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03-Spring3 依赖注入(DI)B_循环依赖【转】 博客分类: Spring JavaSpringBeanDI循环依赖 

程序员文章站 2024-02-06 20:39:04
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循环依赖即循环引用,就是两个或多个Bean相互之间的持有对方,比如CircleA引用CircleBCircleB引用CircleCCircleC引用CircleA,则它们最终反映为一个环

03-Spring3 依赖注入(DI)B_循环依赖【转】
            
    
    博客分类: Spring JavaSpringBeanDI循环依赖 

 

循环引用是无法解决的,除非有终结条件,否则就是死循环,最终导致内存溢出错误。

         Spring容器循环依赖包括构造器循环依赖和setter循环依赖,那Spring容器如何解决循环依赖呢?首先让我们定义循环引用类。

 

CircleA

package com.iflytek.demo;

public class CircleA {
	private CircleB circleB;

	public CircleA() {
	}

	public CircleA(CircleB circleB) {
		this.circleB = circleB;
	}

	public void setCircleB(CircleB circleB) {
		this.circleB = circleB;
	}

	public void a() {
		circleB.b();
	}
}

 

 

CircleB

package com.iflytek.demo;

public class CircleB {
	private CircleC circleC;

	public CircleB() {
	}

	public CircleB(CircleC circleC) {
		this.circleC = circleC;
	}

	public void setCircleC(CircleC circleC) {
		this.circleC = circleC;
	}

	public void b() {
		circleC.c();
	}
}

 

CircleC

package com.iflytek.demo;

public class CircleC {
	private CircleC circleC;

	public CircleC() {
	}

	public CircleC(CircleC circleC) {
		this.circleC = circleC;
	}

	public void setCircleC(CircleC circleC) {
		this.circleC = circleC;
	}

	public void c() {
		circleC.c();
	}
}

 

一、构造器循环依赖

表示通过构造器注入构成的循环依赖,此依赖是无法解决的,只能抛出BeanCurrentlyInCreationException异常表示循环依赖。如在创建CircleA类时,构造器需要CircleB类,那将去创建CircleB,在创建CircleB类时又发现需要CircleC类,则又去创建CircleC,最终在创建CircleC时发现又需要CircleA;从而形成一个环,没办法创建。

Spring容器将每一个正在创建的Bean 标识符放在一个“当前创建Bean池”中,Bean标识符在创建过程中将一直保持在这个池中,因此如果在创建Bean过程中发现自己已经在“当前创建Bean池”里时将抛出BeanCurrentlyInCreationException异常表示循环依赖;而对于创建完毕的Bean将从“当前创建Bean池”中清除掉。

 

circle.xml

<?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:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="
	       http://www.springframework.org/schema/beans
	       http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
		   http://www.springframework.org/schema/context
		   http://www.springframework.org/schema/context/spring-context-3.0.xsd">
	
	<bean id="circleA" class="com.iflytek.demo.CircleA">
		<constructor-arg index="0" ref="circleB" />
	</bean>
	<bean id="circleB" class="com.iflytek.demo.CircleB">
		<constructor-arg index="0" ref="circleC" />
	</bean>
	<bean id="circleC" class="com.iflytek.demo.CircleC">
		<constructor-arg index="0" ref="circleA" />
	</bean>

</beans>

 

CircleTest

package com.iflytek.test;

import org.junit.Test;
import org.springframework.beans.factory.BeanCurrentlyInCreationException;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class CircleTest {

	@Test(expected = BeanCurrentlyInCreationException.class)
	public void testCircleByConstructor() throws Throwable {
		try {
			new ClassPathXmlApplicationContext("circle.xml");
		} catch (Exception e) {
			// 因为要在创建circle3时抛出;
			Throwable e1 = e.getCause().getCause().getCause();
			throw e1;
		}
	}

}

 

org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circleA': Requested bean is currently in creation: Is there an unresolvable circular reference?

 

1Spring容器创建“circleA Bean,首先去“当前创建Bean池”查找是否当前Bean正在创建,如果没发现,则继续准备其需要的构造器参数“circleB”,并将“circleA标识符放到“当前创建Bean池”;

2Spring容器创建“circleB Bean,首先去“当前创建Bean池”查找是否当前Bean正在创建,如果没发现,则继续准备其需要的构造器参数“circleC”,并将“circleB标识符放到“当前创建Bean池”;

3Spring容器创建“circleC Bean,首先去“当前创建Bean池”查找是否当前Bean正在创建,如果没发现,则继续准备其需要的构造器参数“circleA”,并将“circleC标识符放到“当前创建Bean池”;

4、到此为止Spring容器要去创建“circleABean,发现该Bean 标识符在“当前创建Bean池”中,因为表示循环依赖,抛出BeanCurrentlyInCreationException

 

一、Setter循环依赖

表示通过setter注入方式构成的循环依赖。

对于setter注入造成的依赖是通过Spring容器提前暴露刚完成构造器注入但未完成其他步骤(如setter注入)的Bean来完成的,而且只能解决单例作用域的Bean循环依赖。

如下代码所示,通过提前暴露一个单例工厂方法,从而使其他Bean能引用到该Bean

	addSingletonFactory(beanName, new ObjectFactory() {
	    public Object getObject() throws BeansException {
	        return getEarlyBeanReference(beanName, mbd, bean);
	    }
	});

 

具体步骤如下:

       1Spring容器创建单例“circleA Bean,首先根据无参构造器创建Bean,并暴露一个“ObjectFactory ”用于返回一个提前暴露一个创建中的Bean,并将“circleA标识符放到“当前创建Bean池”;然后进行setter注入“circleB”;

       2Spring容器创建单例“circleB Bean,首先根据无参构造器创建Bean,并暴露一个“ObjectFactory”用于返回一个提前暴露一个创建中的Bean,并将“circleB标识符放到“当前创建Bean池”,然后进行setter注入“circleC”;

       3Spring容器创建单例“circleC Bean,首先根据无参构造器创建Bean,并暴露一个“ObjectFactory ”用于返回一个提前暴露一个创建中的Bean,并将“circleC标识符放到“当前创建Bean池”,然后进行setter注入“circleA”;进行注入“circleA”时由于提前暴露了“ObjectFactory”工厂从而使用它返回提前暴露一个创建中的Bean

      4、最后在依赖注入“circleB”和“circleA”,完成setter注入。

 

对于“prototype”作用域BeanSpring容器无法完成依赖注入,因为“prototype”作用域的BeanSpring容器不进行缓存,因此无法提前暴露一个创建中的Bean

 

circle2.xml

<!-- 定义Bean配置文件,注意scope都是“prototype” -->
	<bean id="circleA" class="com.iflytek.demo.CircleA" scope="prototype">
		<property name="circleB" ref="circleB" />
	</bean>
	<bean id="circleB" class="com.iflytek.demo.CircleB" scope="prototype">
		<property name="circleC" ref="circleC" />
	</bean>
	<bean id="circleC" class="com.iflytek.demo.CircleC" scope="prototype">
		<property name="circleA" ref="circleA" />
	</bean>

 

@Test(expected = BeanCurrentlyInCreationException.class)
	public void testCircleBySetterAndPrototype() throws Throwable {
		try {
			ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext(
					"circle2.xml");
			System.out.println(ctx.getBean("circleA"));
		} catch (Exception e) {
			Throwable e1 = e.getCause().getCause().getCause();
			throw e1;
		}
	}

 

org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circleA': Requested bean is currently in creation: Is there an unresolvable circular reference?
					circleA
					org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'circleA': Requested bean is currently in creation: Is there an unresolvable circular reference?
					Error creating bean with name 'circleA': Requested bean is currently in creation: Is there an unresolvable circular reference?
					null

 

对于“singleton”作用域Bean,可以通过“setAllowCircularReferences(false);”来禁用循环引用:

	@Test(expected = BeanCurrentlyInCreationException.class)
	public void testCircleBySetterAndSingleton2() throws Throwable {
		try {
			ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext();
			ctx.setConfigLocation("circle3.xml");
			ctx.refresh();
		} catch (Exception e) {
			Throwable e1 = e.getCause().getCause().getCause();
			throw e1;
		}
	}

 

	org.springframework.beans.NotWritablePropertyException: Invalid property 'circleA' of bean class [com.iflytek.demo.CircleC]: Bean property 'circleA' is not writable or has an invalid setter method. Did you mean 'circleC'?

 

转自:http://jinnianshilongnian.iteye.com/

 

  • 03-Spring3 依赖注入(DI)B_循环依赖【转】
            
    
    博客分类: Spring JavaSpringBeanDI循环依赖 
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