Spring bean的bean的三种实例化方式

Bean 定义

被称作 bean 的对象是构成应用程序的支柱也是由 Spring IoC 容器管理的。bean 是一个被实例化，组装，并通过 Spring IoC 容器所管理的对象。这些 bean 是由用容器提供的配置元数据创建的，例如，已经在先前章节看到的，在 XML 的表单中的定义。

spring bean的实例化方式一共有三种方式：

1.构造器构造bean

Spring可以使用默认构造器或者有参构造器创建Bean实例

有参构造器

UserService.java

package com.zk.spring;
 
public class UserService {
    String name;
    public UserService(String name)
    {
        this.name=name;
    }
    public void addUser(){
        System.out.println(name);
    }
}

TestDemo.java

package com.zk.spring;
 
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class TestDemo {
    @Test
    public void test1(){
        String xmlpath="ApplicationContext.xml";
        ApplicationContext ac=new ClassPathXmlApplicationContext(xmlpath);
        UserService us=(UserService) ac.getBean("UserService");
        us.addUser();
    }
}

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:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">
<bean id="UserService" class="com.zk.spring.UserService">
<constructor-arg index="0" value="Spring"></constructor-arg>
</bean>
</beans>

运行效果图:

有参构造器

UserService.java

package com.zk.spring;
 
public class UserService {
    public UserService(){
         
    }
    public void addUser(){
        System.out.println("addUser");
    }
}

TestDemo.java

package com.zk.spring;
 
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class TestDemo {
    @Test
    public void test1(){
        String xmlpath="ApplicationContext.xml";
        ApplicationContext ac=new ClassPathXmlApplicationContext(xmlpath);
        UserService us=(UserService) ac.getBean("UserService");
        us.addUser();
    }
}

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:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">
<bean id="UserService" class="com.zk.spring.UserService"></bean>
</beans>

运行效果图:

2.使用实例工厂实例化Bean

实例化静态工厂注入bean，需要先实例化一个工厂类，然后通过由实例化工厂对象中的一个方法来创建bean，并注入到容器中。在 bean 的 factory-bean 属性里指定拥有该工厂方法的 Bean；在 factory-method 属性里指定该工厂方法的名称；使用 construtor-arg 元素为工厂方法传递方法参数。

UserService接口

package com.zk.myspring;
 
public interface UserService {
    public void addUser();
}

UserServiceImpl实现类

package com.zk.myspring;
 
public class UserServiceImpl implements UserService{
    @Override
    public void addUser() {
        // TODO Auto-generated method stub
        System.out.println("UserService 实例工厂");
    }
}

MyBeanFactory实例工厂

package com.zk.myspring;
 
public class MyBeanFactory {
    /*
     * 创建实例工厂
     */
    public UserService createService(){
        return new UserServiceImpl();
    }
}

ApplicationContext.xml配置文件

<?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:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">
   
<!-- 将实例工厂实例交给spring
class确定实例工厂全限定类名
factory-method:确定静态方法名
-->
<bean id="MyBeanFactoryId" class="com.zk.myspring.MyBeanFactory"></bean>
<bean id="userserviceId"  factory-bean="MyBeanFactoryId" factory-method="createService"></bean>
</beans>

运行效果图:

3.使用静态工厂创建Bean

利用静态工厂方法可以把bean注入到IOC容器中。在XML文件中配置bean时，要指定class的属性为工厂的类；factory-method属性指定工厂类中工厂方法，用于创建bean；constrctor-arg用于给工厂方法传递参数。

UserService.java

package com.zk.spring;
 
public interface UserService {
    public void addUser();
}

UserServiceImpl.java

package com.zk.spring;
 
public class UserServiceImpl implements UserService{
    @Override
    public void addUser() {
        // TODO Auto-generated method stub
        System.out.println("UserService静态工厂");
    }
}

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:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">
<!--  <bean id="UserService" class="com.zk.spring.UserService"></bean>-->
<bean id="UserServiceId" class="com.zk.spring.MyBeanFactory" factory-method="createService">
</bean>
</beans>

TestDemo.java

package com.zk.spring;
 
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class TestDemo {
    @Test
    public void test1(){
        String xmlpath="ApplicationContext.xml";
        ApplicationContext ac=new ClassPathXmlApplicationContext(xmlpath);
        UserService us=(UserService) ac.getBean("UserServiceId");
        System.out.println(us);
        us.addUser();
    }
}