Spring实例化Bean的五种方式

一，概述

对于我们Java来说，创建对象无非就是new或者通过反射两种方式。

Spring中实例化对象一般是通过反射来实现的，但是Spring的强大之处不仅仅在于它为Java开发者提供了极大便利，更在于它的开放式架构，使得用户可以拥有最大扩展Spring的能力。

我们大致可以总结出以下五种实例化bean的方式：

1. 实现InstantiationAwareBeanPostProcessor接口
2. 实现FactoryBean接口
3. 通过Supplier接口
4. 工厂方法factory-method
5. 通过反射

二，实现InstantiationAwareBeanPostProcessor接口

MyInstantiationAwareBeanPostProcessor.java

public class MyInstantiationAwareBeanPostProcessor implements InstantiationAwareBeanPostProcessor {
    @Override
    public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        if (beanName.equals("a")) {
            return new B();
        }
        return null;
    }
}

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
    <bean id="a" class="com.bobo.A"></bean>
    <bean class="com.bobo.MyInstantiationAwareBeanPostProcessor"></bean>
</beans>

测试代码：

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("boboTest.xml");
        System.out.println(context.getBean("a"));
    }
}

运行输出：

[email protected]

三，实现FactoryBean接口

MyFactoryBean.java

@Component
public class MyFactoryBean implements FactoryBean<A> {
    @Override
    public A getObject() throws Exception {
        return new A();
    }

    @Override
    public Class<?> getObjectType() {
        return A.class;
    }
}

测试代码

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("boboTest.xml");
        System.out.println(context.getBean("myFactoryBean"));
    }
}

运行输出：

[email protected]

原理：请移步到这里查看：

Spring中FactoryBean真相揭秘

四，通过Supplier接口

CreateSupplierB.java

public class CreateSupplierB {
    public static B createB(){
        return new B();
    }
}

MyBeanFactoryPostProcessor.java

@Component
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        BeanDefinition beanDefinition = beanFactory.getBeanDefinition("a");
        GenericBeanDefinition genericBeanDefinition = (GenericBeanDefinition) beanDefinition;
        genericBeanDefinition.setInstanceSupplier(CreateSupplierB::createB);
    }
}

测试代码：

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("boboTest.xml");
        System.out.println(context.getBean("a"));
    }
}

运行输出：

[email protected]

原理：请移步这里

Spring实例化(createBeanInstance)源码解析

五，工厂方法factory-method

1，静态工厂

MyStaticFactory.java

public class MyStaticFactory {
    public static A getA() {
        return new A();
    }
}

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
    <bean id="a" class="com.bobo.MyStaticFactory" factory-method="getA"></bean>
</beans>

测试：

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("boboTest.xml");
        System.out.println(context.getBean("a"));
    }
}

运行输出：

[email protected]

2，实例工厂

MyInstanceFactory.java

public class MyInstanceFactory {
    public A getA() {
        return new A();
    }
}

aoolicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
    <bean id="myInstanceFactory" class="com.bobo.MyInstanceFactory"></bean>
    <bean id="a" class="com.bobo.MyStaticFactory" factory-bean="myInstanceFactory" factory-method="getA"></bean>
</beans>

测试代码：

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("boboTest.xml");
        System.out.println(context.getBean("a"));
    }
}

运行输出：

[email protected]

3，原理

请移步这里：

Spring实例化(createBeanInstance)源码解析

六，通过反射

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd">
    <bean id="a" class="com.bobo.A"></bean>
</beans>

测试代码：

public class Test {
    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("boboTest.xml");
        System.out.println(context.getBean("a"));
    }
}

运行输出：

[email protected]