Spring扩展接口-FactoryBean

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简介

创建对象最简单的方式是直接使用new操作符，如果创建对象比较繁杂，可以采用工厂模式。同样Spring中提供了FactoryBean接口来帮助创建对象。

public interface FactoryBean<T> {
    T getObject() throws Exception;

    Class<?> getObjectType();

    default boolean isSingleton() {
        return true;
    }

}

• T getObject()：返回由FactoryBean创建的bean实例。bean示例可能是singleton还是prototype取决于isSingleton方法的返回。
• boolean isSingleton()：返回由FactoryBean创建的bean实例的作用域是singleton还是prototype。
• Class getObjectType()：返回FactoryBean创建的bean类型。

在Spring中，创建bean时，如果该bean实现了FactoryBean接口，那么就会调用getObject方法获取真正的bean， 通过 getBean()方法返回的不是FactoryBean本身，而是FactoryBean#getObject()方法所返回的bean。如果想要获取FactoryBean本身，在beanName前显示的加上 "&" 前缀, 例如getBean("&myBean")。

用途示例

Spring代理实现-ProxyFactoryBean

Spring代理的实现类ProxyFactoryBean，当实例化ProxyFactoryBean时，对原有的singletonInstance对象进行封装，返回的是一个代理对象。Spring常用的AOP，以及事务注解都是基于Spring代理实现。

public class ProxyFactoryBean extends ProxyCreatorSupport implements FactoryBean<Object>, BeanClassLoaderAware, BeanFactoryAware {

    @Nullable
    private Object singletonInstance;  //原有对象

    @Nullable
    public Object getObject() throws BeansException {
        this.initializeAdvisorChain();
        if (this.isSingleton()) {
            return this.getSingletonInstance();
        } else {
            if (this.targetName == null) {
                this.logger.info("Using non-singleton proxies with singleton targets is often undesirable. Enable prototype proxies by setting the 'targetName' property.");
            }

            return this.newPrototypeInstance();  //返回代理后的对象
        }
    }

    public Class<?> getObjectType() {
        synchronized(this) {
            if (this.singletonInstance != null) {
                return this.singletonInstance.getClass();
            }
        }

        Class<?>[] ifcs = this.getProxiedInterfaces();
        if (ifcs.length == 1) {
            return ifcs[0];
        } else if (ifcs.length > 1) {
            return this.createCompositeInterface(ifcs);
        } else {
            return this.targetName != null && this.beanFactory != null ? this.beanFactory.getType(this.targetName) : this.getTargetClass();
        }
    }

    public boolean isSingleton() {
        return this.singleton;
    }
}

自定义bean工厂创建方法

public class CarFactoryBean implements FactoryBean<Car> { 

    private String carInfo; 

    public Car getObject() throws Exception {
        Car car = new Car();
        String[] infos = carInfo.split(",");
        car.setBrand(infos[0]);
        car.setMaxSpeed(Integer.valueOf(infos[1]));
        car.setPrice(Double.valueOf(infos[2]));
        return car;
    }

    public Class<Car> getObjectType() {
        return Car.class;
    }
    public boolean isSingleton() {
        return false;
    }
}

配置文件添加

<bean id="car" class="com.test.factorybean.CarFactoryBean" carInfo="大众SUV,180,180000"/>

当调用getBean("car") 时，Spring会去调用getObject方法，获取的是Car对象，调用getBean("&car") 时，获取的是该CarFactoryBean示例。

更多信息请移步Spring专栏：www.yuque.com/mrhuang-ire…

```

var code = "89b099c7-7355-41c3-805c-c7f14da1c7eb"

参考：
[1].[https://docs.spring.io/spring-framework/docs/4.3.15.RELEASE/spring-framework-reference/htmlsingle/#beans-factory-extension-factorybean](https://docs.spring.io/spring-framework/docs/4.3.15.RELEASE/spring-framework-reference/htmlsingle/#beans-factory-extension-factorybean)