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 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 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 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

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