The document provides a comprehensive overview of Java agents and their integration with OSGi, including usage scenarios, implementation details, and challenges in integration testing. It covers both static and dynamic agents, class transformations, and various examples of agent applications such as monitoring and profiling. Additionally, it discusses the packaging requirements for agents and outlines the testing strategies needed to effectively use Java agents in OSGi environments.

1. Willitblend?JavaagentsandOSGi
RobertMunteanu,Adobe
Slides revision: 20200829-d098a06
APACHECON NA
Spt. 28th - Oct. 2nd 2020 1

2. Welcome
2

3. Aboutme
3

4. Outline
Quick demo
Java agents primer
Usage scenarios
OSGi integration
Integration testing
Testing demo
4

5. Quickdemo
5

6. Javaagentsprimer
6

7. JavainstrumentationAPIs
java.lang.instrument Javadoc, Java SE 8
Provides services that allow Java programming
language agents to instrument programs running
on the JVM.
7

8. Staticagents
# loaded at application startup
$ java -javaagent:agent.jar -jar app.jar
Premain-Class: org.example.my.Agent
import java.lang.instrument.*;
public class Agent {
public static void premain(String args,⏎
Instrumentation inst) {
inst.addTransformer(new ClassFileTransformer() {
/* implementation elided */
});
}
}
8

9. Dynamicagents
// dynamically attached to a running JVM
VirtualMachine vm = VirtualMachine.attach(vmPid);
vm.loadAgent(agentFilePath);
vm.detach();
Agent-Class: org.example.my.Agent
import java.lang.instrument.*;
public class Agent {
public static void agentmain(String args,⏎
Instrumentation inst) {
inst.addTransformer(new ClassFileTransformer() {
/* implementation elided */
});
}
}
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10. Classtransformation
public interface ClassFileTransformer {
byte[] transform(ClassLoader loader,
String className,
Class<?> classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer)
throws IllegalClassFormatException;
}
10

11. Javabytecode
public static void main(java.lang.String[]);
Code:
0: getstatic #16⏎
// Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #22⏎
// String Hello, world
5: invokevirtual #24⏎
// Method java/io/PrintStream.println:(Ljava/lang/String;
8: return
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12. Bytecodegenerationlibraries
Apache Commons BCEL
ByteBuddy
CGLib
Javassist
ObjectWeb ASM
12

13. BytecodegenerationwithJavassist
public byte[] transform(...) throws ... {
ClassPool classPool = ClassPool.getDefault();
CtMethod method = classPool.getMethod(⏎
Descriptor.toJavaName(className), "main");
method.insertAfter("System.out.println " + ⏎
"("... hello yourself!...");");
byte[] newClass = method.getDeclaringClass()⏎
.toBytecode();
method.getDeclaringClass().detach();
return newClass;
}
13

14. Usagescenarios
14

15. Whentouseagents
1. Code outside of your control
2. No better platform facilities exist
3. (Usually) Cross-cutting concerns
15

16. Agentexamples
1. Monitoring (logging, tracing, error reporting ...)
2. Profiling
3. Debugging
4. Mocking libraries
5. Code reload/Hot swap
16

17. OSGiintegration
17

18. Mindtheclassloader
- ClassPool defaultPool = ClassPool.getDefault();
- CtClass cc = defaultPool.get(⏎
- Descriptor.toJavaName(className));
+ ClassPool classPool = new ClassPool(true);
+ classPool.appendClassPath(new LoaderClassPath(loader));
+ classPool.insertClassPath(new ByteArrayClassPath(⏎
+ Descriptor.toJavaName(className), classfileBuffer));
18

19. Carefullymanagedependencies
New requirements typically fail since they are not
defined by the bundle
Bundles can be processed at build-time
Patching Import-Package
DynamicImport-Package: *
19

20. OSGialternatives-WeavingHooks
 Simplified deployment - OSGi bundle
 Simple registration via OSGi whiteboard
 Handles updated bundle package imports
 OSGi-only solution
20

21. OSGialternatives-WeavingHooks
@Component
public class SimpleWeavingHook implements WeavingHook {
public void weave(WovenClass wovenClass) {
/* checks, exception handling elided */
ClassPool cp = buildClassPool(wovenClass);
CtClass toTransform = cp.get(Descriptor.toJavaName(⏎
wovenClass.getClassName()));
CtMethod getMethod = toTransform.⏎
getDeclaredMethod("doGet");
getMethod.insertBefore(⏎
"System.out.println("doGet invoked");");
wovenClass.setBytes(toTransform.toBytecode());
}
}
21

22. Integrationtesting
22

23. Packagingchallenges
Java agents...
must be packaged as a Jar file, with a specific
manifest
not trivially attached to the current process
usually a one-way deal, no support for rolling
back class changes
23

24. Customtestlaunchers
"unit" tests
launch Java process with custom agents attached
require separate communication channel with
java agent
no out-of-the-box support for code coverage and
other tools
24

25. Bootstrappingthetest
// 1. which java?
String javaHome = System.getProperty("java.home");
Path javaExe = Paths.get(javaHome, "bin", "java");
// 2. which jar?
String ja = findAgentJar();
// 3. which classpath?
String classPath = buildClassPath();
// 4. launch
ProcessBuilder pb = new ProcessBuilder(
javaExe.toString(), "-javaagent:" + ja,
"-cp", classPath,
TestApplication.class.getName()
);
25

26. Verifyingsideeffects
Path stdout = Paths.get("target", "stdout.txt");
Path stderr = Paths.get("target", "stderr.txt");
pb.redirectInput(Redirect.INHERIT);
pb.redirectOutput(stdout.toFile());
pb.redirectError(stderr.toFile());
26

27. Addingcodecoverage
ProcessBuilder pb = new ProcessBuilder(
javaExe.toString(),
"-javaagent:" + codeCoverageAgent,
"-javaagent:" + ja,
"-cp",
classPath,
TestApplication.class.getName()
);
27

28. OSGiintegrationtesting
// note - must run in a forked container
@RunWith(PaxExam.class)
public class OsgiIT {
@Configuration
public Option[] config() throws IOException {
return options( junitBundles(), ⏎
vmOption("-javaagent:" + agentJar) );
}
@Test
public void callTimesOut() throws IOException {
assertTrue(agentReallyWorks());
}
}
28

29. Testingdemo
29

30. Resources
https://docs.oracle.com/javase/8/docs/api/java/lang/instrum
summary.html
https://www.javassist.org/
https://sling.apache.org/documentation/bundles/connectio
agent.html
30