FactoryBean接口
实现类对象将被用作创建Bean实例的工厂,即调用getObject()方法返回的对象才是真正要使用的Bean实例,而不是直接将FactoryBean对象作为暴露的Bean实例。
FactoryBeans可以支持singleton和prototype,并且可以根据需要懒加载或在启动时立即创建对象。
这个接口在编写扫描接口生成代理对象的场景下经常使用,比如Mybatis Mapper接口扫描、Dubbo接口扫描、Feign接口扫描等。
Spring容器只负责管理FactoryBean实例的生命周期,而不是FactoryBean创建的对象的生命周期。因此不会自动调用暴露的Bean对象的destroy方法。FactoryBean应该实现DisposableBean,并将关闭调用委托给底层对象。- public interface FactoryBean<T> {
- /**
- * Return an instance (possibly shared or independent) of the object
- * managed by this factory.
- */
- T getObject() throws Exception;
- /**
- * Return the type of object that this FactoryBean creates,
- * or null if not known in advance.
- */
- Class<?> getObjectType();
- /**
- * Is the object managed by this factory a singleton?
- */
- default boolean isSingleton() {
- return true;
- }
- }
复制代码 我们可以使用正常的getBean(beanName)方式获取通过getObject()方法暴露的Bean实例,也可以使用getBean("&" + beanName)方式获取FactoryBean的实例。
下文将介绍其实现方式。
FactoryBean的实现方式
Spring创建单例Bean的逻辑在DefaultListableBeanFactory的preInstantiateSingletons()方法中:- public void preInstantiateSingletons() throws BeansException {
- List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
- // Trigger initialization of all non-lazy singleton beans...
- for (String beanName : beanNames) {
- RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
- if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
- if (isFactoryBean(beanName)) {
- // 此处使用"& + beanName"作为beanName创建FactoryBean实例
- Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
- if (bean instanceof FactoryBean) {
- FactoryBean<?> factory = (FactoryBean<?>) bean;
- 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);
- }
- }
- }
- // Trigger post-initialization callback for all applicable beans...
- // ...
- }
- public Object getBean(String name) throws BeansException {
- return doGetBean(name, null, null, false);
- }
- protected <T> T doGetBean(
- String name, Class<T> requiredType, Object[] args, boolean typeCheckOnly)
- throws BeansException {
- // 此步骤将&符移除
- String beanName = transformedBeanName(name);
- Object bean;
- Object sharedInstance = getSingleton(beanName);
- if (sharedInstance != null && args == null) {
- bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
- } else {
- if (isPrototypeCurrentlyInCreation(beanName)) {
- throw new BeanCurrentlyInCreationException(beanName);
- }
- BeanFactory parentBeanFactory = getParentBeanFactory();
- if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
- String nameToLookup = originalBeanName(name);
- if (parentBeanFactory instanceof AbstractBeanFactory) {
- return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
- nameToLookup, requiredType, args, typeCheckOnly);
- } else if (args != null) {
- return (T) parentBeanFactory.getBean(nameToLookup, args);
- } else if (requiredType != null) {
- return parentBeanFactory.getBean(nameToLookup, requiredType);
- } else {
- return (T) parentBeanFactory.getBean(nameToLookup);
- }
- }
- if (!typeCheckOnly) {
- markBeanAsCreated(beanName);
- }
- try {
- RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
- checkMergedBeanDefinition(mbd, beanName, args);
- String[] dependsOn = mbd.getDependsOn();
- if (dependsOn != null) {
- for (String dep : dependsOn) {
- if (isDependent(beanName, dep)) {
- throw new BeanCreationException(mbd.getResourceDescription(), beanName,
- "Circular depends-on relationship between");
- }
- registerDependentBean(dep, beanName);
- try {
- getBean(dep);
- } catch (NoSuchBeanDefinitionException ex) {
- throw new BeanCreationException(mbd.getResourceDescription(), beanName,
- "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
- }
- }
- }
- if (mbd.isSingleton()) {
- // getSingleton方法中会先从单例池查找Bean
- // 如果没有则会调用lambda中的createBean(beanName, mbd, args)创建Bean并将其注册到单例池
- // 所以此时放入单例池的是FactoryBean实例
- sharedInstance = getSingleton(beanName, () -> {
- try {
- // 此处的beanName参数是不带&符的字符串
- return createBean(beanName, mbd, args);
- } catch (BeansException ex) {
- destroySingleton(beanName);
- throw ex;
- }
- });
- // beanName参数是不带&符的字符串
- // name是原始的带&符的字符串
- // 该方法里面会判断name是否有&符,如果有直接返回FactoryBean实例
- // 如果没有&符会调用getObject()方法获取真正要暴露的Bean实例
- bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
- } else if (mbd.isPrototype()) {
- Object prototypeInstance = null;
- try {
- beforePrototypeCreation(beanName);
- prototypeInstance = createBean(beanName, mbd, args);
- } finally {
- afterPrototypeCreation(beanName);
- }
- bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
- } else {
- String scopeName = mbd.getScope();
- if (!StringUtils.hasLength(scopeName)) {
- throw new IllegalStateException("No scope name defined for bean");
- }
- Scope scope = this.scopes.get(scopeName);
- if (scope == null) {
- throw new IllegalStateException("No Scope registered for scope name");
- }
- try {
- Object scopedInstance = scope.get(beanName, () -> {
- beforePrototypeCreation(beanName);
- try {
- return createBean(beanName, mbd, args);
- } finally {
- afterPrototypeCreation(beanName);
- }
- });
- bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
- } catch (IllegalStateException ex) {
- throw new BeanCreationException(beanName, "", ex);
- }
- }
- } catch (BeansException ex) {
- cleanupAfterBeanCreationFailure(beanName);
- throw ex;
- }
- }
- return (T) bean;
- }
- protected Object getObjectForBeanInstance(
- Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
- // 判断name有&符,直接返回FactoryBean实例
- if (BeanFactoryUtils.isFactoryDereference(name)) {
- if (beanInstance instanceof NullBean) {
- return beanInstance;
- }
- if (!(beanInstance instanceof FactoryBean)) {
- throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
- }
- if (mbd != null) {
- mbd.isFactoryBean = true;
- }
- return beanInstance;
- }
- if (!(beanInstance instanceof FactoryBean)) {
- return beanInstance;
- }
- Object object = null;
- if (mbd != null) {
- mbd.isFactoryBean = true;
- } else {
- // 从factoryBeanObjectCache池获取
- object = getCachedObjectForFactoryBean(beanName);
- }
- if (object == null) {
- // Return bean instance from factory.
- FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
- // Caches object obtained from FactoryBean if it is a singleton.
- if (mbd == null && containsBeanDefinition(beanName)) {
- mbd = getMergedLocalBeanDefinition(beanName);
- }
- boolean synthetic = (mbd != null && mbd.isSynthetic());
- // 调用getObject()方法获取真正要暴露的Bean实例并将其放入factoryBeanObjectCache池
- object = getObjectFromFactoryBean(factory, beanName, !synthetic);
- }
- return object;
- }
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