說說FactoryBean

寫在前面

FactoryBean有什麼做用：

FactoryBean是以工廠形式生成Bean，在對Bean進行修飾以後返回Bean。

經常使用的使用場景爲： 根據不一樣的配置類型返回不一樣類型的處理Bean，總體上簡化了XML配置等。

Spring自己有70多個FactoryBean的實現，經過它隱藏了一些複雜的實現細節。

理解FactoryBean

舉個例子，看看經過FactoryBean簡化XML配置。

public class Student {
	/** 姓名 */
	private String name;
	/** 年齡 */
	private int age;
	/** 班級名稱 */
	private String className;
}

實現FactoryBeen接口：

public class StudentFactoryBean implements FactoryBean<Student> {

	private String studentInfo;

	@Override
	public Student getObject() throws Exception {
		if (this.studentInfo == null) {
			throw new IllegalArgumentException("'studentInfo' is required");
		}

		String[] splitStudentInfo = studentInfo.split(",");
		if (null == splitStudentInfo || splitStudentInfo.length != 3) {
			throw new IllegalArgumentException("'studentInfo' config error");
		}

		Student student = new Student();
		student.setName(splitStudentInfo[0]);
		student.setAge(Integer.valueOf(splitStudentInfo[1]));
		student.setClassName(splitStudentInfo[2]);
		return student;
	}

	@Override
	public Class<?> getObjectType() {
		return Student.class;
	}

	public void setStudentInfo(String studentInfo) {
		this.studentInfo = studentInfo;
	}
}

建立XML配置：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	   xmlns:p="http://www.springframework.org/schema/p"
	   xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd">

	<!--注意:class不是Student而是StudentFactoryBean-->
	<bean id="student" class="com.lyc.cn.day03.StudentFactoryBean" p:studentInfo="張三,25,三年二班"/>

</beans>

測試：

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("day03.xml");
System.out.println(applicationContext.getBean("student"));
System.out.println(applicationContext.getBean("&student"));
		
Student{name='張三', age=25, className='三年二班'}
org.springframework.beans.factory_bean.StudentFactoryBean@1ae369b7

這樣就簡化了經過BeanFactory接口簡化配置XML的做業了。若是不加&返回的是實例，加了返回的是工廠bean自己。

默認返回單例：

//實例是否單例模式,默認返回true
	default boolean isSingleton() {
		return true;
	}

實現個性化輸出Bean

經過FactoryBean個性化輸出Bean。

public interface Animal {
	void sayHello();
}
public class Cat implements Animal {
	@Override
	public void sayHello() {
		System.out.println("hello, 喵喵喵...");
	}
}
public class Dog implements Animal {
	@Override
	public void sayHello() {
		System.out.println("hello, 汪汪汪...");
	}
}

對於Animal 接口有兩個實現：Cat和Dog。

新建AnimalFactorybean：

public class AnimalFactoryBean implements FactoryBean<Animal> {

	private String animal;

	@Override
	public Animal getObject() throws Exception {
		if (null == animal) {
			throw new IllegalArgumentException("'animal' is required");
		}
		if ("cat".equals(animal)) {
			return new Cat();
		} else if ("dog".equals(animal)) {
			return new Dog();
		} else {
			throw new IllegalArgumentException("animal type error");
		}
	}

	@Override
	public Class<?> getObjectType() {
		if (null == animal) {
			throw new IllegalArgumentException("'animal' is required");
		}
		if ("cat".equals(animal)) {
			return Cat.class;
		} else if ("dog".equals(animal)) {
			return Dog.class;
		} else {
			throw new IllegalArgumentException("animal type error");
		}
	}

	public void setAnimal(String animal) {
		this.animal = animal;
	}
}

增長XML配置：

<bean id="animal" class="com.lyc.cn.day03.AnimalFactoryBean" p:animal="cat"/>

測試：

public void testAnimalFactoryBean() {
	ApplicationContext applicationContext = new ClassPathXmlApplicationContext("day03.xml");
	Animal animal = applicationContext.getBean("animal", Animal.class);
	animal.sayHello();
}
--------------------- 
hello, 喵喵喵...