Spring循環依賴的三種方式

時間 2019-11-17

原文原文鏈接

引言：循環依賴就是N個類中循環嵌套引用，若是在平常開發中咱們用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是若是解決循環依賴的。html

第一種：構造器參數循環依賴

Spring容器會將每個正在建立的Bean 標識符放在一個「當前建立Bean池」中，Bean標識符在建立過程當中將一直保持
在這個池中，所以若是在建立Bean過程當中發現本身已經在「當前建立Bean池」裏時將拋出
BeanCurrentlyInCreationException異常表示循環依賴；而對於建立完畢的Bean將從「當前建立Bean池」中清除掉。java

首先咱們先初始化三個Bean。spring

[java] view plain copy緩存

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

[java] view plain copyapp

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

[java] view plain copy測試

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狀況，ui

咱們都把這三個Bean交給Spring管理，並用有參構造實例化this

[html] view plain copyspa

<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：.net

[java] view plain copy

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：

[java] view plain copy

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?

若是你們理解開頭那句話的話，這個報錯應該不驚訝，Spring容器先建立單例StudentA，StudentA依賴StudentB，而後將A放在「當前建立Bean池」中，此時建立StudentB,StudentB依賴StudentC ,而後將B放在「當前建立Bean池」中,此時建立StudentC，StudentC又依賴StudentA，可是，此時Student已經在池中，因此會報錯，，由於在池中的Bean都是未初始化完的，因此會依賴錯誤，（初始化完的Bean會從池中移除）

第二種：setter方式單例，默認方式

若是要說setter方式注入的話，咱們最好先看一張Spring中Bean實例化的圖

如圖中前兩步驟得知：Spring是先將Bean對象實例化以後再設置對象屬性的

修改配置文件爲set方式注入

[html] view plain copy

<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：

[java] view plain copy

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：

[java] view plain copy

com.zfx.student.StudentA@1fbfd6

爲何用set方式就不報錯了呢？

咱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，而且Spring提供了獲取這個未設置屬性的實例化對象引用的方法。結合咱們的實例來看，，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，。如下的源碼在Spring的Bean包中的DefaultSingletonBeanRegistry.java類中

[java] view plain copy

/** 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.
* 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：

[html] view plain copy

<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建立一個實例對象。二者的區別是：有狀態的bean都使用Prototype做用域，無狀態的通常都使用singleton單例做用域。

測試用例：

[java] view plain copy

public class Test {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("com/zfx/student/applicationContext.xml");
//此時必需要獲取Spring管理的實例，由於如今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：

[java] view plain copy

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?

爲何原型模式就報錯了呢？

對於「prototype」做用域Bean，Spring容器沒法完成依賴注入，由於「prototype」做用域的Bean，Spring容器不進行緩存，所以沒法提早暴露一個建立中的Bean。