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。
