Java中的静态循环依赖 [英] Static Circular Dependency in Java

问题描述

代码如下：

class A
{
    public static int X;
    static { X = B.Y + 1;}
}
public class B
{
    public static int Y = A.X + 1;
    static {}
    public static void main(String[] args) {
        System.out.println("X = "+A.X+", Y = "+B.Y);
    }
}

输出为：
X = 1 ，Y = 2

the output is: X = 1, Y = 2

为什么？和如何???

Why? and How???

-Ivar

PS：代码片段来自JavaCamp.org

P.S: Code snippet taken from JavaCamp.org

推荐答案

这是什么按时间顺序发生：

Here is what happens in chronological order:

1. Class B 包含main方法所以它由类加载器加载。

1. Class B contains the main-method so it is loaded by the class loader.

初始化 B 引用一个，所以加载了<$ p $ c> A 类。

Initialization of B references A, so class A is loaded.

A 有一个静态变量 X 初始化为 BY + 1 。

A has a static variable X initialized to B.Y + 1.

BY 的初始化尚未执行，因此 BY 评估为0，因此1分配给 AX

The initialization of B.Y hasn't been executed yet, so B.Y evaluates to 0, and thus 1 is assigned to A.X

现在 已完成加载，并且可以进行 BY 的初始化。

Now A has finished loading, and the initialization of B.Y can take place.

值 AX + 1 （1 + 1）分配给 BY 。

The value of A.X + 1 (1 + 1) is assigned to B.Y.

AX 和 BY 的值打印为 1 并且分别为 2 。

The values of A.X and B.Y are printed as 1 and 2 respectively.

进一步阅读：

Java语言规范，§12.4.1何时发生初始化

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