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。

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

@Componentpublic 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。

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

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

@Componentpublic 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; }}@Componentpublic 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。