2.54 循环依赖

1. 循环依赖是什么

假设有两个Bean，Bean1和Bean2，如果Bean1依赖Bean2，Bean2依赖Bean1。 那么Spring创建Bean1的时候需要先创建Bean2，创建Bean2又需要先创建Bean1，这就叫做循环依赖

2. 循环依赖会导致什么问题

2.1. 构造器注入会抛出异常

• TestController

@Service
public class TestController
{
    private TestService testService;

    @Autowired
    public TestController(TestService testService)
    {
        this.testService = testService;
    }

    public void test()
    {
        String byId = testService.getById(1);
        System.out.println(byId);
    }
}

• TestService

@Service
public class TestService
{
    private TestController testController;

    @Autowired
    public TestService(TestController testController)
    {
        this.testController = testController;
    }

    public String getById(Integer id)
    {
        return "" + id;
    }
}

启动后会抛出异常：

***************************
APPLICATION FAILED TO START
***************************

Description:

The dependencies of some of the beans in the application context form a cycle:

┌─────┐
|  testController defined in file [C:\Users\[user]\code\test\target\classes\com\zsk\test\controller\TestController.class]
↑     ↓
|  testService defined in file [C:\Users\[user]\code\test\target\classes\com\zsk\test\service\TestService.class]
└─────┘



Process finished with exit code 1

3. 如何解决循环依赖的问题

3.1. 属性/setter方法注入

• TestController

@Service
public class TestController
{
    @Autowired//依赖TestService
    private TestService testService;

    public void test()
    {
        String byId = testService.getById(1);
        System.out.println(byId);
    }
}

• TestService

@Service
public class TestService
{
    @Autowired//依赖TestController
    private TestController testController;

    public String getById(Integer id)
    {
        return "" + id;
    }
}

3.1.1. 原理分析

Spring有三个步骤，

1. createBeanInstance：实例化，其实也就是调用对象的构造方法实例化对象
2. populateBean：填充属性，这一步主要是多bean的依赖属性进行填充
3. initializeBean：调用spring xml中的init 方法。一个创建实例，另一个对属性赋值（属性的注入就是在这个步骤进行）。

用上面的例子说明，Spring创建TestController的时候：

• createBeanInstance创建TestController实例，放到map缓存中
• populateBean对TestController的属性TestService赋值的时候，发现没有TestService实例，因此先要创建TestService实例
  • createBeanInstance创建TestService实例
  • populateBean对TestService的属性TestController赋值的时候，容器中已经有了TestController实例，那么从map缓存中取出来赋值
• 继续回到populateBean对TestController的属性TestService赋值即可

3.2. 使用@PostConstruct

3.3. 实现ApplicationContextAware与InitializingBean

3.4. 在构造函数上都加上@Lazy注解

• TestController

@Service
public class TestController
{
    private TestService testService;

    @Lazy
    @Autowired
    public TestController(TestService testService)
    {
        this.testService = testService;
    }

    public void test()
    {
        String byId = testService.getById(1);
        System.out.println(byId);
    }

• TestService

@Service
public class TestService
{
    private TestController testController;

    @Autowired
    @Lazy
    public TestService(TestController testController)
    {
        this.testController = testController;
    }

    public String getById(Integer id)
    {
        return "" + id;
    }
}

4. 参考

• SpringBoot构造器注入循环依赖及解决_Revivedsun的专栏-CSDN博客
• spring是如何解决循环依赖的？ - 掘金
• 一文说透 Spring 循环依赖问题 - 知乎