Spring解决循环依赖问题的四种方法汇总_Java

spring解决循环依赖问题的四种方法

@lazy方式
使用setter/field injection
使用@postconstruct
实现 applicationcontextaware 和 initializingbean

不管使用那种方式，最佳的还是通过调整代码结构，从根上设计从而达到避免。

定义两个相互依赖的 bean（通过构造函数注入）

package icu.kevin.springpart.circular.constructor;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.stereotype.component;
@component
public class circulardependencya {
    private circulardependencyb circb;
    @autowired
    public circulardependencya(circulardependencyb circb) {
        this.circb = circb;
    }
}

package icu.kevin.springpart.circular.constructor;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.stereotype.component;
@component
public class circulardependencyb {
    private circulardependencya circa;
    @autowired
    public circulardependencyb(circulardependencya circa) {
        this.circa = circa;
    }
}

package icu.kevin.springpart.circular.constructor;
import org.springframework.context.annotation.componentscan;
import org.springframework.context.annotation.configuration;
@configuration
@componentscan(basepackages = { "icu.kevin.springpart.circular.constructor" })
public class testconfig {
}

编写测试用例，测试是否存在循环依赖

package icu.kevin.springpart.circular.constructor;
import org.junit.test;
import org.junit.runner.runwith;
import org.springframework.test.context.contextconfiguration;
import org.springframework.test.context.junit4.springjunit4classrunner;
@runwith(springjunit4classrunner.class)
@contextconfiguration(classes = { testconfig.class })
public class circulardependencyintegrationtest {
    // error creating bean with name 'circulardependencya': requested bean is currently in creation: is there an unresolvable circular reference?
    @test
    public void givencirculardependency_whenconstructorinjection_thenitfails() {
        // 测试可以为空，因为在上下文加载期间将检测到循环依赖关系
    }
}

@lazy方式

@lazy告诉 spring 延迟初始化其中一个 bean。因此，它不会完全初始化 bean，而是创建一个代理将其注入另一个 bean。注入的 bean 只有在第一次需要时才会完全创建。

@lazy 注解是通过建立一个中间代理层，来破解循环依赖的。

package icu.kevin.springpart.circular.lazy;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.context.annotation.lazy;
import org.springframework.stereotype.component;
@component
public class circulardependencya {
    private circulardependencyb circb;
    @autowired
    public circulardependencya(@lazy circulardependencyb circb) {
        this.circb = circb;
    }
}

package icu.kevin.springpart.circular.lazy;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.stereotype.component;
@component
public class circulardependencyb {
    private circulardependencya circa;
    @autowired
    public circulardependencyb(circulardependencya circa) {
        this.circa = circa;
    }
}

package icu.kevin.springpart.circular.lazy;
import org.springframework.context.annotation.componentscan;
import org.springframework.context.annotation.configuration;
@configuration
@componentscan(basepackages = { "icu.kevin.springpart.circular.lazy" })
public class testconfig {
}

package icu.kevin.springpart.circular.lazy;
import org.junit.test;
import org.junit.runner.runwith;
import org.springframework.test.context.contextconfiguration;
import org.springframework.test.context.junit4.springjunit4classrunner;
@runwith(springjunit4classrunner.class)
@contextconfiguration(classes = { testconfig.class })
public class circulardependencyintegrationtest {
    // error creating bean with name 'circulardependencya': requested bean is currently in creation: is there an unresolvable circular reference?
    @test
    public void givencirculardependency_whenconstructorinjection_thenitfails() {
        // 测试可以为空，因为在上下文加载期间将检测到循环依赖关系
    }
}

启动可以看出，在上下文检查循环依赖是可以正常通过的

查看源码contextannotationautowirecandidateresolver#getlazyresolutionproxyifnecessary首先会调用 islazy 去判断一下是否需要延迟加载，
如果需要，则调用 buildlazyresolutionproxy 方法构建一个延迟加载的对象；

使用setter/field injection

在setter注入的时候发生的依赖循环是可以被解决的，但只能解决单例情况的循环依赖。
对于setter注入造成的依赖是通过spring容器提前暴露刚刚完成构造器但还没有进行注入的bean来完成的。通过提前暴露一个单例工厂让其他的bean可以引用到该bean。

package icu.kevin.springpart.circular.setter;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.stereotype.component;
@component
public class circulardependencya {
    private circulardependencyb circb;
    public circulardependencyb getcircb() {
        return circb;
    }
    @autowired
    public void setcircb(circulardependencyb circb) {
        this.circb = circb;
    }
}

package icu.kevin.springpart.circular.setter;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.stereotype.component;
@component
public class circulardependencyb {
    private circulardependencya circa;
    private string message = "hi!";
    public circulardependencya getcirca() {
        return circa;
    }
    @autowired
    public void setcirca(circulardependencya circa) {
        this.circa = circa;
    }
    public string getmessage() {
        return message;
    }
    public void setmessage(string message) {
        this.message = message;
    }
}

package icu.kevin.springpart.circular.setter;
import org.springframework.context.annotation.componentscan;
import org.springframework.context.annotation.configuration;
@configuration
@componentscan(basepackages = { "icu.kevin.springpart.circular.setter" })
public class testconfig {
}

package icu.kevin.springpart.circular.setter;
import org.junit.assert;
import org.junit.test;
import org.junit.runner.runwith;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.context.applicationcontext;
import org.springframework.test.context.contextconfiguration;
import org.springframework.test.context.junit4.springjunit4classrunner;
@runwith(springjunit4classrunner.class)
@contextconfiguration(classes = { testconfig.class })
public class circulardependencyintegrationtest {
    @autowired
    private applicationcontext context;
    @test
    public void givencirculardependency_whenconstructorinjection_thenitfails() {
        // empty test; we just want the context to load
        circulardependencya circa = context.getbean(circulardependencya.class);
        assert.assertequals("hi!", circa.getcircb().getmessage());
    }
}

使用@postconstruct

package icu.kevin.springpart.circular.postconstruct;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.stereotype.component;
import javax.annotation.postconstruct;
@component
public class circulardependencya {
    @autowired
    private circulardependencyb circb;
    @postconstruct
    public void init(){
        circb.setcirca(this);
    }
    public circulardependencyb getcircb() {
        return circb;
    }
}

package icu.kevin.springpart.circular.postconstruct;
import org.springframework.stereotype.component;
@component
public class circulardependencyb {
    private circulardependencya circa;
    private string message = "hi!";
    public circulardependencya getcirca() {
        return circa;
    }
    public void setcirca(circulardependencya circa) {
        this.circa = circa;
    }
    public string getmessage() {
        return message;
    }
    public void setmessage(string message) {
        this.message = message;
    }
}

package icu.kevin.springpart.circular.postconstruct;
import org.springframework.context.annotation.componentscan;
import org.springframework.context.annotation.configuration;
@configuration
@componentscan(basepackages = { "icu.kevin.springpart.circular.postconstruct" })
public class testconfig {
}

package icu.kevin.springpart.circular.postconstruct;
import org.junit.assert;
import org.junit.test;
import org.junit.runner.runwith;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.context.applicationcontext;
import org.springframework.test.context.contextconfiguration;
import org.springframework.test.context.junit4.springjunit4classrunner;
@runwith(springjunit4classrunner.class)
@contextconfiguration(classes = { testconfig.class })
public class circulardependencyintegrationtest {
    @autowired
    private applicationcontext context;
    @test
    public void givencirculardependency_whenconstructorinjection_thenitfails() {
        //
        circulardependencya circa = context.getbean(circulardependencya.class);
        assert.assertequals("hi!", circa.getcircb().getmessage());
    }
}

实现 applicationcontextaware 和 initializingbean

如果其中一个 bean 实现了 applicationcontextaware，则该 bean 可以访问 spring 上下文，并可以从那里提取另一个 bean。

通过实现 initializingbean，我们指示该 bean 在设置完所有属性后必须执行一些操作。在这种情况下，我们要手动设置依赖项。

package icu.kevin.springpart.circular.initializingbean;
import org.springframework.beans.beansexception;
import org.springframework.beans.factory.initializingbean;
import org.springframework.context.applicationcontext;
import org.springframework.context.applicationcontextaware;
import org.springframework.stereotype.component;
@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;
    }
}

package icu.kevin.springpart.circular.initializingbean;
import org.springframework.stereotype.component;
@component
public class circulardependencyb {
    private circulardependencya circa;
    private string message = "hi!";
    public circulardependencya getcirca() {
        return circa;
    }
    public void setcirca(circulardependencya circa) {
        this.circa = circa;
    }
    public string getmessage() {
        return message;
    }
    public void setmessage(string message) {
        this.message = message;
    }
}

package icu.kevin.springpart.circular.initializingbean;
import org.springframework.context.annotation.componentscan;
import org.springframework.context.annotation.configuration;
@configuration
@componentscan(basepackages = { "icu.kevin.springpart.circular.initializingbean" })
public class testconfig {
}

package icu.kevin.springpart.circular.initializingbean;
import org.junit.assert;
import org.junit.test;
import org.junit.runner.runwith;
import org.springframework.beans.factory.annotation.autowired;
import org.springframework.context.applicationcontext;
import org.springframework.test.context.contextconfiguration;
import org.springframework.test.context.junit4.springjunit4classrunner;
@runwith(springjunit4classrunner.class)
@contextconfiguration(classes = { testconfig.class })
public class circulardependencyintegrationtest {
    @autowired
    private applicationcontext context;
    @test
    public void givencirculardependency_whenconstructorinjection_thenitfails() {
        // empty test; we just want the context to load
        circulardependencya circa = context.getbean(circulardependencya.class);
        assert.assertequals("hi!", circa.getcircb().getmessage());
    }
}

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