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CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project Overview

BTrace is a safe, dynamic tracing tool for the Java platform that instruments running Java applications to inject tracing code ("bytecode tracing"). It's similar to DTrace for OpenSolaris but designed specifically for Java.

Build Commands

Primary Build

# Build entire project with distribution packages
./gradlew :btrace-dist:build

# Output locations:
# - Distributions: btrace-dist/build/distributions/ (*.tar.gz, *.zip, *.rpm, *.deb)
# - Exploded binary (BTRACE_HOME): btrace-dist/build/resources/main/

Testing

# Run all tests
./gradlew test

# Run tests for specific module
./gradlew :btrace-instr:test
./gradlew :btrace-compiler:test

# Run specific test class
./gradlew :btrace-instr:test --tests "*CompilerTest"

# Run specific test method
./gradlew :btrace-instr:test --tests "org.openjdk.btrace.compiler.CompilerTest.testSimple"

# Integration tests (requires dist built first)
./gradlew :btrace-dist:build
./gradlew :integration-tests:test -Pintegration

Golden Files (Test Data)

Some instrumentation tests use golden files to verify bytecode correctness. When you intentionally change instrumented bytecode:

# Regenerate all golden files
./gradlew test -PupdateTestData

# Then commit the updated files in btrace-instr/src/test/resources/instrumentorTestData/

Code Formatting

# Check formatting (runs during build)
./gradlew spotlessCheck

# Auto-format all code (Google Java Format)
./gradlew spotlessApply

Module-Specific Build

# Build single module
./gradlew :<module-name>:build

# Build without tests (faster)
./gradlew :<module-name>:build -x test

Architecture Overview

BTrace follows a clear separation between compile-time safety verification and runtime instrumentation:

Core Flow: Script → Compilation → Instrumentation → Execution

BTrace Script (.java)
    ↓
[btrace-compiler] Compile & verify safety
    ↓
Compiled Script (.class)
    ↓
[btrace-agent] Load into target JVM
    ↓
[btrace-instr] Transform target classes
    ↓
[btrace-runtime] Execute probe code

Module Responsibilities

btrace-compiler - Script compilation and safety verification

  • Entry: Compiler.java - JSR 199 compilation pipeline
  • Safety: Verifier.java - Enforces BTrace restrictions (no loops, no allocations, no exceptions, no field assignment)
  • Post-processing: Postprocessor.java - Bytecode transformations after compilation
  • Location: btrace-compiler/src/main/java/org/openjdk/btrace/compiler/

btrace-agent - Java agent for attaching to target JVMs

  • Entry: Main.java - Implements premain() and agentmain() for instrumentation
  • Client management: Client.java, RemoteClient.java, FileClient.java
  • Starts server on port 2020 by default to accept client connections
  • Location: btrace-agent/src/main/java/org/openjdk/btrace/agent/

btrace-instr - Bytecode instrumentation using ASM

  • Transformer: BTraceTransformer.java - ClassFileTransformer implementation
  • Probe factory: BTraceProbeFactory.java - Creates BTraceProbe instances
  • Instrumentor: Instrumentor.java - Main ASM-based bytecode injection logic
  • 15 specialized instrumentors for different probe types (method entry/exit, field access, allocation, etc.)
  • Location: btrace-instr/src/main/java/org/openjdk/btrace/instr/

btrace-runtime - Runtime services for BTrace scripts

  • Factory: BTraceRuntimes.java - Loads version-specific implementations (JDK 8, 9, 11, 15+)
  • Base: BTraceRuntimeImplBase.java - Core runtime services (print, profiling, JMX, etc.)
  • Multi-version support: Separate implementations in src/main/java9/, src/main/java11/, src/main/java15/
  • Location: btrace-runtime/src/main/java/org/openjdk/btrace/runtime/

btrace-client - Command-line client tool

  • Main: Main.java - CLI for connecting to agent and submitting scripts
  • Client: Client.java - Manages socket connection to agent
  • Location: btrace-client/src/main/java/org/openjdk/btrace/client/

btrace-core - Core abstractions and communication

  • Wire protocols: WireProtocol.java - V1 (Java serialization) and V2 (binary, 3-6x faster)
  • Commands: 20+ command types in comm/ package for agent-client communication
  • Annotations: @BTrace, @OnMethod, @OnTimer, etc. in annotations/ package
  • Location: btrace-core/src/main/java/org/openjdk/btrace/core/

Key Architectural Patterns

Safety-First Design: Scripts are verified at compile-time to prevent:

  • Loops and recursion (no infinite loops in instrumented code)
  • Object allocation (minimize GC pressure on target app)
  • Exception throwing (don't crash target app)
  • Field assignment (maintain immutability)

Instrumentation Pipeline: When a class loads:

  1. BTraceTransformer.transform() checks registered probes
  2. For matching classes, Instrumentor creates ASM visitor chain
  3. Each specialized instrumentor handles specific probe types (entry, exit, call, field access, etc.)
  4. Modified bytecode with injected probe calls is returned

Multi-Version Runtime: The runtime has version-specific implementations to handle JDK differences:

  • Base in src/main/java/ (Java 8)
  • JDK 9+ features in src/main/java9/
  • JDK 11+ features in src/main/java11/
  • JDK 15+ features in src/main/java15/

Communication: Agent and client communicate via pluggable WireProtocol:

  • V1: Java Object Serialization (backward compatible)
  • V2: Custom binary protocol (higher performance, 2-5x smaller payloads)
  • Auto-negotiation ensures compatibility

Code Conventions

Source Compatibility: Java 8 for main code, compiled with Java 11 toolchain

  • Set in common.gradle: sourceCompatibility = 8, targetCompatibility = 8
  • Uses Java 11 compiler for better optimization while maintaining Java 8 compatibility

Formatting: Google Java Format via Spotless

  • Import order: java, javax, org.openjdk.btrace, *, then static imports
  • Always run ./gradlew spotlessApply before committing

Testing:

  • Unit tests in src/test/java/ named *Test.java
  • BTrace instrumentation scripts in src/test/btrace/ (compiled with btracec)
  • Golden files in src/test/resources/instrumentorTestData/ for bytecode verification
  • Integration tests require full distribution build first

Important Implementation Details

BTrace Script Restrictions (enforced by Verifier.java):

  • No new keyword (object allocation)
  • No loops (for, while, do-while)
  • No throw statements
  • No field assignment (can read fields, but not write)
  • Only whitelisted method calls allowed (BTraceUtils.*, etc.)
  • Scripts must be annotated with @BTrace

Instrumentation Probe Types (@OnMethod locations):

  • Kind.ENTRY - Method entry
  • Kind.RETURN - Method return (normal)
  • Kind.ERROR - Method return (exception)
  • Kind.CALL - Before method call
  • Kind.LINE - Specific line number
  • Kind.FIELD_GET / FIELD_SET - Field access
  • Kind.ARRAY_GET / ARRAY_SET - Array access
  • Kind.NEW - Object/array allocation
  • Plus more (see Location.java)

ClassFileTransformer Thread Safety: BTraceTransformer uses ReentrantReadWriteLock because:

  • Multiple threads can load classes concurrently
  • Probe registration (write) vs. class transformation (read) must be coordinated

Performance Considerations:

  • Filter-based class matching to avoid unnecessary transforms
  • Class metadata caching (ClassCache.java)
  • Binary protocol V2 for reduced serialization overhead
  • Async command queue with configurable backoff

Module Dependencies

btrace-core (annotations, wire protocol, commands)
    ↓
btrace-compiler, btrace-instr, btrace-runtime
    ↓
btrace-agent (orchestrates everything)
    ↓
btrace-client, btrace-dist

Working with BTrace Scripts

BTrace scripts are Java files with special annotations. They're compiled separately from regular Java code:

Compilation: Use btracec tool or Gradle tasks

# In tests
./gradlew :btrace-instr:compileTestProbes
./gradlew :integration-tests:compileTestProbes

# In benchmarks
./gradlew :benchmarks:agent-benchmark:btracec

Example Script Structure:

@BTrace
public class MyTrace {
    @OnMethod(clazz="java.io.FileInputStream", method="<init>")
    public static void onFileOpen(@ProbeClassName String className, String fileName) {
        BTraceUtils.println("File opened: " + fileName);
    }
}

Debugging Tips

Enable Debug Output: BTrace has built-in debug support via DebugSupport.java

  • Set system properties or use agent debug flags
  • Check Main.java for available agent arguments

Test Isolation: Each test should be independent

  • Tests run with cleanTest dependency (always run, never cached)
  • Integration tests require fresh distribution build
  • BTrace agent defaults to port 2020; ManifestLibsTests uses port 2022 to avoid contention
  • Port is configured via btrace.port system property, passed to spawned BTrace processes via -Dbtrace.port and -p CLI flag

Bytecode Verification: If instrumentation tests fail:

  1. Check if bytecode generation changed
  2. Regenerate golden files with -PupdateTestData
  3. Verify the changes are intentional
  4. Commit updated golden files

ClassLoader Issues: BTrace uses multiple classloaders:

  • Bootstrap classpath for agent core (bootstrap section of masked btrace.jar)
  • System classpath for client tools
  • Target application classloaders remain isolated
  • See Main.java for bootstrap setup

Contributing Notes

From Readme.md: Pull requests can only be accepted from signers of the Oracle Contributor Agreement