基于SpringBoot構(gòu)造器注入循環(huán)依賴(lài)及解決方式

更新時(shí)間：2020年04月26日 16:23:10 作者：FserSuN

這篇文章主要介紹了基于SpringBoot構(gòu)造器注入循環(huán)依賴(lài)及解決方式，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。一起跟隨小編過(guò)來(lái)看看吧

1. 循環(huán)依賴(lài)是什么？

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

Bean A → Bean B → Bean A

更復(fù)雜的間接依賴(lài)造成的循環(huán)依賴(lài)如下。

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

2. 循環(huán)依賴(lài)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(lài):

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(lài)時(shí)，Spring將無(wú)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(lài)問(wèn)題。如果使用其它類(lèi)型的注入方式能夠避免這種問(wèn)題。

3. 構(gòu)造器注入循環(huán)依賴(lài)實(shí)例

首先定義兩個(gè)相互通過(guò)構(gòu)造器注入依賴(lài)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(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(wèn)題目前有如下幾種常見(jiàn)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(lài)。

4.2 使用注解 @Lazy

一種最簡(jiǎn)單的消除循環(huán)依賴(lài)的方式是通過(guò)延遲加載。在注入依賴(lài)時(shí)，先注入代理對(duì)象，當(dāng)首次使用時(shí)再創(chuàng)建對(duì)象完成注入。

@Component
public 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(lài)最終被使用時(shí)才進(jìn)行注入。對(duì)前文的樣例代碼少做修改，來(lái)觀察測(cè)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(shí)現(xiàn)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(jié)

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

以上這篇基于SpringBoot構(gòu)造器注入循環(huán)依賴(lài)及解決方式就是小編分享給大家的全部?jī)?nèi)容了，希望能給大家一個(gè)參考，也希望大家多多支持腳本之家。

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