浅谈Spring解决循环依赖的三种方式

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; 
 } 
} 

<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> 

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)); 
 } 
} 

<!--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> 

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)); 
 } 
} 

/** 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); 
  } 
 } 
 } 

<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> 

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)); 
 } 
}