实例化Bean的三种方式

一、构造方法方式

1.1 BookDaoImpl

package com.lin.dao.daoimpl;

import com.lin.dao.BookDao;

public class BookDaoImpl implements BookDao {

    public BookDaoImpl() {
        System.out.println("BookDao的无参构造器");
    }

    /**
     * 数据层实现
     */
    public String queryBooks() {
        System.out.println("数据层查询书本方法");
        return "数据层查询书本方法";
    }
}

1.2 applicationContext.xml配置

   <!--
           bean标签：表示配置bean,
          id属性:bean的id,使用容器可以通过id值获取对应的bean,在一个容器中id值唯一
          class属性：表示给bean定义类型,即配置的bean的全路径类名
         -->
       <!--方式一：构造方法实例化bean-->
    <bean id="bookDao"  class="com.lin.dao.daoimpl.BookDaoImpl"></bean>

1.3 ConstructorMethodForInstance

package com.lin.beaninstance;

import com.lin.dao.BookDao;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * bean实例化方式一：构造方法方式
 */
public class ConstructorMethodForInstance {
    public static void main(String[] args) {
        //1.创建IoC容器对象，加载spring核心配置文件
        ApplicationContext springcontext = new ClassPathXmlApplicationContext("applicationContext.xml");

        //2 从IOC容器中获取Bean对象(BookDao对象)
        BookDao bookDao = (BookDao) springcontext.getBean("bookDao");
        //3 调用Bean对象(BookDao对象)的方法
        bookDao.queryBooks();


    }
}

运行结果

注意：无参构造方法如果不存在，将抛出异常BeanCreationException

二、静态工厂方式

2.1 BookDaoStaticFactory静态工厂

package com.lin.factory;

import com.lin.dao.BookDao;
import com.lin.dao.daoimpl.BookDaoImpl;

public class BookDaoStaticFactory {
    //静态工厂创建对象
    public static BookDao getBookDao(){
        return new BookDaoImpl();
    }

}

2.2 applicationContext.xml配置

 <!--方式二：使用静态工厂实例化bean-->
<bean id="bookDao" class="com.lin.factory.BookDaoStaticFactory" factory-method="getBookDao"></bean>

2.3 关系图

2.4 StaticFactoryForInstance

package com.lin.beaninstance;

import com.lin.dao.BookDao;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * bean实例化方式二：静态工厂方式
 */
public class StaticFactoryForInstance {

    public static void main(String[] args) {
        //1.创建IoC容器对象，加载spring核心配置文件
        ApplicationContext springContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        //2 从IOC容器中获取Bean对象(BookDao对象)
        BookDao bookDao = (BookDao) springContext.getBean("bookDao");
        //3 调用Bean对象(BookDao对象)的方法
        bookDao.queryBooks();

    }
}

2.4 运行结果

三 、实例工厂方式

3.1 BookDaoFactory工厂类

package com.lin.factory;

import com.lin.dao.BookDao;
import com.lin.dao.daoimpl.BookDaoImpl;

public class BookDaoFactory {
    //实例工厂创建对象
    public BookDao getBookDao(){
        return new BookDaoImpl();
    }
}

3.2 applicationContext.xml配置

     <!--方式三：使用实例工厂实例化bean-->
    <bean id="bookDaoFactory" class="com.lin.factory.BookDaoFactory"></bean>

    <bean id="bookDao" factory-bean="bookDaoFactory" factory-method="getBookDao"></bean>

3.3 关系图

3.4 实现FactoryBean<T>方式

定义BookDaoFactoryBean实现FactoryBean

package com.lin.beaninstance;

import com.lin.dao.BookDao;
import com.lin.dao.daoimpl.BookDaoImpl;
import org.springframework.beans.factory.FactoryBean;

public class BookDaoFactoryBean implements FactoryBean<BookDao> {
    @Override
    public BookDao getObject() throws Exception {
        return new BookDaoImpl();
    }

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

applicationContext.xml配置

    <bean id="bookDao" class="com.lin.beaninstance.BookDaoFactoryBean"></bean>

getObjectType() {
return BookDao.class;
}
}

####  applicationContext.xml配置

~~~xml
    <bean id="bookDao" class="com.lin.beaninstance.BookDaoFactoryBean"></bean>