Spring循环依赖解决原理

循环依赖问题如果不解决, 会导致栈溢出异常, 循环圈内的实例无法创建

例如: 有3个类: A, B, C. 其中A依赖B, B依赖C, C依赖A.

public class A {
    private B b;
    public B getB () {
        return b;
    }
    public void setB (B b) {
        this.b = b;
    }
}

public class B {
    private C c;
    public C getC () {
        return c;
    }
    public void setC (C c) {
        this.c = c;
    }
}

public class C {
    private A a;
    public A getA () {
        return a;
    }
    public void setA (A a) {
        this.a = a;
    }
}
Util类定义了获取它们各自实例的getBean方法, 包含两步: 1. 获取被依赖的类的实例  2. 注入到当前实例

public class Util {
    public static A getBeanA(){
        A a = new A();
        B b = getBeanB();
        a.setB(b);
        return a;
    }
    private static B getBeanB () {
        B b = new B();
        C c = getBeanC();
        b.setC(c);
        return b;
    }
    private static C getBeanC () {
        C c = new C();
        A a = getBeanA();
        c.setA(a);
        return c;
    }
}

public class App {
    public static void main (String[] args) {
        A beanA = Util.getBeanA();
    }
}
当调用getBeanA时,就引发栈溢出异常,其根因是: 实例A时,发现需要实例B, 而实例B时发现需要实例C, 而实例C时又发现需要先实例A,可是A的实例还在等待B实例的完成, 彼此间像死锁一样缠住了.

解决方法: 提前暴露半成品的实例 : 在被需要时, 直接将这个半成品给需要者.

public class Util {

　　// 缓存半成品的实例
    public static Map<String, Object> earlyBeanMap = new HashMap<String, Object>(3);
    public static A getBeanA(){

　　　　　// 先从半成品中拿依赖的实例, 如果没有才新建
        if(earlyBeanMap.containsKey("a")){
            return (A)earlyBeanMap.get("a");
        }
        A a = new A();
        earlyBeanMap.put("a",a);
        B b = getBeanB();
        a.setB(b);
        return a;
    }
    private static B getBeanB () {
        if(earlyBeanMap.containsKey("b")){
            return (B)beanMap.get("b");
        }
        B b = new B();
        earlyBeanMap.put("b",b);
        C c = getBeanC();
        b.setC(c);
        return b;
    }
    private static C getBeanC () {
        if(earlyBeanMap.containsKey("c")){
            return (C)earlyBeanMap.get("c");
        }
        C c = new C();
        earlyBeanMap.put("c",c);
        A a = getBeanA();
        c.setA(a);
        return c;
    }
}