引言:循环依赖就是n个类中循环嵌套引用,如果在日常开发中我们用new 对象的方式发生这种循环依赖的话程序会在运行时一直循环调用,直至内存溢出报错。下面说一下spring是如果解决循环依赖的。
第一种:构造器参数循环依赖
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beancurrentlyin creationexception异常表示循环依赖。
如在创建testa类时,构造器需要testb类,那将去创建testb,在创建testb类时又发现需要testc类,则又去创建testc,最终在创建testc时发现又需要testa,从而形成一个环,没办法创建。
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。
首先我们先初始化三个bean。
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public class studenta {
private studentb studentb ;
public void setstudentb(studentb studentb) {
this .studentb = studentb;
}
public studenta() {
}
public studenta(studentb studentb) {
this .studentb = studentb;
}
}
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public class studentb {
private studentc studentc ;
public void setstudentc(studentc studentc) {
this .studentc = studentc;
}
public studentb() {
}
public studentb(studentc studentc) {
this .studentc = studentc;
}
}
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public class studentc {
private studenta studenta ;
public void setstudenta(studenta studenta) {
this .studenta = studenta;
}
public studentc() {
}
public studentc(studenta studenta) {
this .studenta = studenta;
}
}
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ok,上面是很基本的3个类,,studenta有参构造是studentb。studentb的有参构造是studentc,studentc的有参构造是studenta ,这样就产生了一个循环依赖的情况,我们都把这三个bean交给spring管理,并用有参构造实例化
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<bean id= "a" class = "com.zfx.student.studenta" >
<constructor-arg index= "0" ref= "b" ></constructor-arg>
</bean>
<bean id= "b" class = "com.zfx.student.studentb" >
<constructor-arg index= "0" ref= "c" ></constructor-arg>
</bean>
<bean id= "c" class = "com.zfx.student.studentc" >
<constructor-arg index= "0" ref= "a" ></constructor-arg>
</bean>
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下面是测试类:
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public class test {
public static void main(string[] args) {
applicationcontext context = new classpathxmlapplicationcontext( "com/zfx/student/applicationcontext.xml" );
//system.out.println(context.getbean("a", studenta.class));
}
}
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执行结果报错信息为:
caused by: org.springframework.beans.factory.beancurrentlyincreationexception:
error creating bean with name 'a': requested bean is currently in creation: is there an unresolvable circular reference?
如果大家理解开头那句话的话,这个报错应该不惊讶,spring容器先创建单例studenta,studenta依赖studentb,然后将a放在“当前创建bean池”中,此时创建studentb,studentb依赖studentc ,然后将b放在“当前创建bean池”中,此时创建studentc,studentc又依赖studenta, 但是,此时student已经在池中,所以会报错,,因为在池中的bean都是未初始化完的,所以会依赖错误 ,(初始化完的bean会从池中移除)
第二种:setter方式单例,默认方式
如果要说setter方式注入的话,我们最好先看一张spring中bean实例化的图
如图中前两步骤得知:spring是先将bean对象实例化之后再设置对象属性的
修改配置文件为set方式注入
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<!--scope= "singleton" (默认就是单例方式) -->
<bean id= "a" class = "com.zfx.student.studenta" scope= "singleton" >
<property name= "studentb" ref= "b" ></property>
</bean>
<bean id= "b" class = "com.zfx.student.studentb" scope= "singleton" >
<property name= "studentc" ref= "c" ></property>
</bean>
<bean id= "c" class = "com.zfx.student.studentc" scope= "singleton" >
<property name= "studenta" ref= "a" ></property>
</bean>
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下面是测试类:
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public class test {
public static void main(string[] args) {
applicationcontext context = new classpathxmlapplicationcontext( "com/zfx/student/applicationcontext.xml" );
system.out.println(context.getbean( "a" , studenta. class ));
}
}
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打印结果为:
com.zfx.student.studenta@1fbfd6
为什么用set方式就不报错了呢 ?
我们结合上面那张图看,spring先是用构造实例化bean对象 ,此时spring会将这个实例化结束的对象放到一个map中,并且spring提供了获取这个未设置属性的实例化对象引用的方法。 结合我们的实例来看,,当spring实例化了studenta、studentb、studentc后,紧接着会去设置对象的属性,此时studenta依赖studentb,就会去map中取出存在里面的单例studentb对象,以此类推,不会出来循环的问题喽、
下面是spring源码中的实现方法,。以下的源码在spring的bean包中的defaultsingletonbeanregistry.java类中
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/** cache of singleton objects: bean name --> bean instance(缓存单例实例化对象的map集合) */
private final map<string, object> singletonobjects = new concurrenthashmap<string, object>( 64 );
/** cache of singleton factories: bean name --> objectfactory(单例的工厂bean缓存集合) */
private final map<string, objectfactory> singletonfactories = new hashmap<string, objectfactory>( 16 );
/** cache of early singleton objects: bean name --> bean instance(早期的单身对象缓存集合) */
private final map<string, object> earlysingletonobjects = new hashmap<string, object>( 16 );
/** set of registered singletons, containing the bean names in registration order(单例的实例化对象名称集合) */
private final set<string> registeredsingletons = new linkedhashset<string>( 64 );
/**
* 添加单例实例
* 解决循环引用的问题
* add the given singleton factory for building the specified singleton
* if necessary.
* <p>to be called for eager registration of singletons, e.g. to be able to
* resolve circular references.
* @param beanname the name of the bean
* @param singletonfactory the factory for the singleton object
*/
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);
this .earlysingletonobjects.remove(beanname);
this .registeredsingletons.add(beanname);
}
}
}
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第三种:setter方式原型,prototype
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。
修改配置文件为:
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<bean id= "a" class = "com.zfx.student.studenta" <span style= "color:#ff0000;" >scope= "prototype" </span>>
<property name= "studentb" ref= "b" ></property>
</bean>
<bean id= "b" class = "com.zfx.student.studentb" <span style= "color:#ff0000;" >scope= "prototype" </span>>
<property name= "studentc" ref= "c" ></property>
</bean>
<bean id= "c" class = "com.zfx.student.studentc" <span style= "color:#ff0000;" >scope= "prototype" </span>>
<property name= "studenta" ref= "a" ></property>
</bean>
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scope="prototype" 意思是 每次请求都会创建一个实例对象。两者的区别是:有状态的bean都使用prototype作用域,无状态的一般都使用singleton单例作用域。
测试用例:
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public class test {
public static void main(string[] args) {
applicationcontext context = new classpathxmlapplicationcontext( "com/zfx/student/applicationcontext.xml" );
<strong> //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</strong>
system.out.println(context.getbean( "a" , studenta. class ));
}
}
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打印结果:
caused by: org.springframework.beans.factory.beancurrentlyincreationexception:
error creating bean with name 'a': requested bean is currently in creation: is there an unresolvable circular reference?
为什么原型模式就报错了呢 ?
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。
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持服务器之家。
原文链接:http://blog.csdn.net/u010644448/article/details/59108799