Kotlin incremental compilation in gradle allows gradle to recompile only changed and related files.

Enable kotlin incremental compilation
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In gradle.properties

kotlin.build.report.output=file
kotlin.incremental.useClasspathSnapshot=true
kotlin.incremental=true
kotlin.incremental.java=true

Run the project and check report file in build/reports/kotlin-build/

Build attributes:
  REBUILD_REASON: Unknown Gradle changes

Total time for Kotlin tasks: 1,924.09 s (93.8 % of all tasks time)
Time      |% of Kotlin time|Task
1,924.09 s|100.0 %         |:app:compileDevelopDebugKotlin

Task ':app:compileDevelopDebugKotlin' finished in 1,924.09 s
Compilation log for task ':app:compileDevelopDebugKotlin':
  Use module detection: true
  Non-incremental compilation will be performed: UNKNOWN_CHANGES_IN_GRADLE_INPUTS
  Cleaning outputs on rebuild
    Deleting contents of directory '/Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug'
    Deleting contents of directory '/Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/kotlin/compileDevelopDebugKotlin/cach
eable'
    Deleting contents of directory '/Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/kotlin/compileDevelopDebugKotlin/loca
l-state'
  Non-incremental compilation will be performed: UNKNOWN_CHANGES_IN_GRADLE_INPUTS

The project’s app module was completly rebuild because something changed in our gradle scripts.

Modify some source file BlowfishApp.kt and build again, the report become

Build attributes:
  REBUILD_REASON: Incremental compilation is not enabled

Total time for Kotlin tasks: 1,125.33 s (93.9 % of all tasks time)
Time      |% of Kotlin time|Task
1,118.77 s|99.4 %          |:app:compileDevelopDebugKotlin
6.56 s    |0.6 %           |:app:kaptGenerateStubsDevelopDebugKotlin

But also in the report file says Performing incremental compilation

  Performing incremental compilation
  Deleting /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug/com/blowfish/member/BlowfishApp$When
Mappings.class on clearing cache for /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/BlowfishApp.kt
  Deleting /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug/com/blowfish/member/BlowfishApp$blowfishBroadcastReceiver.class on clearing cache for /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/BlowfishApp.kt
  Deleting /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug/com/blowfish/member/BlowfishApp$onCreate$1.class on clearing cache for /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/BlowfishApp.kt
  Deleting /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug/com/blowfish/member/BlowfishApp$spec
ial$$inlined$inject$default$1.class on clearing cache for /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/BlowfishApp.kt
  Deleting /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug/com/blowfish/member/BlowfishApp$onCreate$4.class on clearing cache for /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/BlowfishApp.kt
  Deleting /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/build/tmp/kotlin-classes/developDebug/com/blowfish/member/BlowfishApp$init
AutoTrackerDCS$1.class on clearing cache for /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/BlowfishApp.kt

Reasons incremental compilation not working
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Unknown Gradle changes
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Gradle changes includes:

add/remove dependency library, change library version
change version name, version code of an Android project.
Any other file changes of gradle, or files included by gradle by apply(from = "../config/config.gradle.kts")
gradle.properties changes.
Change of gradlew arguments, such as run with ./gradlew --debug

Changes above will also invalidate gradle build caches

Remove of .java file
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 REBUILD_REASON:
    Incremental compilation is not enabled(1)
    Untracked Java file is removed(2)

 Could not get changed for untracked removed java file /Users/blowfish/Desktop/git/blowfish/blowfish-android-member/app/src/main/java/com/blowfish/member/util/Point.java
  Non-incremental compilation will be performed: JAVA_CHANGE_UNTRACKED_FILE_IS_REMOVED

Incremental compilation is not enabled
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People discuss about this, but no further information.

From kotlin source code, incremental compilation flag is from config isIncremental.

More gradle build info
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If run gradlew with --debug, you get more information about whether build is incremental or not.

./gradlew --debug  app:assembleDevelopDebug |tee app.8.1-2.log

Check log file

$ grep -i "incremental" app.8.1-2.log|less

...
[DEBUG] [org.gradle.api.Task] [KOTLIN] Set task ':app:compileDevelopDebugKotlin'.incremental=true
[DEBUG] [org.gradle.api.Task] [KOTLIN] Set task ':app:compileDevelopDebugKotlin'.incremental=false
[DEBUG] [org.gradle.api.Task] [KOTLIN] Set task ':app:compileDevelopDebugKotlin'.incremental=true
...

[INFO] [org.gradle.internal.execution.steps.ResolveInputChangesStep] The input changes require a full rebuild for incremental task ':app:compileDevelopDebugKotlin'.

At first .incremental config is set to true, but if you see something like The input changes require a full rebuild, that task will not run incrementally.

We could not identify the root cause, because only the :app module in project has this problem, other modules could build incrementally.

This occurs after upgrading gradle from 7.6 to 8.2. After revert gradle back to latest 7.x and a fully rebuild. We could build :app module incrementally again.

Unconfirmed
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:app modules contains aboud 2000 source files
The use of both kapt (dagger, autovalue, reductor) and ksp (epoxy)

TODO
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Decouple dependency in :app module, separate to different gradle modules, hopefully they can build in parallel.
Check kotlin build report on jenkins,if any changes cause incremental build to fail, we would notice at first time.

Enable kotlin incremental compilation #

Reasons incremental compilation not working #

Unknown Gradle changes #

Remove of .java file #

Incremental compilation is not enabled #

More gradle build info #

Unconfirmed #

TODO #