Spring中的Bean——实例化

Bean的实例化

    1.     构造器实例化：Spring容器通过Bean对应类中默认的无参构造方法来实例化Bean

package com.itheima.instance.constructor;
 
public class Bean1 {
 
}

<?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">
<bean id="bean1" class="com.itheima.instance.constructor.Bean1"></bean>
</beans>

在beans1.xml文件中，定义了一个id为bean1的Bean，并通过class属性指定其对应的实现类Bean1

package com.itheima.instance.constructor;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class InstanceTest1 {
    public static void main(String[] args) {
        //定义配置文件路径
        String xmlPath = "com/itheima/instance/constructor/beans1.xml";
        //ApplicationContext在加载配置文件时，对Bean进行实例化
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext(xmlPath);
        Bean1 bean = (Bean1) applicationContext.getBean("bean1");
        System.out.println(bean);
    }
}

在InstanceTest1类中，首先定义了配置文件的路径，然后Spring容器ApplicationContext会加载配置文件。在加载时，Spring容器会通过id为bean1的实现类Bean1中默认的无参构造方法对Bean进行实例化。

   2.     静态工厂方法实例化

package com.itheima.instance.static_factory;
 
public class Bean2 {
 
}

package com.itheima.instance.static_factory;
 
public class MyBean2Factory {
    //使用自己的方法创建Bean2实例
    public static Bean2 createBean(){
        return new Bean2();
    }
}

<?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">
<bean id="bean2" class="com.itheima.instance.static_factory.MyBean2Factory" factory-method="createBean"></bean>
</beans>

定义id为bean2的Bean，通过class属性指定其对应的工厂实现类（MyBean2Factory.java），需要增加factory-method属性来告诉Spring容器其方法名称为createBean。

package com.itheima.instance.static_factory;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class InstanceTest2 {
    public static void main(String[] args) {
        //定义配置文件路径
        String xmlPath = "com/itheima/instance/static_factory/beans2.xml";
        //ApplicationContext在加载配置文件时，对Bean进行实例化
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext(xmlPath);
        System.out.println(applicationContext.getBean("bean2"));
    }
}

 3.     实例工厂方式实例化

package com.itheima.instance.factory;
 
public class Bean3 {
 
}

package com.itheima.instance.factory;
 
public class MyBean3Factory {
    public MyBean3Factory(){
        System.out.println("bean3工厂实例化中");
    }
    //创建Bean3实例的方法
    public Bean3 createBean(){
        return new Bean3();
    }
}

<?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">
<bean id="myBean3Factory" class="com.itheima.instance.factory.MyBean3Factory"></bean>
<!-- 使用 factory-bean属性指向配置的实例工厂
     使用factory-method属性确定使用工厂中的哪个方法 -->
     <bean id="bean3" factory-bean="myBean3Factory" factory-method="createBean"></bean>
</beans>

首先配置了一个工厂Bean，然后配置了需要实例化的Bean。在id为bean3的Bean中，使用factory-bean属性指向配置的实例工厂，使用factory-method属性来确定使用工厂中的createBean()方法

package com.itheima.instance.factory;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class InstanceTest3 {
    public static void main(String[] args) {
        //指定配置文件路径
        String xmlPath = "com/itheima/instance/factory/beans3.xml";
        //ApplicationContext加载配置文件时，对Bean进行实例化
        @SuppressWarnings("resource")
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext(xmlPath);
        System.out.println(applicationContext.getBean("bean3"));
    }
}