场景设计和推测

情况:
1. 在类A中的初始化中实例化B
2. 在类B的初始化中实例化A

类设计
- A类:
  - 静态变量a=new B();静态变量a1=1(之后在静态初始化块里赋值为2);
  - 实例变量a2=11(之后再初始化块中赋值为12);
  - 构造函数;
- B类:
  - 静态变量b=new A();静态变量b1=3(之后在静态初始化块里赋值为4);
  - 实例变量b2=21(之后再初始化块中赋值为22);
  - 构造函数;

猜想执行结果: 由于类初始化之后类实例化,所以A类初始化需要B实例化,B实例化又需要A初始化,造成循环依赖,最终结果为死锁

打点位置:
1. 类加载结束点(text: Loaded Main2 from file)
2. 类初始化开始点/结束点(text: Class A2 init)
3. 实例初始化开始点/结束点(text: Instance A2 init)
4. 构造函数结束点(text: Instance A2 new)

场景代码

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. \ta=" + a + " \ta1=" + a1 + " \ta.b2=" + (a == null ? "NPE" : a.b2) + " \tb=" + B2.b + " \tb1=" + B2.b1 + " \tb.a2=" + (B2.b == null ? "NPE" : B2.b.a2));    }    public int a2 = 11;    static {        a1 = 2;        System.out.println("Class A2 init end. \ta=" + a + " \ta1=" + a1 + " \ta.b2=" + (a == null ? "NPE" : a.b2) + " \tb=" + B2.b + " \tb1=" + B2.b1 + " \tb.a2=" + (B2.b == null ? "NPE" : B2.b.a2));    }    {        a2 = 12;        System.out.println("Instance A2 init end. \ta=" + a + " \ta1=" + a1 + " \ta.b2=" + (a == null ? "NPE" : a.b2) + " \tb=" + B2.b + " \tb1=" + B2.b1 + " \tb.a2=" + (B2.b == null ? "NPE" : B2.b.a2));    }    public A2() {        System.out.println("Instance A2 new. \ta=" + a + " \ta1=" + a1 + " \ta.b2=" + (a == null ? "NPE" : a.b2) + " \tb=" + B2.b + " \tb1=" + B2.b1 + " \tb.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. \tb=" + b + " \tb1=" + b1 + " \tb.a2=" + (b == null ? "NPE" : b.a2) + " \ta=" + A2.a + " \ta1=" + A2.a1 + " \ta.b2=" + (A2.a == null ? "NPE" : A2.a.b2));    }    public int b2 = 21;    static {        b1 = 4;        System.out.println("Class B2 init end. \tb=" + b + " \tb1=" + b1 + " \tb.a2=" + (b == null ? "NPE" : b.a2) + " \ta=" + A2.a + " \ta1=" + A2.a1 + " \ta.b2=" + (A2.a == null ? "NPE" : A2.a.b2));    }    {        b2 = 22;        System.out.println("Instance B2 init end. \tb=" + b + " \tb1=" + b1 + " \tb.a2=" + (b == null ? "NPE" : b.a2) + " \ta=" + A2.a + " \ta1=" + A2.a1 + " \ta.b2=" + (A2.a == null ? "NPE" : A2.a.b2));    }    public B2() {        System.out.println("Instance B2 new. \tb=" + b + " \tb1=" + b1 + " \tb.a2=" + (b == null ? "NPE" : b.a2) + " \ta=" + A2.a + " \ta1=" + A2.a1 + " \ta.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);    }}

执行结果分析

程序输出结果:

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 start4. [Loaded B2 from file:/Users/jiadongy/JVM_Learning_Sample/out/production/JVM_Learning_Sample/]5. Class B2 init start6. Instance A2 init start. a=null a1=0 a.b2=NPE b=null b1=0 b.a2=NPE7. Instance A2 init end.      a=null a1=0 a.b2=NPE b=null b1=0 b.a2=NPE8. Instance A2 new.         a=null a1=0 a.b2=NPE b=null b1=0 b.a2=NPE9. Class B2 init end.              b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE10. Instance B2 init start.       b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE11. Instance B2 init end.          b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE12. Instance B2 new.             b=A2@61bbe9ba b1=4 b.a2=12 a=null a1=0 a.b2=NPE13. Class A2 init end.      a=B2@610455d6 a1=2 a.b2=22 b=A2@61bbe9ba b1=4 b.a2=1214. A2 a=B2@610455d615. A2 a1=216. A2 a2=1217. B2 b=A2@61bbe9ba18. B2 b1=419. B2 b2=22

把它转化为下面的表格,更加清晰地描述A/B各个阶段执行的过程:

A	B
A类加载完成
A类初始化 - 开始
	B类加载完成
	B类初始化 - 开始
A类实例初始化 - 开始
A类实例初始化 - 结束
A类实例构造函数执行完成
	B类初始化 - 结束
	B类实例初始化 - 开始
	B类实例初始化 - 结束
	B类实例构造函数执行完成
A类初始化 - 结束

可以看到在A类初始化的过程中,A类被实例化了(并且该阶段正常结束了),也就是说类的初始化阶段并不是原子的/排他的.

如在本例中,A类实例化阶段的结束早于其类初始化阶段,A类实例化完成时,A类的静态变量还未被初始化.

Reference.1中已经描述了这种情况:

_加载/验证/准备/初始化和卸载这5个阶段的顺序是确定的_,类的加载过程必须按照这种顺序按部就班的开始...注意,这里笔者写的是按部就班的"开始",而不是按部就班的"进行"或"完成",强调这点是因为这些阶段通都是相互交叉混合式进行的,通常会在一个阶段执行的过程中调用/激活另外一个阶段
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     <深入理解java虚拟机>
      >P210
     