引言:循環依賴就是N個類中循環嵌套引用,如果在日常開發中我們用new 對象的方式發生這種循環依賴的話程序會在運行時一直循環調用,直至內存溢出報錯。下面說一下Spring是如果解決循環依賴的。
第一種:構造器參數循環依賴
Spring容器會將每一個正在創建的Bean 標識符放在一個“當前創建Bean池”中,Bean標識符在創建過程中將一直保持在這個池中。
因此如果在創建Bean過程中發現自己已經在“當前創建Bean池”裏時將拋出BeanCurrentlyInCreationException異常表示循環依賴;而對於創建完畢的Bean將從“當前創建Bean池”中清除掉。
首先我們先初始化三個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個類,,StudentA有參構造是StudentB。StudentB的有參構造是StudentC,StudentC的有參構造是StudentA ,這樣就產生了一個循環依賴的情況,
我們都把這三個Bean交給Spring管理,並用有參構造實例化。
<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>
下面是測試類:
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));
}
}
執行結果報錯信息爲:
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?
如果大家理解開頭那句話的話,這個報錯應該不驚訝,Spring容器先創建單例StudentA,StudentA依賴StudentB,然後將A放在“當前創建Bean池”中,此時創建StudentB,StudentB依賴StudentC ,然後將B放在“當前創建Bean池”中,此時創建StudentC,StudentC又依賴StudentA, 但是,此時Student已經在池中,所以會報錯,,因爲在池中的Bean都是未初始化完的,所以會依賴錯誤 ,(初始化完的Bean會從池中移除)
第二種:setter方式單例,默認方式
如果要說setter方式注入的話,我們最好先看一張Spring中Bean實例化的圖
如圖中前兩步驟得知:Spring是先將Bean對象實例化之後再設置對象屬性的
修改配置文件爲set方式注入
<!--scope="singleton"(默認就是單例方式) -->
<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>
下面是測試類:
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));
}
}
打印結果爲:
com.zfx.student.StudentA@1fbfd6
爲什麼用set方式就不報錯了呢 ?
我們結合上面那張圖看,Spring先是用構造實例化Bean對象 ,此時Spring會將這個實例化結束的對象放到一個Map中,並且Spring提供了獲取這個未設置屬性的實例化對象引用的方法。
結合我們的實例來看,,當Spring實例化了StudentA、StudentB、StudentC後,緊接着會去設置對象的屬性,此時StudentA依賴StudentB,就會去Map中取出存在裏面的單例StudentB對象,以此類推,不會出來循環的問題嘍、
下面是Spring源碼中的實現方法。以下的源碼在Spring的Bean包中的DefaultSingletonBeanRegistry.java類中
/** Cache of singleton objects: bean name --> bean instance(緩存單例實例化對象的Map集合) */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
/** Cache of singleton factories: bean name --> ObjectFactory(單例的工廠Bean緩存集合) */
private final Map<String, ObjectFactory> singletonFactories = new HashMap<String, ObjectFactory>(16);
/** Cache of early singleton objects: bean name --> bean instance(早期的單身對象緩存集合) */
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);
/** Set of registered singletons, containing the bean names in registration order(單例的實例化對象名稱集合) */
private final Set<String> registeredSingletons = new LinkedHashSet<String>(64);
/**
* 添加單例實例
* 解決循環引用的問題
* 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);
}
}
}
第三種:setter方式原型,prototype
修改配置文件爲:
<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 每次請求都會創建一個實例對象。
兩者的區別是:有狀態的bean都使用Prototype作用域,無狀態的一般都使用singleton單例作用域。
測試用例:
public class Test {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("com/zfx/student/applicationContext.xml");
//此時必須要獲取Spring管理的實例,因爲現在scope="prototype" 只有請求獲取的時候纔會實例化對象
System.out.println(context.getBean("a", StudentA.class));
}
}
打印結果:
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?
爲什麼原型模式就報錯了呢?
對於“prototype”作用域Bean,Spring容器無法完成依賴注入,因爲“prototype”作用域的Bean,Spring容器不進行緩存,因此無法提前暴露一個創建中的Bean。