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"htmlcode">

@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;
 }
}

@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;
 }
}

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。

以上这篇基于SpringBoot构造器注入循环依赖及解决方式就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持。