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Instrument to remove (JavaForum Stuttgart 2024)

Instrument to remove (JavaForum Stuttgart 2024)

Johannes Bechberger

September 02, 2024
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  1. web!</artifactId> !</dependency> <dependency> <groupId>org.springframework.boot!</groupId> <artifactId>spring-boot-starter- validation!</artifactId> !</dependency> <dependency> <groupId>org.springframework.boot!</groupId> <artifactId>spring-boot-starter-

    thymeleaf!</artifactId> !</dependency> <dependency> <groupId>org.springframework.boot!</groupId> <artifactId>spring-boot-starter- test!</artifactId> <scope>test!</scope> !</dependency> !<!-- Databases - Uses H2 by default --> <dependency> <groupId>com.h2database!</groupId>
  2. 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
  3. So let’s write our own * as a proof of

    concept for a blog post *
  4. 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
  5. Static initializers in interfaces interface I { static { Store.getInstance().processClassUsage("I");

    } public void method(); } Forbidden in Java Allowed in Byte code
  6. Classes found in byte code Recorded classes Bloat classes Dynamically

    generated classes (e.g. for lambdas) Remove
  7. class UnusedClass { static { LOG.error("Class UnusedClass is used "

    + "which is not allowed"); } private int field; public void method() {...} }
  8. 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
  9. package java.lang; public final class String implements ... { static

    { Store.getInstance().processClassUsage("java.lang.String"); } @Stable private final byte[] value; ... public String() {...} ... }
  10. 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/
  11. Normally public static void main(String[] args) { ... } public

    static void main(String args[]) { ... } void main() { ... }
  12. Main Class public static void agentmain(String agentArgs) { ... }

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

    ... } public static void premain(String agentArgs, Instrumentation inst) { ... }
  14. 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
  15. 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
  16. ClassTransformer#transform byte[] transform( ClassLoader loader, String className, Class<?> klass, ProtectionDomain

    domain, byte[] classfileBuffer) throws IllegalClassFormatException { ... }
  17. 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)); } ?
  18. 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 ...
  19. @ExtendWith(MockitoExtension.class) public class MockitoTest { @Mock List<String> 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);
  20. public interface Collection<E> extends Iterable<E> { !// !!... default Stream<E>

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

    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; } !//!!... }
  22. var exprEditor = new ExprEditor() { @Override public void edit(MethodCall

    m) throws CannotCompileException { if (!isAddTransformerMethod(m)) { return; } m.replace( "me.bechberger.meta.runtime.InstrumentationHandler" + ".addTransformer($0, $1);"); } };
  23. 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); } }