java - java字节码中的局部变量

Question

我正在尝试学习 Java 字节码，我偶然发现了这个:我使用 -g 选项编译了这段非常简单的代码:

public class Test
{
 public static void main(String args[])
 {
  double a = 1.0;
  int b = (int)a;
 }
}

主要代码原来是:

0 dconst_1
1 dstore_1
2 dload_1
3 d2i
4 istore_3
5 return

另外，main的最大局部变量是4个，而LocalVariableTable只有3个条目(args,a,b)。我很想知道为什么编译器保留了 4 个局部变量，而显然只有 3 个，并且 localvariable[2] 没有被使用。有什么我想念的吗？谢谢

Answer 1

是4是因为long和double在frame中占据了2个连续的entry。 Java JVM specification 明确指定了这一点:

A value of type long or type double occupies two consecutive local variables. Such a value may only be addressed using the lesser index. For example, a value of type double stored in the local variable array at index n actually occupies the local variables with indices n and n +1; however, the local variable at index n +1 cannot be loaded from. It can be stored into. However, doing so invalidates the contents of local variable n.