spring如何快速稳定解决循环依赖问题
循环依赖其实就是循环引用,很多地方都说需要两个或则两个以上的bean互相持有对方最终形成闭环才是循环依赖,比如a依赖于b,b依赖于c,c又依赖于a。其实一个bean持有自己类型的属性也会产生循环依赖。
setter singleton循环依赖
使用
singlesetterbeana依赖singlesetterbeanb,singlesetterbeanb依赖singlesetterbeana。
@data public class singlesetterbeana { @autowired private singlesetterbeanb singlesetterbeanb; }
@data public class singlesetterbeanb { @autowired private singlesetterbeana singlesetterbeana; }
源码分析
spring是通过三级缓存来解决循环依赖的,那么三级缓存是怎么工作的呢?
三级缓存对应org.springframework.beans.factory.support.defaultsingletonbeanregistry类的三个属性:
/** cache of singleton objects: bean name to bean instance. */ private final map<string, object> singletonobjects = new concurrenthashmap<>(256); // 一级缓存 /** cache of singleton factories: bean name to objectfactory. */ private final map<string, objectfactory<?>> singletonfactories = new hashmap<>(16); // 二级缓存 /** cache of early singleton objects: bean name to bean instance. */ private final map<string, object> earlysingletonobjects = new concurrenthashmap<>(16); // 三级缓存
对于setter注入造成的依赖是通过spring容器提前暴露刚完成实例化但未完成初始化的bean来完成的,而且只能解决单例作用域的bean循环依赖。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,关键源码如下所示:
org.springframework.beans.factory.support.abstractautowirecapablebeanfactory#docreatebean
// 处理循环依赖,实例化后放入三级缓存 boolean earlysingletonexposure = (mbd.issingleton() && this.allowcircularreferences && issingletoncurrentlyincreation(beanname)); if (earlysingletonexposure) { if (logger.istraceenabled()) { logger.trace("eagerly caching bean '" + beanname + "' to allow for resolving potential circular references"); } addsingletonfactory(beanname, () -> getearlybeanreference(beanname, mbd, bean)); }
bean实例化后放入三级缓存中:
org.springframework.beans.factory.support.defaultsingletonbeanregistry#addsingletonfactory
protected void addsingletonfactory(string beanname, objectfactory<?> singletonfactory) { assert.notnull(singletonfactory, "singleton factory must not be null"); synchronized (this.singletonobjects) { if (!this.singletonobjects.containskey(beanname)) { this.singletonfactories.put(beanname, singletonfactory); // 三级缓存 this.earlysingletonobjects.remove(beanname); this.registeredsingletons.add(beanname); } } }
放入三级缓存中的是objectfactory类型的lambda表达式:
org.springframework.beans.factory.support.abstractautowirecapablebeanfactory#getearlybeanreference
protected object getearlybeanreference(string beanname, rootbeandefinition mbd, object bean) { object exposedobject = bean; if (!mbd.issynthetic() && hasinstantiationawarebeanpostprocessors()) { for (beanpostprocessor bp : getbeanpostprocessors()) { if (bp instanceof smartinstantiationawarebeanpostprocessor) { smartinstantiationawarebeanpostprocessor ibp = (smartinstantiationawarebeanpostprocessor) bp; /** * @see org.springframework.aop.framework.autoproxy.abstractautoproxycreator#getearlybeanreference(java.lang.object, java.lang.string) */ // 使用abstractautoproxycreator#getearlybeanreference创建代理对象 exposedobject = ibp.getearlybeanreference(exposedobject, beanname); } } } return exposedobject; }
构造器参数循环依赖
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。
使用
@data public class singleconstrutorbeana { public singleconstrutorbeana(singleconstrutorbeanb singleconstrutorbeanb) { } }
@data public class singleconstrutorbeanb { public singleconstrutorbeanb(singleconstrutorbeana singleconstrutorbeana) { } }
上面的代码运行时会抛出如下异常:
... ...
caused by: org.springframework.beans.factory.unsatisfieddependencyexception: error creating bean with name 'singleconstrutorbeanb': unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.beancurrentlyincreationexception: error creating bean with name 'singleconstrutorbeana': requested bean is currently in creation: is there an unresolvable circular reference?
at org.springframework.beans.factory.support.constructorresolver.createargumentarray(constructorresolver.java:805)
at org.springframework.beans.factory.support.constructorresolver.autowireconstructor(constructorresolver.java:228)
at org.springframework.beans.factory.support.abstractautowirecapablebeanfactory.autowireconstructor(abstractautowirecapablebeanfactory.java:1403)
at org.springframework.beans.factory.support.abstractautowirecapablebeanfactory.createbeaninstance(abstractautowirecapablebeanfactory.java:1245)
at org.springframework.beans.factory.support.abstractautowirecapablebeanfactory.docreatebean(abstractautowirecapablebeanfactory.java:579)
at org.springframework.beans.factory.support.abstractautowirecapablebeanfactory.createbean(abstractautowirecapablebeanfactory.java:538)
at org.springframework.beans.factory.support.abstractbeanfactory.lambda$dogetbean$0(abstractbeanfactory.java:329)
at org.springframework.beans.factory.support.defaultsingletonbeanregistry.getsingleton(defaultsingletonbeanregistry.java:234)
at org.springframework.beans.factory.support.abstractbeanfactory.dogetbean(abstractbeanfactory.java:323)
at org.springframework.beans.factory.support.abstractbeanfactory.getbean(abstractbeanfactory.java:202)
at org.springframework.beans.factory.config.dependencydescriptor.resolvecandidate(dependencydescriptor.java:276)
at org.springframework.beans.factory.support.defaultlistablebeanfactory.doresolvedependency(defaultlistablebeanfactory.java:1321)
at org.springframework.beans.factory.support.defaultlistablebeanfactory.resolvedependency(defaultlistablebeanfactory.java:1240)
at org.springframework.beans.factory.support.constructorresolver.resolveautowiredargument(constructorresolver.java:892)
at org.springframework.beans.factory.support.constructorresolver.createargumentarray(constructorresolver.java:796)
... 76 more
caused by: org.springframework.beans.factory.beancurrentlyincreationexception: error creating bean with name 'singleconstrutorbeana': requested bean is currently in creation: is there an unresolvable circular reference?
at org.springframework.beans.factory.support.defaultsingletonbeanregistry.beforesingletoncreation(defaultsingletonbeanregistry.java:355)
at org.springframework.beans.factory.support.defaultsingletonbeanregistry.getsingleton(defaultsingletonbeanregistry.java:227)
at org.springframework.beans.factory.support.abstractbeanfactory.dogetbean(abstractbeanfactory.java:323)
at org.springframework.beans.factory.support.abstractbeanfactory.getbean(abstractbeanfactory.java:202)
at org.springframework.beans.factory.config.dependencydescriptor.resolvecandidate(dependencydescriptor.java:276)
at org.springframework.beans.factory.support.defaultlistablebeanfactory.doresolvedependency(defaultlistablebeanfactory.java:1321)
at org.springframework.beans.factory.support.defaultlistablebeanfactory.resolvedependency(defaultlistablebeanfactory.java:1240)
at org.springframework.beans.factory.support.constructorresolver.resolveautowiredargument(constructorresolver.java:892)
at org.springframework.beans.factory.support.constructorresolver.createargumentarray(constructorresolver.java:796)
... 90 more
源码分析
spring容器会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。
protected void beforesingletoncreation(string beanname) { if (!this.increationcheckexclusions.contains(beanname) && !this.singletonscurrentlyincreation.add(beanname)) { throw new beancurrentlyincreationexception(beanname); } }
@lazy打破循环依赖
在上面的例子中只需要在singleconstrutorbeana或者singleconstrutorbeanb的构造方法上面加上@lazy注解,就会发现不会抛出异常了,这又是为什么呢?
下面假设在singleconstrutorbeana的构造方法上面加了@lazy注解,在构造b时,发现参数a时被@lazy注解修饰时,那么就不会调用getbean来获取对象,而是创建了一个代理对象,所以不会构成真正的循环依赖,不会抛出beancurrentlyincreationexception异常。
/** * 处理懒加载对象 * 懒加载返回的又是一个代理对象,不会真正的调用getbean,所以如果构造方法依赖中有循环依赖,那么不会报错 * @see org.springframework.context.annotation.contextannotationautowirecandidateresolver#getlazyresolutionproxyifnecessary(org.springframework.beans.factory.config.dependencydescriptor, java.lang.string) */ object result = getautowirecandidateresolver().getlazyresolutionproxyifnecessary( descriptor, requestingbeanname); if (result == null) { // 调用beanfactory.getbean(beanname)从容器中获取依赖对象 result = doresolvedependency(descriptor, requestingbeanname, autowiredbeannames, typeconverter); } return result;
setter prototype循环依赖
对于prototype作用域bean,spring容器无法完成依赖注入,因为spring容器不进行缓存"prototype"作用域的bean,因此无法提前暴露一个创建中的bean。
使用
@data @component @scope(configurablebeanfactory.scope_prototype) public class prototypebeana { @autowired private prototypebeanb prototypebeanb; }
@data @component @scope(configurablebeanfactory.scope_prototype) public class prototypebeanb { @autowired private prototypebeana prototypebeana; }
@test public void test3() { annotationconfigapplicationcontext applicationcontext = new annotationconfigapplicationcontext(); applicationcontext.register(prototypebeana.class); applicationcontext.register(prototypebeanb.class); applicationcontext.refresh(); applicationcontext.getbean(prototypebeana.class); // 此时必须要获取spring管理的实例,因为现在scope="prototype" 只有请求获取的时候才会实例化对象 }
运行结果如下:
caused by: org.springframework.beans.factory.beancurrentlyincreationexception: error creating bean with name 'prototypebeana': requested bean is currently in creation: is there an unresolvable circular reference?
at org.springframework.beans.factory.support.abstractbeanfactory.dogetbean(abstractbeanfactory.java:269)
at org.springframework.beans.factory.support.abstractbeanfactory.getbean(abstractbeanfactory.java:202)
at org.springframework.beans.factory.config.dependencydescriptor.resolvecandidate(dependencydescriptor.java:276)
at org.springframework.beans.factory.support.defaultlistablebeanfactory.doresolvedependency(defaultlistablebeanfactory.java:1322)
at org.springframework.beans.factory.support.defaultlistablebeanfactory.resolvedependency(defaultlistablebeanfactory.java:1240)
at org.springframework.beans.factory.annotation.autowiredannotationbeanpostprocessor$autowiredfieldelement.resolvefieldvalue(autowiredannotationbeanpostprocessor.java:668)
... 89 more
源码分析
org.springframework.beans.factory.support.abstractbeanfactory#dogetbean
... ... // 判断是否在当前创建bean池中 if (isprototypecurrentlyincreation(beanname)) { throw new beancurrentlyincreationexception(beanname); } ... ...
异常就是在上面的代码中抛出来的,那么beanname是什么时候添加至当前创建bean池中的呢?
org.springframework.beans.factory.support.abstractbeanfactory#dogetbean
else if (mbd.isprototype()) { // it's a prototype -> create a new instance. // prototype类型的bean的实例化 object prototypeinstance = null; try { beforeprototypecreation(beanname); prototypeinstance = createbean(beanname, mbd, args); } finally { afterprototypecreation(beanname); } bean = getobjectforbeaninstance(prototypeinstance, name, beanname, mbd); }
org.springframework.beans.factory.support.abstractbeanfactory#beforeprototypecreation
protected void beforeprototypecreation(string beanname) { // threadlocal object curval = this.prototypescurrentlyincreation.get(); if (curval == null) { this.prototypescurrentlyincreation.set(beanname); } else if (curval instanceof string) { set<string> beannameset = new hashset<>(2); beannameset.add((string) curval); beannameset.add(beanname); this.prototypescurrentlyincreation.set(beannameset); } else { set<string> beannameset = (set<string>) curval; beannameset.add(beanname); } }
其根本原因就是spring容器不会对prototype类型的bean进行缓存,因此无法提前利用三级缓存暴露一个代理对象。
循环依赖开关
可以通过allowcircularreferences来禁止循环依赖,这样的话,singleton bean的setter循环依赖也会报错。
applicationcontext.setallowcircularreferences(false);
二级缓存可行?
缓存 | 说明 |
---|---|
singletonobjects | 第一级缓存,存放可用的成品bean。 |
earlysingletonobjects | 第二级缓存,存放半成品的bean,半成品的bean是已创建对象,但是未注入属性和初始化,用以解决循环依赖。 |
singletonfactories | 第三级缓存,存的是bean工厂对象,用来生成半成品的bean并放入到二级缓存中,用以解决循环依赖。 |
理论上二级缓存时可行的,只需要将三级缓存中beanfactory创建的对象提前放入二级缓存中,这样三级缓存就可以移除了。
那么spring中为什么还要使用三级缓存呢?如果要使用二级缓存解决循环依赖,意味着所有bean在实例化后就要完成aop代理,这样违背了spring设计的原则,spring在设计之初就是通过annotationawareaspectjautoproxycreator这个后置处理器来在bean生命周期的最后一步来完成aop代理,而不是在实例化后就立马进行aop代理。
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