spring-FactoryBean

package org.springframework.beans.factory;

import org.springframework.lang.Nullable;

public interface FactoryBean<T> {
    @Nullable
    T getObject() throws Exception;

    @Nullable
    Class<?> getObjectType();

    default boolean isSingleton() {
        return true;
    }
}

如果你的类实现了FactoryBean,那么spring IOC容器中存在两个对象

一个是getObject方法返回的对象（当前类的名字首字母小写）

一个是当前对象（怎么存的？ &+当前类的名字首字母小写)

package com.test2;

public class TestMyFactoryBean {

    public void test(){
        System.out.println("test TestMyFactoryBean");

    }
}

package com.test2;

import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Component;

@Component
public class MyFactoryBean implements FactoryBean {

    public void test(){
        System.out.println("test MyFactoryBean");
    }
    public Object getObject() throws Exception {
        return new TestMyFactoryBean();
    }

    public Class<?> getObjectType() {
        return TestMyFactoryBean.class;
    }

    public boolean isSingleton() {
        return true;
    }
}

package com.test2;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class MyTest {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context =
                new AnnotationConfigApplicationContext(MyFactoryBean.class);
        System.out.println(context.getBean(MyFactoryBean.class));
        System.out.println(context.getBean("myFactoryBean"));
        System.out.println(context.getBean(TestMyFactoryBean.class));

　　　　　　System.out.println(context.getBean("&myFactoryBean"));

} }

输出结果：

com.test2.MyFactoryBean@131276c2
com.test2.TestMyFactoryBean@26aa12dd
com.test2.TestMyFactoryBean@26aa12dd
com.test2.MyFactoryBean@131276c2

什么时候用呢？

一个类的依赖关系比较复杂，可以通过FactoryBean类向外部提供一个简单的方式让人快速获取到。

比如mybatis中的:

public class SqlSessionFactoryBean implements FactoryBean<SqlSessionFactory>, InitializingBean, ApplicationListener<ApplicationEvent> {

public SqlSessionFactory getObject() throws Exception {
    if (this.sqlSessionFactory == null) {
        this.afterPropertiesSet();
    }

    return this.sqlSessionFactory;
}