Brown Bag talk I gave on 1st March 2012

1. Byteman - Carving up your Java code
Chris Sinjakli

2. A tracing/debugging tool
Runtime manipulation of Java code

3. Java Agent API
Muck around with the bytecode to
your heart’s content

4. Java Agent API
package java.lang.instrument;
interface ClassFileTransformer
{
byte[] transform(ClassLoader loader, String
className, Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer);
}

5. Who does this?
• About a million tutorials
– Usually print the class name
– At best adding simple trace code to methods
• Newer, cooler mocking frameworks
– Powermock, Mockito, JMockit
– Better approach then JMock & friends (which use
Proxy + CGLIB)
– Mock final classes/methods, plus other stuff
• Pure Java profilers?

6. Not that convenient
Who wants to spend their time
dealing with bytecode?

7. Byteman
Muck around with the Java code to
your heart’s content

8. How?
• Specify byteman.jar as a Java Agent when you
start your app
• Provide a Byteman rule script
• Can provide these scripts later (to a running
app) – app must be started with Byteman
listener

9. Simple program
package org.my
class AppMain
{
public static void main(String[] args)
{
for (int i = 0; i < args.length; i++) {
System.out.println(args[i]);
}
}
}

10. Run it
> java org.my.AppMain foo bar baz
foo
bar
baz
>

11. Byteman rule script
RULE trace main entry
CLASS AppMain
METHOD main
AT ENTRY
IF true
DO traceln("entering main")
ENDRULE

12. Byteman rule script
RULE trace main exit
CLASS AppMain
METHOD main
AT EXIT
IF true
DO traceln("exiting main")
ENDRULE

13. Run it
> java -
javaagent:byteman.jar=script:appmain.btm org.m
y.AppMain foo bar baz
entering main
foo
bar
baz
exiting main
>

14. Worth mentioning
• You can use it on classes you don’t have the
source for
• You can even use this on core Java classes
(java.lang) – Byteman makes you confirm
• Works on interfaces too (all implementing classes
get your injected code)
• Much more sophisticated rule scripts
– Different “AT” points in method
– “IF” statements that do something
– Access to local variables (eg “AFTER WRITE $foo”)

15. Testing

16. Testing support
• Comes with a JUnit Runner
• Define the rules (from earlier scripts) as test
annotations
• Useful for fault injection (integration tests
where you’re not mocking everything out?)

17. Testing support
@Test
@BMRule(name=“foo”,
targetClass = “FileInputStream”,
targetMethod=“<init>(File)”,
condition=“$1.getName().equals(“doesnt_exist.txt”)”,
action=“throw new FileNotFoundException(“It’s not there!”)”)
public void testReadFile() {
...
myClass.processFile(“doesnt_exist.txt”); // Throws an exception
...
}

18. Testing support
• Rule in previous example will only be in place
for that test
– Can annotate class instead to apply to all tests
• There’s also @BMScript to import external
rule scripts
• Relatively simple to run with maven (config
samples on website)

19. Summary
• Seems cool
• Not sure what I’d actually use it for
• Integration tests most likely candidate

20. Questions?