摘要：前置知識類的生命周期場景設計和推測情況在類中的初始化中實例化在類的初始化中實例化類設計類靜態(tài)變量靜態(tài)變量之后在靜態(tài)初始化塊里賦值為實例變量之后再初始化塊中賦值為構造函數(shù)類靜態(tài)變量靜態(tài)變量之后在靜態(tài)初始化塊里賦值為實例變量之后再初始化塊中賦值

前置知識: 類的生命周期

情況:

在類A中的初始化中實例化B

在類B的初始化中實例化A

類設計

A類:

靜態(tài)變量a=new B();靜態(tài)變量a1=1(之后在靜態(tài)初始化塊里賦值為2);

實例變量a2=11(之后再初始化塊中賦值為12);

構造函數(shù);

B類:

靜態(tài)變量b=new A();靜態(tài)變量b1=3(之后在靜態(tài)初始化塊里賦值為4);

實例變量b2=21(之后再初始化塊中賦值為22);

構造函數(shù);

猜想執(zhí)行結果: 由于類初始化之后類實例化,所以A類初始化需要B實例化,B實例化又需要A初始化,造成循環(huán)依賴,最終結果為死鎖

打點位置:

類加載結束點(text: Loaded Main2 from file)

類初始化開始點/結束點(text: Class A2 init)

實例初始化開始點/結束點(text: Instance A2 init)

構造函數(shù)結束點(text: Instance A2 new)

class A2 {
    static {
        System.out.println("Class A2 init start");
    }

    static B2 a = new B2();
    static int a1 = 1;

    {
        System.out.println("Instance A2 init start. 	a=" + a + " 	a1=" + a1 + " 	a.b2=" + (a == null ? "NPE" : a.b2) + " 	b=" + B2.b + " 	b1=" + B2.b1 + " 	b.a2=" + (B2.b == null ? "NPE" : B2.b.a2));
    }

    public int a2 = 11;

    static {
        a1 = 2;
        System.out.println("Class A2 init end. 	a=" + a + " 	a1=" + a1 + " 	a.b2=" + (a == null ? "NPE" : a.b2) + " 	b=" + B2.b + " 	b1=" + B2.b1 + " 	b.a2=" + (B2.b == null ? "NPE" : B2.b.a2));
    }

    {
        a2 = 12;
        System.out.println("Instance A2 init end. 	a=" + a + " 	a1=" + a1 + " 	a.b2=" + (a == null ? "NPE" : a.b2) + " 	b=" + B2.b + " 	b1=" + B2.b1 + " 	b.a2=" + (B2.b == null ? "NPE" : B2.b.a2));
    }

    public A2() {
        System.out.println("Instance A2 new. 	a=" + a + " 	a1=" + a1 + " 	a.b2=" + (a == null ? "NPE" : a.b2) + " 	b=" + B2.b + " 	b1=" + B2.b1 + " 	b.a2=" + (B2.b == null ? "NPE" : B2.b.a2));
    }
}

class B2 {
    static {
        System.out.println("Class B2 init start");
    }

    static A2 b = new A2();
    static int b1 = 3;

    {
        System.out.println("Instance B2 init start. 	b=" + b + " 	b1=" + b1 + " 	b.a2=" + (b == null ? "NPE" : b.a2) + " 	a=" + A2.a + " 	a1=" + A2.a1 + " 	a.b2=" + (A2.a == null ? "NPE" : A2.a.b2));
    }

    public int b2 = 21;

    static {
        b1 = 4;
        System.out.println("Class B2 init end. 	b=" + b + " 	b1=" + b1 + " 	b.a2=" + (b == null ? "NPE" : b.a2) + " 	a=" + A2.a + " 	a1=" + A2.a1 + " 	a.b2=" + (A2.a == null ? "NPE" : A2.a.b2));
    }

    {
        b2 = 22;
        System.out.println("Instance B2 init end. 	b=" + b + " 	b1=" + b1 + " 	b.a2=" + (b == null ? "NPE" : b.a2) + " 	a=" + A2.a + " 	a1=" + A2.a1 + " 	a.b2=" + (A2.a == null ? "NPE" : A2.a.b2));
    }

    public B2() {
        System.out.println("Instance B2 new. 	b=" + b + " 	b1=" + b1 + " 	b.a2=" + (b == null ? "NPE" : b.a2) + " 	a=" + A2.a + " 	a1=" + A2.a1 + " 	a.b2=" + (A2.a == null ? "NPE" : A2.a.b2));
    }
}

class Main2 {
    static public void main(String... args) {
        System.out.println("A2 a=" + A2.a);
        System.out.println("A2 a1=" + A2.a1);
        System.out.println("A2 a2=" + B2.b.a2);
        System.out.println("B2 b=" + B2.b);
        System.out.println("B2 b1=" + B2.b1);
        System.out.println("B2 b2=" + A2.a.b2);
    }
}

程序輸出結果:

1. [Loaded Main2 from file:/Users/jiadongy/JVM_Learning_Sample/out/production/JVM_Learning_Sample/]
2. [Loaded A2 from file:/Users/jiadongy/JVM_Learning_Sample/out/production/JVM_Learning_Sample/]
3. Class A2 init start
4. [Loaded B2 from file:/Users/jiadongy/JVM_Learning_Sample/out/production/JVM_Learning_Sample/]
5. Class B2 init start
6. Instance A2 init start. a=null a1=0 a.b2=NPE b=null b1=0 b.a2=NPE
7. Instance A2 init end.      a=null a1=0 a.b2=NPE b=null b1=0 b.a2=NPE
8. Instance A2 new.         a=null a1=0 a.b2=NPE b=null b1=0 b.a2=NPE
9. Class B2 init end.              b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE
10. Instance B2 init start.       b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE
11. Instance B2 init end.          b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE
12. Instance B2 new.             b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE
13. Class A2 init end.      a=B2@610455d6 a1=2 a.b2=22 b=A2@61bbe9ba b1=4 b.a2=12
14. A2 a=B2@610455d6
15. A2 a1=2
16. A2 a2=12
17. B2 b=A2@61bbe9ba
18. B2 b1=4
19. B2 b2=22

把它轉化為下面的表格,更加清晰地描述A/B各個階段執(zhí)行的過程:

可以看到在A類初始化的過程中,A類被實例化了(并且該階段正常結束了),也就是說類的初始化階段并不是原子的/排他的.
如在本例中,A類實例化階段的結束早于其類初始化階段,A類實例化完成時,A類的靜態(tài)變量還未被初始化.

Reference.1中已經(jīng)描述了這種情況:

_加載/驗證/準備/初始化和卸載這5個階段的順序是確定的_,類的加載過程必須按照這種順序按部就班的開始...注意,這里筆者寫的是按部就班的"開始",而不是按部就班的"進行"或"完成",強調這點是因為這些階段通都是相互交叉混合式進行的,通常會在一個階段執(zhí)行的過程中調用/激活另外一個階段
                                                          <<深入理解Java虛擬機>>P210

類的循環(huán)初始化不會引起死鎖

5個階段的開始是有順序的,結束則不一定

階段不是排他的/臨界的

循環(huán)初始化可能引起意料之外的情況,盡量避免

eg.類在初始化過程中修改另一個類的變量,導致另一個類得到了意料之外的初始值

深入理解Java虛擬機