本文参考:《Reverse Engineering for Beginners》Dennis Yurichev著
异常
由之前月份处理修改的例子
//清单1IncorrectMonthException.java
public class IncorrectMonthException extends Exception { private int index; public IncorrectMonthException(int index) { this.index = index; } public int getIndex() { return index; } }
//清单2Month2.java
class Month2 { public static String[] months = { "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" }; public static String get_month(int i) throws IncorrectMonthException { if (i < 0 || i > 11) throw new IncorrectMonthException(i); return months[i]; }; public static void main(String[] args) { try { System.out.println(get_month(100)); } catch (IncorrectMonthException e) { System.out.println("incorrect month index: " + e.getIndex()); e.printStackTrace(); } }; }
编译
javac Month2.java
只写一个Month2.java,这里是IncorrectMonthException.java也会自动编译
反编译
javap -c -verbose Month2.class javap -c -verbose IncorrectMonthException.class
IncorrectMonthException方法
public IncorrectMonthException(int); descriptor: (I)V flags: ACC_PUBLIC Code: stack=2, locals=2, args_size=2 0: aload_0 1: invokespecial #1 // Method java/lang/Exception."<init>":()V 4: aload_0 5: iload_1 6: putfield #2 // Field index:I 9: return
本质上,IncorrectMonthException类只是做了对象构造,还有访问器方法。IncorrectMonthException类是继承于Exception类,所以,IncorrectMonthException类构造之前,构造父类Exception,然后传递整数给IncorrectMonthException类作为唯一的属性值
getIndex方法
public int getIndex(); descriptor: ()I flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #2 // Field index:I 4: ireturn
getIndex()只是一个访问器,引用到IncorrectMonthException类,被传到局部变量的索引值0的位置(this指针),用aload_0指令取得, 用getfield指令取得对象的整数值,用ireturn指令将其返回。
再看Month2.class的get_month方法
public static java.lang.String get_month(int) throws IncorrectMonthException; descriptor: (I)Ljava/lang/String; flags: ACC_PUBLIC, ACC_STATIC Code: stack=3, locals=1, args_size=1 0: iload_0 1: iflt 10 4: iload_0 5: bipush 11 7: if_icmple 19 10: new #2 // class IncorrectMonthException 13: dup 14: iload_0 15: invokespecial #3 // Method IncorrectMonthException."<init>":(I)V 18: athrow 19: getstatic #4 // Field months:[Ljava/lang/String; 22: iload_0 23: aaload 24: areturn
部分指令解释
0: iload_0 //第0个变量(即变量i)压入操作数栈
1: iflt 10 //当栈顶int型数值小于0时跳转到偏移块10(这里是从栈顶弹出一个值进行比较)
4: iload_0 //第0个变量(即变量i)压入操作数栈
5: bipush 11 //将11压入操作数栈
7: if_icmple 19 //会从栈中弹出两个值进行比较,如果第二个(变量i)小于或者等于第一个(数字11),那么跳转到偏移位19
10: new #2 // class IncorrectMonthException //创建IncorrectMonthException对象的引用并压入栈顶
13: dup //复制栈顶值压入栈顶
14: iload_0 //第0个变量(即变量i)压入操作数栈
15: invokespecial #3 // Method IncorrectMonthException."<init>":(I)V //调用IncorrectMonthException的"<init>"方法,这里需要从栈顶弹出两个值,1是IncorrectMonthException对象的引用,2是变量i
18: athrow //将栈顶的异常抛出
19: getstatic #4 // Field months:[Ljava/lang/String; //获取静态成员months数组的引用压入栈
22: iload_0 //第0个变量(即变量i)压入操作数栈
23: aaload //栈顶弹出两个值,将引用型数组指定索引的值推送至栈顶,这里索引i为第一个弹出值,数组引用months为第二个弹出值,即months[i]的值送入栈顶
24: areturn //返回栈顶值
Month2.class的main方法
public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=3, locals=2, args_size=1 0: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 3: bipush 100 5: invokestatic #6 // Method get_month:(I)Ljava/lang/String; 8: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 11: goto 47 14: astore_1 15: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream; 18: new #8 // class java/lang/StringBuilder 21: dup 22: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V 25: ldc #10 // String incorrect month index: 27: invokevirtual #11 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 30: aload_1 31: invokevirtual #12 // Method IncorrectMonthException.getIndex:()I 34: invokevirtual #13 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder; 37: invokevirtual #14 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 40: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 43: aload_1 44: invokevirtual #15 // Method IncorrectMonthException.printStackTrace:()V 47: return Exception table: from to target type 0 11 14 Class IncorrectMonthException
注意Exception table:文字以及下面的表数据
异常表的第一行数据表示从0到11行若发生异常,则跳转到偏移块14的位置,同时也指明异常的类型为IncorrectMonthException类
异常从偏移块14开始执行,如果没有这个异常表的数据,则偏移块14后的指令不会被执行
这有个例子,IDA是如何显示异常表:
原作者用的自己电脑里的random.class,这里用刚才的Month2.class替代
.catch IncorrectMonthException from met003_begin to met003_11 using met003_14
原文地址:http://blog.51cto.com/7317859/2105935