基於SpringBoot構造器注入迴圈依賴及解決方式
1. 迴圈依賴是什麼?
Bean A 依賴 B,Bean B 依賴 A這種情況下出現迴圈依賴。
Bean A → Bean B → Bean A
更復雜的間接依賴造成的迴圈依賴如下。
Bean A → Bean B → Bean C → Bean D → Bean E → Bean A
2. 迴圈依賴會產生什麼結果?
當Spring正在載入所有Bean時,Spring嘗試以能正常建立Bean的順序去建立Bean。
例如,有如下依賴:
Bean A → Bean B → Bean C
Spring先建立beanC,接著建立bean B(將C注入B中),最後建立bean A(將B注入A中)。
但當存在迴圈依賴時,Spring將無法決定先建立哪個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迴圈依賴問題。如果使用其它型別的注入方式能夠避免這種問題。
3. 構造器注入迴圈依賴例項
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 } }
執行方法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,消除迴圈依賴。
4.2 使用註解 @Lazy
一種最簡單的消除迴圈依賴的方式是通過延遲載入。在注入依賴時,先注入代理物件,當首次使用時再建立物件完成注入。
@Component public class CircularDependencyA { private CircularDependencyB circB; @Autowired public CircularDependencyA(@Lazy CircularDependencyB circB) { this.circB = circB; } }
使用@Lazy後,執行程式碼,可以看到異常消除。
4.3 使用Setter/Field注入
Spring文件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。對前文的樣例程式碼少做修改,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
處理迴圈依賴有多種方式。首先考慮是否能夠通過重新設計依賴來避免迴圈依賴。如果確實需要迴圈依賴,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。
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