FactoryBean的作用

Spring 中有两种类型的Bean，一种是普通Bean，另一种是工厂Bean 即 FactoryBean。FactoryBean跟普通Bean不同，其返回的对象不是指定类的一个实例，而是该FactoryBean的getObject方法所返回的对象。创建出来的对象是否属于单例由isSingleton中的返回决定。

一般情况下，Spring通过反射机制利用<bean>的class属性指定实现类实例化Bean，在某些情况下，实例化Bean过程比较复杂，如果按照传统的方式，则需要在<bean>中提供大量的配置信息。配置方式的灵活性是受限的，这时采用编码的方式可能会得到一个简单的方案。Spring为此提供了一个org.springframework.bean.factory.FactoryBean的工厂类接口，用户可以通过实现该接口定制实例化Bean的逻辑。FactoryBean接口对于Spring框架来说占用重要的地位，Spring自身就提供了70多个FactoryBean的实现。它们隐藏了实例化一些复杂Bean的细节，给上层应用带来了便利。从Spring3.0开始，FactoryBean开始支持泛型，即接口声明改为FactoryBean<T>的形式

以Bean结尾，表示它是一个Bean，不同于普通Bean的是：它是实现了FactoryBean<T>接口的Bean，根据该Bean的ID从BeanFactory中获取的实际上是FactoryBean的getObject()返回的对象，而不是FactoryBean本身，如果要获取FactoryBean对象，请在id前面加一个&符号来获取。

FactoryBean接口定义

package org.springframework.beans.factory;

public interface FactoryBean<T> {
T getObject() throws Exception;

Class<?> getObjectType();

boolean isSingleton();
}

应用场景
FactoryBean 通常是用来创建比较复杂的bean，一般的bean 直接用xml配置即可，但如果一个bean的创建过程中涉及到很多其他的bean 和复杂的逻辑，用xml配置比较困难，这时可以考虑用FactoryBean。

很多开源项目在集成Spring 时都使用到FactoryBean，比如 MyBatis3 提供 mybatis-spring项目中的 org.mybatis.spring.SqlSessionFactoryBean：

<!-- spring和MyBatis整合，不需要mybatis的配置映射文件 -->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource"/>
<!-- 自动扫描mapping.xml文件 -->
<property name="mapperLocations" value="classpath:mapper/*.xml"></property>
</bean>

public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {
private static final Log LOGGER = LogFactory.getLog(SqlSessionFactoryBean.class);
...
public SqlSessionFactory getObject() throws Exception {
if (this.sqlSessionFactory == null) {
this.afterPropertiesSet();
}

return this.sqlSessionFactory;
}
...
}

 下面看一下具体的源码：

AbstractBeanFactory通过getBean向IoC容器获取被管理的Bean：

AbstractBeanFactory的getBean相关方法的源码如下：

protected <T> T doGetBean(String name, Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = this.transformedBeanName(name);
Object sharedInstance = this.getSingleton(beanName);
Object bean;
if (sharedInstance != null && args == null) {
if (this.logger.isDebugEnabled()) {
if (this.isSingletonCurrentlyInCreation(beanName)) {
this.logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference");
} else {
this.logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}

bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, (RootBeanDefinition)null);
} else {
if (this.isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}

BeanFactory parentBeanFactory = this.getParentBeanFactory();
if (parentBeanFactory != null && !this.containsBeanDefinition(beanName)) {
String nameToLookup = this.originalBeanName(name);
if (args != null) {
return parentBeanFactory.getBean(nameToLookup, args);
}

return parentBeanFactory.getBean(nameToLookup, requiredType);
}

if (!typeCheckOnly) {
this.markBeanAsCreated(beanName);
}

try {
final RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
this.checkMergedBeanDefinition(mbd, beanName, args);
String[] dependsOn = mbd.getDependsOn();
String[] var11;
if (dependsOn != null) {
var11 = dependsOn;
int var12 = dependsOn.length;

for(int var13 = 0; var13 < var12; ++var13) {
String dependsOnBean = var11[var13];
if (this.isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
}

this.registerDependentBean(dependsOnBean, beanName);
this.getBean(dependsOnBean);
}
}

if (mbd.isSingleton()) {
sharedInstance = this.getSingleton(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
try {
return AbstractBeanFactory.this.createBean(beanName, mbd, args);
} catch (BeansException var2) {
AbstractBeanFactory.this.destroySingleton(beanName);
throw var2;
}
}
});
bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
} else if (mbd.isPrototype()) {
var11 = null;

Object prototypeInstance;
try {
this.beforePrototypeCreation(beanName);
prototypeInstance = this.createBean(beanName, mbd, args);
} finally {
this.afterPrototypeCreation(beanName);
}

bean = this.getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
} else {
String scopeName = mbd.getScope();
Scope scope = (Scope)this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}

try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
AbstractBeanFactory.this.beforePrototypeCreation(beanName);

Object var1;
try {
var1 = AbstractBeanFactory.this.createBean(beanName, mbd, args);
} finally {
AbstractBeanFactory.this.afterPrototypeCreation(beanName);
}

return var1;
}
});
bean = this.getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
} catch (IllegalStateException var21) {
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", var21);
}
}
} catch (BeansException var23) {
this.cleanupAfterBeanCreationFailure(beanName);
throw var23;
}
}

if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return this.getTypeConverter().convertIfNecessary(bean, requiredType);
} catch (TypeMismatchException var22) {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Failed to convert bean '" + name + "' to required type [" + ClassUtils.getQualifiedName(requiredType) + "]", var22);
}

throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
} else {
return bean;
}
}

 我们可以看到，无论是直接取单例的bean，还是创建单例、多例、自定义生命周期的bean，都会经过bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);这个方法，我们现在就来看看这里到底是做了什么。

protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(this.transformedBeanName(name), beanInstance.getClass());
} else if (beanInstance instanceof FactoryBean && !BeanFactoryUtils.isFactoryDereference(name)) {
Object object = null;
if (mbd == null) {
object = this.getCachedObjectForFactoryBean(beanName);
}

if (object == null) {
FactoryBean<?> factory = (FactoryBean)beanInstance;
if (mbd == null && this.containsBeanDefinition(beanName)) {
mbd = this.getMergedLocalBeanDefinition(beanName);
}

boolean synthetic = mbd != null && mbd.isSynthetic();
object = this.getObjectFromFactoryBean(factory, beanName, !synthetic);
}

return object;
} else {
return beanInstance;
}
}

这里有必要单独说一下解引用：

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，“&”是引用符号。

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。

上面的代码可以看到，对于大多数bean的getBean，一般走到第二步就返回了，也就是说我们创建的Bean对象就是想要的bean，但对于FactoryBean的创建，如果是对内存地址的引用，那么取到的是它生产的bean，而不是它本身。所以我们继续看怎么取到生产的对象的：

FactoryBeanRegistrySupport类的getObjectFromFactoryBean方法

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && this.containsSingleton(beanName)) {
synchronized(this.getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
object = this.doGetObjectFromFactoryBean(factory, beanName);
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
if (alreadyThere != null) {
object = alreadyThere;
} else {
if (object != null && shouldPostProcess) {
try {
object = this.postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable var9) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's singleton object failed", var9);
}
}

this.factoryBeanObjectCache.put(beanName, object != null ? object : NULL_OBJECT);
}
}

return object != NULL_OBJECT ? object : null;
}
} else {
Object object = this.doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
object = this.postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable var11) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", var11);
}
}

return object;
}
}
doGetObjectFromFactoryBean：
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, String beanName) throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = this.getAccessControlContext();

try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
return factory.getObject();
}
}, acc);
} catch (PrivilegedActionException var6) {
throw var6.getException();
}
} else {
object = factory.getObject();
}
} catch (FactoryBeanNotInitializedException var7) {
throw new BeanCurrentlyInCreationException(beanName, var7.toString());
} catch (Throwable var8) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", var8);
}

if (object == null && this.isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName, "FactoryBean which is currently in creation returned null from getObject");
} else {
return object;
}
}

第一个方法就是区分单例还是多例，第二个方法是真真的调用getObject的方法获得FactoryBean生产的对象。从代码中可以看到，具体产生Bean的地方时这个getObject方法，Spring为这个FactoryBean提供了70多个实现，比如Poxy、JDNI、RMI等等。
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版权声明：本文为CSDN博主「liuhmmjj」的原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/u014082714/article/details/81166648