femtojar

femtojar shrinks executable JARs by bundling .class files and resources into a single compressed blob and loading them through a tiny bootstrap classloader at runtime. If you want, it runs ProGuard before hand to further optimize/shrink the JAR.

It's for executable JARs, like jstall, not libraries.

Femtojar is still an early proof-of-concept, but initial results show promising size reductions of 15-30% for tested shaded/uber JARs, going up to 70% in some cases with ProGuard.

Features

• Novel single-blob class compression for better cross-class redundancy
• Zopfli compression mode to squeeze out extra bytes at the cost of longer build times
• Bundling of non-META-INF/* resources into the blob for better compression (with some caveats, see below)
• Optional ProGuard shrinking/optimization before reencoding
• Maven plugin and standalone CLI

Important: Always black box test the resulting JARs, especially if using ProGuard, as bytecode transformations can break edge cases.

Maven Plugin

Goal:

• femtojar:reencode-jars

Important:

• Use this for executable (typically shaded/uber) JARs.
• It is not designed for library JAR publishing.

Minimal Plugin Configuration

If your project produces a single JAR named ${project.build.finalName}.jar (the default for shaded/uber JARs), no <configuration> block is needed. The JAR is rewritten in place in ${project.build.directory}/${project.build.finalName}.jar (typically target/<artifactId>-<version>.jar):

<plugin>
  <groupId>me.bechberger</groupId>
  <artifactId>femtojar</artifactId>
  <version>0.1.6</version>
  <executions>
    <execution>
      <goals>
        <goal>reencode-jars</goal>
      </goals>
    </execution>
  </executions>
</plugin>

If you need to target a different JAR or customise behaviour, add a <configuration> block:

<plugin>
  <groupId>me.bechberger</groupId>
  <artifactId>femtojar</artifactId>
  <version>0.1.6</version>
  <executions>
    <execution>
      <goals>
        <goal>reencode-jars</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <jars>
      <jar>
        <in>${project.build.finalName}.jar</in>
      </jar>
    </jars>
  </configuration>
</plugin>

Full Configuration Example

<plugin>
  <groupId>me.bechberger</groupId>
  <artifactId>femtojar</artifactId>
  <version>0.1.6</version>
  <executions>
    <execution>
      <id>recompress-jar</id>
      <phase>package</phase>
      <goals>
        <goal>reencode-jars</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <failOnError>true</failOnError>
    <skip>false</skip>
    <compressionMode>MAX</compressionMode>
    <bundleResources>true</bundleResources>
    <jars>
      <jar>
        <in>${project.build.finalName}.jar</in>
        <out>${project.build.finalName}-optimized.jar</out>
      </jar>
      <jar>
        <in>other.jar</in>
      </jar>
    </jars>
  </configuration>
</plugin>

Example Project

See example-project/pom.xml for a complete usage example that shades dependencies, then re-encodes the output JAR.

Plugin Parameters

Parameter	Description	Default
jars	List of JAR entries to reencode. If omitted, defaults to ${project.build.finalName}.jar rewritten in place.	Auto-detected
jars[i].in	Input JAR path (relative to ${project.build.directory} if not absolute)	Required per entry
jars[i].out	Optional output JAR path. If omitted, input JAR is rewritten in place.	Not set
compressionMode	Compression preset: DEFAULT (deflate), ZOPFLI (7 iterations), MAX (100 iterations).	DEFAULT
bundleResources	Bundle non-META-INF/* resources into the blob.	true
failOnError	Fail build immediately on rewrite errors.	true
skip	Skip plugin execution.	false
proguard.enabled	Run ProGuard before reencoding.	false
proguard.prependDefaultConfig	Prepend the bundled default ProGuard config.	true
proguard.configFile	Path to a user ProGuard .pro config file.	Not set
proguard.options	Inline ProGuard options (e.g. -dontobfuscate).	Not set
proguard.out	Separate ProGuard output path. If omitted, a temp file is used.	Not set
proguard.libraryJars	Additional -libraryjars paths for ProGuard.	Not set

All proguard.* parameters can also be specified per-JAR inside <jar><proguard>...</proguard></jar>. Per-JAR values override the global setting; null fields fall back to the global config.

ProGuard Configuration

femtojar can optionally run ProGuard before reencoding to shrink/optimize the JAR. This typically yields an additional ~30% size reduction on top of femtojar's normal compression. ProGuard runs in-process via the proguard.ProGuard API. All ProGuard settings are grouped under a <proguard> element, available both globally and per-JAR.

A bundled default config (proguard-default.pro) ships with femtojar and is prepended automatically unless disabled. It includes java.base as a library JAR and standard keep rules for main classes, native methods, enums, serialization, and annotations.

Caveats:

• ProGuard modifies bytecode — the resulting JAR should be thoroughly tested before shipping.
• Reflection-heavy code (e.g. CLI frameworks like picocli, serialization libraries) typically requires additional -keep rules to work correctly.
• ProGuard may be sensitive to the target Java version; verify with the JDK you deploy on.
• No aggressive optimizations are applied by the default config, but ProGuard is still an invasive transform compared to plain reencoding.

<configuration>
  <proguard>
    <enabled>true</enabled>
    <prependDefaultConfig>true</prependDefaultConfig>
    <configFile>${basedir}/proguard.conf</configFile>
    <options>
      <option>-dontobfuscate</option>
    </options>
  </proguard>
  <jars>
    <jar>
      <in>${project.build.finalName}.jar</in>
      <!-- per-JAR ProGuard overrides -->
      <proguard>
        <options>
          <option>-keep class com.example.api.cli.** { *; }</option>
        </options>
      </proguard>
    </jar>
  </jars>
</configuration>

Per-JAR Overrides

In addition to ProGuard, the following settings can be overridden per-JAR:

Per-JAR Parameter	Description
jars[i].compressionMode	Override global compressionMode for this JAR.
jars[i].bundleResources	Override global bundleResources.
jars[i].proguard	Override global ProGuard config (same sub-elements).

Resource Bundling Caveats

In the default case when not bundleResources=false, non-META-INF/* resources are packed into the compressed blob for better compression.

The bootstrap handler registers a handler for the custom "femtojar:" URL protocol via URL.setURLStreamHandlerFactory, but this only works with when no other code uses URL.setURLStreamHandlerFactory. This is usually the case, but if not, then set bundleResources=false.

But there might be other issues related to frameworks that expect specific URLs or else, so test resources properly in your particular use case and disable this feature when you see any problems.

<configuration>
  <bundleResources>false</bundleResources>
  <!-- ... rest of config -->
</configuration>

Or per-JAR:

<jar>
  <in>${project.build.finalName}.jar</in>
  <bundleResources>false</bundleResources>
</jar>

CLI

A standalone CLI jar is built as an attached shaded artifact.

Build it:

mvn package

Run it:

java -jar target/femtojar-0.1.2-cli.jar app.jar app-optimized.jar

With ProGuard:

java -jar target/femtojar-0.1.2-cli.jar app.jar app-optimized.jar --proguard --proguard-config proguard.conf

Benchmark form:

java -jar target/femtojar-0.1.2-cli.jar app.jar --benchmark --benchmark-format markdown

Benchmark with custom ProGuard rules used for all proguard* benchmark rows:

java -jar target/femtojar-0.1.2-cli.jar app.jar --benchmark --benchmark-format json \
  --proguard-config proguard.conf \
  --proguard-options "-dontwarn"

CLI options:

• First positional arg: input JAR path
• Second positional arg (optional): output JAR path (defaults to in-place rewrite)
• --compression <default|zopfli|max>: compression preset (default=deflate, zopfli=7 iterations, max=100 iterations)
• --no-bundle-resources: disable resource bundling, keep the resource files separate in the JAR
• --proguard: run ProGuard before reencoding
• --proguard-config <path>: path to a ProGuard .pro config file
• --proguard-options <option>: inline ProGuard option (repeatable)
• --proguard-out <path>: write ProGuard output to a separate path instead of a temp file
• --no-proguard-default-config: do not prepend the bundled default ProGuard config
• --verbose: print verbose processing output
• --benchmark: run a non-destructive benchmark matrix and print size/time comparisons
• --benchmark-format <text|markdown|json>: optional benchmark output format (default: text)
• -h, --help: show usage

Bytecode Size Guide

For source-level bytecode size reduction techniques (with measured before/after numbers and javap snippets), see:

• guide/BYTECODE_SIZE_GUIDE.md
• guide/code/README.md

Comparison with Other Tools

Tool	Approach	Strengths	Limitations
JarTighten	ZIP entry optimization	Minimal changes to JAR structure, focused recompression	Limited to ZIP compression, no bytecode optimization
Pack200	Native compression with cross-class deduplication	Achieved 4-10× reduction, follows Java standards	Deprecated in Java 11, removed in Java 14, maintenance burden
jlink	Custom runtime images (JDK 9+)	Reduces runtime size by excluding unused modules	Requires modular JDK, creates platform-specific artifacts

Feel free to suggest additions or corrections to this comparison.

Build and Test

mvn test
mvn package
mvn -Prun-its verify

Integration fixtures are documented in src/it/README.md.

Release Script

Release automation is in release.py.

Dry run:

./release.py --dry-run

Patch release without deploy:

./release.py --patch

Default release flow:

• require release notes in CHANGELOG.md under ## [Unreleased]
• roll ## [Unreleased] notes into ## [<version>] - <YYYY-MM-DD>
• bump version in root pom.xml
• update version references in README.md
• update femtojar plugin version in example-project/pom.xml
• run tests and build artifacts
• optional mvn clean deploy -P release
• commit + tag (and optional push)

Support, Feedback, Contributing

This project is open to feature requests/suggestions, bug reports etc. via GitHub issues. Contribution and feedback are encouraged and always welcome.

License

MIT, Copyright 2026 SAP SE or an SAP affiliate company, Johannes Bechberger and contributors