Skip to content
<awesome-gh-repos>
vic

lightbend-labs/jardiff

A tool for comparing JAR files, including method bodies and Scala 2 pickled signatures
Source code at GitHub https://github.com/lightbend-labs/jardiff
lightbend-labs/jardiff.json
{
  "createdAt": "2017-05-31T06:50:55Z",
  "defaultBranch": "main",
  "description": "A tool for comparing JAR files, including method bodies and Scala 2 pickled signatures",
  "fullName": "lightbend-labs/jardiff",
  "homepage": "",
  "language": "Scala",
  "name": "jardiff",
  "pushedAt": "2025-11-17T19:11:05Z",
  "stargazersCount": 185,
  "topics": [
    "command-line-tool",
    "diff",
    "jvm-bytecode",
    "scala"
  ],
  "updatedAt": "2025-11-15T14:45:24Z",
  "url": "https://github.com/lightbend-labs/jardiff"
}

JAR differ

Section titled “JAR differ”

A tool for generating bytecode diffs

Requires JDK 8+

About

Section titled “About”

JarDiff is a tool for generating detailed but comprehensible diffs of sets (JAR or directory) of Java classfiles.

Class files are rendered with ASM’s Textifier and with scalap. Other files are rendered as-is.

The rendered files are committed into a Git repository, one commit per provided command line argument.

The diffs between these are rendered to standard out (unless --quiet is provided). If only a single argument is provided, the initial commit is rendered.

By default, a temporary git repository is used and deleted on exit. Use --git to provide a custom location for the repository that can be inspected after the tool has run.

Installing

Section titled “Installing”

macOS

Section titled “macOS”

macOS users may install with:

brew install retronym/formulas/jardiff

Other Platforms

Section titled “Other Platforms”

Usage

Section titled “Usage”
% jardiff -h
usage: jardiff [-c] [-g <dir>] [-h] [-i <arg>] [-p] [-q] [-r] [-U <n>] VERSION1 [VERSION2 ...]


Each VERSION may designate a single file, a directory, JAR file or a
`:`-delimited classpath


 -c,--suppress-code       Suppress method bodies
 -g,--git <dir>           Directory to output a git repository containing the
                          diff
 -h,--help                Display this message
 -i,--ignore <arg>        File pattern to ignore rendered files in gitignore
                          format
 -p,--suppress-privates   Display only non-private members
 -q,--quiet               Don't output diffs to standard out
 -r,--raw                 Disable sorting and filtering of classfile contents
 -U,--unified <n>         Number of context lines in diff


% jardiff dir1 dir2


% jardiff v1/A.class v2/A.class


% jardiff --git /tmp/diff-repo --quiet v1.jar v2.jar v3.jar

Building

Section titled “Building”

After cloning this project, use sbt clean core/assembly.

Sample Output

Section titled “Sample Output”

Scala 2.11 vs 2.12 trait encoding changes

Section titled “Scala 2.11 vs 2.12 trait encoding changes”
test.scala
trait T { def foo = 4 }
class C extends T
% ~/scala/2.11/bin/scalac -d /tmp/v1 test.scala && ~/scala/2.12/bin/scalac -d /tmp/v2 test.scala


% jardiff /tmp/v1 /tmp/v2
diff --git a/C.class.asm b/C.class.asm
index f3a33f1..33b9282 100644
--- a/C.class.asm
+++ b/C.class.asm
@@ -1,4 +1,4 @@
-// class version 50.0 (50)
+// class version 52.0 (52)
 // access flags 0x21
 public class C implements T  {


@@ -8,7 +8,7 @@
     ALOAD 0
     INVOKESPECIAL java/lang/Object.<init> ()V
     ALOAD 0
-    INVOKESTATIC T$class.$init$ (LT;)V
+    INVOKESTATIC T.$init$ (LT;)V
     RETURN
     MAXSTACK = 1
     MAXLOCALS = 1
@@ -16,7 +16,7 @@
   // access flags 0x1
   public foo()I
     ALOAD 0
-    INVOKESTATIC T$class.foo (LT;)I
+    INVOKESTATIC T.foo$ (LT;)I
     IRETURN
     MAXSTACK = 1
     MAXLOCALS = 1
diff --git a/T.class.asm b/T.class.asm
index 9180093..fcac19f 100644
--- a/T.class.asm
+++ b/T.class.asm
@@ -1,8 +1,28 @@
-// class version 50.0 (50)
+// class version 52.0 (52)
 // access flags 0x601
 public abstract interface T {




-  // access flags 0x401
-  public abstract foo()I
+  // access flags 0x9
+  public static $init$(LT;)V
+    // parameter final synthetic  $this
+    RETURN
+    MAXSTACK = 0
+    MAXLOCALS = 1
+
+  // access flags 0x1
+  public default foo()I
+    ICONST_4
+    IRETURN
+    MAXSTACK = 1
+    MAXLOCALS = 1
+
+  // access flags 0x1009
+  public static synthetic foo$(LT;)I
+    // parameter final synthetic  $this
+    ALOAD 0
+    INVOKESPECIAL T.foo ()I
+    IRETURN
+    MAXSTACK = 1
+    MAXLOCALS = 1
 }

Scala standard library changes during 2.11 minor releases

Section titled “Scala standard library changes during 2.11 minor releases”
% jardiff --quiet --git /tmp/scala-library-diff /Users/jz/scala/2.11.*/lib/scala-library.jar

Browsable Repo: scala-library-diff

API changes visible in bytecode descriptors / generic signature / scalap output

Section titled “API changes visible in bytecode descriptors / generic signature / scalap output”
V1.scala
trait C {
  def m1(a: String)
  def m2(a: Option[String])
  def m3(a: String)
}
V2.scala
trait C {
  def m1(a: AnyRef)
  def m2(a: Option[AnyRef])
  def m3(a: scala.collection.immutable.StringOps) // value class
}
diff --git a/C.class.asm b/C.class.asm
index 52a43d5..bdb0541 100644
--- a/C.class.asm
+++ b/C.class.asm
@@ -4,12 +4,12 @@




   // access flags 0x401
-  public abstract m1(Ljava/lang/String;)V
+  public abstract m1(Ljava/lang/Object;)V
     // parameter final  a


   // access flags 0x401
-  // signature (Lscala/Option<Ljava/lang/String;>;)V
-  // declaration: void m2(scala.Option<java.lang.String>)
+  // signature (Lscala/Option<Ljava/lang/Object;>;)V
+  // declaration: void m2(scala.Option<java.lang.Object>)
   public abstract m2(Lscala/Option;)V
     // parameter final  a


diff --git a/C.class.scalap b/C.class.scalap
index 78f7d91..637cd98 100644
--- a/C.class.scalap
+++ b/C.class.scalap
@@ -1,5 +1,5 @@
 trait C extends scala.AnyRef {
-  def m1(a: scala.Predef.String): scala.Unit
-  def m2(a: scala.Option[scala.Predef.String]): scala.Unit
-  def m3(a: scala.Predef.String): scala.Unit
+  def m1(a: scala.AnyRef): scala.Unit
+  def m2(a: scala.Option[scala.AnyRef]): scala.Unit
+  def m3(a: scala.collection.immutable.StringOps): scala.Unit
 }

Validating a Scala compiler nightly

Section titled “Validating a Scala compiler nightly”
% git clone --quiet --depth 1 akka/akka; cd akka


% cat ~/.sbt/0.13/resolver.sbt
resolvers ++= (
  if (scalaVersion.value.contains("-bin"))
     List("scala-integration" at "https://scala-ci.typesafe.com/artifactory/scala-integration/")
  else Nil
)


% for v in 2.12.2 2.12.3-bin-bd6294d; do \
  echo $v; \
  mkdir -p /tmp/akka-$v; \
  time sbt ++$v clean package &>/dev/null; \
  for f in $(find . -name '*.jar'); do cp $f /tmp/akka-$v/; done; \
done


2.12.2


real  3m19.468s
user  8m56.947s
sys  0m17.343s
2.12.3-bin-bd6294d


real  2m57.631s
user  8m40.168s
sys  0m16.172s


% jardiff -q --git /tmp/akka-diff $(find /tmp/akka-2.12.2 | paste -s -d : -) $(find /tmp/akka-2.12.3-bin-bd6294d | paste -s -d : -)

The resulting diff shows that the only change in the generated bytecode is due to scala/scala#5857, “Fix lambda deserialization in classes with 252+ lambdas”

% git --git-dir /tmp/akka-diff/.git log -p -1
commit 83f0e0a5dabbdf1b7677363bf238d81e4c5b032e (HEAD -> master)
Author: Jason Zaugg <jzaugg@gmail.com>
Date:   Mon Jun 5 17:08:17 2017 +1000


    jardiff textified output of: /tmp/akka-2.12.3-bin-bd6294d:/tmp/akka-2.12.3-bin-bd6294d/akka-2.5-SNAPSHOT.jar:...


diff --git a/akka/stream/scaladsl/GraphApply.class.asm b/akka/stream/scaladsl/GraphApply.class.asm
index 5672560..4d1fafb 100644
--- a/akka/stream/scaladsl/GraphApply.class.asm
+++ b/akka/stream/scaladsl/GraphApply.class.asm
@@ -2802,6 +2802,8 @@ public abstract interface akka/stream/scaladsl/GraphApply {


   // access flags 0x100A
   private static synthetic $deserializeLambda$(Ljava/lang/invoke/SerializedLambda;)Ljava/lang/Object;
    TRYCATCHBLOCK L0 L1 L1 java/lang/IllegalArgumentException
   L0
     ALOAD 0
     INVOKEDYNAMIC lambdaDeserialize(Ljava/lang/invoke/SerializedLambda;)Ljava/lang/Object; [
       // handle kind 0x6 : INVOKESTATIC
@@ -3308,12 +3310,20 @@ public abstract interface akka/stream/scaladsl/GraphApply {
       // handle kind 0x6 : INVOKESTATIC
       akka/stream/scaladsl/GraphApply.$anonfun$create$250(Lscala/Function1;Ljava/lang/Object;)Ljava/lang/Object;,
       // handle kind 0x6 : INVOKESTATIC
      akka/stream/scaladsl/GraphApply.$anonfun$create$251(Lscala/Function1;Ljava/lang/Object;)Ljava/lang/Object;,
      akka/stream/scaladsl/GraphApply.$anonfun$create$251(Lscala/Function1;Ljava/lang/Object;)Ljava/lang/Object;
    ]
    ARETURN
   L1
    ALOAD 0
    INVOKEDYNAMIC lambdaDeserialize(Ljava/lang/invoke/SerializedLambda;)Ljava/lang/Object; [
      // handle kind 0x6 : INVOKESTATIC
      scala/runtime/LambdaDeserialize.bootstrap(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;[Ljava/lang/invoke/MethodHandle;)Ljava/lang/invoke/CallSite;
      // arguments:
       // handle kind 0x6 : INVOKESTATIC
       akka/stream/scaladsl/GraphApply.$anonfun$create$252(Lscala/Function1;Ljava/lang/Object;)Ljava/lang/Object;
     ]
     ARETURN
    MAXSTACK = 1
    MAXSTACK = 2
     MAXLOCALS = 1


   // access flags 0x9