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This article introduces the knowledge of "extension points in the process of creating Bean by Spring". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Introduction

I'm tired at work.

Everything is annoying.

Write an article

Keep one's promise

One picture wins all.

The green part is generally used for Spring internal extension, and the yellow part can be used for custom instantiation. This article only talks about InitializingBean. For the green part, I suggest you check out the blog https://my.oschina.net/xiaolyuh/blog/3113215 written by others.

InitializingBean API Note / * 1. The implemented interface is triggered by a Bean,BeanFactory after setting all its properties. * 2. Can be used to perform custom instantiation or to verify that necessary properties are set. * Note: another way to implement InitializingBean is to * create a custom init method, * through the init-method of the element or using @ PostConstruct annotations * / public interface InitializingBean {void afterPropertiesSet () throws Exception;} application examples

When looking for a specific example, I came across the SqlSessionFactoryBean class used in the combination of mybatis and Spring, which is implemented as follows:

Public class SqlSessionFactoryBean implements FactoryBean, InitializingBean, ApplicationListener {public void afterPropertiesSet () throws Exception {notNull (dataSource, "Property 'dataSource' is required"); notNull (sqlSessionFactoryBuilder, "Property' sqlSessionFactoryBuilder' is required"); state ((configuration = = null & & configLocation = = null) | | (configuration! = null & & configLocation! = null), "Property 'configuration' and' configLocation' can not specified with together"); this.sqlSessionFactory = buildSqlSessionFactory () } / / omit other implementations.}

The implementation of afterPropertiesSet is very simple, which is to create SqlSessionFactory. I was more curious than the afterPropertiesSet,FactoryBean interface, and when I read FactoryBean's comments carefully, I found that the above diagram was missing an important extension point.

Another important extension FactoryBean

To solve the complicated problem of instantiating Bean, the logic of instantiating Bean can be realized by customizing the FactoryBean interface.

Although FactoryBean is defined in the style of bean, it always exposes objects created by getObject (). Wen Anshi summed up in 20191013

The programming contract for FactoryBean is as follows

Its implementation should not rely on annotation injection or other reflection tools

The invocation of getObjectType (), getObject () predates the service startup, even before any post-processor

If you need to get other Bean, you need to implement the BeanFactoryAware interface and manually program to get other bean.

Finally, the FactoryBean object participates in the process of synchronizing BeanFactory to create the bean, which usually does not require internal synchronization, except when lazily loading the FactoryBean itself.

Note translation / * * 1. This interface is implemented by a bean, which is used in conjunction with BeanFactory. * 2. It itself is the factory of a single object. * 3.FactoryBean supports singletons and prototypes, and can provide lazy loading or early exposure at startup as needed. * 4. SmartFactoryBean allows you to expose more fine-grained behavior metadata * this interface is widely used by the 5.Spring framework itself, * for example, {@ link org.springframework.aop.framework.ProxyFactoryBean}, * {@ link org.springframework.jndi.JndiObjectFactoryBean}. * / public interface FactoryBean {/ * returns an instance (which may be shared or independent) managed by the current factory * combined with BeanFactory to support singleton and prototype mode. * if the FactoryBean is not fully loaded when called (for example, there is a circular reference) * FactoryBeanNotInitializedException will be thrown. * after Spring 2.0, FactoryBeans is allowed to return null. * FactoryBeanNotInitializedException will not be thrown when null is returned. The implementation of * FactoryBean will depend on the situation. * whether to throw FactoryBeanNotInitializedException * BeanFactory implementation must consider this situation. * / @ Nullable T getObject () throws Exception; / * returns the type of the object created by FactoryBean. If you don't know to return null in advance, it allows you to check for a specific type of bean when instantiating the object (for example, for automatic assembly). * when creating an implementation of a singleton object, this method should avoid singleton creation as far as possible, and it should estimate the type in advance. * when creating a prototype type object, it is also recommended to return meaningful type information. * this method can be called before the FactoryBean is fully instantiated. It cannot rely on initialization. * Note: automatic injection simply ignores factoryBean, and this method returns null. * therefore, it is strongly recommended that you implement this method correctly using the current state of factorybean. When * / @ Nullable Class getObjectType (); / * returns true, getobject () * always returns the same object. * Note: if a factorybean holds a singleton object, * the object returned from getObject () * may be owned by its BeanFaFactory cache. * the singleton status of factorybean itself is usually determined by the BeanFactory that owns it; * / default boolean isSingleton () {return true;}} BeanFactory differs from FactoryBean API name BeanFactoryFactoryBean usage BeanFactory is the top-level interface of the IOC container. Its responsibilities include instantiating, locating, configuring objects in the application and establishing dependencies between these objects to solve the complex problems in the process of instantiating Bean. Through the implementation of this interface to customize and instantiate Bean's logically managed objects, all the objects created by the beangetObject method, "Spring creates extension points in the process of Bean", this is the end of the introduction. Thank you for reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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