nightOfStreet

菜瓜:水稻,这次我特意去看了java的循环依赖

水稻:哟,有什么收获

菜瓜:两种情况,构造器循环依赖,属性循环依赖

  • 构造器循环依赖在逻辑层面无法通过。对象通过构造函数创建时如果需要创建另一个对象,就会存在递归调用。栈内存直接溢出
  • 属性循环依赖可以解决。在对象创建完成之后通过属性赋值操作。
  • package club.interview.base;
    
    /**
     * 构造器循环依赖 - Exception in thread "main" java.lang.*Error
     * toString()循环打印也会异常 - Exception in thread "main" java.lang.*Error
     * @author QuCheng on 2020/6/18.
     */
    public class Circular {
    
        class A {
            B b;
    
    //        public A() {
    //            b = new B();
    //        }
    
    //        @Override
    //        public String toString() {
    //            return "A{" +
    //                    "b=" + b +
    //                    '}';
    //        }
        }
    
        class B {
            A a;
    
    //        public B() {
    //            a = new A();
    //        }
    
    //        @Override
    //        public String toString() {
    //            return "B{" +
    //                    "a=" + a +
    //                    '}';
    //        }
        }
    
        private void test() {
            B b = new B();
            A a = new A();
            a.b = b;
            b.a = a;
            System.out.println(a);
            System.out.println(b);
        }
    
        public static void main(String[] args) {
            new Circular().test();
        }
    }

水稻:厉害啊,Spring也不支持构造函数的依赖注入,而且也不支持多例的循环依赖。同样的,它支持属性的依赖注入。

  • 看效果 - 如果toString()打印同样会出现栈内存溢出。
  • package com.vip.qc.circular;
    
    import org.springframework.stereotype.Component;
    
    import javax.annotation.Resource;
    
    /**
     * @author QuCheng on 2020/6/18.
     */
    @Component("a")
    public class CircularA {
    
        @Resource
        private CircularB circularB;
    
    //    @Override
    //    public String toString() {
    //        return "CircularA{" +
    //                "circularB=" + circularB +
    //                '}';
    //    }
    }
    
    
    
    
    package com.vip.qc.circular;
    
    import org.springframework.stereotype.Component;
    
    import javax.annotation.Resource;
    
    /**
     * @author QuCheng on 2020/6/18.
     */
    @Component("b")
    public class CircularB {
    
        @Resource
        private CircularA circularA;
    
    //    @Override
    //    public String toString() {
    //        return "CircularB{" +
    //                "circularA=" + circularA +
    //                '}';
    //    }
    }
    
    
        @Test
        public void testCircular() {
            String basePackages = "com.vip.qc.circular";
            new AnnotationConfigApplicationContext(basePackages);
        }

菜瓜:看来spring的实现应该也是通过属性注入的吧

水稻:你说的对。先给思路和demo,之后带你扫一遍源码,follow me !

  • spring的思路是给已经初始化的bean标记状态,假设A依赖B,B依赖A,先创建A
    • 先从缓存容器(总共三层,一级拿不到就拿二级,二级拿不到就从三级缓存中拿正在创建的)中获取A,未获取到就执行创建逻辑
    • 对象A在创建完成还未将属性渲染完之前标记为正在创建中,放入三级缓存容器。渲染属性populateBean()会获取依赖的对象B。
    • 此时B会走一次getBean逻辑,B同样会先放入三级缓存,然后渲染属性,再次走getBean逻辑注入A,此时能从三级缓存中拿到A,并将A放入二级容器。B渲染完成放入一级容器
    • 回到A渲染B的方法populateBean(),拿到B之后能顺利执行完自己的创建过程。放入一级缓存
  •  为了证实结果,我把源码给改了一下,看结果

    • package com.vip.qc.circular;
      
      import org.springframework.stereotype.Component;
      
      import javax.annotation.Resource;
      
      /**
       * @author QuCheng on 2020/6/18.
       */
      @Component("a")
      public class CircularA {
      
          @Resource
          private CircularB circularB;
      
          @Override
          public String toString() {
              return "CircularA{" +
                      "circularB=" + circularB +
                      '}';
          }
      }
      
      
      package com.vip.qc.circular;
      
      import org.springframework.stereotype.Component;
      
      import javax.annotation.Resource;
      
      /**
       * @author QuCheng on 2020/6/18.
       */
      @Component("b")
      public class CircularB {
      
          @Resource
          private CircularA circularA;
      
          @Override
          public String toString() {
              return "CircularB{" +
                      "circularA=" + circularA +
                      '}';
          }
      }
      
      
      测试代码
      @Test
          public void testCircular() {
              String basePackages = "com.vip.qc.circular";
              new AnnotationConfigApplicationContext(basePackages);
      }
      
      测试结果(我改过源码了)
      ---- 
      将a放入三级缓存
      将b放入三级缓存
      将a放入二级缓存
      将b放入一级缓存
      从二级缓存中拿到了a
      将a放入一级缓存

       

        
  • 再看源码
    • 关键类处理getSingleton逻辑 - 缓存容器
      • public class DefaultSingletonBeanRegistry 
        
          /** Cache of singleton objects: bean name to bean instance. */
          // 一级缓存
            private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
        
            /** Cache of singleton factories: bean name to ObjectFactory. */
          // 三级缓存
            private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
        
            /** Cache of early singleton objects: bean name to bean instance. */
          // 二级缓存
            private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
    • 主流程 AbstractApplicationContext#refresh() -> DefaultListableBeanFactory#preInstantiateSingletons() -> AbstractBeanFactory#getBean() & #doGetBean()
      • protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
              @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
           /**
            * 处理FactoryBean接口名称转换 {@link BeanFactory#FACTORY_BEAN_PREFIX }
            */
           final String beanName = transformedBeanName(name);
                ...
           // ①从缓存中拿对象(如果对象正在创建中且被依赖注入,会放入二级缓存)
           Object sharedInstance = getSingleton(beanName);
           if (sharedInstance != null && args == null) {
              ...
           }else {      
                    ...
                 if (mbd.isSingleton()) {
                   // ② 将创建的对象放入一级缓存
                    sharedInstance = getSingleton(beanName, () -> {
                       try {
                             // ③ 具体创建的过程,每个bean创建完成之后都会放入三级缓存,然后渲染属性
                          return createBean(beanName, mbd, args);
                       }catch (BeansException ex) {
                         ...
           ...
           return (T) bean;
        } 
    • ①getSingleton(beanName) - 二级缓存操作
      • protected Object getSingleton(String beanName, boolean allowEarlyReference) {
           // 实例化已经完成了的放在singletonObjects
           Object singletonObject = this.singletonObjects.get(beanName);
           // 解决循环依赖
           if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
              synchronized (this.singletonObjects) {
                 singletonObject = this.earlySingletonObjects.get(beanName);
                 if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                       singletonObject = singletonFactory.getObject();
                       this.earlySingletonObjects.put(beanName, singletonObject);
                       if(beanName.equals("a")||beanName.equals("b")||beanName.equals("c"))
                          System.out.println("将"+beanName+"放入二级缓存");;
                       this.singletonFactories.remove(beanName);
                    }
                 }else if(singletonObject != null){
                    System.out.println("从二级缓存中拿到了"+beanName);
                 }
              }
           }
           return singletonObject;
        }
    • ② getSingleton(beanName,lamdba) - 一级缓存操作
      • public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
              Assert.notNull(beanName, "Bean name must not be null");
              synchronized (this.singletonObjects) {
                 Object singletonObject = this.singletonObjects.get(beanName);
                 if (singletonObject == null) {
                    if (this.singletonsCurrentlyInDestruction) {
                    ...
                    // 正在创建的bean加入singletonsCurrentlyInCreation - 保证只有一个对象创建,阻断循环依赖
                    beforeSingletonCreation(beanName);
                              ...
                    try {
                       singletonObject = singletonFactory.getObject();
                    ...
                    finally {
                    ...
                       // 从singletonsCurrentlyInCreation中移除
                       afterSingletonCreation(beanName);
                    }
                    if (newSingleton) {
                       // 对象创建完毕 - 放入一级缓存(从其他缓存移除)
                       addSingleton(beanName, singletonObject);
                    }
                 }
                 return singletonObject;
              }
           }
                
         //  -----  内部调用一级缓存操作
            protected void addSingleton(String beanName, Object singletonObject) {
                synchronized (this.singletonObjects) {
                    if(beanName.equals("a")||beanName.equals("b")||beanName.equals("c"))
                        System.out.println("将"+beanName+"放入一级缓存");;
                    this.singletonObjects.put(beanName, singletonObject);
                    this.singletonFactories.remove(beanName);
                    this.earlySingletonObjects.remove(beanName);
                    this.registeredSingletons.add(beanName);
                }
            }        
           
    • ③createBean(beanName, mbd, args) - 三级缓存操作
      • protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)throws BeanCreationException {
             ...
           if (instanceWrapper == null) {
              // 5* 实例化对象本身
              instanceWrapper = createBeanInstance(beanName, mbd, args);
           }
           ...
           boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                 isSingletonCurrentlyInCreation(beanName));
           if (earlySingletonExposure) {
              ...
              // 将创建好还未渲染属性的bean 放入三级缓存
              addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
           }
        
           Object exposedObject = bean;
           try {
              // 渲染bean自身和属性
              populateBean(beanName, mbd, instanceWrapper);
              // 实例化之后的后置处理 - init
              exposedObject = initializeBean(beanName, exposedObject, mbd);
           }
           catch (Throwable ex) {
           ...
           return exposedObject;
        }
          
          
           // ------------- 内部调用三级缓存操作 
           protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
                Assert.notNull(singletonFactory, "Singleton factory must not be null");
                synchronized (this.singletonObjects) {
                    if (!this.singletonObjects.containsKey(beanName)) {
                        if(beanName.equals("a")||beanName.equals("b")||beanName.equals("c"))
                            System.out.println("将"+beanName+"放入三级缓存");;
                        this.singletonFactories.put(beanName, singletonFactory);
                        this.earlySingletonObjects.remove(beanName);
                        this.registeredSingletons.add(beanName);
                    }
                }
            }       

菜瓜:demo比较简单,流程大致明白,源码我还需要斟酌一下,整体有了概念。这个流程好像是掺杂在bean的创建过程中,结合bean的生命周期整体理解可能会更深入一点

水稻:是的。每个知识点都不是单一的,拿着bean的生命周期再理解一遍可能会更有收获。

 

讨论

  • 为什么是三级缓存,两级不行吗?
    • 猜测:理论上两级也可以实现。多一个二级缓存可能是为了加快获取的速度。加入A依赖B,A依赖C,B依赖A,C依赖A,那么C在获取A的时候只需要从二级缓存中就能拿到A了

总结

  • Spring的处理方式和java处理的思想一致,构造器依赖本身是破坏语义和规范的
  • 属性赋值--> 依赖注入 。 先创建对象,再赋值属性,赋值的时候发现需要创建便生成依赖对象,被依赖对象需要前一个对象就从缓存容器中拿取即可

 

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