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



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。

public class StudentA {

    private StudentB studentB ;

    public void setStudentB(StudentB studentB) {
        this.studentB = studentB;
    }

    public StudentA() {
    }
    
    public StudentA(StudentB studentB) {
        this.studentB = studentB;
    }
}

public class StudentB {

    private StudentC studentC ;

    public void setStudentC(StudentC studentC) {
        this.studentC = studentC;
    }
    
    public StudentB() {
    }

    public StudentB(StudentC studentC) {
        this.studentC = studentC;
    }
}
public class StudentC {

    private StudentA studentA ;

    public void setStudentA(StudentA studentA) {
        this.studentA = studentA;
    }

    public StudentC() {
    }
 
    public StudentC(StudentA studentA) {
        this.studentA = studentA;
    }
}

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

 <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> 

下面是测试类:

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));
    }
}

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

	<!--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>

下面是测试类:

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));
    }
}

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

/** 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);
			}
		}
	}


第三种:setter方式原型,prototype


修改配置文件为:

<bean id="a" class="com.zfx.student.StudentA" scope="prototype">
		<property name="studentB" ref="b"></property>
	</bean>
	<bean id="b" class="com.zfx.student.StudentB" scope="prototype">
		<property name="studentC" ref="c"></property>
	</bean>
	<bean id="c" class="com.zfx.student.StudentC" scope="prototype">
		<property name="studentA" ref="a"></property>
	</bean>

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:

public class Test {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("com/zfx/student/applicationContext.xml");
        //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
        System.out.println(context.getBean("a", StudentA.class));
    }
}

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。










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