SpringBoot构造器注入循环依赖及解决

- 日理万妓 2024-04-17 15:11 70阅读 0赞

## 1. 循环依赖是什么？ ##

Bean A 依赖 B，Bean B 依赖 A这种情况下出现循环依赖。  
Bean A → Bean B → Bean A  
更复杂的间接依赖造成的循环依赖如下。  
Bean A → Bean B → Bean C → Bean D → Bean E → Bean A

## 2. 循环依赖会产生什么结果？ ##

当Spring正在加载所有Bean时，Spring尝试以能正常创建Bean的顺序去创建Bean。  
例如，有如下依赖:  
Bean A → Bean B → Bean C  
Spring先创建beanC，接着创建bean B（将C注入B中)，最后创建bean A(将B注入A中)。

但当存在循环依赖时，Spring将无法决定先创建哪个bean。这种情况下，Spring将产生异常BeanCurrentlyInCreationException。

当使用构造器注入时经常会发生循环依赖问题。如果使用其它类型的注入方式能够避免这种问题。

## 3. 构造器注入循环依赖实例 ##

首先定义两个相互通过构造器注入依赖的bean。

@Component
    public class CircularDependencyA {
     
        private CircularDependencyB circB;
     
        @Autowired
        public CircularDependencyA(CircularDependencyB circB) {
            this.circB = circB;
        }
    }

@Component
    public class CircularDependencyB {
     
        private CircularDependencyA circA;
     
        @Autowired
        public CircularDependencyB(CircularDependencyA circA) {
            this.circA = circA;
        }
    }

@Configuration
    @ComponentScan(basePackages = { "com.baeldung.circulardependency" })
    public class TestConfig {
    }

@RunWith(SpringJUnit4ClassRunner.class)
    @ContextConfiguration(classes = { TestConfig.class })
    public class CircularDependencyTest {
     
        @Test
        public void givenCircularDependency_whenConstructorInjection_thenItFails() {
            // Empty test; we just want the context to load
        }
    }

运行方法givenCircularDependency\_whenConstructorInjection\_thenItFails将会产生异常：**BeanCurrentlyInCreationException: Error creating bean with name ‘circularDependencyA’:  
Requested bean is currently in creation: Is there an unresolvable circular reference?**

## 4.解决方法 ##

处理这种问题目前有如下几种常见方式。

### 4.1 重新设计 ###

重新设计结构，消除循环依赖。

### 4.2 使用注解 @Lazy ###

一种最简单的消除循环依赖的方式是通过延迟加载。在注入依赖时，先注入代理对象，当首次使用时再创建对象完成注入。

@Component
    public class CircularDependencyA {
     
        private CircularDependencyB circB;
     
        @Autowired
        public CircularDependencyA(@Lazy CircularDependencyB circB) {
            this.circB = circB;
        }
    }

使用@Lazy后，运行代码，可以看到异常消除。

### 4.3 使用Setter/Field注入 ###

Spring文档建议的一种方式是使用setter注入。当依赖最终被使用时才进行注入。对前文的样例代码少做修改，来观察测试效果。

@Component
    public class CircularDependencyA {
     
        private CircularDependencyB circB;
     
        @Autowired
        public void setCircB(CircularDependencyB circB) {
            this.circB = circB;
        }
     
        public CircularDependencyB getCircB() {
            return circB;
        }
    }

@Component
    public class CircularDependencyB {
     
        private CircularDependencyA circA;
     
        private String message = "Hi!";
     
        @Autowired
        public void setCircA(CircularDependencyA circA) {
            this.circA = circA;
        }
     
        public String getMessage() {
            return message;
        }
    }

@RunWith(SpringJUnit4ClassRunner.class)
    @ContextConfiguration(classes = { TestConfig.class })
    public class CircularDependencyTest {
     
        @Autowired
        ApplicationContext context;
     
        @Bean
        public CircularDependencyA getCircularDependencyA() {
            return new CircularDependencyA();
        }
     
        @Bean
        public CircularDependencyB getCircularDependencyB() {
            return new CircularDependencyB();
        }
     
        @Test
        public void givenCircularDependency_whenSetterInjection_thenItWorks() {
            CircularDependencyA circA = context.getBean(CircularDependencyA.class);
    
            Assert.assertEquals("Hi!", circA.getCircB().getMessage());
        }
    }

### 4.4 使用@PostConstruct ###

@Component
    public class CircularDependencyA {
     
        @Autowired
        private CircularDependencyB circB;
     
        @PostConstruct
        public void init() {
            circB.setCircA(this);
        }
     
        public CircularDependencyB getCircB() {
            return circB;
        }
    }

@Component
    public class CircularDependencyB {
     
        private CircularDependencyA circA;
         
        private String message = "Hi!";
     
        public void setCircA(CircularDependencyA circA) {
            this.circA = circA;
        }
         
        public String getMessage() {
            return message;
        }
    }

### 4.5 实现ApplicationContextAware与InitializingBean ###

@Component
    public class CircularDependencyA implements ApplicationContextAware, InitializingBean {
     
        private CircularDependencyB circB;
     
        private ApplicationContext context;
     
        public CircularDependencyB getCircB() {
            return circB;
        }
     
        @Override
        public void afterPropertiesSet() throws Exception {
            circB = context.getBean(CircularDependencyB.class);
        }
     
        @Override
        public void setApplicationContext(final ApplicationContext ctx) throws BeansException {
            context = ctx;
        }
    }

# 5.总结 #

处理循环依赖有多种方式。首先考虑是否能够通过重新设计依赖来避免循环依赖。如果确实需要循环依赖，那么可以通过前文提到的方式来处理。优先建议使用setter注入来解决。