循环引用

问题的产生

• 代码出现两个bean在创建时都相互引用了

  @Component
  public class A {
      private B b;
  
      @Autowired
      public A(B b){
          this.b = b;
      }
  }
  
  @Component
  public class B {
      private A a;
  
      @Autowired
      public B(A a){
          this.a = a;
      }
  }
  
  public static void main(String[] args) {
          ConfigurableApplicationContext run = SpringApplication.run(Springboot.class, args);
  
          Object a = run.getBean("a");
      }

• 我们在向容器获取A对象是出错了

  The dependencies of some of the beans in the application context form a cycle:
  
  ┌─────┐
  |  a defined in file [D:\download\SpringBoot\springboot_demo05\target\classes\com\panther\springboot\mycompoent\A.class]
  ↑     ↓
  |  b defined in file [D:\download\SpringBoot\springboot_demo05\target\classes\com\panther\springboot\mycompoent\B.class]
  └─────┘
  
  
  Action:
  
  Relying upon circular references is discouraged and they are prohibited by default. Update your application to remove the dependency cycle between beans. As a last resort, it may be possible to break the cycle automatically by setting spring.main.allow-circular-references to true.

  解决办法

  方案一

  自己创建对象，不使用IOC自动注入

  public class A {
      private B b;
      public A(){
  
      }
  
      public A(B b){
          this.b = b;
      }
  
      public void setB(B b) {
          this.b = b;
      }
  
      public B getB() {
          return b;
      }
  }
  
  public class B {
      private A a;
  
      public B(){
  
      }
  
      public B(A a){
          this.a = a;
      }
  
      public void setA(A a) {
          this.a = a;
      }
  
      public A getA() {
          return a;
      }
  }
  
  private static final Logger logger = Logger.getLogger("logger");
  
      public static void main(String[] args) {
          SpringApplication.run(Springboot.class, args);
  
  //        Object a = run.getBean("a");
          A a = new A();
          B b = new B();
  
          b.setA(a);
          a.setB(b);
          logger.info(String.valueOf(a.getB() == b)); //true
          logger.info(String.valueOf(b.getA() == a)); //true
          logger.info(String.valueOf(a.getB()); //com.panther.test@B5947
          logger.info(String.valueOf(b.getA()); //com.panther.test@A5945
      }

  DEBUG分析

  
• 发现在new a和b时 属性都是空的
  
• 向下继续运行时，好像还是产生了循环依赖，但为什么a.getB()能输出B的实体类
• 有点懵逼。。。

  百度的拼凑版本解释： jvm在new一个对象时，并不会创建这个对象，只是向内存申请了一段空间，属于半初始化的实体类。

  方案二

  Spring框架提供的解决办法

  @Component
  public class A {
      private B b;
  
      @Autowired
      public void setB(B b) {
          this.b = b;
      }
  }
  
  @Component
  public class B {
      private A a;
  
      @Autowired
      public void setA(A a) {
          this.a = a;
      }
  }
  
  public static void main(String[] args) {
          ConfigurableApplicationContext run = SpringApplication.run(Springboot.class, args);
  
          A a = run.getBean(A.class);
          B b = run.getBean(B.class);
  
          logger.info(String.valueOf(a.getB() == b)); //true
          logger.info(String.valueOf(b.getA() == a)); //true
          logger.info(b.getA());//com.panther.springboot.test.A@27e7c77f
          logger.info(a.getB());//com.panther.springboot.test.B@6f70a21b
      }
      
      

• 开启循环引用配置

  spring.main.allow-circular-references: true

• spring 引入三级缓存去解决循环引用

  //一级缓存
      private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
  
  // 三级缓存 map中存放早期半初始化的的bean，并提前暴露
      private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
  
  //二级缓存
      private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

• 可以发现b实体类里的A属性是一个代理对象

  if (specificInterceptors != DO_NOT_PROXY) {
              this.advisedBeans.put(cacheKey, Boolean.TRUE);
              Object proxy = createProxy(
                      bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
              this.proxyTypes.put(cacheKey, proxy.getClass());
              return proxy;
          }

  方案三

  自己写一个代理去解决

  @Component
  @Aspect
  public class MyAspectJ {
  
      @Before("execution(* com.panther.springboot.mycompoent.*.*(..))")
      public void before(){
          System.out.println("---------");
      }
  }