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Spring注册bean的三种方式

Spring是通过IoC容器对Bean进行管理的，而Bean的初始化主要分为两个过程：Bean的注册和Bean实例化。Bean的注册主要是指Spring通过读取配置文件获取各个bean的声明信息，并且对这些信息进行注册的过程。本文将介绍bean的三种注册方式。

1、无参构造器注册bean

实体类

public class User {
    private String username;
    private String password;

    public User() {
        System.out.println("User类的无参构造方法User()执行了");
    }
    
    public User(String username, String password) {
        System.out.println("User类的有参构造方法User()执行了");
        this.username = username;
        this.password = password;
    }
}

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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="user" class="entity.User"></bean>

</beans>

测试类

public class TestSpring {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        User user = applicationContext.getBean("user", User.class);
        System.out.println(user);
    }
}

输出结果

User类的无参构造方法User()执行了
entity.User@29774679

2、静态工厂方法注册bean

实体类

public class User {
    private String username;
    private String password;

    public User() {
        System.out.println("User类的无参构造方法User()执行了");
    }
    
    public User(String username, String password) {
        System.out.println("User类的有参构造方法User()执行了");
        this.username = username;
        this.password = password;
    }
}

工厂类

public class UserFactory {
    private static User user;

    static {
        user = new User();
    }

    public static User getInstance() {
        return user;
    }
}

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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

    <?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">

    <!-- factory-method：用于指定一个静态工厂方法，Spring容器会调用这个静态方法创建个对象。-->
    <bean id="user" class="entity.UserFactory" factory-method="getInstance"></bean>
    
</beans>

</beans>

测试类

public class TestSpring {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        User user = applicationContext.getBean("user", User.class);
        System.out.println(user);
    }
}

输出结果

User类的无参构造方法User()执行了
entity.User@5622fdf

3、实例工厂方法注册bean

实体类

java.util.Calendar  
    public final Date getTime()

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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">

    <!-- 静态工厂方法注册bean -->
    <bean id="calendar" class="java.util.Calendar" factory-method="getInstance"></bean>

    <!-- 
        实例工厂方法注册bean：
            factory-bean：用于指定一个bean的id
            factory-method：用于指定一个方法
    -->
    <bean id="time" factory-bean="calendar" factory-method="getTime"></bean>

</beans>

测试类

@Test
public void test03() {
    ApplicationContext ac = new ClassPathXmlApplicationContext("applicationContext.xml");
    Date time = ac.getBean("time",Date.class);
    System.out.println(time);
}