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Will it blend? Java agents and OSGi

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Java agents are a little-known but extremely powerful part of the Java ecosystem. Agents are able to transform existing classes at runtime, allowing scenarios such as logging and monitoring, hot reload or gathering code coverage. However, their usage presents a number of pitfalls as well.

In this talk we will present the steps of writing a java agent from scratch, indicate various common mistakes and pain points and draw conclusions on best practices. Special care will be taken to discuss how running in an OSGi environment affects Java agents and how we can best approach integration testing in a modular environment.

After this talk participants will have a better understanding of the Java instrumentation API, how it fits in with OSGi runtimes and about should / should not be done with it.

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Will it blend? Java agents and OSGi

  1. 1. Willitblend?JavaagentsandOSGi Slides revision: 20190923-ea7c311 1
  2. 2. Welcome 2
  3. 3. Aboutme 3
  4. 4. Outline Quick demo Java agents primer Usage scenarios OSGi integration Integration testing Testing demo 4
  5. 5. Quickdemo 5
  6. 6. Javaagentsprimer 6
  7. 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. 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. 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 */ }); } } 9
  10. 10. Classtransformation public interface ClassFileTransformer { byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException; } 10
  11. 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 11
  12. 12. Bytecodegenerationlibraries Apache Commons BCEL ByteBuddy CGLib Javassist ObjectWeb ASM 12
  13. 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. 14. Usagescenarios 14
  15. 15. Whentouseagents 1. Code outside of your control 2. No better platform facilities exist 3. (Usually) Cross-cutting concerns 15
  16. 16. Agentexamples 1. Monitoring (logging, tracing, error reporting ...) 2. Profiling 3. Debugging 4. Mocking libraries 5. Code reload/Hot swap 16
  17. 17. OSGiintegration 17
  18. 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. 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. 20. OSGialternatives-WeavingHooks  Simplified deployment - OSGi bundle  Simple registration via OSGi whiteboard  Handles updated bundle package imports  OSGi-only solution 20
  21. 21. Integrationtesting 21
  22. 22. 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 22
  23. 23. 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 23
  24. 24. 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() ); 24
  25. 25. 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()); 25
  26. 26. Addingcodecoverage ProcessBuilder pb = new ProcessBuilder( javaExe.toString(), "-javaagent:" + codeCoverageAgent, "-javaagent:" + ja, "-cp", classPath, TestApplication.class.getName() ); 26
  27. 27. 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()); } } 27
  28. 28. Testingdemo 28
  29. 29. 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 29
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