Spring 实例化bean的三种方式：

方法一：使用构造器实例化bean

java代码：

package com.model;

public class User {

    private String username;

    public User(){

    }

    public User(String username){
        this.setUsername(username);
    }

    public void sayHello(){
        System.out.println("hello ");
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

配置文件：

 <!--方法一：使用构造器实例化bean  -->
  <bean id="user" class="com.model.User"></bean>

测试代码：

ApplicationContext applicationContext = new ClassPathXmlApplicationContext(new String[]{"applicationContext.xml"});
        //方法一：使用构造器实例化bean
        User user = (User)applicationContext.getBean("user");
        user.sayHello();

方法二：使用静态工厂方法实例化bean
java代码：

package com.test;

public class StaticFactory {

    private static StaticFactory staticFactory  = new StaticFactory();
    public static StaticFactory getBean() {
        return staticFactory;
    }

    public void say() {
        System.out.println("new Instance!");
    }
}

配置文件：

<!--方法二：使用静态工厂方法实例化bean  -->
  <bean id="staticFactory" class="com.test.StaticFactory" factory-method="getBean"/>

测试代码：

ApplicationContext applicationContext = new ClassPathXmlApplicationContext(new String[]{"applicationContext.xml"});

        //方法二：使用静态工厂方法实例化bean
        StaticFactory staticFactory = (StaticFactory)applicationContext.getBean("staticFactory");
        staticFactory.say();

方法三：使用实例工厂方法实例化bean

java代码：

package com.test;

import com.dao.UserDao;
import com.dao.impl.UserDaoImpl;

public class InstanceBean {
    private static UserDao userDaoA = new UserDaoImpl();

    //该方法必须是非静态的
    public UserDao getBeanInstance() {
        return userDaoA;
    }

}

接口UserDao：
package com.dao;

import com.model.User;

public interface UserDao {
    public void add(User user);
    public void findByid(String id);
}

实现类UserDaoImpl：
package com.dao.impl;

import com.dao.UserDao;
import com.model.User;

public class UserDaoImpl implements UserDao {

    private String hello;
    @Override
    public void add(User user) {
        System.out.println("方法三：qwqwqwq!!!");
    }

    @Override
    public void findByid(String id) {

    }

    public String getHello() {
        return hello;
    }

    public void setHello(String hello) {
        this.hello = hello;
    }

}

配置文件：

<!--方法三：使用实例工厂方法实例化bean  -->
  <bean id="instanceBean" class="com.test.InstanceBean"/>
  <bean id="userDaoA" factory-bean="instanceBean" factory-method="getBeanInstance"></bean>

测试代码：

ApplicationContext applicationContext = new ClassPathXmlApplicationContext(new String[]{"applicationContext.xml"});
        //方法三：使用实例工厂方法实例化bean
        UserDao userDao = (UserDao)applicationContext.getBean("userDao");
        String username = "pppp";
        userDao.add(new User(username));