Spring 实例化Bean的两种方式

使用Spring管理Bean也称依赖注入( Dependency Injection, DI ),通过这种方式将Bean的控制权交给Spring

在使用Spring实例化一个对象时，无论类是否有参数都会默认调用对象类的无参构造,对于有参数的情况，Spring有两种方式可以带参实例化

示例类 Shape

public class Shape {
    private Integer width;
    private Integer height;

    public Shape() {
        System.out.println("运行了Shape的无参构造");
    }
    
    public Shape(Integer width, Integer height) {
        this.width = width;
        this.height = height;
        System.out.println("运行了Shape的有参构造");
    }

    public Integer getHeight() {
        return height;
    }

    public Integer getWidth() {
        return width;
    }

    public void setHeight(Integer height) {
        this.height = height;
    }

    public void setWidth(Integer width) {
        this.width = width;
    }

    @Override
    public String toString() {
        return "Width: " + this.width + "\tHeight:" + this.height;
    }
}

主函数

public class Demo {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        Shape shape = (Shape)applicationContext.getBean("Shape");
        System.out.println(shape);
    }
}

applicationContext.xml

通过Setter实例化

<?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="Shape" class="learn.test.Shape">
        <!-- property标签会自动调用Setter   -->
        <property name="width" value="200"></property>
        <property name="height" value="500"></property>
    </bean>
    
</beans>

运行结果

运行了Shape的无参构造
Width: 200  Height:500

通过类带参构造实例化

<?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="Shape" class="learn.test.Shape">
        <!-- constructor-arg标签调用带参构造   -->
        <constructor-arg name="width" value="200"></constructor-arg>
        <constructor-arg name="height" value="500"></constructor-arg>
    </bean>
    
</beans>

运行结果：

运行了Shape的有参构造
Width: 200  Height:500