Instrument to remove (JavaForum Stuttgart 2024)

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Instrument to Remove Using Java agents for fun and profit Johannes Bechberger

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web! ! org.springframework.boot! spring-boot-starter- validation! ! org.springframework.boot! spring-boot-starter- thymeleaf! ! org.springframework.boot! spring-boot-starter- test! test! ! ! com.h2database!

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Do you need all?

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Probably not

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Software systems have a natural tendency to grow in size and complexity over time. A part of this growth comes with new features or bug fixes, while another part is due to useless code that accumulates over time. The problem of safely debloating real-world applications remains a long- standing software engineering endeavor today. — Soto-Valero et. al “ Source: https://arxiv.org/pdf/2008.08401.pdf

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How do we debloat?

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Debloating tools Dynamic Static Coverage Profiling Code Analysis Dependency Analysis ./gradlew lintGradle GraalVM native image

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You know the JVM? Could you help us debloat programs using agents or profilers?

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Existing tools have problems...

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So let’s write our own * as a proof of concept for a blog post *

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Reduce the Problem

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Find unused classes Source: https://arxiv.org/pdf/2008.08401.pdf

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Find unused classes class A { static { Store.getInstance().processClassUsage("A"); } private int field; public void method() {...} }

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When are static initializers called “ • T is a class and an instance of A is created. • A static method declared by A is invoked. • A static field declared by A is assigned. • A static field declared by A is used and the field is not a constant variable – JLS SE17 §12.4.2. Detailed Initialization Procedure

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Static initializers in interfaces interface I { static { Store.getInstance().processClassUsage("I"); } public void method(); } Forbidden in Java Allowed in Byte code

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Classes found in byte code Recorded classes Bloat classes Dynamically generated classes (e.g. for lambdas)

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What to do with this information?

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Classes found in byte code Recorded classes Bloat classes Dynamically generated classes (e.g. for lambdas) Remove

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class UnusedClass { static { LOG.error("Class UnusedClass is used " + "which is not allowed"); } private int field; public void method() {...} }

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And now for something complete different Source: Monty Python

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Implementation

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In come Java Agents

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They are essentially like bike computers https://commons.wikimedia.org/wiki/File:Cyclocomputer.jpg

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JVM Agent attach

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JVM Agent

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Structure JVM with Agent Instrumenter Used Classes Instrumenter JVM Instr. JAR Used Classes Reduced JAR JAR with Error Messages JAR Due to Spring classloader magic

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https://github.com/parttimenerd/dead-code-agent

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Agent Structure Main premain(args) ClassTF transform Store Class State start store

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package my.app; class A { static { Store.getInstance().processClassUsage("my.app.A"); } private int field; public void method() {...} } Only one problem...

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package java.lang; public final class String implements ... { static { Store.getInstance().processClassUsage("java.lang.String"); } @Stable private final byte[] value; ... public String() {...} ... }

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Class Loader Hierarchies platform app bootstrap X Y class reference delegates to

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Agent Structure Main premain(args) ClassTF transform Store Class State start store Runtime JAR

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Class Loader Hierarchies platform app bootstrap X Y class reference delegates to searches in runtime.jar https://mostlynerdless.de/blog/2023/06/02/class-loader-hierarchies/

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Implemented via inst.appendToBootstrapClassLoaderSearch(new JarFile( getExtractedJARPath().toFile())); instrumentation Instrumentation { void appendToBootstrapClassLoaderSearch(JarFile jarfile); }

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Normally public static void main(String[] args) { ... } public static void main(String args[]) { ... } void main() { ... }

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Main Class public static void agentmain(String agentArgs) { ... } JVM start later attach attach public static void premain(String agentArgs) { ... }

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Main Class public static void agentmain(String agentArgs, Instrumentation inst) { ... } public static void premain(String agentArgs, Instrumentation inst) { ... }

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Main Class public class Main { public static void premain(String agentArgs, Instrumentation inst) { AgentOptions options = new AgentOptions(agentArgs); ... inst.appendToBootstrapClassLoaderSearch( new JarFile(getExtractedJARPath().toFile())); ... inst.addTransformer(new ClassTransformer(options), true); } ... } Instrumentation.retransformClasses

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ClassTransformer extends ClassFileTransformer “ An agent registers an implementation of this interface using the addTransformer method so that the transformer’s transform method is invoked when classes are loaded, redefined, or retransformed – ClassFileTransformer Documentation

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ClassTransformer#transform byte[] transform( ClassLoader loader, String className, Class> klass, ProtectionDomain domain, byte[] classfileBuffer) throws IllegalClassFormatException { ... }

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ClassTransformer#transform if (className.startsWith("me/bechberger/runtime/Store") || className.startsWith("me/bechberger/ClassTransformer") || className.startsWith("java/") || className.startsWith("jdk/internal") || className.startsWith("sun/")) { return classfileBuffer; } How to actually transform the bytecode?

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https://www.javassist.org/ https://bytebuddy.net/#/

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ClassTransformer#transform private void transform(String className, CtClass cc) { String cn = formatClassName(className); Store.getInstance() .processClassLoad(cn,cc.getClassFile().getInterfaces()); cc.makeClassInitializer() .insertBefore( String.format("me.bechberger.runtime.Store.getInstance()" + ".processClassUsage(\"%s\");", cn)); } ?

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With Spring-PetClinic Class org.apache.tomcat.util.net.SocketBufferHandler is used which is not allowed Class org.apache.tomcat.util.net.SocketBufferHandler$1 is used which is not allowed Class org.apache.tomcat.util.net.NioChannel is used which is not allowed Class org.apache.tomcat.util.net.NioChannel$1 is used which is not allowed ... java -javaagent:./target/dead-code.jar=output=classes.txt \ -jar petclinic.jar u ch.qos.logback.classic.encoder.PatternLayoutEncoder l ch.qos.logback.classic.joran.JoranConfigurator u ch.qos.logback.classic.jul.JULHelper u ch.qos.logback.classic.jul.LevelChangePropagator ...

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Who has wrote an agent before?

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Who has used an agent before?

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Other applications of agents

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Main agentmain(args) premain(args) Profiler sample sleep Store start store Write your own profiler

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Who instruments the instrumenter?

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@ExtendWith(MockitoExtension.class) public class MockitoTest { @Mock List mockedList; @Test public void whenNotUseMockAnnotation_thenCorrect() throws InterruptedException { mockedList.add("one"); Mockito.verify(mockedList).add("one"); assertEquals(0, mockedList.size()); Mockito.when(mockedList.size()).thenReturn(100); assertEquals(100, mockedList.size()); } } Thread.sleep(10000000L);

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This uses ByteBuddy

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You know the JVM? Could you help us debloat programs using agents or profilers?

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Agent design process Choose prior agent(s) Copy agent code Modify code transformer Test and debug

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public interface Collection extends Iterable { !// !!... default Stream stream() { return StreamSupport.stream(this.spliterator(), false); } !//!!... }

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public interface Collection extends Iterable { !// !!... default Stream stream() { MockMethodDispatcher var1 = MockMethodDispatcher.get("APErU6j7", this); Callable var4 = var1 !!= null !&& var1.isMocked(this) !&& !var1.isOverridden(this, Collection.class.getMethod("stream")) ? var1.handle(this, Collection.class.getMethod("stream"), new Object[0]) : null; Stream var2 = var4 !!= null ? null : StreamSupport.stream(this.spliterator(), false); if (var4 !!= null) { var2 = (Stream)var4.call(); } return var2; } !//!!... }

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Instrumenting agent for instrumenting instrumenting agents

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private void transform(String className, CtClass cc) throws CannotCompileException { var exprEditor = 1!/* !.. !*/; for (CtConstructor constructor : cc.getDeclaredConstructors()) { constructor.instrument(exprEditor); } for (CtMethod method : cc.getDeclaredMethods()) { method.instrument(exprEditor); } }

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https://github.com/parttimenerd/meta-agent

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@parttimen3rd on Twitter parttimenerd on GitHub mostlynerdless.de @SweetSapMachine sapmachine.io