FactoryBean详解
一、首先来看看FactoryBean与BeanFactory:
这个两个拼写起来很相似,比较容易搞混,是反过来的。
FactoryBean ： 是一个Java Bean，但是它是一个能生产出当前对象的工厂Bean,它的实现和工厂模式及修饰器模式很像。
- BeanFactory:这就是一个Factory，是整个Spring IOC容器的核心内容,生产并存储很多的bean。
二、应用
- 现在就建一个对象实现FactoryBean接口试试：
package test;
import org.springframework.beans.factory.FactoryBean;
/**
*
* @author
*/
public class MyTestBean implements FactoryBean<MyTestBean> {
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
//返回由FactoryBean创建的bean实例，如果isSingleton()返回true，则该实例会放到Spring容器中单实例缓存池中。
@Override
public MyTestBean getObject() throws Exception {
MyTestBean bean = new MyTestBean();
bean.setName("Tom");
return bean;
}
//返回FactoryBean创建的bean类型。
@Override
public Class<?> getObjectType() {
return MyTestBean.class;
}
//返回由FactoryBean创建的bean实例的作用域是singleton还是prototype。
@Override
public boolean isSingleton() {
return true;
}
}- xml配置
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd " >
<bean id ="myTestBean" class= "test.MyTestBean" > </bean>
</beans>- 运行：
ApplicationContext xmlBeanFactory = new ClassPathXmlApplicationContext("spring-base.xml");
MyTestBean myTestBean = (MyTestBean) xmlBeanFactory.getBean("myTestBean");
System.out.println(myTestBean + "--" + myTestBean.getName());这里MyTestBean实现了FactoryBean接口，这个时候我们getBean(“myTestBean”)得到的就是getObject()返回的对象(
注意:这个返回的对象不一定就是当前类的这个对象,也就是说可以不是MyTestBean对象,根据自己的需求可以是其他的)，走FactoryBean它定义的getObject()方法;所以最后打印的name自然是Tom。如果非要得到MyTestBean对象(就是不取getObject()返回的那个对象)可以在getBean的时候前面多加一个
"&"。
三、源码是如何实现的
- 既然是getBean方法开始的,那么就从getBean看起。我的源码版本比较老,是spring 3.2.18.RELEASE,找到AbstractBeanFactory 229行左右doGetBean方法是从getBean方法调用过来的,这个方法也是初始化bean的过程
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 获取一个 “正统的” beanName，处理两种情况，一个是前面说的 FactoryBean(前面带 "&")，
// 一个是别名问题，因为这个方法是 getBean，获取 Bean 用的，你要是传一个别名进来，是完全可以的
final String beanName = transformedBeanName(name);//转换name
// 注意跟着这个，这个是返回值
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 检查下是不是已经创建过了
Object sharedInstance = getSingleton(beanName);
// 这里说下 args 呗，虽然看上去一点不重要。前面我们一路进来的时候都是 getBean(beanName)，
// 所以 args 其实是 null 的，但是如果 args 不为空的时候，那么意味着调用方不是希望获取 Bean，而是创建 Bean
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 下面这个方法：如果是普通 Bean 的话，直接返回 sharedInstance，
// 如果是 FactoryBean 的话，返回它创建的那个实例对象
// 此次FactoryBean 重点
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
// 当前线程已经创建过了此 beanName 的 prototype 类型的 bean，那么抛异常
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 检查一下这个 BeanDefinition 在容器中是否存在
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 如果当前容器不存在这个 BeanDefinition，试试父容器中有没有
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
// 返回父容器的查询结果
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
// typeCheckOnly 为 false，将当前 beanName 放入一个 alreadyCreated 的 Set 集合中。
markBeanAsCreated(beanName);
}
/*
* 到这里的话，要准备创建 Bean 了，对于 singleton 的 Bean 来说，容器中还没创建过此 Bean；
* 对于 prototype 的 Bean 来说，本来就是要创建一个新的 Bean。
*/
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 先初始化依赖的所有 Bean
// 注意，这里的依赖指的是 depends-on 中定义的依赖
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
// if (isDependent(beanName, dep)) {
// 先初始化被依赖项
getBean(dependsOnBean);
// 注册一下依赖关系
registerDependentBean(dependsOnBean, beanName);
}
}
// Create bean instance.
// 创建 singleton 的实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
try {
// 执行创建 Bean
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;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 创建 prototype 的实例
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
// 执行创建 Bean
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
// 如果不是 singleton 和 prototype 的话，需要委托给相应的实现类来处理
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
// 执行创建 Bean
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; " +
"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
// 最后，检查一下类型对不对，不对的话就抛异常，对的话就返回了
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type [" +
ClassUtils.getQualifiedName(requiredType) + "]", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
} - 实现了FactoryBean的bean对象重点就在getObjectForBeanInstance方法 在 AbstractBeanFactory 1475行左右
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
//如果是对FactoryBean的解引用，但bean对象不是FactoryBean，抛出异常
//Dereference(解引用)：一个在C/C++中应用的比较多术语，在C++中，"*"是解引用符号,得到对象的值;"&"是引用符号,得到对象内存地址。
//解引用：变量所指向的是所引用对象的本身数据，而不是对象的内存地址。
if (BeanFactoryUtils.isFactoryDereference(name) && !(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Bean实例不是FactoryBean，或者指定名称是FactoryBean的解引用，也就是普通的bean调用，则直接返回当前的Bean实例
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
//处理对FactoryBean的调用
Object object = null;
if (mbd == null) {
//从Bean工厂缓存中获取给定名称的实例对象
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.
//如果从Bean工厂生产的Bean是单态模式的，则缓存
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
//调用FactoryBeanRegistrySupport类的getObjectFromFactoryBean方法，实现FactoryBean生产Bean对象实例的过程
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}- 再进入FactoryBeanRegistrySupport的getObjectFromFactoryBean 大概在98行
// Bean工厂生产Bean实例对象
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
// Bean工厂是单态模式，并且Bean工厂缓存中存在指定名称的Bean实例对象
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// 直接从Bean工厂缓存中获取指定名称的Bean实例对象
Object object = this.factoryBeanObjectCache.get(beanName);
// Bean工厂缓存中没有指定名称的实例对象，则生产该实例对象
if (object == null) {
// 调用Bean工厂的getObject方法生产指定Bean的实例对象
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)
// 将生产的实例对象添加到Bean工厂缓存中
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
}
return (object != NULL_OBJECT ? object : null);
}
}
else {
// 调用Bean工厂的getObject方法生产指定Bean的实例对象
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}- 来到doGetObjectFromFactoryBean方法 154行左右
//调用FactoryBean接口实现类的创建对象方法的地方
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
//实现PrivilegedExceptionAction接口的匿名内置类
//根据JVM检查权限，然后决定BeanFactory创建实例对象
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
//调用FactoryBean接口实现类的创建对象方法
return factory.getObject();
}
}, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//调用FactoryBean接口实现类的创建对象方法
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);
}
//创建出来的实例对象为null，或者因为单态对象正在创建而返回null
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
return object;
}到这里就是一个实现了FactoryBean接口初始化不带”&”符号的一个基本过程。
另外文章代码或者我理解有误的地方,希望能批评指出。
