测试一下25道Spring经典面试题你会几道？循环依赖面试详解

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));
    }
}

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?

    <!--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));
    }
}

com.zfx.student.StudentA@1fbfd6

/** 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" 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>

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));
    }
}

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?