spring Bean的实例化：无参构造器、静态工厂方法和实例工厂方法

Bean的实例化

1.spring的三种实例化方式

1.默认无参的构造器实例化
2.静态工厂方法实例化
3.实例工厂方法实例化
构造器实例化是指spring容器通过Bean对应类中默认的无参构造方法来实例化。
静态工厂要求开发者创建一个静态工厂的方法来创建Bean的实例，器Bean配置中的class属性所指定的不在是Bean实例的实现类，而是静态工程类，同时还需要使用factory-method属性来制定所创建的静态工程方法。
实例工厂不在使用静态方法创建Bean实例，而是采用直接创建Bean实例的方式；在配置文件中，需要实例化的Bean也不是通过class属性直接指向的实例化类，而是通过factory-bean属性指向配置的实例工厂，然后使用factory-method属性确定使用工厂中的哪个方法。

2.例子

2.1创建一个空的spring工程

2.2新建Java类

package client;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import domain.def.DefaultBean;
import domain.inst.InstanceBean;
import domain.stat.StaticBean;

public class Main {

  @SuppressWarnings("resource")
  public static void main(String[] args) {

    String defaultPath = "default";
    String staticPath = "static";
    String instancePath = "instance";
    String before = "resource/beans-";
    String after = ".xml";
    
    ApplicationContext applicationContext = null;
    
    applicationContext = new ClassPathXmlApplicationContext(before
        + defaultPath + after);
    DefaultBean defaultBean = (DefaultBean) applicationContext
        .getBean(defaultPath + "Bean");
    System.out.println(defaultBean);
    
    applicationContext = new ClassPathXmlApplicationContext(before
        + staticPath + after);
    StaticBean staticBean = (StaticBean) applicationContext
        .getBean(staticPath + "Bean");
    System.out.println(staticBean);
    
    applicationContext = new ClassPathXmlApplicationContext(before
        + instancePath + after);
    InstanceBean instanceBean = (InstanceBean) applicationContext
        .getBean(instancePath + "Bean");
    System.out.println(instanceBean);
    
  }

}

package domain.def;

public class DefaultBean {

}

package domain.inst;

public class InstanceBean {

}

package domain.inst.factory;

import domain.inst.InstanceBean;

public class InstanceBeanFactory {

  public InstanceBean createInstanceBean(){
    return new InstanceBean();
  }
  
}

package domain.stat;

public class StaticBean {

}

package domain.stat.factory;

import domain.stat.StaticBean;

public class StaticBeanFactory {

  public static StaticBean createStaticBean(){
    return new StaticBean();
  }
  
}

2.3xml文件

beans-default.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">
  <bean id="defaultBean" class="domain.def.DefaultBean"></bean>
</beans>

beans-instance.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">
  <bean id="instanceBeanFactory" 
  class="domain.inst.factory.InstanceBeanFactory"></bean>
  <bean id="instanceBean" factory-bean="instanceBeanFactory"
  factory-method="createInstanceBean"></bean>
</beans>

beans-static.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">
  <bean id="staticBean" 
  class="domain.stat.factory.StaticBeanFactory"
  factory-method="createStaticBean"></bean>
</beans>

2.4运行结果

domain.def.DefaultBean@ae45eb6
domain.stat.StaticBean@4dfa3a9d
domain.inst.InstanceBean@6500df86

3.总结

spring的三种实例化方式
1.默认无参的构造器实例化
2.静态工厂方法实例化
3.实例工厂方法实例化