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CircleB,CircleB引用CircleC,CircleC引用CircleA,则它们最终反映为一个环
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?
1、Spring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”;
2、Spring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”;
3、Spring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“circleA”Bean,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:
1、Spring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”;
2、Spring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”;
3、Spring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Bean,Spring容器无法完成依赖注入,因为“prototype”作用域的Bean,Spring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'?
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