spring怎么避免循环依赖

1、循环依赖

（1）概念

对象依赖分为强依赖和弱依赖：

强依赖指的是一个对象包含了另外一个对象的引用，例如：学生类中包含了课程类，在学生类中存在课程类的引用

创建课程类：

@Data
public class Course {
    private String cname;
    private Integer credit;
}

创建学生类：

@Data
public class Student {
    private String sname;
    private int age;
    private Course course;
}

测试类：

public class Test {
    public static void main(String[] args) {
        Student student=new Student();
        Course course=new Course();
        student.setCourse(course);
    }
}

弱依赖指的是一个对象里面调用了另外一个对象

循环依赖：A引用B，而B又引用A

创建课程类，课程类中有学生类的引用：

@Data
public class Course {
    private String cname;
    private Integer credit;
    private Student student;
}

创建学生类，学生类中存在课程类中的引用：

@Data
public class Student {
    private String sname;
    private int age;
    private Course course;
}

创建测试类，测试类中学生类调用课程类的对象，课程类的对象又调用学生类的对象：

public class Test {
    public static void main(String[] args) {
        Student student=new Student();
        Course course=new Course();
        student.setCourse(course);
        course.setStudent(student);
    }
}

测试类可以正常地获取对象，也就是说这种方式是支持循环依赖的。

（2）spring的循环依赖

    <bean id="student" class="com.zhb.bean.Student" scope="singleton">
        <property name="course" ref="course"></property>
    </bean>
    <bean id="course" class="com.zhb.bean.Course" scope="singleton">
        <property name="student" ref="student"></property>
    </bean>

单例模式下支持，多例模式下不支持

（3）spring是否支持循环依赖

支持

（4）spring的所有对象是不是都支持循环依赖

单例模式下支持：单例模式下是首先创建出两个对象，然后进行依赖的注入。A注入B，B注入A。

多例模式下不支持：A创建的时候会去判断是否依赖于其他对象，如果依赖的话就不会去先创建A。创建B的时候也会去判断是否依赖其他对象，如果A依赖于B，而B又依赖于A的话就会造成一个死循环，两个对象都不会创建。

2、spring循环依赖的过程

（1）流程

 

 （2）源码

ClassPathXmlApplicationContext方法：

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, @Nullable ApplicationContext parent) throws BeansException {
        super(parent);
        this.setConfigLocations(configLocations);
        if (refresh) {
            this.refresh();//refresh方法是真正的创建对象的方法
        }

    }

refresh方法：

public void refresh() throws BeansException, IllegalStateException {
        synchronized(this.startupShutdownMonitor) {
            this.prepareRefresh();
            ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
            this.prepareBeanFactory(beanFactory);

            try {
                this.postProcessBeanFactory(beanFactory);
                this.invokeBeanFactoryPostProcessors(beanFactory);//Bean工厂的后置处理器
                this.registerBeanPostProcessors(beanFactory);
                this.initMessageSource();
                this.initApplicationEventMulticaster();
                this.onRefresh();
                this.registerListeners();
                this.finishBeanFactoryInitialization(beanFactory);//此方法是真正的创建对象的方法，能够实例化所有的单例bean
                this.finishRefresh();
            } catch (BeansException var9) {
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
                }

                this.destroyBeans();
                this.cancelRefresh(var9);
                throw var9;
            } finally {
                this.resetCommonCaches();
            }

        }
    }

finishBeanFactoryInitialization

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        if (beanFactory.containsBean("conversionService") && beanFactory.isTypeMatch("conversionService", ConversionService.class)) {
            beanFactory.setConversionService((ConversionService)beanFactory.getBean("conversionService", ConversionService.class));
        }

        if (!beanFactory.hasEmbeddedValueResolver()) {
            beanFactory.addEmbeddedValueResolver((strVal) -> {
                return this.getEnvironment().resolvePlaceholders(strVal);
            });
        }

        String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
        String[] var3 = weaverAwareNames;
        int var4 = weaverAwareNames.length;

        for(int var5 = 0; var5 < var4; ++var5) {
            String weaverAwareName = var3[var5];
            this.getBean(weaverAwareName);
        }

        beanFactory.setTempClassLoader((ClassLoader)null);
        beanFactory.freezeConfiguration();
        beanFactory.preInstantiateSingletons();
    }

 接口：

void preInstantiateSingletons() throws BeansException;

public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Pre-instantiating singletons in " + this);
        }

        List<String> beanNames = new ArrayList(this.beanDefinitionNames);
        Iterator var2 = beanNames.iterator();//取出名字

        while(true) {
            String beanName;
            Object bean;
            do {
                while(true) {
                    RootBeanDefinition bd;
                    do {
                        do {
                            do {
                                if (!var2.hasNext()) {
                                    var2 = beanNames.iterator();

                                    while(var2.hasNext()) {
                                        beanName = (String)var2.next();
                                        Object singletonInstance = this.getSingleton(beanName);
                                        if (singletonInstance instanceof SmartInitializingSingleton) {
                                            SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton)singletonInstance;
                                            if (System.getSecurityManager() != null) {
                                                AccessController.doPrivileged(() -> {
                                                    smartSingleton.afterSingletonsInstantiated();
                                                    return null;
                                                }, this.getAccessControlContext());
                                            } else {
                                                smartSingleton.afterSingletonsInstantiated();
                                            }
                                        }
                                    }

                                    return;
                                }

                                beanName = (String)var2.next();
                                bd = this.getMergedLocalBeanDefinition(beanName);
                            } while(bd.isAbstract());//单例抽象懒加载
                        } while(!bd.isSingleton());
                    } while(bd.isLazyInit());

                    if (this.isFactoryBean(beanName)) {
                        bean = this.getBean("&" + beanName);
                        break;
                    }

                    this.getBean(beanName);
                }
            } while(!(bean instanceof FactoryBean));

            FactoryBean<?> factory = (FactoryBean)bean;
            boolean isEagerInit;
            if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                SmartFactoryBean var10000 = (SmartFactoryBean)factory;
                ((SmartFactoryBean)factory).getClass();
                isEagerInit = (Boolean)AccessController.doPrivileged(var10000::isEagerInit, this.getAccessControlContext());
            } else {
                isEagerInit = factory instanceof SmartFactoryBean && ((SmartFactoryBean)factory).isEagerInit();
            }

            if (isEagerInit) {
                this.getBean(beanName);
            }
        }
    }

getSingleton方法：

 @Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);//
        if (singletonObject == null && this.isSingletonCurrentlyInCreation(beanName)) {
            synchronized(this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory<?> singletonFactory = (ObjectFactory)this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }

        return singletonObject;
    }

3、怎么避免循环依赖？

三级缓存

• 一级缓存：singletonObjects，存放完全实例化属性赋值完成的Bean，直接可以使用。
• 二级缓存：earlySingletonObjects，存放早期Bean的引用，尚未属性装配的Bean
• 三级缓存：singletonFactories，三级缓存，存放实例化完成的Bean工厂。

假设A依赖B，B依赖A（注意：这里是set属性依赖）分以下步骤执行：

　　A依次执行doGetBean、查询缓存、createBean创建实例，实例化完成放入三级缓存singletonFactories中，接着执行populateBean方法装配属性，但是发现有一个属性是B的对象。因此再次调用doGetBean方法创建B的实例，依次执行doGetBean、查询缓存、createBean创建实例，实例化完成之后放入三级缓存singletonFactories中，执行populateBean装配属性，但是此时发现有一个属性是A对象。

　　因此再次调用doGetBean创建A的实例，但是执行到getSingleton查询缓存的时候，从三级缓存中查询到了A的实例(早期引用，未完成属性装配)，此时直接返回A，不用执行后续的流程创建A了，那么B就完成了属性装配，此时是一个完整的对象放入到一级缓存singletonObjects中。

　　B创建完成了，则A自然完成了属性装配，也创建完成放入了一级缓存singletonObjects中。