python/tools/java_blob_emitter.py

#!/usr/bin/env python3
# Copyright (C) 2026 The Android Open Source Project
#
# 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.

import argparse
import base64
import os
import textwrap

# The JVM specification strictly limits the bytecode of any single method (including
# static initializers `<clinit>`) to 65535 bytes. A simple `{ (byte)x, ... }` byte array
# generates push and store bytecode instructions per element. While the absolute
# mathematical threshold for standard javac is ~10,943 elements, we conservatively cap
# this at 8192 (8 KB) to guarantee safety across different compiler versions and pipelines.
JAVA_ARRAY_BYTE_LIMIT = 8192

TEMPLATE = """/*
 * Copyright (C) 2026 The Android Open Source Project
 *
 * 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.
 */

package {package};

/**
 * Autogenerated by java_blob_emitter.py
 */
public class {classname} {{
{class_body}
}}
"""


def generate_code(package: str, classname: str, data: bytes) -> str:
  # Store data as byte array literal
  if len(data) <= JAVA_ARRAY_BYTE_LIMIT:
    bytes = [f"(byte)0x{b:02x}" for b in data]
    bytes_joined = ", ".join(bytes)
    bytes_wrapped = textwrap.fill(
        bytes_joined,
        width=100,
        initial_indent="        ",
        subsequent_indent="        ")

    class_body = "    private static final byte[] BYTES = {\n"
    class_body += bytes_wrapped + "\n"
    class_body += "    };\n\n"
    class_body += "    public static byte[] getBytes() {\n"
    class_body += "        return BYTES;\n"
    class_body += "    }"
  # Store data as base64-encoded string
  else:
    bytes_base64 = base64.b64encode(data).decode('utf-8')
    class_body = f"    private static final String BYTES_BASE64 = \"{bytes_base64}\";\n\n"
    class_body += "    public static byte[] getBytes() {\n"
    class_body += "        return android.util.Base64.decode(BYTES_BASE64, android.util.Base64.DEFAULT);\n"
    class_body += "    }"

  return TEMPLATE.format(
      package=package, classname=classname, class_body=class_body)


def main():
  parser = argparse.ArgumentParser()
  parser.add_argument('--output', required=True)
  parser.add_argument('--package', required=True)
  parser.add_argument('input')
  args = parser.parse_args()

  with open(args.input, 'rb') as f:
    data = f.read()

  classname = os.path.basename(args.output).split('.', 1)[0]
  code = generate_code(args.package, classname, data)

  with open(args.output, 'w') as f:
    f.write(code)


if __name__ == '__main__':
  main()