Spring 静态工厂与实例工厂

1. 静态工厂创建Bean

@Data
public class Car {
    private String name;
    private Double price;
}

/** 静态工厂类 */
public class CarStaticFactory {
    /** 无参 */
    public static Car getCar1() {
        System.out.println("static Car getCar1()");
        Car car = new Car();
        car.setName("car1");
        car.setPrice(3.14);
        return car;
    }
    /** 有参 */
    public static Car getCar2(String name, Double price) {
        System.out.println("static Car getCar2()");
        Car car = new Car();
        car.setName(name);
        car.setPrice(price);
        return car;
    }
}

<bean id="car1" class="bean.CarStaticFactory"
    factory-method="getCar1" />

<bean id="car2" class="bean.CarStaticFactory"
    factory-method="getCar2">
    <!-- 传入参数 -->
    <constructor-arg name="name" value="benz"/>
    <constructor-arg name="price" value="500.0"/>
</bean>

@Test
public void testIoc() throws Exception {
    ApplicationContext ioc = new ClassPathXmlApplicationContext("applicationContext.xml");
    Car car1 = (Car) ioc.getBean("car1");
    Car car2 = (Car) ioc.getBean("car2");
    System.out.println(car1);
    System.out.println(car2);
}

运行。可知通过静态工厂类创建Bean的原理是：直接调用静态方法

• 调用CarStaticFactory.getCart1()
• 调用CarStaticFactory.getCart2(name, price)

2. 实例工厂

public class CarInstanceFactory {
    /** 无参 */
    public Car getCar1() {
        System.out.println("Car getCar1()");
        Car car = new Car();
        car.setName("car1");
        car.setPrice(3.14);
        return car;
    }
    /** 有参 */
    public Car getCar2(String name, Double price) {
        System.out.println("Car getCar2()");
        Car car = new Car();
        car.setName(name);
        car.setPrice(price);
        return car;
    }
}

<!-- 先注册实例工厂类 -->
<bean id="carInstanceFactory" class="bean.CarInstanceFactory" />

<!-- factory-bean指定工厂类对应的Bean组件 -->
<bean id="car1" factory-bean="carInstanceFactory"
    factory-method="getCar1" />

<bean id="car2" factory-bean="carInstanceFactory"
    factory-method="getCar2">
    <constructor-arg name="name" value="benz" />
    <constructor-arg name="price" value="500.0" />
</bean>

@Test
public void testIoc() throws Exception {
    ApplicationContext ioc = new ClassPathXmlApplicationContext("applicationContext.xml");
    Car car1 = (Car) ioc.getBean("car1");
    Car car2 = (Car) ioc.getBean("car2");
    System.out.println(car1);
    System.out.println(car2);
}

运行。可知通过实例工厂类创建Bean的原理是：

• 先注册实例工厂类到IOC容器，即IOC容器创建时，会先创建一个工厂类实例
• 有了工厂类实例之后，会调用工厂方法，创建对应的Bean

3. FactoryBean工厂接口

package bean;

import org.springframework.beans.factory.FactoryBean;

public class CarFactoryBeanImpl implements FactoryBean<Car>{
    /** 工厂方法，返回要创建的对象 */
    @Override
    public Car getObject() throws Exception {
        System.out.println("getObject()");
        Car car = new Car();
        car.setName("benz");
        car.setPrice(300.0);
        return car;
    }
    /** 返回创建的对象的类型 */
    @Override
    public Class<?> getObjectType() {
        return Car.class;
    }
    /** 创建的对象是单例还是多例 */
    @Override
    public boolean isSingleton() {
        return true;
    }
}

<bean id="carFactoryBeanImpl" class="bean.CarFactoryBeanImpl"/>

@Test
public void test() throws Exception {
    ApplicationContext ioc = new ClassPathXmlApplicationContext("applicationContext.xml");
    // 取出的不是工厂类，而是工厂创建的Bean
    Car car = (Car) ioc.getBean("carFactoryBeanImpl");
    System.out.println(car);
}

3.1. 单例与多例

实现FactoryBean接口时，isSingleton() 决定了对象是单例还是多例

/** 创建的对象是单例还是多例 */
@Override
public boolean isSingleton() {
    return true;
}

运行测试方法，可知单例模式下，从工厂中取出的是同一对象。但是与scope="singleton" 不同之处在于，只有你去取对象时，才会被实例化

@Test
public void test() throws Exception {
    // 在ioc创建时，car没有被实例化
    ApplicationContext ioc = new ClassPathXmlApplicationContext("applicationContext.xml");
    // 在第1次获取Bean时，car才被实例化
    Car car1 = (Car) ioc.getBean("carFactoryBeanImpl");
    Car car2 = (Car) ioc.getBean("carFactoryBeanImpl");
    // true
    System.out.println(car1 == car2);
}

修改为多例

@Override
public boolean isSingleton() {
    return false;
}

@Test
public void test() throws Exception {
    // 在ioc创建时，car没有被实例化
    ApplicationContext ioc = new ClassPathXmlApplicationContext("applicationContext.xml");
    // 每一次获取时，都会实例化新的car
    Car car1 = (Car) ioc.getBean("carFactoryBeanImpl");
    Car car2 = (Car) ioc.getBean("carFactoryBeanImpl");
    // false
    System.out.println(car1 == car2);
}