Spring循環依賴解決原理

循環依賴問題如果不解決, 會導致棧溢出異常, 循環圈內的實例無法創建

例如: 有3個類: A, B, C. 其中A依賴B, B依賴C, C依賴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類定義了獲取它們各自實例的getBean方法, 包含兩步: 1. 獲取被依賴的類的實例  2. 注入到當前實例

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();
    }
}
當調用getBeanA時,就引發棧溢出異常,其根因是: 實例A時,發現需要實例B, 而實例B時發現需要實例C, 而實例C時又發現需要先實例A,可是A的實例還在等待B實例的完成, 彼此間像死鎖一樣纏住了.

解決方法: 提前暴露半成品的實例 : 在被需要時, 直接將這個半成品給需要者.

public class Util {

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

　　　　　// 先從半成品中拿依賴的實例, 如果沒有才新建
        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;
    }
}