Add jcmp plugin to ease up spotting api differences for muzzle

Author: amarziali

build.gradle.kts

@@ -12,6 +12,7 @@ plugins {
   id("dd-trace-java.ci-jobs")
 
   id("com.diffplug.spotless") version "8.1.0"
+  id("me.champeau.gradle.japicmp") version "0.4.3"
   id("com.github.spotbugs") version "6.4.7"
   id("de.thetaphi.forbiddenapis") version "3.10"
   id("io.github.gradle-nexus.publish-plugin") version "2.0.0"
@@ -156,3 +157,25 @@ testAggregate(
     ":dd-java-agent:agent-debugger"
   )
 )
+
+// JApiCmp configuration example
+// Usage: ./gradlew japicmp -Partifact=groupId:artifactId -Pbaseline=1.0.0 -Ptarget=2.0.0
+tasks.register<me.champeau.gradle.japicmp.JapicmpTask>("japicmp") {
+  val artifact = providers.gradleProperty("artifact").orNull
+  val baseline = providers.gradleProperty("baseline").orNull
+  val target = providers.gradleProperty("target").orNull
+
+  if (artifact != null && baseline != null && target != null) {
+    oldClasspath.from(configurations.detachedConfiguration(
+      dependencies.create("$artifact:$baseline")
+    ))
+    newClasspath.from(configurations.detachedConfiguration(
+      dependencies.create("$artifact:$target")
+    ))
+    onlyModified.set(true)
+    failOnModification.set(false)
+    ignoreMissingClasses.set(true)
+    txtOutputFile.set(layout.buildDirectory.file("reports/japicmp.txt"))
+    htmlOutputFile.set(layout.buildDirectory.file("reports/japicmp.html"))
+  }
+}

docs/how_instrumentations_work.md

@@ -91,14 +91,45 @@ To run muzzle on your instrumentation, run:
 
 > [!WARNING]
 > Muzzle does _not_ run tests.
-> It checks that the types and methods used by the instrumentation are present in particular versions of libraries. 
-> It can be subverted with `MethodHandle` and reflection -- in other words, having the `muzzle` task passing is not enough 
+> It checks that the types and methods used by the instrumentation are present in particular versions of libraries.
+> It can be subverted with `MethodHandle` and reflection -- in other words, having the `muzzle` task passing is not enough
 > to validate an instrumentation.
 
 By default, all the muzzle directives are checked against all the instrumentations included in a module.
-However, there can be situations in which it’s only needed to check one specific directive on an instrumentation.
+However, there can be situations in which it's only needed to check one specific directive on an instrumentation.
 At this point the instrumentation should override the method `muzzleDirective()` by returning the name of the directive to execute.
 
+### Identifying Breaking Changes with JApiCmp
+
+Before defining muzzle version ranges, you can use the JApiCmp plugin to compare different versions of a library and
+identify breaking API changes. This helps determine where to split version ranges in your muzzle directives.
+
+The `japicmp` task compares two versions of a Maven artifact and reports:
+- Removed classes and methods (breaking changes)
+- Added classes and methods (non-breaking changes)
+- Modified methods with binary compatibility status
+- Suggestions for muzzle version ranges based on breaking changes
+
+#### Usage
+
+Compare two versions of any Maven artifact:
+
+```shell
+./gradlew japicmp -Partifact=groupId:artifactId -Pbaseline=oldVersion -Ptarget=newVersion
+```
+
+For example, to compare MongoDB driver versions:
+
+```shell
+./gradlew japicmp -Partifact=org.mongodb:mongodb-driver-sync -Pbaseline=3.11.0 -Ptarget=4.0.0
+```
+
+#### Output
+
+The task generates two reports:
+
+- **Text report**: `build/reports/japicmp.txt` - Detailed line-by-line comparison
+- **HTML report**: `build/reports/japicmp.html` - Browsable visual report
 
 ## Instrumentation classes