Java API Discovery

Efficiently discover actual API signatures from compiled JARs using javap. This approach eliminates guessing and trial-and-error by examining the compiled bytecode directly.

When to Use This Skill

• Encountering compilation errors from incorrect method assumptions
• Working with unfamiliar Java SDKs or libraries
• Need to understand pagination patterns (offset vs cursor)
• Dealing with union types or polymorphic responses
• Verifying field existence before writing code
• Documentation is incomplete, outdated, or ambiguous

Core Workflow

1. Find the JAR

# Gradle projects
find ~/.gradle/caches -name "*library-name*.jar" -type f 2>/dev/null | head -5

# Maven projects
find ~/.m2/repository -name "*library-name*.jar" -type f 2>/dev/null | head -5

# Project libs
find . -name "*.jar" -type f 2>/dev/null

2. List Package Contents

# List all classes in a package
jar tf /path/to/library.jar | grep "com/example/package" | head -20

# Find specific class
jar tf /path/to/library.jar | grep -i "ClassName"

3. Examine Class API

# Show all public methods
javap -cp /path/to/library.jar com.example.ClassName

# Filter for getters
javap -cp /path/to/library.jar com.example.ClassName | grep -E "public.*get"

# Filter for setters/builders
javap -cp /path/to/library.jar com.example.ClassName | grep -E "public.*(set|with|build)"

# Show full signatures including generics
javap -s -cp /path/to/library.jar com.example.ClassName

4. Verify Before Coding

Before writing any API call:

1. Find the exact method name with javap
2. Check return type (especially for collections: List vs Set)
3. Verify parameter types
4. Look for builder patterns vs constructors

Common Patterns to Discover

Pagination

# Check for pagination methods
javap -cp /path/to/jar com.example.ApiClient | grep -iE "(page|offset|cursor|limit)"

Common patterns:

• Offset-based: pageOffset(Long), pageLimit(Long)
• Cursor-based: pageCursor(String), nextCursor()
• Token-based: pageToken(String), nextPageToken()

Union Types

# Look for getActualInstance pattern
javap -cp /path/to/jar com.example.Response | grep -E "(getActualInstance|instanceof)"

Handling union types:

Object actual = response.getActualInstance();
if (actual instanceof TypeA) {
    TypeA typed = (TypeA) actual;
}

Builder vs Constructor

# Check construction options
javap -cp /path/to/jar com.example.Model | grep -E "(public.*\(|builder|Builder)"

Enum Constants

# List enum values
javap -cp /path/to/jar com.example.TokenType | grep -E "public static final"

Quick Reference

Common Pitfalls

• ❌ Guessing method names → Always verify with javap
• ❌ Assuming collection types → Check if List, Set, or Collection
• ❌ Trusting old documentation → Bytecode is truth
• ❌ Ignoring return types → Union types need getActualInstance()

Progressive Context

• For helper scripts: see scripts/discover-api.sh
• For detailed patterns: see reference.md