TFLite Flex Delegate for Android (16KB Page Size)

CI repository for building libtensorflowlite_flex_jni.so with 16KB page size support for Android.

Why

Starting with Android 15, arm64 devices may use 16KB page size instead of 4KB. Google Play requires 16KB compatibility for new apps.

The pre-built libtensorflowlite_flex_jni.so shipped in the Maven artifact org.tensorflow:tensorflow-lite-select-tf-ops does not support 16KB page alignment. This repository builds the JNI library from source with the correct linker flags:

• -Wl,-z,max-page-size=16384
• -Wl,-z,common-page-size=16384

Architectures

ABI	Description
arm64-v8a	64-bit ARM (modern devices)
armeabi-v7a	32-bit ARM (legacy devices)
x86_64	x86_64 (emulators, ChromeOS)

Running the Build

Manual: Actions → Build TFLite Flex Delegate for Android → Run workflow

You can specify a TensorFlow git ref (branch, tag, or SHA). Defaults to master.

Automatic: triggers on changes to patches/ or the workflow file.

Artifacts & Releases

Each build creates a GitHub Release tagged tf-<commit-hash> with two files attached:

• libtensorflowlite_flex_jni.so-arm64-v8a
• libtensorflowlite_flex_jni.so-armeabi-v7a
• libtensorflowlite_flex_jni.so-x86_64

Replacing .so in a project using flutter_litert_flex

There are two ways to replace it with our 16KB-aligned build:

---

Option A — Automatic download via build.gradle.kts (recommended)

Add the following to your app-level android/app/build.gradle.kts. Gradle will download the .so files from GitHub Releases on first build and cache them in the build directory.

import java.net.URI

// --- Flex Delegate 16KB config ---
val flexDelegateRepo = "arxdeus/tflite_flex_16kb_android"  // ← your repo
val flexDelegateTag = "latest"  // or a specific tag like "tf-73ef2dec449a"
val flexDelegateAbis = listOf("arm64-v8a", "armeabi-v7a", "x86_64")
val flexDelegateCacheDir = layout.buildDirectory.dir("flex-delegate")

val downloadFlexDelegate by tasks.registering {
    val cacheDir = flexDelegateCacheDir.get().asFile
    outputs.dir(cacheDir)

    onlyIf {
        flexDelegateAbis.any { abi ->
            !File(cacheDir, "$abi/libtensorflowlite_flex_jni.so").exists()
        }
    }

    doLast {
        val baseUrl = if (flexDelegateTag == "latest") {
            "https://github.com/$flexDelegateRepo/releases/latest/download"
        } else {
            "https://github.com/$flexDelegateRepo/releases/download/$flexDelegateTag"
        }

        flexDelegateAbis.forEach { abi ->
            val target = File(cacheDir, "$abi/libtensorflowlite_flex_jni.so")
            if (!target.exists()) {
                val url = "$baseUrl/libtensorflowlite_flex_jni.so-$abi"
                logger.lifecycle("⬇ Downloading flex delegate ($abi)...")
                target.parentFile.mkdirs()
                URI(url).toURL().openStream().use { input ->
                    target.outputStream().use { output -> input.copyTo(output) }
                }
                logger.lifecycle("  ${target.length() / 1_048_576} MB → $target")
            }
        }
    }
}

android {
    // ... your existing config ...

    defaultConfig {
        ndk {
            abiFilters += flexDelegateAbis
        }
    }

    sourceSets.getByName("main") {
        jniLibs.srcDir(flexDelegateCacheDir)
    }

    packaging {
        jniLibs {
            pickFirsts += flexDelegateAbis.map { "lib/$it/libtensorflowlite_flex_jni.so" }
        }
    }
}

tasks.configureEach {
    if (name.startsWith("merge") && name.endsWith("NativeLibs")) {
        dependsOn(downloadFlexDelegate)
    }
}

How it works:

• On first flutter build apk, Gradle downloads both .so files into build/flex-delegate/
• Subsequent builds reuse the cache (files survive until flutter clean)
• sourceSets.jniLibs.srcDir adds the cache dir as a native library source
• pickFirsts resolves the conflict with the Maven-bundled .so
• Nothing is committed to git — the build directory is already in .gitignore

To force re-download: cd android && ./gradlew clean

---

Option B — Manual download

Step 1 — Add the custom .so files

Download both files from the latest release, rename them (drop the ABI suffix), and place into your app:

your_app/android/app/src/main/jniLibs/
├── arm64-v8a/libtensorflowlite_flex_jni.so
├── armeabi-v7a/libtensorflowlite_flex_jni.so
└── x86_64/libtensorflowlite_flex_jni.so

Step 2 — Configure Gradle

In android/app/build.gradle.kts:

android {
    defaultConfig {
        ndk {
            abiFilters += listOf("arm64-v8a", "armeabi-v7a", "x86_64")
        }
    }

    packaging {
        jniLibs {
            pickFirsts += listOf(
                "lib/arm64-v8a/libtensorflowlite_flex_jni.so",
                "lib/armeabi-v7a/libtensorflowlite_flex_jni.so",
                "lib/x86_64/libtensorflowlite_flex_jni.so",
            )
        }
    }
}

Groovy version (build.gradle)

android {
    defaultConfig {
        ndk {
            abiFilters 'arm64-v8a', 'armeabi-v7a', 'x86_64'
        }
    }

    packagingOptions {
        pickFirst 'lib/arm64-v8a/libtensorflowlite_flex_jni.so'
        pickFirst 'lib/armeabi-v7a/libtensorflowlite_flex_jni.so'
        pickFirst 'lib/x86_64/libtensorflowlite_flex_jni.so'
    }
}

Step 3 — Add to .gitignore (optional)

The .so files are ~110 MB each:

android/app/src/main/jniLibs/**/libtensorflowlite_flex_jni.so

---

Verifying the replacement

Build the APK and confirm 16KB alignment:

flutter build apk

# Check the .so inside the APK
unzip -l build/app/outputs/flutter-apk/app-release.apk | grep libtensorflowlite_flex_jni

# Verify 16KB alignment
unzip -o build/app/outputs/flutter-apk/app-release.apk \
  lib/arm64-v8a/libtensorflowlite_flex_jni.so -d /tmp
readelf -l /tmp/lib/arm64-v8a/libtensorflowlite_flex_jni.so | grep -i align

The LOAD segment alignment should show 0x4000 (16384).

How it works

The flutter_litert_flex plugin declares a Maven dependency on org.tensorflow:tensorflow-lite-select-tf-ops, which provides:

• Java class org.tensorflow.lite.flex.FlexDelegate — still needed, kept as-is
• Native library libtensorflowlite_flex_jni.so — replaced by our 16KB-aligned build

The pickFirsts directive resolves the duplicate: Gradle picks our copy and discards the one from the AAR.

Patches

fix-android-flex-delegate-build.patch

Fixes duplicate symbol linker errors when building the flex delegate for Android.

Root cause: portable_tensorflow_lib_lite unconditionally depends on onednn_env_vars (Intel MKL — useless on ARM), which pulls in //tensorflow/core:framework (the desktop framework library). On Android, framework_internal_impl gets linked via if_static(), duplicating symbols already present in portable_tensorflow_lib_lite (graph_def_util.cc, kernel_def_builder.cc, graph_to_functiondef.cc, etc.).

Fix:

• Guard onednn_env_vars behind if_not_mobile()
• Add explicit protobuf and executor deps that were previously transitive through framework_internal_impl

Versions

Component	Version
Android NDK	r28c (28.2.13676358)
Android SDK	API 36
Build Tools	36.1.0
Bazel	7.7.0 (from TF .bazelversion)
Python	3.12 (hermetic)