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基于SpringBoot构造器注入循环依赖及解决方式

2020-04-27 12:04 2136 查看

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。

@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
}
}

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。

@Component
public 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。

@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 实现ApplicationContextAware与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.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。

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