Spring BeanFactory和FactoryBean

Spring BeanFactory和FactoryBean

一、BeanFactory

BeanFactory定义了 IOC 容器的最基本形式，并提供了 IOC 容器应遵照的的最基本的接口，也就是 Spring IOC所遵照的最底层和最基本的编程规范。在  Spring 代码中， BeanFactory 只是个接口，并非 IOC 容器的具体实现，可是 Spring 容器给出了不少种实现，如 DefaultListableBeanFactory 、 XmlBeanFactory 、 ApplicationContext等，都是附加了某种功能的实现。

public interface BeanFactory {

	String FACTORY_BEAN_PREFIX = "&";

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 */
	Object getBean(String name) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 */
	<T> T getBean(String name, Class<T> requiredType) throws BeansException;

	/**
	 * Return the bean instance that uniquely matches the given object type, if any.
	 * @param requiredType type the bean must match; can be an interface or superclass.
	 * {@code null} is disallowed.
	 */
	<T> T getBean(Class<T> requiredType) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	
	 */
	Object getBean(String name, Object... args) throws BeansException;

	/**
	 * Does this bean factory contain a bean definition or externally registered singleton
	 * instance with the given name?
	 */
	boolean containsBean(String name);

	/**
	 * Is this bean a shared singleton? That is, will {@link #getBean} always
	 * return the same instance?
	 */
	boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Is this bean a prototype? That is, will {@link #getBean} always return
	 * independent instances?
	 */
	boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Check whether the bean with the given name matches the specified type.
	 * More specifically, check whether a {@link #getBean} call for the given name
	 * would return an object that is assignable to the specified target type.
	 */
	boolean isTypeMatch(String name, Class<?> targetType) throws NoSuchBeanDefinitionException;

	/**
	 */
	Class<?> getType(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Return the aliases for the given bean name, if any.
	 * All of those aliases point to the same bean when used in a {@link #getBean} call.
	 */
	String[] getAliases(String name);

}

 

二、FactoryBean

通常状况下，Spring 经过反射机制利用 <bean> 的 class 属性指定实现类实例化 Bean。在某些状况下，实例化Bean 过程比较复杂，若是按照传统的方式，则须要在 <bean> 中提供大量的配置信息。配置方式的灵活性是受限的，这时采用编码的方式可能会获得一个简单的方案。Spring 为此提供了一个org.springframework.bean.factory.FactoryBean 的工厂类接口，用户能够经过实现该接口定制实例化 Bean 的逻辑。

FactoryBean接口对于 Spring 框架来讲占用重要的地位， Spring 自身就提供了 70 多个 FactoryBean 的实现。它们隐藏了实例化一些复杂 Bean 的细节，给上层应用带来了便利。从 Spring 3.0 开始， FactoryBean 开始支持泛型，即接口声明改成 FactoryBean<T> 的形式：

public interface FactoryBean<T> {

	/**
	 * Return an instance (possibly shared or independent) of the object
	 * managed by this factory.
	 */
	T getObject() throws Exception;

	/**
	 * Return the type of object that this FactoryBean creates,
	 * or <code>null</code> if not known in advance.
	 */
	Class<?> getObjectType();

	/**
	 * Is the object managed by this factory a singleton? That is,
	 * will {@link #getObject()} always return the same object
	 * (a reference that can be cached)?
	 */
	boolean isSingleton();

}

在该接口中还定义了如下3 个方法：

T getObject()：返回由 FactoryBean 建立的 Bean 实例，若是 isSingleton() 返回 true ，则该实例会放到 Spring容器中单实例缓存池中；

boolean isSingleton()：返回由 FactoryBean 建立的 Bean 实例的做用域是 singleton 仍是 prototype ；

Class<T> getObjectType()：返回 FactoryBean 建立的 Bean 类型。

当配置文件中<bean> 的 class 属性配置的实现类是 FactoryBean 时，经过 getBean() 方法返回的不是FactoryBean 自己，而是 FactoryBean#getObject() 方法所返回的对象，至关于 FactoryBean#getObject() 代理了getBean() 方法。

例：若是使用传统方式配置下面Car 的 <bean> 时， Car 的每一个属性分别对应一个 <property> 元素标签。

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

    public int getMaxSpeed() {
        return this.maxSpeed;
    }

    public void setMaxSpeed(int maxSpeed) {
        this.maxSpeed = maxSpeed;
    }

    public String getBrand() {
        return this.brand;
    }

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

    public double getPrice() {
        return this.price;
    }

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

若是用FactoryBean 的方式实现就灵活点，下例经过逗号分割符的方式一次性的为 Car 的全部属性指定配置值：

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;
    }

    public String getCarInfo() {
        return this.carInfo;
    }

    // 接受逗号分割符设置属性信息  
    public void setCarInfo(String carInfo) {
        this.carInfo = carInfo;
    }
}

有了这个CarFactoryBean 后，就能够在配置文件中使用下面这种自定义的配置方式配置 Car Bean 了：

<bean id="car" class="com.baobaotao.factorybean.CarFactoryBean" p:carInfo="法拉利,400,2000000"/>

当调用getBean("car") 时， Spring 经过反射机制发现 CarFactoryBean 实现了 FactoryBean 的接口，这时 Spring容器就调用接口方法 CarFactoryBean#getObject() 方法返回。若是但愿获取 CarFactoryBean 的实例，则须要在使用getBean(beanName) 方法时在 beanName 前显示的加上 "&" 前缀：如 getBean("&car");

 

三、区别

BeanFactory是个 Factory ，也就是 IOC 容器或对象工厂， FactoryBean 是个 Bean 。在 Spring 中，全部的Bean 都是由 BeanFactory( 也就是 IOC 容器 ) 来进行管理的。但对 FactoryBean 而言，这个 Bean 不是简单的 Bean，而是一个能生产或者修饰对象生成的工厂 Bean, 它的实现与设计模式中的工厂模式和修饰器模式相似。

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