Spring中实例化Bean的三种方式

目录直通车

1、构造器方法实例化 Bean

2、静态工厂实例化 Bean

3、实例工厂实例化 Bean

静态工厂实例化Bean与实例工厂实例化Bean的区别

---

实例化 Bean 是 Bean 生命周期的第一步。

假设现在有一个下面这样的实体类

public class Car {
      private String brand;
      private int price;

    public Car(){}

    public Car(String brand, int price) {
        this.brand = brand;
        this.price = price;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }


    @Override
    public String toString() {
        return "Car{" +
                "brand='" + brand + '\'' +
                ", price='" + price + '\'' +
                '}';
    }
}

1、构造器方法实例化 Bean

构造器方法分有参和无参两种，在XML中对上面这个实体进行实例化。

<?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" profile="dev">

    <!-- 无参构造实例化 -->
    <bean id="car1" class="com.fun.entity.Car"/>

    <!-- 有参构造器实例化 -->
    <bean id="car2" class="com.fun.entity.Car">
        <!-- 指定构造器参数 index对应构造器中参数的位置 -->
        <!-- 也可以通过指定参数类型，指定参数名称来注入属性-->
        <constructor-arg index="0" value="大众"/>
        <constructor-arg index="1" value="300000"/>
    </bean>
</beans>

之后再Main函数中，通过 ApplicationContext applicationContext = new ClassPathXmlApplicationContext()创建出IOC容器并 将xml文件加载到容器，通过applicationContext对象即可获取到 person1 和 person2了。

2、静态工厂实例化 Bean

  <!-- 通过静态工厂方法来配置bean
    此处配置的是car，而不是StaticFactory -->
    <bean id="car1" class="com.fun.StaticFactory" factory-method="getCar">
        <!-- constructor-args 传入value参数到getCar函数 -->
        <constructor-arg value="Audi"/>
    </bean>

/**
 * 静态工厂方法
 */
public class StaticFactory {
    private static Map<String,Car> cars = new HashMap<>();
    static {
        cars.put("Audi",new Car("Audi",300000));
        cars.put("Benz",new Car("Benz",660000));
    }
    public static Car getCar(String name){
        return cars.get(name);
    }
}

 通过xml中传入的 name，获取到对应 car 对象。

public class StaticFactoryApplication {
    public static void main(String[] args) {

        ApplicationContext ctx = new 
ClassPathXmlApplicationContext("com/spring/beans/InstantiateBean/InstanceAndStaticFactory/bean-factory.xml");
        Car car1 = (Car) ctx.getBean("car1");
        System.out.println(car1);
}

3、实例工厂实例化 Bean

 <!-- 实例工厂方法 -->
    <bean id="carFactory" class="com.fun.InstanceFactory"/>
    <bean id="car2" factory-bean="carFactory" factory-method="getCar">
        <constructor-arg value="benz"/>
    </bean>

/**
 * 实例工厂方法
 * 现需要创建工厂本身再调用工厂的实例方法来返回bean的实例
 */
public class InstanceFactory {
    private Map<String,Car> cars = new HashMap<>();
    public InstanceFactory(Map<String, Car> cars) {
        this.cars = cars;
    }
    public InstanceFactory() {
        cars = new HashMap<>();
        cars.put("audi",new Car("audi",300000));
        cars.put("benz",new Car("benz",660000));
    }

    public Car getCar(String name) {
        return cars.get(name);
    }

}

public class FactoryApplication {
    public static void main(String[] args) {

        ApplicationContext ctx = new 
ClassPathXmlApplicationContext("com/spring/beans/InstantiateBean/InstanceAndStaticFactory/bean-factory.xml");
        Car car2 = (Car) ctx.getBean("car2");
        System.out.println(car2);
    }
}

静态工厂实例化Bean与实例工厂实例化Bean的区别

对比如下两者的 XML。

  <!-- 静态工厂方法 -->
    <bean id="car1" class="com.fun.StaticFactory" factory-method="getCar">
        <constructor-arg value="Audi"/>
    </bean>

 <!-- 实例工厂方法 -->
    <bean id="carFactory" class="com.fun.InstanceFactory"/>
    <bean id="car2" factory-bean="carFactory" factory-method="getCar">
        <constructor-arg value="benz"/>
    </bean>