Spring註冊bean的三種方式

時間 2020-04-24

原文原文鏈接

Spring是經過IoC容器對Bean進行管理的，而Bean的初始化主要分爲兩個過程：Bean的註冊和Bean實例化。Bean的註冊主要是指Spring經過讀取配置文件獲取各個bean的聲明信息，而且對這些信息進行註冊的過程。本文將介紹bean的三種註冊方式。java

一、無參構造器註冊bean

實體類spring

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

    public User() {
        System.out.println("User類的無參構造方法User()執行了");
    }
    
    public User(String username, String password) {
        System.out.println("User類的有參構造方法User()執行了");
        this.username = username;
        this.password = password;
    }
}

配置文件app

<?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>

測試類測試

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);
    }
}

輸出結果this

User類的無參構造方法User()執行了
entity.User@29774679

二、靜態工廠方法註冊bean

實體類code

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

    public User() {
        System.out.println("User類的無參構造方法User()執行了");
    }
    
    public User(String username, String password) {
        System.out.println("User類的有參構造方法User()執行了");
        this.username = username;
        this.password = password;
    }
}

工廠類xml

public class UserFactory {
    private static User user;

    static {
        user = new User();
    }

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

配置文件對象

<?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：用於指定一個靜態工廠方法，Spring容器會調用這個靜態方法建立個對象。-->
    <bean id="user" class="entity.UserFactory" factory-method="getInstance"></bean>
    
</beans>

</beans>

測試類get

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);
    }
}

輸出結果it

User類的無參構造方法User()執行了
entity.User@5622fdf

三、實例工廠方法註冊bean

實體類

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">

    <!-- 靜態工廠方法註冊bean -->
    <bean id="calendar" class="java.util.Calendar" factory-method="getInstance"></bean>

    <!-- 
        實例工廠方法註冊bean：
            factory-bean：用於指定一個bean的id
            factory-method：用於指定一個方法
    -->
    <bean id="time" factory-bean="calendar" factory-method="getTime"></bean>

</beans>

測試類

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

測試結果