考虑这两种方法:
public static void forLoop(int start, int limit) {
for (int i = start; i < limit; i++) {
}
}
public static void whileLoop(int start, int limit) {
int i = start;
while (i < limit) {
i++;
}
}
编译后,它们会生成字节码(这是使用javap
的详细输出):
public static void forLoop(int, int);
descriptor: (II)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=2
0: iload_0
1: istore_2
2: iload_2
3: iload_1
4: if_icmpge 13
7: iinc 2, 1
10: goto 2
13: return
LineNumberTable:
line 6: 0
line 9: 13
LocalVariableTable:
Start Length Slot Name Signature
2 11 2 i I
0 14 0 start I
0 14 1 limit I
public static void whileLoop(int, int);
descriptor: (II)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=2
0: iload_0
1: istore_2
2: iload_2
3: iload_1
4: if_icmpge 13
7: iinc 2, 1
10: goto 2
13: return
LineNumberTable:
line 12: 0
line 13: 2
line 14: 7
line 16: 13
LocalVariableTable:
Start Length Slot Name Signature
0 14 0 start I
0 14 1 limit I
2 12 2 i I
正如您所看到的,这两种方法的代码部分完全相同。然而,当我使用JD反编译此类时,它会正确地生成:
public static void forLoop(int start, int limit) {
for (int i = start; i < limit; i++) {}
}
public static void whileLoop(int start, int limit)
{
int i = start;
while (i < limit) {
i++;
}
}
它是如何做到这一点的?这些方法的字节码完全相同!尽管每个方法的LineNumberTable
和LocalVariableTable
属性不同,但我不愿意相信这是原因,因为这些属性不是Code
属性必须包含的属性(参见Java语言规范第8版第4.7节)。