1. 循環(huán)依賴是什么？

Bean A 依賴 B，Bean B 依賴 A這種情況下出現(xiàn)循環(huán)依賴。

Bean A → Bean B → Bean A

更復(fù)雜的間接依賴造成的循環(huán)依賴如下。

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

2. 循環(huán)依賴會(huì)產(chǎn)生什么結(jié)果？

當(dāng)Spring正在加載所有Bean時(shí)，Spring嘗試以能正常創(chuàng)建Bean的順序去創(chuàng)建Bean。

例如，有如下依賴:

Bean A → Bean B → Bean C

Spring先創(chuàng)建beanC，接著創(chuàng)建bean B（將C注入B中)，最后創(chuàng)建bean A(將B注入A中)。

但當(dāng)存在循環(huán)依賴時(shí)，Spring將無法決定先創(chuàng)建哪個(gè)bean。這種情況下，Spring將產(chǎn)生異常BeanCurrentlyInCreationException。

當(dāng)使用構(gòu)造器注入時(shí)經(jīng)常會(huì)發(fā)生循環(huán)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(gòu)造器注入循環(huán)依賴實(shí)例

首先定義兩個(gè)相互通過構(gòu)造器注入依賴的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 }}

運(yùn)行方法givenCircularDependency_whenConstructorInjection_thenItFails將會(huì)產(chǎn)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(shè)計(jì)

重新設(shè)計(jì)結(jié)構(gòu)，消除循環(huán)依賴。

4.2 使用注解 @Lazy

一種最簡單的消除循環(huán)依賴的方式是通過延遲加載。在注入依賴時(shí)，先注入代理對象，當(dāng)首次使用時(shí)再創(chuàng)建對象完成注入。

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

使用@Lazy后，運(yùn)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(dāng)依賴最終被使用時(shí)才進(jìn)行注入。對前文的樣例代碼少做修改，來觀察測試效果。

@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 實(shí)現(xiàn)ApplicationContextAware與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.總結(jié)

處理循環(huán)依賴有多種方式。首先考慮是否能夠通過重新設(shè)計(jì)依賴來避免循環(huán)依賴。如果確實(shí)需要循環(huán)依賴，那么可以通過前文提到的方式來處理。優(yōu)先建議使用setter注入來解決。

以上這篇基于SpringBoot構(gòu)造器注入循環(huán)依賴及解決方式就是小編分享給大家的全部內(nèi)容了，希望能給大家一個(gè)參考，也希望大家多多支持好吧啦網(wǎng)。