摘要：前置知識類的生命周期場景設(shè)計(jì)和推測情況在類中的初始化中實(shí)例化在類的初始化中實(shí)例化類設(shè)計(jì)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(shí)例變量之后再初始化塊中賦值為構(gòu)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(shí)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(shí)例化B

在類B的初始化中實(shí)例化A

類設(shè)計(jì)

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(shí)例變量a2=11(之后再初始化塊中賦值為12);

構(gòu)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(shí)例變量b2=21(之后再初始化塊中賦值為22);

構(gòu)造函數(shù);

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

打點(diǎn)位置:

類加載結(jié)束點(diǎn)(text: Loaded Main2 from file)

類初始化開始點(diǎn)/結(jié)束點(diǎn)(text: Class A2 init)

實(shí)例初始化開始點(diǎn)/結(jié)束點(diǎn)(text: Instance A2 init)

構(gòu)造函數(shù)結(jié)束點(diǎn)(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(jié)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(zhuǎn)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(shí)例化了(并且該階段正常結(jié)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(shí)例化階段的結(jié)束早于其類初始化階段,A類實(shí)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(yàn)證/準(zhǔn)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(jìn)行"或"完成",強(qiáng)調(diào)這點(diǎn)是因?yàn)檫@些階段通都是相互交叉混合式進(jìn)行的,通常會在一個階段執(zhí)行的過程中調(diào)用/激活另外一個階段
                                                          <<深入理解Java虛擬機(jī)>>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(jié)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(dǎo)致另一個類得到了意料之外的初始值

深入理解Java虛擬機(jī)