聊聊Spring的FactoryBean其实没那么难
2020-12-15 09:16 爱研究源码的javaer 阅读(512) 评论(3) 编辑 收藏 举报前言
谈到Spring的FactoryBean，就会知道Spring中经典的面试题：FactoryBean和BeanFactory的区别。我们这里就简单概括下： 、
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BeanFactory是接口，提供了OC容器最基本的形式，给具体的IOC容器的实现提供了规范，FactoryBean也是接口，为IOC容器中Bean的实现提供了更加灵活的方式，FactoryBean在IOC容器的基础上给Bean的实现加上了一个简单工厂模式和装饰模式(如果想了解装饰模式参考：修饰者模式(装饰者模式，Decoration) 我们可以在getObject()方法中灵活配置。其实在Spring源码中有很多FactoryBean的实现类.
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BeanFactory是个Factory，也就是IOC容器或对象工厂，FactoryBean是个Bean。在Spring中，所有的Bean都是由BeanFactory(也就是IOC容器)来进行管理的。但对FactoryBean而言，这个Bean不是简单的Bean，而是一个能生产或者修饰对象生成的工厂Bean,它的实现与设计模式中的工厂模式和修饰器模式类似 。
示例
首先我们定义个普通的java pojo对象名叫Car
public class Car {
private int maxSpeed ;
private String brand ;
private double price ;
public double getPrice() {
return price;
}
public int getMaxSpeed() {
return maxSpeed;
}
public String getBrand() {
return brand;
}
public void setBrand(String brand) {
this.brand = brand;
}
public void setMaxSpeed(int maxSpeed) {
this.maxSpeed = maxSpeed;
}
public void setPrice(double price) {
this.price = price;
}
@Override
public String toString() {
return "Car{" +
"maxSpeed=" + maxSpeed +
", brand='" + brand + '\'' +
", price=" + price +
'}';
}
}
再定义一个实现FactoryBean接口的实现类CarFactoryBean
public class CarFactoryBean implements FactoryBean<Car> {
private String carInfo;
public Car getObject() throws Exception {
System.out.println("start new car");
Car car = new Car();
String[] infos = carInfo.split(",");
car.setBrand(infos[0]);
car.setMaxSpeed(Integer.valueOf(infos[1]));
car.setPrice(Double.valueOf(infos[2]));
return car;
}
public Class<Car> getObjectType(){
return Car.class ;
}
//返回的实例是同一个
public boolean isSingleton() {
return true;
}
public String getCarInfo() {
return this.carInfo ;
}
public void setCarInfo(String carInfo) {
this.carInfo = carInfo;
}
}
再来看看我们的Xml配置是如何配置Bean的
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:aop="http://www.springframework.org/schema/aop"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop.xsd">
<bean id="car" name="car1 car2" class="com.john.factorybean.CarFactoryBean">
<property name="carInfo" value="超级跑车,400,2000000"/>
</bean>
</beans>
最后我们加载这个配置文件，启动Spring容器来获取car这个Bean
public static void main( String[] args ) {
ClassPathXmlApplicationContext ac =new ClassPathXmlApplicationContext();
ac.setConfigLocations("spring-config-locatorfactory.xml");
ac.refresh();
//getBean的时候才会去getObject
ac.getBean("car", Car.class);
}
成功打印出
start new car
Car{maxSpeed=400, brand='超级跑车', price=2000000.0}
原理
如果我们把ac.getBean("car", Car.class);这行代码去掉，我们会发现start new car这行是不会打印的，那就是说明getObject是延迟加载的，只有显示调用才能触发，spring在初始化启动单例bean实例化的时候并不会触发(SmartFactoryBean除外，之后我们会单独分析下SmartFactoryBean)。
那我们就到源码级别证明下Spring的FactoryBean的getObject是延迟初始化的
DefaultListableBeanFactory
preInstantiateSingletons
@Override
public void preInstantiateSingletons() throws BeansException {
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
//todo 遍历所有不是延迟初始化的 单例 bean 2020-08-28
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//不是抽象的 且是单例的 且不是懒加载的
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//todo 如果是factorybean 那还得去判断 是不是eagerInit
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
//判断是不是渴望初始化的。。
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}
}
我们看到Spring会在实例化单例Bean的时候会去判断是否是Factorybean,如果是的话就会去根据FACTORY_BEAN_PREFIX + beanName获取Factorybean本身，然后再判断这个FactoryBean是SmartFactoryBean且isEagerInit设置为true才会去获取beanName对应的Bean，也就是实例化这个bean。
那既然是延迟初始化的，那我们再获取实际bean的时候Spring内部是怎么做的呢？
AbstractBeanFactory
doGetBean
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// Create bean instance.
//从头开始加载 bean，这个过程由 getSingleton() 实现。
if (mbd.isSingleton()) {
//https://www.cnblogs.com/leihuazhe/p/9481018.html
////todo 如果有单例那直接返回了 ， args参数也就没啥用处了 2020-08-30
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly(显式) 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);
}
}
这里我们从getObjectForBeanInstance方法可以看出来这里就是获取真正bean的方法。
getObjectForBeanInstance
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// 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.
//我们使用它来创建一个bean实例，除非调用者实际上想要引用工厂
//todo 如果是获取FactoryBean本身 那就直接返回这个工厂
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
}
也就是说不是FactoryBean本身或者根据name判断就是获取FactoryBean本身的话，那就直接返回这个beanInstance。
那我们在getBean("car")的时候其实获取到的beanInstance是FactoryBean的实例，所以走到这边的时候不会返回。
下面Spring就会走到抽象类FactoryBeanRegistrySupport中的getObjectFromFactoryBean函数，因为AbstractBeanFactory继承自FactoryBeanRegistrySupport，从名字就可以看出这个函数的意义就是调用FactoryBean的getObject方法。
FactoryBeanRegistrySupport
getObjectFromFactoryBean
/**
* Obtain an object to expose from the given FactoryBean.
* @param factory the FactoryBean instance
* @param beanName the name of the bean
* @param shouldPostProcess whether the bean is subject to post-processing
* @return the object obtained from the FactoryBean
* @throws BeanCreationException if FactoryBean object creation failed
* @see org.springframework.beans.factory.FactoryBean#getObject()
*/
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) {
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(暂时) 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);
}
}
//object放入factoryBeanObjectCache缓存
if (containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
//非单例
省略部分代码
}
}
函数内部还会调用doGetObjectFromFactoryBean函数
doGetObjectFromFactoryBean
/**
* Obtain an object to expose from the given FactoryBean.
* @param factory the FactoryBean instance
* @param beanName the name of the bean
* @return the object obtained from the FactoryBean
* @throws BeanCreationException if FactoryBean object creation failed
* @see org.springframework.beans.factory.FactoryBean#getObject()
*/
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 {
//最重要的调用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;
}
从这个函数中我们看到了FactoryBean自身的getObject方法。调用完getObject方法后，Spring就会把这个对象放入factoryBeanObjectCache缓存对象中。之后再获取的时候直接从factoryBeanObjectCache缓存中拿就是了。
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