sbt-javap

As above, so below.

overview

sbt-javap is an SBT plugin to call javap directly from SBT.

Java provides the javap tool to disassemble and inspect Java bytecode. This allows authors to see how their code operates at a low level, to understand how high-level Scala concepts are encoded, and to potentially spot performance issues.

One challenge with using javap is correctly specifying a classpath. Since SBT knows your project's classpath, sbt-javap can automatically invoke javap for you.

quick start

Currently, sbt-javap is published for SBT 1.x.

To use sbt-javap, add the following to project/plugins.sbt:

addSbtPlugin("org.spire-math" % "sbt-javap" % "0.0.1")

usage

sbt-javap provides one new SBT task: javap.

This command takes the fully-qualified name of a class, dissassembles that class into a file, and then displays that file interactively via a pager (by default less).

For example, running javap scala.Unit produces the following:

sbt:myproject> javap scala.Unit
decompiling scala.Unit to /Users/erik/t/sbt-javap-test/target/scala-2.12/javap/scala.Unit.bytecode

At that point, less will be used to view that file:

Compiled from "Unit.scala"
public abstract class scala.Unit {
  public static java.lang.String toString();
    Code:
       0: getstatic     #16                 // Field scala/Unit$.MODULE$:Lscala/Unit$;
       3: invokevirtual #18                 // Method scala/Unit$.toString:()Ljava/lang/String;
       6: areturn

  public static void unbox(java.lang.Object);
    Code:
       0: getstatic     #16                 // Field scala/Unit$.MODULE$:Lscala/Unit$;
       3: aload_0
       4: invokevirtual #22                 // Method scala/Unit$.unbox:(Ljava/lang/Object;)V
       7: return

  public static scala.runtime.BoxedUnit box(scala.runtime.BoxedUnit);
    Code:
       0: getstatic     #16                 // Field scala/Unit$.MODULE$:Lscala/Unit$;
       3: aload_0
       4: invokevirtual #26                 // Method scala/Unit$.box:(Lscala/runtime/BoxedUnit;)Lscala/runtime/BoxedUnit;
       7: areturn

  public scala.Unit();
    Code:
       0: aload_0
       1: invokespecial #30                 // Method java/lang/Object."<init>":()V
       4: return
}
(END)

Note that you can decompile any Scala or Java class that's on your classpath, not just classes that you defined. Decompiling classes from your dependencies (or from the standard library) can be very illuminating.

name-mangling

Scala uses name-mangling to encode various types of names into the single Java class namespace. Here are some examples:

Scala objects (object Foo) have a $ suffix appended (Foo$).
Scala traits (trait Bar) have a $class suffix appended (Bar$class).
Scala classes (class Qux) use their names as normal (Qux).

(In all of the above cases the other name-mangling rules may still apply.)

Scala types are often found inside of object values as a form of namespacing. Scala uses a $ delimiter to mangle these names. For example, given object Kennel { class Dog } the inner class name would become Kennel$Dog.

Finally, Scala also allows a wider range of names than Java. For example, + is a valid name (e.g. object + { ... }), and would be encoded in Java as $plus. If your class/trait/object has a name containing these characters, you'll need to determine how the name was mangled.

It's often useful to use the REPL to see how particular characters of a name are encoded:

scala> object +*%
defined object $plus$times$percent

future work

Error handling and reporting could be a lot better.

It's likely that the SBT-related code could be improved.

It should be possible to cross-publish this plugin for SBT 0.13.x as well as SBT 1.x.

This README could easily be expanded to document SBT options, expand on name-mangling, and explain how to interpet bytecode (or link to other references).

It might be nice to parse and/or highlight bytecode to make it easier to read. It would also be nice to support an option to decompile only a particular method, rather than an entire class.

copyright & license

All code is available to you under the Apache 2 license, available at https://opensource.org/licenses/Apache-2.0.

non / sbt-javap 0.0.1