Spring扩展点之FactoryBean接口
- 2019 年 10 月 4 日
- 筆記
首先看一下接口定义
public interface FactoryBean<T> { /** * 返回对象实例 */ @Nullable T getObject() throws Exception; /** * 返回对象类型, */ @Nullable Class<?> getObjectType(); /** * 该工厂管理的对象是否为单例? */ default boolean isSingleton() { return true; } }
由接口定义可以看出来，实现这个接口的bean不是主要功能，getObject()创建的对象才是重点。那么在这我们就可以猜到了，可以是使用FactoryBean创建一些实例化过程比较复杂的bean
FactoryBean的注册
FactoryBean的处理逻辑在AbstractBeanFactory.doGetBean方法内
protected <T> T doGetBean( final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException { //获取bean名称 final String beanName = transformedBeanName(name); Object bean; //省略部分内容 //这里就是FactoryBean的相关处理，下面会展开说 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); //省略部分内容 return (T) bean; }
看一下具体的逻辑，这里需要注意Spring关于bean的name有个潜规则，凡是以&开头的bean名称都默认为FactoryBean
protected Object getObjectForBeanInstance( Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) { // 如果beanName以工厂引用&开头 if (BeanFactoryUtils.isFactoryDereference(name)) { if (beanInstance instanceof NullBean) { return beanInstance; } // 如果name以&开头，而beanInstance不是FactoryBean类型，则抛异常 if (!(beanInstance instanceof FactoryBean)) { throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass()); } } // 如果beanInstance不是FactoryBean类型，则直接返回beanInstance // 或者name以&开头，也直接返回beanInstance，说明我们就想获取FactoryBean实例 if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) { return beanInstance; } Object object = null; if (mbd == null) { object = getCachedObjectForFactoryBean(beanName); } if (object == null) { // 此时beanInstance是FactoryBean类型，而name又不是以&开头; 这是我们示例工程的情况，也是最普通、用的最多的情况 // 将beanInstance强转成FactoryBean类型 FactoryBean<?> factory = (FactoryBean<?>) beanInstance; // 从缓存中获取我们需要的实例对象 if (mbd == null && containsBeanDefinition(beanName)) { mbd = getMergedLocalBeanDefinition(beanName); } boolean synthetic = (mbd != null && mbd.isSynthetic()); // 调用FactoryBean的getObject方法创建我们需要的实例对象 object = getObjectFromFactoryBean(factory, beanName, !synthetic); } return object; } protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) { //针对单例的处理 if (factory.isSingleton() && containsSingleton(beanName)) { synchronized (getSingletonMutex()) { Object object = this.factoryBeanObjectCache.get(beanName); if (object == null) { //通过factory.getObject获取 object = doGetObjectFromFactoryBean(factory, beanName); Object alreadyThere = this.factoryBeanObjectCache.get(beanName); if (alreadyThere != null) { object = alreadyThere; } else { if (shouldPostProcess) { if (isSingletonCurrentlyInCreation(beanName)) { // Temporarily 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); } } if (containsSingleton(beanName)) { //将获取到的对象放到factoryBeanObjectCache单例缓存map进行存储 this.factoryBeanObjectCache.put(beanName, object); } } } return object; } } else { //非单例的处理，直接通过factory.getObejct获取，然后再返回给用户 Object object = doGetObjectFromFactoryBean(factory, beanName); if (shouldPostProcess) { try { object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex); } } return object; } }
生成bean对象的方法：
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 { object = factory.getObject();//生成对象 } } 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;}
Spring的实现
Spring中实现这个接口的bean有很多，但是我们最熟悉也是最重要的就是在我之前文章中提到过得ProxyFactoryBean这个bean是实现AOP技术的重点，简单回顾一下
public Object getObject() throws BeansException { initializeAdvisorChain(); if (isSingleton()) { return getSingletonInstance(); } else { if (this.targetName == null) { logger.warn("Using non-singleton proxies with singleton targets is often undesirable. " + "Enable prototype proxies by setting the 'targetName' property."); } return newPrototypeInstance(); } } private synchronized Object getSingletonInstance() { if (this.singletonInstance == null) { this.targetSource = freshTargetSource(); if (this.autodetectInterfaces && getProxiedInterfaces().length == 0 && !isProxyTargetClass()) { // Rely on AOP infrastructure to tell us what interfaces to proxy. Class<?> targetClass = getTargetClass(); if (targetClass == null) { throw new FactoryBeanNotInitializedException("Cannot determine target class for proxy"); } setInterfaces(ClassUtils.getAllInterfacesForClass(targetClass, this.proxyClassLoader)); } // Initialize the shared singleton instance. super.setFrozen(this.freezeProxy); this.singletonInstance = getProxy(createAopProxy()); } return this.singletonInstance; }
