Powerful Java Hacking made easy - use good judgement!

A very powerful hack - use with caution!

Here's a very quick tutorial on how you can use Byteman to change a running application on the fly. You can insert arbitrary code into a running application server. Caveat CodeMonkeytor!

Let's say we have a JEE 6 application that's misbehaving. You can easily attach Byteman to the app server, then inject the code you want-- without bringing down the server!

I like to use 2 scripts with Byteman, one to attach to the server (since this should be done only once), and one to check and install the Byteman rules I want to run. This way, you can incrementally add to your Byteman rules.

There's an example of a misbehaving Servlet at the bottom of this post. It's bad code. Let's pick on a single fault. The user is asked to provide two numbers, the servlet is supposed to divide them and provide the result. But if the user enters a zero for the second argument, a divide by zero exception results! Please install the servlet and try for yourself.

Making JEE apps is trivial now that JEE 6 is here. I'd recommend using JBoss AS 7, and building with Maven. (I used to hate Maven, but since I've started using it, I hate it a little less. Still, it seems better than Ant in easy cases.) So please find the Maven pom.xml at the bottom of this post, with the Servlet code.

But this is a post about Byteman, so I'm going to put that code first. Here are the scripts:

# InstallByteman.sh - run this only once, after JBoss is already running. It doesn't matter if JBoss has been running 7 months or 7 seconds.

# There should be 4 lines to this script-- in case your browser line-wraps.....

#!/bin/bash

export JBOSS_HOME=/home/rick/Tools/JBoss/AS7/jboss-as-7.1.0.Beta1b

export BYTEMAN_HOME=/home/rick/Tools/Byteman/byteman-2.0

export BYTEMAN_BIN=$BYTEMAN_HOME/bin

$BYTEMAN_BIN/bminstall.sh -b -Dorg.jboss.byteman.transform.all $JBOSS_HOME/jboss-modules.jar

# InstallRules.sh - This script validates and installs your Rules. You can run this as many times as you need, to incrementally build your rules up.

# There should be 10 lines to this script-- in case your browser line-wraps.....

#!/bin/bash

export JBOSS_HOME=/home/rick/Tools/JBoss/AS7/jboss-as-7.1.0.Beta1b

export BYTEMAN_HOME=/home/rick/Tools/Byteman/byteman-2.0

export BYTEMAN_BIN=$BYTEMAN_HOME/bin

# Export this so Byteman can validate your classes. This is wherever you have compiled your classes to.

export APP_TGT=/home/rick/Blog_Temp/Byteman2/simpleappservlet30/target/classes

# check it

$BYTEMAN_BIN/bmcheck.sh -cp $APP_TGT/HackFix.btm

# add the rule

$BYTEMAN_BIN/bmsubmit.sh HackFix.btm

So we can see in the above Install script that we have a Byteman rule named 'HackFix.btm'. Here it is.

(BTW, what this script is doing is changing the second argument passed to the method 'makeDivision' into a 1 if it is a zero. To prevent divide-by-zero...)

RULE Hack Fix for Divide Servlet

CLASS com.flyingdog.SimpleServlet

METHOD makeDivision

AT ENTRY

IF 0 == $2

DO $2 = 1;

ENDRULE

--------------------------------------------------------------------------------------------

So that's all the Byteman stuff above. You can use those samples to attach to a running application server (ANY application server, not just JBoss) and inject whatever arbitrary code you want. Cool (and dangerous), huh?

Ok, if you're like me, you'd love a quick way to try that out. So here's the pom.xml for the SimpleServlet...

 <project xmlns="http://maven.apache.org/POM/4.0.0"  

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">

<modelVersion>4.0.0</modelVersion>

<groupId>com.flyingdog</groupId>

<artifactId>notsogoodapp</artifactId>

<packaging>war</packaging>

<version>1.0</version>

<name>notsogoodapp</name>

<url>http://maven.apache.org</url>

<build>

<finalName>notsogoodapp</finalName>

<plugins>

<plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-war-plugin</artifactId>

<version>2.1-beta-1</version>

<configuration>

<failOnMissingWebXml>false</failOnMissingWebXml>

</configuration>

</plugin>

<plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-compiler-plugin</artifactId>

<configuration>

<source>1.6</source>

<target>1.6</target>

</configuration>

</plugin>

</plugins>

</build>

<repositories>

<repository>

<id>java.net</id>

<url>http://download.java.net/maven/2</url>

</repository>

</repositories>

<dependencies>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<version>3.8.1</version>

<scope>test</scope>

</dependency>

<dependency>

<groupId>javax</groupId>

<artifactId>javaee-api</artifactId>

<version>6.0</version>

<scope>provided</scope>

</dependency>

</dependencies>

</project>

Now the bad Servlet.. Install it, have it divide a few numbers. Especially give it a zero to divide by, and notice the ugly blowup! Then use the above Byteman script to 'fix' the problem.

 package com.flyingdog;
import java.io.IOException;
import java.io.PrintWriter;
import java.lang.StringBuffer;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(urlPatterns = {"/simpleservlet", "*.foo"})
public class SimpleServlet extends HttpServlet {
@Override
protected void doPost(HttpServletRequest request,
HttpServletResponse response) {
doGet(request, response);
}
@Override
protected void doGet(HttpServletRequest request,
HttpServletResponse response) {
try {
response.setContentType("text/html");
PrintWriter printWriter = response.getWriter();
printWriter.println("<h2>");
printWriter.println("</h2>");
// if there is a value there, try to provide an answer
if (null != request.getParameter("firstValue")){
printWriter.println(makeAnswer(request));
}
// print the form that requests numbers
printWriter.println(makeForm());
} catch (IOException ioException) {
ioException.printStackTrace();
}
}
private String makeAnswer(HttpServletRequest request){
String first = request.getParameter("firstValue");
String second = request.getParameter("secondValue");
int iFirst = Integer.parseInt(first);
int iSecond = Integer.parseInt(second);
int answer = makeDivision(iFirst, iSecond);
String response = first + " divided by " + second + " equals " + answer;
return response;
}
private int makeDivision(int first, int second){
return first / second;
}
private String makeForm(){
StringBuffer sb = new StringBuffer();
sb.append("<form method=\"post\" action=\"simpleservlet\">\n");
sb.append("<table cellpadding=\"0\" cellspacing=\"0\" border=\"0\">\n");
sb.append(" <tr>\n");
sb.append("  <td>Please enter two numbers to divide:</td>\n");
sb.append("  <td><input type=\"text\" name=\"firstValue\" /></td>\n");
sb.append("  <td><input type=\"text\" name=\"secondValue\" /></td>\n");
sb.append(" </tr>\n");
sb.append(" <tr>\n");
sb.append("  <td></td>\n");
sb.append("  <td><input type=\"submit\" value=\"Submit\"></td>\n");
sb.append(" </tr>\n");
sb.append("</table>\n");
sb.append("</form>\n");
return sb.toString();
}
}

So there you have it. To recap:

1. Use the pom.xml and the Servlet code to make the .war
2. Deploy the .war to a JEE 6 app server, like JBoss AS 7.
3. Access the servlet at http://localhost:8080/notsogoodapp/simpleservlet
4. Observe how it divides two numbers. Let it try to divide by zero, see blowup.
5. Run the Byteman scripts, see how blowup stops.

Happy Hacking!