Better error message on class uniqueness lock file failure

changelog/@unreleased/pr-1452.v2.yml

@@ -0,0 +1,6 @@
+type: fix
+fix:
+  description: When `checkClassUniqueness` fails, it will show the difference between
+    the actual and expected lock file.
+  links:
+  - https://github.com/palantir/gradle-baseline/pull/1452

gradle-baseline-java/build.gradle

@@ -17,6 +17,7 @@ dependencies {
     implementation 'com.palantir.javaformat:palantir-java-format-spi'
     // Add an explicit dependency to ensure consumers can use JDK14 source compat
     implementation 'org.ow2.asm:asm'
+    implementation 'com.googlecode.java-diff-utils:diffutils'
 
     runtimeOnly 'com.palantir.javaformat:gradle-palantir-java-format'
 

gradle-baseline-java/src/main/groovy/com/palantir/baseline/tasks/CheckClassUniquenessLockTask.java

@@ -16,11 +16,15 @@
 
 package com.palantir.baseline.tasks;
 
+import com.google.common.base.Splitter;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSortedMap;
+import difflib.DiffUtils;
+import difflib.Patch;
 import java.io.File;
 import java.util.Collection;
 import java.util.Comparator;
+import java.util.List;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
@@ -159,9 +163,26 @@ private void ensureLockfileContains(String expected) {
 
         String onDisk = GFileUtils.readFile(lockFile);
         if (!onDisk.equals(expected)) {
-            throw new GradleException(lockFile
-                    + " is out of date, please run `./gradlew "
-                    + "checkClassUniqueness --write-locks` to update this file");
+            List<String> onDiskLines = Splitter.on('\n').splitToList(onDisk);
+            Patch<String> diff = DiffUtils.diff(onDiskLines, Splitter.on('\n').splitToList(expected));
+
+            throw new GradleException(String.join(
+                    "\n",
+                    String.format(
+                            "%s is out of date, please run `./gradlew checkClassUniqueness --write-locks` "
+                                    + "to update this file. The diff is:",
+                            lockFile),
+                    "",
+                    String.join(
+                            "\n",
+                            DiffUtils.generateUnifiedDiff("on disk", "expected", onDiskLines, diff, Integer.MAX_VALUE)),
+                    "",
+                    "On disk was:",
+                    "",
+                    onDisk,
+                    "",
+                    "Expected was:",
+                    expected));
         }
     }
 

versions.lock

@@ -35,7 +35,7 @@ com.google.protobuf:protobuf-java:3.4.0 (1 constraints: f4102eb8)
 com.google.testing.compile:compile-testing:0.18 (1 constraints: 3114b84c)
 com.google.truth:truth:0.45 (2 constraints: 1d266427)
 com.googlecode.concurrent-trees:concurrent-trees:2.6.1 (1 constraints: 761166da)
-com.googlecode.java-diff-utils:diffutils:1.3.0 (2 constraints: 3a1dc41d)
+com.googlecode.java-diff-utils:diffutils:1.3.0 (3 constraints: 3f226bf4)
 com.googlecode.javaewah:JavaEWAH:1.1.7 (1 constraints: 490e7f50)
 com.jcraft:jsch:0.1.55 (1 constraints: 7b0ef35e)
 com.jcraft:jzlib:1.1.1 (1 constraints: 430e7950)

versions.props

@@ -13,6 +13,7 @@ org.jooq:jooq = 3.13.2
 org.slf4j:slf4j-api = 1.7.30
 org.immutables:* = 2.8.8
 org.ow2.asm:asm = 8.0.1
+com.googlecode.java-diff-utils:diffutils = 1.3.0
 
 # test deps
 com.fasterxml.jackson.*:* = 2.11.1