Troubleshooting Common One-JAR ProblemsOne-JAR is a convenient tool for packaging a Java application and its dependencies into a single executable JAR. It simplifies distribution and deployment, but like any packaging mechanism, it can introduce problems that are often unfamiliar to developers used to standard classpath arrangements. This article covers the most frequent One-JAR issues, how to diagnose them, and practical fixes and workarounds.
1. Understanding One-JAR’s structure and runtime behavior
Before troubleshooting, it helps to know how One-JAR works internally. One-JAR embeds multiple JARs (your application and dependency JARs) inside a single wrapper JAR. At runtime, One-JAR uses a custom ClassLoader to locate and load classes and resources from these nested JARs. This behavior differs from the JVM’s standard classpath and can affect:
- Resource lookup (resources packaged inside nested JARs may be found differently).
- Class loading order (One-JAR’s ClassLoader may load classes in an order that differs from a normal classpath).
- Services and ServiceLoader behavior.
- Tools and libraries that rely on direct file-system access to JAR contents.
2. Common problem: ClassNotFoundException / NoClassDefFoundError
Symptoms
- JVM throws ClassNotFoundException or NoClassDefFoundError for classes that are present in dependency JARs.
Causes and fixes
- Incorrect packaging: Verify that the dependency JARs are actually included inside the One-JAR. Open the One-JAR with a zip tool and confirm the nested JARs are present (usually under the lib/ or main/ directory).
- Manifest Main-Class misconfiguration: Ensure the One-JAR wrapper’s Main-Class points to the One-JAR bootstrapper (commonly com.simontuffs.onejar.Boot) and that your application entry point is specified correctly using One-JAR’s configuration (one-jar.properties or equivalent).
- ClassLoader conflicts: Some libraries expect to be loaded by the system or context ClassLoader. Try setting the thread context ClassLoader to One-JAR’s ClassLoader in your bootstrap code:
Thread.currentThread().setContextClassLoader(OneJarClassLoader.getInstance());(Adapt to your One-JAR version’s API.)
- Duplicate classes: Conflicts from multiple JARs providing the same class can cause NoClassDefFoundError at runtime. Use a dependency analyzer (Maven’s dependency:tree or Gradle’s dependencies) to identify duplicates and exclude or reconcile versions.
- Shaded or relocated packages: If you used shading/relocation, ensure the relocated classes are referenced correctly.
3. Resources not found (missing configuration files, images, etc.)
Symptoms
- getResource or getResourceAsStream returns null; configuration files or images aren’t loaded.
Causes and fixes
- Resource path differences: When resources are inside nested JARs, the ClassLoader lookup path can differ. Always use Class.getResourceAsStream(“/path/to/resource”) or the context ClassLoader to load resources.
- Resource case-sensitivity: ZIP/JARs are case-sensitive; verify exact paths.
- Resource packaging: Confirm resources are included in the dependency JARs inside One-JAR.
- File-based code expecting real files: Libraries that call new File(“/…”) on a resource will fail because nested JAR entries aren’t files on disk. Extract the resource to a temporary file before use:
InputStream in = MyClass.class.getResourceAsStream("/config.xml"); File tmp = File.createTempFile("config", ".xml"); try (FileOutputStream out = new FileOutputStream(tmp)) { in.transferTo(out); } // pass tmp.getAbsolutePath() to the library
4. ServiceLoader and META-INF/services failures
Symptoms
- ServiceLoader.load(…) returns no providers, or libraries relying on SPI don’t find implementations.
Causes and fixes
- One-JAR may merge or nest service files differently. Ensure that META-INF/services files from dependency JARs are accessible to the One-JAR ClassLoader.
- If services are lost during packaging, merge service files at build time (use the Maven Shade plugin’s ServicesResourceTransformer or Gradle’s equivalent) so the combined services file lists all implementations.
- Alternatively, implement a custom provider registration mechanism if ServiceLoader isn’t functioning.
5. Native library (JNI) issues
Symptoms
- UnsatisfiedLinkError for native libraries, or native libraries not found.
Causes and fixes
- JNI libraries (.so, .dll) cannot be loaded directly from inside nested JARs. Extract native libraries to a temporary directory and load them with System.load(path).
- Ensure correct architecture and OS-specific native library versions are packaged and selected at runtime.
- Example extraction pattern:
InputStream lib = MyClass.class.getResourceAsStream("/native/libexample.so"); File tmp = File.createTempFile("libexample", ".so"); try (FileOutputStream out = new FileOutputStream(tmp)) { lib.transferTo(out); } System.load(tmp.getAbsolutePath());
6. Performance and memory overhead
Symptoms
- Slow startup, high memory usage, GC pauses.
Causes and fixes
- One-JAR’s ClassLoader may read and unpack nested JARs at startup. Minimize the number and size of bundled dependencies.
- Use dependency pruning: remove unused libraries or use tools like ProGuard to shrink bytecode.
- Increase JVM memory settings (Xms/Xmx) if large dependency sets need memory during startup.
- Consider lazy-loading resources or using a different packaging approach (e.g., modular runtime images with jlink or native images) for large apps.
7. Troubles with build tools (Maven/Gradle integration)
Symptoms
- Build fails, or the produced One-JAR lacks expected contents.
Causes and fixes
- Plugin configuration errors: Double-check plugin versions and configuration snippets. For Maven, ensure the One-JAR plugin runs in the package phase and that dependencies are marked with the correct scopes.
- Incorrect dependency scopes: Test and runtime dependencies must be included; provided/optional won’t be packaged.
- Exclude unwanted files: Use plugin excludes to avoid packaging transient files (tests, docs).
- Reproducible build: Clean the build directory before packaging to avoid stale artifacts.
8. Debugging tips and tools
- Inspect the One-JAR: Treat it as a zip file. List entries with:
jar tf onejar.jarConfirm paths of nested jars and resources.
- Enable verbose class loading:
- Use -verbose:class to log classes loaded and their source JARs; this helps locate which nested JAR supplies a class.
- Add logging in bootstrap code: Temporarily log classpath entries and resource lookup attempts to understand where lookups fail.
- Create a minimal reproducer: Strip down the app to the smallest example that reproduces the issue — this isolates whether One-JAR or your code causes the problem.
- Use dependency analysis tools (Maven’s dependency:tree, Gradle’s dependencies) to find version conflicts.
9. Alternatives and migration considerations
If One-JAR’s peculiarities are blocking you or causing hard-to-fix runtime issues, consider alternatives:
- Fat JARs created by the Maven Shade plugin or Gradle’s shadowJar — these merge classes/resources into one JAR rather than nesting JARs (watch for resource merging issues).
- jlink to create a custom Java runtime image (Java 9+), reducing dependency complexity.
- Native images (GraalVM) for fast startup and single-file executables, though they require extra build steps.
- Docker containers to package the entire runtime and dependencies while keeping standard classpath layout.
10. Quick checklist for troubleshooting
- Confirm nested JARs and resources exist inside the One-JAR.
- Run with -verbose:class to see class loading sources.
- Verify Main-Class and One-JAR bootstrap configuration.
- Check for duplicate or conflicting classes.
- Extract resources/native libs when code expects file paths.
- Merge META-INF/services files if ServiceLoader fails.
- Rebuild with clean workspace and correct plugin configuration.
- If problems persist, create a minimal reproducible example.
Troubleshooting One-JAR issues is mostly about understanding how nested JARs, the custom ClassLoader, and resource access differ from the JVM’s default behavior. With systematic inspection of the packaged JAR, classloading logs, and minimal repro cases, most problems can be identified and resolved.
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