Java bytecode manipulation (help!)
I would want to do the following but do not know which tool is able to provide the facilities.
1) A class with 2 methods with instructions inside e.g. a=b+c; c=2+b; a=5+c;
2) Randomly choose one of the instructions from each method
3) Swap the 2 instructions
4) Create a new class that contains the 2 new methods (with one of their instructions swapped)
Is objectweb's asm able to achieve this? Or cglib would be easier? I am still researching the feasibility of implementing such cases.
Thanks in advance!
Stan
Hi, I agree it is a good model for building expressions (tree like structure), but in this case there will be alot of instances being created thus consumes alot of memory for genetic programming. This is why I wanted to go into explore the ways of manupilating at the bytecode level (the idea a class is a generation, its methods are the individuals). I wanted to operate without having to consume too much memory. But of course, to do this there is a high trade off in terms of extensibility.
Nevertheless thanks for the reply, now I had learned it is also possible to do it using Composite.
Recently I had managed to find out how to swap the instruction codes using asm. Next is to see whether I can group the bytecode instructions as one expression in the source level e.g.
at source level
a = 3.5 + b
at bytecode level
LDC 3.5
DLOAD 2
DADD
DSTORE 1
and do the swaping by group. It seems like there is no easy way on this if I still want to enforce to build a lightweight engine.
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Edited by: orsvon on Sep 10, 2010 5:02 AMThanks for your useful link!
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Error:
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I have heard rumors of a way to add a comment block inside java bytecode. Can anyone point me to a reference that spells this out more clearly?
Thanks for the help Owen.
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http://www.eecs.umich.edu/~bchandra/publications/phd.pdf
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I'm sure they're talking about java source code, not
byte code.
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code instructions.
What you "might" be able to do (but only you could
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Hi guys,
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i am having a slight problem as i am constructing a method --> menu() which handles displaying
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i am having issues with switch statement which processes the return from menu() methd,,,
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here is my code...
import java.text.*;
import java.io.*;
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for(;;)
/* Switch statement for menu manipulation */
switch(menu())
case A: System.out.println("You have selected option A");
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break;
case D: System.out.println("You have selected option D");
break;
case S: System.out.println("You have selected option S");
break;
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exit(0);
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char option;
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System.out.println("\n\n");
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System.out.println("\t\t| |");
System.out.println("\t\t| Student Manager Menu |");
System.out.println("\t\t|_______________________|");
System.out.println("\n\t________________________________________");
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System.out.println("\t| Add new student\t\t A \t|");
System.out.println("\t| Add credits\t\t\t B \t|");
System.out.println("\t| Display one record\t\t C \t|");
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System.out.println("\t| \t\t\t\t\t|");
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System.out.println("\t| Quit\t\t\t\t Q \t|");
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khalidHi,
There are few changes which u need to make for making ur code work.
1) In main method, in switch case change case A: to case 'A':
Characters should be represented in single quotes.
2) in case 'Q' change exit(0) to System.exit(0);
3) The method static char menu() { should be changed to static char menu() throws IOException {
4) Change option = stdin.readLine(); to
option = (char)stdin.read();
Then compile and run ur code. This will work.
Or else just copy the below code
import java.text.*;
import java.io.*;
public class StudentDriver{
public static void main(String[] args) throws IOException {
for(;;) {
/* Switch statement for menu manipulation */
switch(menu()) {
case 'A': System.out.println("You have selected option A");
break;
case 'B': System.out.println("You have selected option B");
break;
case 'C': System.out.println("You have selected option C");
break;
case 'D': System.out.println("You have selected option D");
break;
case 'S': System.out.println("You have selected option S");
break;
case 'Q':
System.out.println("\n\n\n\n\n\n\t\t Thank you for using our system..." +
"\n\n\t\t\t Good Bye \n\n\n\n");
System.exit(0);
static char menu() throws IOException {
char option;
BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
System.out.println("\n\n");
System.out.println("\n\t\t_________________________");
System.out.println("\t\t| |");
System.out.println("\t\t| Student Manager Menu |");
System.out.println("\t\t|_______________________|");
System.out.println("\n\t________________________________________");
System.out.println("\t| \t\t\t\t\t|");
System.out.println("\t| Add new student\t\t A \t|");
System.out.println("\t| Add credits\t\t\t B \t|");
System.out.println("\t| Display one record\t\t C \t|");
System.out.println("\t| Show the average credits\t D \t|");
System.out.println("\t| \t\t\t\t\t|");
System.out.println("\t| Save the changes\t\t S \t|");
System.out.println("\t| Quit\t\t\t\t Q \t|");
System.out.println("\t|_______________________________________|\n");
System.out.print("\t Your Choice: ");
option = (char)stdin.read();
return option;
regards
Karthik -
Hi,
I more or less wrote a small binary instrumentation toolkit which is needed for special purposes. I create a class with two methods at runtime. First method is the default constructor ( needed for instance creation ), and single, not static, method containing whatever.
The bytecode of the second method was cutted out from another class. All needed references in the bytecode are fixed correctly. I pass an additional parameter when i call the method. This parameter is the this-instance of the original method. My idea was to replace all
opcodes which are using the this reference by the additional passed reference of the original this-instance.
For not beeing forced to change all related this-instance opcodes, i thought maybe it is possible to overwrite the this instance like that:
0 wide
1 aload 5
4 wide
5 astore 0What this code does is it pushed the reference of the 6th element of the method stack and stores it in the first slot on the stack, which is reserved for the this instance if the method is a non-static one.
Currently i get following error when i try to call the method:
"Exception in thread "main" java.lang.VerifyError: (class: OHP1239475266A, method: OHP1239475266AM signature: (ILjava/lang/String;D[Ljava/lang/Integer;Lmuh/AClassWithAMethod;)Ljava/lang/Boolean;) Incompatible type for getting or setting field"
Any idea whether this error could be connected the my technique to overwrite the this instance of a method ? Maybe this is considered to be an illegal technique.
Thank you for your help,
bgnahmhttp://jakarta.apache.org/bcel/
The Byte Code Engineering Library is intended to give users a convenient possibility to analyze, create, and manipulate (binary) Java class files (those ending with .class). Classes are represented by objects which contain all the symbolic information of the given class: methods, fields and byte code instructions, in particular.
Such objects can be read from an existing file, be transformed by a program (e.g. a class loader at run-time) and dumped to a file again. An even more interesting application is the creation of classes from scratch at run-time. The Byte Code Engineering Library (BCEL) may be also useful if you want to learn about the Java Virtual Machine (JVM) and the format of Java .class files.
BCEL contains a byte code verifier named JustIce, which usually gives you much better information about what's wrong with your code than the standard JVM message.
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