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這篇文章將為大家詳細講解有關(guān)Spring循環(huán)依賴的方式有哪些,小編覺得挺實用的,因此分享給大家做個參考,希望大家閱讀完這篇文章后可以有所收獲。
第一種:構(gòu)造器參數(shù)循環(huán)依賴
Spring容器會將每一個正在創(chuàng)建的Bean 標(biāo)識符放在一個“當(dāng)前創(chuàng)建Bean池”中,Bean標(biāo)識符在創(chuàng)建過程中將一直保持
在這個池中,因此如果在創(chuàng)建Bean過程中發(fā)現(xiàn)自己已經(jīng)在“當(dāng)前創(chuàng)建Bean池”里時將拋出
BeanCurrentlyInCreationException異常表示循環(huán)依賴;而對于創(chuàng)建完畢的Bean將從“當(dāng)前創(chuàng)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(gòu)造是StudentB。StudentB的有參構(gòu)造是StudentC,StudentC的有參構(gòu)造是StudentA ,這樣就產(chǎn)生了一個循環(huán)依賴的情況,
我們都把這三個Bean交給Spring管理,并用有參構(gòu)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(zhí)行結(jié)果報錯信息為:
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(yīng)該不驚訝,Spring容器先創(chuàng)建單例StudentA,StudentA依賴StudentB,然后將A放在“當(dāng)前創(chuàng)建Bean池”中,此時創(chuàng)建StudentB,StudentB依賴StudentC ,然后將B放在“當(dāng)前創(chuàng)建Bean池”中,此時創(chuàng)建StudentC,StudentC又依賴StudentA, 但是,此時Student已經(jīng)在池中,所以會報錯,,因為在池中的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對象實例化之后再設(shè)置對象屬性的
修改配置文件為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(jié)果為:
com.zfx.student.StudentA@1fbfd6
為什么用set方式就不報錯了呢 ?
我們結(jié)合上面那張圖看,Spring先是用構(gòu)造實例化Bean對象 ,此時Spring會將這個實例化結(jié)束的對象放到一個Map中,并且Spring提供了獲取這個未設(shè)置屬性的實例化對象引用的方法。 結(jié)合我們的實例來看,,當(dāng)Spring實例化了StudentA、StudentB、StudentC后,緊接著會去設(shè)置對象的屬性,此時StudentA依賴StudentB,就會去Map中取出存在里面的單例StudentB對象,以此類推,不會出來循環(huán)的問題嘍、
下面是Spring源碼中的實現(xiàn)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(huán)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
修改配置文件為:
<bean id="a" class="com.zfx.student.StudentA" <span >scope="prototype"</span>> <property name="studentB" ref="b"></property> </bean> <bean id="b" class="com.zfx.student.StudentB" <span >scope="prototype"</span>> <property name="studentC" ref="c"></property> </bean> <bean id="c" class="com.zfx.student.StudentC" <span >scope="prototype"</span>> <property name="studentA" ref="a"></property> </bean>
scope="prototype" 意思是 每次請求都會創(chuàng)建一個實例對象。兩者的區(qū)別是:有狀態(tài)的bean都使用Prototype作用域,無狀態(tài)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"); <strong>//此時必須要獲取Spring管理的實例,因為現(xiàn)在scope="prototype" 只有請求獲取的時候才會實例化對象</strong> System.out.println(context.getBean("a", StudentA.class)); } }
打印結(jié)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?
為什么原型模式就報錯了呢 ?
對于“prototype”作用域Bean,Spring容器無法完成依賴注入,因為“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(chuàng)建中的Bean。
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