基於SpringBoot構造器注入循環依賴及解決方式

1. 循環依賴是什麼？

Bean A 依賴 B，Bean B 依賴 A這種情況下出現循環依賴。

Bean A → Bean B → Bean A

更復雜的間接依賴造成的循環依賴如下。

Bean A → Bean B → Bean C → Bean D → Bean E → Bean A

2. 循環依賴會產生什麼結果？

當Spring正在加載所有Bean時，Spring嘗試以能正常創建Bean的順序去創建Bean。

例如，有如下依賴:

Bean A → Bean B → Bean C

Spring先創建beanC，接著創建bean B（將C注入B中)，最後創建bean A(將B注入A中)。

但當存在循環依賴時，Spring將無法決定先創建哪個bean。這種情況下，Spring將產生異常BeanCurrentlyInCreationException。

當使用構造器注入時經常會發生循環依賴問題。如果使用其它類型的注入方式能夠避免這種問題。

3. 構造器注入循環依賴實例

首先定義兩個相互通過構造器注入依賴的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 } }

運行方法givenCircularDependency_whenConstructorInjection_thenItFails將會產生異常：

BeanCurrentlyInCreationException: Error creating bean with name ‘circularDependencyA': Requested bean is currently in creation: Is there an unresolvable circular reference?

4.解決方法

處理這種問題目前有如下幾種常見方式。

4.1 重新設計

重新設計結構，消除循環依賴。

4.2 使用註解 @Lazy

一種最簡單的消除循環依賴的方式是通過延遲加載。在注入依賴時，先注入代理對象，當首次使用時再創建對象完成注入。

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

使用@Lazy後，運行代碼，可以看到異常消除。

4.3 使用Setter/Field注入

Spring文檔建議的一種方式是使用setter注入。當依賴最終被使用時才進行注入。對前文的樣例代碼少做修改，來觀察測試效果。

@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 實現ApplicationContextAware與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.總結

處理循環依賴有多種方式。首先考慮是否能夠通過重新設計依賴來避免循環依賴。如果確實需要循環依賴，那麼可以通過前文提到的方式來處理。優先建議使用setter注入來解決。