Will it blend? Java agents and OSGi Slides revision: 20190923-ea7c311 1 Welcome 2 About me 3 Outline Quick demo Java agents primer Usage scenarios OSGi integration Integration testing Testing demo 4 Quick demo 5 Java agents primer 6 Java instrumentation APIs Provides services that allow Java programming language agents to instrument programs running on the JVM. java.lang.instrument Javadoc, Java SE 8 7 Static agents # 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 Dynamic agents // 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 Class transformation public interface ClassFileTransformer { byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException; } 10 Java bytecode 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 Bytecode generation libraries Apache Commons BCEL ByteBuddy CGLib Javassist ObjectWeb ASM 12 Bytecode generation with Javassist 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 Usage scenarios 14 When to use agents 1. Code outside of your control 2. No better platform facilities exist 3. (Usually) Cross-cutting concerns 15 Agent examples 1. Monitoring (logging, tracing, error reporting ...) 2. Profiling 3. Debugging 4. Mocking libraries 5. Code reload/Hot swap 16 OSGi integration 17 Mind the classloader - 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 Carefully manage dependencies 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 OSGi alternatives - Weaving Hooks Simplified deployment - OSGi bundle Simple registration via OSGi whiteboard Handles updated bundle package imports OSGi-only solution 20 Integration testing 21 Packaging challenges 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 Custom test launchers "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 Bootstrapping the test // 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 Verifying side effects 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 Adding code coverage ProcessBuilder pb = new ProcessBuilder( javaExe.toString(), "-javaagent:" + codeCoverageAgent, "-javaagent:" + ja, "-cp", classPath, TestApplication.class.getName() ); 26 OSGi integration testing // 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 Testing demo 28 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 30.