简介

在介绍实例化非懒加载的单例Bean之前，先了解一下FactoryBean

这是spring提供的一个非常重要的功能，是一个小型的工厂，可以灵活的创建出需要的Bean，在很多框架与spring整合的过程中都会用到，例如Mybatis-plus，需求是写一个Mapper注解就能成为Spring的Bean，那么这种批量动态生产Bean的功能就需要用到FactoryBean

源码解析

spring如何判断是一个FactoryBean

// 首先把Bean的class弄出来，并不实例化Bean
beanType = isFactoryBean -> predictBeanType-> resolveBeanClass-> doResolveBeanClass -> Class.forName// 有了class就可以判断了
FactoryBean.class.isAssignableFrom(beanType)其实就是使用的 Class.forName 生成了class

如何获取FactoryBean

以下是实例化非懒加载单例Bean的部分源码

// 通过class去判断，如果现在要实例化的Bean是一个FactoryBean，进入判断
if (isFactoryBean(beanName)) {// 带&表示要实例化FactoryBean,不管前面有多少个&,都会只变成一个Object bean = getBean("&" + beanName);if (bean instanceof FactoryBean) {FactoryBean<?> factory = (FactoryBean<?>) bean;boolean isEagerInit;// 如果是SmartFactoryBean 会有一个isEagerInit，如果isEagerInit返回true，那么会马上实例化真正的Bean，调用getObjectif (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) {// SmartFactoryBean && isEagerInit返回truegetBean(beanName);}}
}

从上面代码可以看出，如果是一个FactoryBean，在实例化非懒加载的单例Bean的时候FactoryBean就会实例出来，如果实现的是SmartFactoryBean，那么真正的Bean也会在这个步骤实例化出来

还可以看出要获取FactoryBean，需要在beanName前面加一个&

下面就来看下这两个接口

FactoryBean接口

public interface FactoryBean<T> {String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";// 创建出来的Bean对象@NullableT getObject() throws Exception;@NullableClass<?> getObjectType();// 默认是创建单例beandefault boolean isSingleton() {return true;}}

SmartFactoryBean接口

SmartFactoryBean继承了FactoryBean，可以决定是否提前实例化对象

public interface SmartFactoryBean<T> extends FactoryBean<T> {// 是否是多例default boolean isPrototype() {return false;}// 提前实例化default boolean isEagerInit() {return false;}}

使用

public class UserBean {public UserBean() {System.out.println("实例化UserBean");}}@Component
public class UserFactoryBean implements FactoryBean<UserBean> {@Overridepublic UserBean getObject() throws Exception {return new UserBean();}@Overridepublic Class<?> getObjectType() {return UserBean.class;}
}public class Application {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);}
}

上面代码运行的时候控制台什么都不会打出，也就是并没有实例化UserBean

修改为 SmartFactoryBean，并且isEagerInit返回true

@Component
public class UserFactoryBean implements SmartFactoryBean<UserBean> {@Overridepublic UserBean getObject() throws Exception {return new UserBean();}@Overridepublic Class<?> getObjectType() {return UserBean.class;}@Overridepublic boolean isEagerInit() {return true;}
}

控制台输出

实例化UserBean

获取Bean

既然FactoryBean可以通过getObject生成一个新的Bean，那么这个新的Bean如何获得，FactoryBean又如何获取，前面也介绍了，可以加一个&，下面来获取一下

public class Application {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);System.out.println(context.getBean("userFactoryBean"));System.out.println(context.getBean("&userFactoryBean"));System.out.println(context.getBean("&&&userFactoryBean"));}
}实例化UserBean
com.shura.beans.UserBean@77556fd
com.shura.beans.UserFactoryBean@368239c8
com.shura.beans.UserFactoryBean@368239c8

为什么加多个&也可以

spring通过一个循环

public static String transformedBeanName(String name) {// 没有&直接返回if (!name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {return name;}return transformedBeanNameCache.computeIfAbsent(name, beanName -> {do {// 循环截取掉 &beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());}while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)); // 如果还有&继续截取return beanName;});
}

通过上面例子我们知道，对于一个FactoryBean,其实会实例化两个Bean，一个是FactoryBean本身，一个是通过getObject方法获取出来的Bean

通过beanName获取的是getObject方法获取出来的Bean，如果beanName前面加上一个或者多个&符号，那么获取的是FactoryBean

何时调用getObject

在spring中每实例化完Bean后，都会接着这么一行代码

beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);

sharedInstance为实例化出来的对象，name为传入进来的名字如&userFactoryBean，beanName便是处理后的name为 userFactoryBean，

// 一个缓存，缓存了通过FactoryBean的getObject获得的对象，key为不带&的
private final Map<String, Object> factoryBeanObjectCache = new ConcurrentHashMap<>(16);protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {// 判断是不是应该获得一个FactoryBeanif (BeanFactoryUtils.isFactoryDereference(name)) {// 进入这表示beanInstance应该要是一个FactoryBeanif (beanInstance instanceof NullBean) {return beanInstance;}// 不是的话就报错if (!(beanInstance instanceof FactoryBean)) {throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());}if (mbd != null) {mbd.isFactoryBean = true;}return beanInstance;}// 能执行到这里，那么期望beanInstance不是一个FactoryBeanif (!(beanInstance instanceof FactoryBean)) {// 确实不是，那么就返回，通常情况我们的Bean就在这返回return beanInstance;}// 后面逻辑表示，期望不是FactoryBean，是现在得到的实例是一个FactoryBean，那么应该要调用getObject获得真正对象Object object = null;if (mbd != null) {mbd.isFactoryBean = true;} else {// 从缓存中拿一下object = getCachedObjectForFactoryBean(beanName);}if (object == null) {// 到这肯定确定beanInstance是一个FactoryBean，直接强转FactoryBean<?> factory = (FactoryBean<?>) beanInstance;// Caches object obtained from FactoryBean if it is a singleton.if (mbd == null && containsBeanDefinition(beanName)) {mbd = getMergedLocalBeanDefinition(beanName);}// synthetic为true，表示是由硬编码创建的可以不用进行PostProcessor，其实也表示不允许改动boolean synthetic = (mbd != null && mbd.isSynthetic());object = getObjectFromFactoryBean(factory, beanName, !synthetic);}return object;
}

上面源码的最重要的部分就是，我希望获得的是getObject返回的bean，但是现在得到的是一个FactoryBean，那么就调用getObject返回真正的Bean

getObjectFromFactoryBean源码分析

protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {if (factory.isSingleton() && containsSingleton(beanName)) {synchronized (getSingletonMutex()) {// 如果是单例会在 factoryBeanObjectCache 缓存，那么就要先从缓存中取，这里很多判断是用来保证bean只实例化一次的逻辑，可以不用看Object object = this.factoryBeanObjectCache.get(beanName);if (object == null) {// 执行getObject()方法，返回的object不可能为null（会返回NullBean）object = doGetObjectFromFactoryBean(factory, beanName);// 可能另一个线程已经创建Object alreadyThere = this.factoryBeanObjectCache.get(beanName);if (alreadyThere != null) {object = alreadyThere;}else {if (shouldPostProcess) {// 执行后置处理postProcessobject = postProcessObjectFromFactoryBean(object, beanName);}if (containsSingleton(beanName)) {// 缓存起来this.factoryBeanObjectCache.put(beanName, object);}}}return object;}}else {// 表示不是单例，那么直接调用getObject返回就行了Object object = doGetObjectFromFactoryBean(factory, beanName);if (shouldPostProcess) {// 执行后置处理postProcessobject = postProcessObjectFromFactoryBean(object, beanName);}return object;}
}private Object doGetObjectFromFactoryBean(FactoryBean<?> factory, String beanName) throws BeanCreationException {// 去掉一些异常判断逻辑，就是调用的getObject方法return factory.getObject();
}

以上就是FactoryBean的所有逻辑了，最终是调用的getObject获取Bean

Mybatis-plus应用FactoryBean

例如 MapperFactoryBean,Mybaties将在后面介绍,源码先展示

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {private Class<T> mapperInterface;private boolean addToConfig = true;public MapperFactoryBean() {}public MapperFactoryBean(Class<T> mapperInterface) {this.mapperInterface = mapperInterface;}protected void checkDaoConfig() {super.checkDaoConfig();Assert.notNull(this.mapperInterface, "Property 'mapperInterface' is required");Configuration configuration = this.getSqlSession().getConfiguration();if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {try {configuration.addMapper(this.mapperInterface);} catch (Exception var6) {this.logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", var6);throw new IllegalArgumentException(var6);} finally {ErrorContext.instance().reset();}}}public T getObject() throws Exception {return this.getSqlSession().getMapper(this.mapperInterface);}public Class<T> getObjectType() {return this.mapperInterface;}public boolean isSingleton() {return true;}public void setMapperInterface(Class<T> mapperInterface) {this.mapperInterface = mapperInterface;}public Class<T> getMapperInterface() {return this.mapperInterface;}public void setAddToConfig(boolean addToConfig) {this.addToConfig = addToConfig;}public boolean isAddToConfig() {return this.addToConfig;}
}

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