是单独缓存factorybean类型实例的map
this.factorybeanObjectCache.put(beanName, object);
}
}
}
return object;
}
} else {
Object object = doGetobjectFromfactorybean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromfactorybean(object, beanName);
} catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of factorybean's object Failed", ex);
}
}
return object;
}
}该方法执行2个比较重要的操作
1.doGetobjectFromfactorybean()
2.将执行doGetobjectFromfactorybean()方法返回的bean的实例放入factorybeanObjectCache缓存中
doGetobjectFromfactorybean()方法
private Object doGetobjectFromfactorybean(final factorybean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getobject, acc);
} catch (PrivilegedActionException pae) {
throw pae.getException();
}
} else {
//调用getobject方法
object = factory.getobject();
}
} catch (factorybeannotinitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
} catch (Throwable ex) {
throw new BeanCreationException(beanName, "factorybean threw exception on object creation", ex);
}
// Do not accept a null value for a factorybean that's not fully
// initialized yet: Many factorybeans just return null then.
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "factorybean which is currently in creation returned null from getobject");
}
object = new NullBean();
}
return object;
}上述代码可以看到最终调用到了factorybean的getobject()方法
回到示例代码factorybean的getobject()的方法返回的是FactoryB的实例
通过上述的源码分析就能解释我们通过beanName=[factorybeanDemo]拿到的bean不是factorybeanDemo的实例，而是FactoryB 的实例
总结
1.factorybean接口的调用入口是在bean的实例化完成以后
2.bean在实现factorybean接口后通过beanName拿到的不是bean本身的实例而是factorybean接口中getobject()方法返回的实例
3.bean在实现factorybean接口后要拿到bean本身的实例，则是通过&+beanName拿到
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