factorybean 声明_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。

后期文章会首发于本专栏，欢迎关注JAVA高级进阶​zhuanlan.zhihu.com

一般情况下，Spring通过反射机制利用的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的形式

FactoryBean接口定义以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 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：

public class SqlSessionFactoryBean implements FactoryBean, InitializingBean, ApplicationListener {

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 doGetBean(String name, Class 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() {

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() {

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() {

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。

我这儿整理了比较全面的JAVA相关的面试资料，

需要领取面试资料的同学，请加群：473984645