Java中的静态循环依赖
对于以下代码：
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
为什么？又如何呢？
PS：代码片段取自 JavaCamp.org
for the following code:
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);
}
}
the output is:
X = 1, Y = 2
Why? And How?
P.S: Code snippet taken from JavaCamp.org
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评论（3）
以下是按时间顺序发生的情况：
Class
B包含 main-method，因此它由类加载器加载。B的初始化引用A，因此加载A类。A有一个静态变量X，初始化为BY + 1。BY的初始化尚未执行，因此BY的计算结果为0，因此1被分配给AX现在 < code>A已完成加载，可以进行
BY的初始化。将
AX + 1（1 + 1）的值赋给BY。AX和BY的值分别打印为1和2。进一步阅读：
Java语言规范，§12.4.1 初始化发生时
Here is what happens in chronological order:
Class
Bcontains the main-method so it is loaded by the class loader.Initialization of
BreferencesA, so classAis loaded.Ahas a static variableXinitialized toB.Y + 1.The initialization of
B.Yhasn't been executed yet, soB.Yevaluates to 0, and thus 1 is assigned toA.XNow
Ahas finished loading, and the initialization ofB.Ycan take place.The value of
A.X + 1(1 + 1) is assigned toB.Y.The values of
A.XandB.Yare printed as1and2respectively.Further reading:
Java Language Specification, §12.4.1 When Initialization Occurs
这只是我的猜测：
B是因为它包含您请求执行的main。B需要A进行操作（它通过访问其静态字段来使用A）A需要类B，而该类恰好已经加载，但尚未初始化A不小心加载了BY（已初始化）默认情况下为 0），因为该类看起来已加载（语言设计缺陷？）AX = 0 + 1A现在已加载，类加载器可以继续加载并初始化BBY = 1 + 1。This is only my guess:
Bis loaded because it containsmainwhich you have requested to be executed.BrequiresAto operate (it usesAby accessing its static field)Bis loaded.Arequires classBwhich happens to be already loaded, but not yet initializedAcarelessly loadsB.Y(initialized to 0 by that time by default), since the class looks like loaded (language design flaw?)A.X = 0 + 1Ais now loaded, class loader can continue to load and initializeBB.Y = 1 + 1.B 是公开的，开始自行加载。
看到 AX = BY +1 == 是从 0 开始（默认 int 对象值），因此是 1；
初始化 B = 1 + 1 = 2；
因此就有了答案。
B being public starts to load itself.
Sees A.X = B.Y +1 == which is 0 to start with (default int object value) hence 1;
initializes B = 1 + 1 = 2;
hence the answer.