Bazel doesn't support aab generation as it can be seen here: https://github.com/bazelbuild/bazel/issues/11497 . Based on this comment https://github.com/bazelbuild/bazel/issues/11497#issuecomment-1012090106, I tried to modify my build process to generate aab. But I am getting NullPointerException. Does any one know how to solve this issue?

wsluser@DESKTOP-1:/mnt/c/Users/user1/Documents/Misc/Dev/custom-mediapipe-fork$ bazel build -c opt --copt -DMESA_EGL_NO_X11_HEADERS --copt -DEGL_NO_X11 mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/hairsegmentationgpu:hairlight_application
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'com_google_absl' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'com_google_benchmark' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'pybind11_bazel' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'com_google_protobuf' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'com_google_googletest' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'com_github_gflags_gflags' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'build_bazel_rules_apple' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'build_bazel_rules_swift' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'build_bazel_apple_support' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'xctestrunner' because it already exists.
DEBUG: /home/wsluser/.cache/bazel/_bazel_wsluser/27576fa3ed0872646a64b44d0d0f4d1d/external/org_tensorflow/third_party/repo.bzl:122:14:
Warning: skipping import of repository 'pybind11' because it already exists.
WARNING: /mnt/c/Users/user1/Documents/Misc/Dev/custom-mediapipe-fork/mediapipe/framework/tool/BUILD:182:24: in cc_library rule //mediapipe/framework/tool:field_data_cc_proto: target '//mediapipe/framework/tool:field_data_cc_proto' depends on deprecated target '@com_google_protobuf//:cc_wkt_protos': Only for backward compatibility. Do not use.
WARNING: /mnt/c/Users/user1/Documents/Misc/Dev/custom-mediapipe-fork/mediapipe/framework/BUILD:54:24: in cc_library rule //mediapipe/framework:calculator_cc_proto: target '//mediapipe/framework:calculator_cc_proto' depends on deprecated target '@com_google_protobuf//:cc_wkt_protos': Only for backward compatibility. Do not use.
Analyzing: target //mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/hairsegmentationgpu:hairlight_application (171 packages loaded, 12409 targets configured)
    currently loading: @org_tensorflow//tensorflow/core/kernels
FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'ConfiguredTargetKey{label=//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/hairsegmentationgpu:hairlight_application_binary, config=BuildConfigurationValue.Key[24c3d57be09409ec5b3d2c93fae5528ea0f5267d0a2f463c174db0f2ce0825d8]}' (requested by nodes 'ConfiguredTargetKey{label=//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/hairsegmentationgpu:hairlight_application_binary.apk, config=BuildConfigurationValue.Key[24c3d57be09409ec5b3d2c93fae5528ea0f5267d0a2f463c174db0f2ce0825d8]}')
        at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:563)
        at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:398)
        at java.base/java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(Unknown Source)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
        at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)
Caused by: java.lang.NullPointerException
        at com.google.devtools.build.lib.rules.java.JavaCompilationHelper.usesAnnotationProcessing(JavaCompilationHelper.java:530)
        at com.google.devtools.build.lib.rules.java.JavaCompilationHelper.createOutputs(JavaCompilationHelper.java:187)
        at com.google.devtools.build.lib.rules.android.AndroidCommon.initJava(AndroidCommon.java:580)
        at com.google.devtools.build.lib.rules.android.AndroidCommon.init(AndroidCommon.java:517)
        at com.google.devtools.build.lib.rules.android.AndroidBinary.init(AndroidBinary.java:311)
        at com.google.devtools.build.lib.rules.android.AndroidBinary.create(AndroidBinary.java:118)
        at com.google.devtools.build.lib.rules.android.AndroidBinary.create(AndroidBinary.java:96)
        at com.google.devtools.build.lib.analysis.ConfiguredTargetFactory.createRule(ConfiguredTargetFactory.java:385)
        at com.google.devtools.build.lib.analysis.ConfiguredTargetFactory.createConfiguredTarget(ConfiguredTargetFactory.java:195)
        at com.google.devtools.build.lib.skyframe.SkyframeBuildView.createConfiguredTarget(SkyframeBuildView.java:940)
        at com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction.createConfiguredTarget(ConfiguredTargetFunction.java:1031)
        at com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction.compute(ConfiguredTargetFunction.java:371)
        at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:477)

hairlight_android_application.bzl

"""
Macros pertaining to building & managing Android app bundles.
"""

def _convert_apk_to_aab_module_impl(ctx):
    output_file = ctx.outputs.output_file
    input_file = ctx.attr.input_file.files.to_list()[0]

    # See aapt2 help documentation for details on the arguments passed here.
    arguments = [
        "convert",
        "--output-format",
        "proto",
        "-o",
        output_file.path,
        input_file.path,
    ]

    # Reference: https://docs.bazel.build/versions/master/skylark/lib/actions.html#run.
    ctx.actions.run(
        outputs = [output_file],
        inputs = ctx.files.input_file,
        tools = [ctx.executable._aapt2_tool],
        executable = ctx.executable._aapt2_tool.path,
        arguments = arguments,
        mnemonic = "GenerateAndroidAppBundleModuleFromApk",
        progress_message = "Generating deployable AAB",
    )
    return DefaultInfo(
        files = depset([output_file]),
        runfiles = ctx.runfiles(files = [output_file]),
    )

def _convert_module_aab_to_structured_zip_impl(ctx):
    output_file = ctx.outputs.output_file
    input_file = ctx.attr.input_file.files.to_list()[0]

    command = """
    # Extract AAB to working directory.
    WORKING_DIR=$(mktemp -d)
    unzip -d $WORKING_DIR {0}
    # Create the expected directory structure for an app bundle.
    # Reference for copying all other files to root: https://askubuntu.com/a/951768.
    mkdir -p $WORKING_DIR/assets $WORKING_DIR/dex $WORKING_DIR/manifest $WORKING_DIR/root
    mv $WORKING_DIR/*.dex $WORKING_DIR/dex/
    mv $WORKING_DIR/AndroidManifest.xml $WORKING_DIR/manifest/
    ls -d $WORKING_DIR/* | grep -v -w -E "res|assets|dex|manifest|root|resources.pb" | xargs -I{{}} sh -c "mv \\$0 $WORKING_DIR/root/ || exit 255" {{}} 2>&1 || exit $?
    # Zip up the result--this will be used by bundletool to build a deployable AAB. Note that these
    # strange file path bits are needed because zip will always retain the directory structure
    # passed via arguments (necessitating changing into the working directory).
    DEST_FILE_PATH="$(pwd)/{1}"
    cd $WORKING_DIR
    zip -r $DEST_FILE_PATH .
    """.format(input_file.path, output_file.path)

    # Reference: https://docs.bazel.build/versions/main/skylark/lib/actions.html#run_shell.
    ctx.actions.run_shell(
        outputs = [output_file],
        inputs = ctx.files.input_file,
        tools = [],
        command = command,
        mnemonic = "ConvertModuleAabToStructuredZip",
        progress_message = "Generating deployable AAB",
    )
    return DefaultInfo(
        files = depset([output_file]),
        runfiles = ctx.runfiles(files = [output_file]),
    )

def _bundle_module_zip_into_deployable_aab_impl(ctx):
    output_file = ctx.outputs.output_file
    input_file = ctx.attr.input_file.files.to_list()[0]
    config_file = ctx.attr.config_file.files.to_list()[0]

    # Reference: https://developer.android.com/studio/build/building-cmdline#build_your_app_bundle_using_bundletool.
    arguments = [
        "build-bundle",
        "--modules=%s" % input_file.path,
        "--config=%s" % config_file.path,
        "--output=%s" % output_file.path,
    ]

    # Reference: https://docs.bazel.build/versions/master/skylark/lib/actions.html#run.
    ctx.actions.run(
        outputs = [output_file],
        inputs = ctx.files.input_file + ctx.files.config_file,
        tools = [ctx.executable._bundletool_tool],
        executable = ctx.executable._bundletool_tool.path,
        arguments = arguments,
        mnemonic = "GenerateDeployAabFromModuleZip",
        progress_message = "Generating deployable AAB",
    )
    return DefaultInfo(
        files = depset([output_file]),
        runfiles = ctx.runfiles(files = [output_file]),
    )

def _package_metadata_into_deployable_aab_impl(ctx):
    output_aab_file = ctx.outputs.output_aab_file
    input_aab_file = ctx.attr.input_aab_file.files.to_list()[0]
    proguard_map_file = ctx.attr.proguard_map_file.files.to_list()[0]

    command = """
    # Extract deployable AAB to working directory.
    WORKING_DIR=$(mktemp -d)
    echo $WORKING_DIR
    cp {0} $WORKING_DIR/temp.aab || exit 255
    # Change the permissions of the AAB copy so that it can be overwritten later.
    chmod 755 $WORKING_DIR/temp.aab || exit 255
    # Create directory needed for storing bundle metadata.
    mkdir -p $WORKING_DIR/BUNDLE-METADATA/com.android.tools.build.obfuscation
    # Copy over the Proguard map file.
    cp {1} $WORKING_DIR/BUNDLE-METADATA/com.android.tools.build.obfuscation/proguard.map || exit 255
    $ Repackage the AAB file into the destination.
    DEST_FILE_PATH="$(pwd)/{2}"
    cd $WORKING_DIR
    zip -Dur temp.aab BUNDLE-METADATA || exit 255
    cp temp.aab $DEST_FILE_PATH || exit 255
    """.format(input_aab_file.path, proguard_map_file.path, output_aab_file.path)

    # Reference: https://docs.bazel.build/versions/main/skylark/lib/actions.html#run_shell.
    ctx.actions.run_shell(
        outputs = [output_aab_file],
        inputs = ctx.files.input_aab_file + ctx.files.proguard_map_file,
        tools = [],
        command = command,
        mnemonic = "PackageMetadataIntoDeployableAAB",
        progress_message = "Generating deployable AAB",
    )
    return DefaultInfo(
        files = depset([output_aab_file]),
        runfiles = ctx.runfiles(files = [output_aab_file]),
    )

def _generate_apks_and_install_impl(ctx):
    input_file = ctx.attr.input_file.files.to_list()[0]
    debug_keystore_file = ctx.attr.debug_keystore.files.to_list()[0]
    apks_file = ctx.actions.declare_file("%s_processed.apks" % ctx.label.name)
    deploy_shell = ctx.actions.declare_file("%s_run.sh" % ctx.label.name)

    # Reference: https://developer.android.com/studio/command-line/bundletool#generate_apks.
    # See also the Bazel BUILD file for the keystore for details on its password and alias.
    generate_apks_arguments = [
        "build-apks",
        "--bundle=%s" % input_file.path,
        "--output=%s" % apks_file.path,
        "--ks=%s" % debug_keystore_file.path,
        "--ks-pass=pass:android",
        "--ks-key-alias=androiddebugkey",
        "--key-pass=pass:android",
    ]

    # Reference: https://docs.bazel.build/versions/master/skylark/lib/actions.html#run.
    ctx.actions.run(
        outputs = [apks_file],
        inputs = ctx.files.input_file + ctx.files.debug_keystore,
        tools = [ctx.executable._bundletool_tool],
        executable = ctx.executable._bundletool_tool.path,
        arguments = generate_apks_arguments,
        mnemonic = "BuildApksFromDeployAab",
        progress_message = "Preparing AAB deploy to device",
    )

    # References: https://github.com/bazelbuild/bazel/issues/7390,
    # https://developer.android.com/studio/command-line/bundletool#deploy_with_bundletool, and
    # https://docs.bazel.build/versions/main/skylark/rules.html#executable-rules-and-test-rules.
    # Note that the bundletool can be executed directly since Bazel creates a wrapper script that
    # utilizes its own internal Java toolchain.
    ctx.actions.write(
        output = deploy_shell,
        content = """
        #!/bin/sh
        {0} install-apks --apks={1}
        echo The APK should now be installed
        """.format(ctx.executable._bundletool_tool.short_path, apks_file.short_path),
        is_executable = True,
    )

    # Reference for including necessary runfiles for Java:
    # https://github.com/bazelbuild/bazel/issues/487#issuecomment-178119424.
    runfiles = ctx.runfiles(
        files = [
            ctx.executable._bundletool_tool,
            apks_file,
        ],
    ).merge(ctx.attr._bundletool_tool.default_runfiles)
    return DefaultInfo(
        executable = deploy_shell,
        runfiles = runfiles,
    )

_convert_apk_to_module_aab = rule(
    attrs = {
        "input_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "output_file": attr.output(
            mandatory = True,
        ),
        "_aapt2_tool": attr.label(
            executable = True,
            cfg = "host",
            default = "@androidsdk//:aapt2_binary",
        ),
    },
    implementation = _convert_apk_to_aab_module_impl,
)

_convert_module_aab_to_structured_zip = rule(
    attrs = {
        "input_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "output_file": attr.output(
            mandatory = True,
        ),
    },
    implementation = _convert_module_aab_to_structured_zip_impl,
)

_bundle_module_zip_into_deployable_aab = rule(
    attrs = {
        "input_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "config_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "output_file": attr.output(
            mandatory = True,
        ),
        "_bundletool_tool": attr.label(
            executable = True,
            cfg = "host",
            default = "//third_party:android_bundletool",
        ),
    },
    implementation = _bundle_module_zip_into_deployable_aab_impl,
)

_package_metadata_into_deployable_aab = rule(
    attrs = {
        "input_aab_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "proguard_map_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "output_aab_file": attr.output(
            mandatory = True,
        ),
    },
    implementation = _package_metadata_into_deployable_aab_impl,
)

_generate_apks_and_install = rule(
    attrs = {
        "input_file": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "debug_keystore": attr.label(
            allow_single_file = True,
            mandatory = True,
        ),
        "_bundletool_tool": attr.label(
            executable = True,
            cfg = "host",
            default = "//third_party:android_bundletool",
        ),
    },
    executable = True,
    implementation = _generate_apks_and_install_impl,
)

def hairlight_android_application(name, config_file, proguard_generate_mapping, **kwargs):
    """
    Creates an Android App Bundle (AAB) binary with the specified name and arguments.
    Args:
        name: str. The name of the Android App Bundle to build. This will corresponding to the name
            of the generated .aab file.
        config_file: target. The path to the .pb.json bundle configuration file for this build.
        proguard_generate_mapping: boolean. Whether to perform a Proguard optimization step &
            generate Proguard mapping corresponding to the obfuscation step.
        **kwargs: additional arguments. See android_binary for the exact arguments that are
            available.
    """
    binary_name = "%s_binary" % name
    module_aab_name = "%s_module_aab" % name
    module_zip_name = "%s_module_zip" % name
    deployable_aab_name = "%s_deployable" % name
    native.android_binary(
        name = binary_name,
        tags = ["manual"],
        proguard_generate_mapping = proguard_generate_mapping,
        **kwargs
    )
    _convert_apk_to_module_aab(
        name = module_aab_name,
        input_file = ":%s.apk" % binary_name,
        output_file = "%s.aab" % module_aab_name,
        tags = ["manual"],
    )
    _convert_module_aab_to_structured_zip(
        name = module_zip_name,
        input_file = ":%s.aab" % module_aab_name,
        output_file = "%s.zip" % module_zip_name,
        tags = ["manual"],
    )
    # No extra package step is needed if there's no Proguard map file.
    _bundle_module_zip_into_deployable_aab(
        name = name,
        input_file = ":%s.zip" % module_zip_name,
        config_file = config_file,
        output_file = "%s.aab" % name,
        tags = ["manual"],
    )

def declare_deployable_application(name, aab_target):
    """
    Creates a new target that can be run with 'bazel run' to install an AAB file.
    Example:
        declare_deployable_application(
            name = "install_oppia_prod",
            aab_target = "//:oppia_prod",
        )
        $ bazel run //:install_oppia_prod
    This will build (if necessary) and install the correct APK derived from the Android app bundle
    on the locally attached emulator or device. Note that this command does not support targeting a
    specific device so it will not work if more than one device is available via 'adb devices'.
    Args:
        name: str. The name of the runnable target to install an AAB file on a local device.
        aab_target: target. The target (declared via oppia_android_application) that should be made
            installable.
    """
    _generate_apks_and_install(
        name = name,
        input_file = aab_target,
        debug_keystore = "@bazel_tools//tools/android:debug_keystore",
        tags = ["manual"],
    )

BUILD

# Copyright 2019 The MediaPipe Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

licenses(["notice"])
load("//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/hairsegmentationgpu:hairlight_android_application.bzl", "declare_deployable_application", "hairlight_android_application")
package(default_visibility = ["//visibility:private"])

cc_binary(
    name = "libmediapipe_jni.so",
    linkshared = 1,
    linkstatic = 1,
    deps = [
        "//mediapipe/graphs/hair_segmentation:mobile_calculators",
        "//mediapipe/java/com/google/mediapipe/framework/jni:mediapipe_framework_jni",
    ],
)

cc_library(
    name = "mediapipe_jni_lib",
    srcs = [":libmediapipe_jni.so"],
    alwayslink = 1,
)

android_binary(
    name = "hairlight_binary",
    srcs = glob(["*.java"]),
    assets = [
        "//mediapipe/graphs/hair_segmentation:mobile_gpu.binarypb",
        "//mediapipe/models:hair_segmentation.tflite",
    ],
    assets_dir = "",
    manifest = "//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/basic:AndroidManifest.xml",
    manifest_values = {
        "applicationId": "com.hairlight.hairsegmentation.apps.hairsegmentationgpu",
        "mainActivity": "com.hairlight.hairsegmentation.apps.basic.MainActivity"
    },
    multidex = "native",
    deps = [
        ":mediapipe_jni_lib",
        "//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/basic:basic_lib",
    ]
)

hairlight_android_application(
    name = "hairlight_application",
    custom_package = "org.oppia.android",
    testonly = False,
    enable_data_binding = True,
    config_file = ":bundle_config.pb.json",
    manifest = "//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/basic:AndroidManifest.xml",
    manifest_values = {
        "applicationId": "com.hairlight.hairsegmentation.apps.hairsegmentationgpu",
        "mainActivity": "com.hairlight.hairsegmentation.apps.basic.MainActivity"
    },
    proguard_generate_mapping = False,
    proguard_specs = [],
    shrink_resources = False,
    multidex = "native",
    deps = [
        ":mediapipe_jni_lib",
        "//mediapipe/examples/android/src/java/com/hairlight/hairsegmentation/apps/basic:basic_lib",
    ],
)

I am on bazel version 4.2.1