Java Jackknife

Jackknife

Instalação

Adicione o grupo do plugin ao ~/.m2/settings.xml para usar o comando curto mvn jackknife:* sem prefixo de grupo:

<settings>
  <pluginGroups>
    <pluginGroup>org.tomitribe.jackknife</pluginGroup>
  </pluginGroups>
</settings>

Sem essa configuração, os comandos precisam do prefixo completo: mvn org.tomitribe.jackknife:jackknife-maven-plugin:instrument ...

Uso

Two capabilities for working with Java dependencies:

1. Explore — Find classes, read decompiled source, understand APIs. No more digging through ~/.m2/repository or guessing at method signatures.
2. Debug — Instrument methods to capture arguments, return values, exceptions, and timing as structured JSON. No more adding println statements to tests.

Exploring Dependencies

Find a class in project dependencies

Manifests list every class in every dependency jar. One class name per line.

Grep "CustomField" .jackknife/manifest/

This tells you which jar contains the class and its full package name.

Find a class anywhere in ~/.m2

If the class isn't a project dependency, search the local Maven repository:

mvn jackknife:index -Dclass=com.fasterxml.jackson.databind.ObjectMapper

This searches ~/.m2/repository using the package name to narrow the search, finds the latest version, and indexes the jar. The class is then available in the manifest and can be decompiled.

For broader searches across a library family:

mvn jackknife:index -Dscope=repo -Dfilter="**jackson**"

This indexes the latest version of every jar whose path matches the filter. Use ** to match across directory boundaries.

Read source code

Check if the class has already been decompiled:

Glob .jackknife/source/**/<ClassName>.java

If found, read it directly. If not, decompile the entire jar (one-time, ~3-5s):

mvn jackknife:decompile -Dclass=com.example.MyClass

Every class in that jar is now available as a .java file. All subsequent reads are direct file access — no Maven invocation needed.

Find resources

Grep "META-INF/services" .jackknife/manifest/

Directory structure

.jackknife/
├── manifest/            Class listings (all jars, sub-second)
│   └── <groupId>/
│       └── <artifact>-<version>.jar.manifest
├── source/              Decompiled source (per-jar, on demand)
│   └── <groupId>/
│       └── <artifact>-<version>/
│           └── com/example/MyClass.java
├── instrument/          Pending instrumentation configs
├── modified/            Patched jars (applied on next build)
└── USAGE.md             This file

Debugging with Instrumentation

The workflow

1. A test fails or you need to understand what a method receives and returns
2. Instrument the method — jackknife injects debug output, no source changes
3. Run mvn test — structured JSON output shows exactly what happened
4. Extract values from the JSON to fix assertions or understand behavior
5. Clean up when done: mvn jackknife:clean -Dpath=modified

Instrument a method

mvn jackknife:instrument -Dmethod="com.example.Foo.bar(java.lang.String,int)"

Matching granularity

Modes

• debug (default) — args, return value, exceptions, and timing
• timing — elapsed time and status only, no args or return values

mvn jackknife:instrument -Dmethod="com.example.Foo.bar" -Dmode=timing

Run the build

mvn test

The next build automatically applies the instrumentation. No special flags.

Instrumenting project code

Jackknife can also instrument your own classes in src/main/ or src/test/. If the class isn't found in dependency manifests, it checks your project source:

mvn jackknife:instrument -Dclass=com.example.MyService -Dmethod=process

The enhance goal (bound to process-test-classes) modifies the compiled bytecode in target/classes/ and target/test-classes/ in place. Tests run against the enhanced bytecode automatically.

Reading the output

Every instrumented call produces one line prefixed with JACKKNIFE:

JACKKNIFE {"event":"register","mode":"debug","method":"org.tomitribe.util.Join.join"}
JACKKNIFE {"event":"call","time":"12.3us","class":"Join","method":"join","args":[", ",["x","y","z"]],"return":"x, y, z"}

The register line confirms instrumentation is active. If you see the register event but no call events, the method was never called by your test.

Grep for all instrumented calls:

Grep "JACKKNIFE" target/surefire-reports/

JSON fields

Extracting values for assertions

Strings are JSON-quoted, numbers are bare, null is null, booleans are bare. Copy values directly from the "return" field into assertEquals:

// From output: "return":"a and b and c"
assertEquals("a and b and c", result);

// From output: "return":42
assertEquals(42, result);

// From output: "return":null
assertNull(result);

Capture files

When arguments or return values are too large for a single line, the full JSON event is written to a capture file:

JACKKNIFE {"event":"call","time":"2.3ms","class":"Join","method":"join","status":"returned","file":"target/jackknife/captures/capture-0012.txt"}

The console line shows timing, method, and status for quick scanning. Read the capture file for the complete JSON event with all argument and return values.

Cleaning up

mvn jackknife:clean                                # remove all of .jackknife/
mvn jackknife:clean -Dpath=modified                # stop instrumentation, keep manifests
mvn jackknife:clean -Dpath=modified/org.tomitribe  # remove one groupId's patches
mvn jackknife:clean -Dpath=source                  # clear decompile cache

After cleaning modified/, the next build uses original unmodified dependencies.