Android protobuf nano documentation

Question

Looking at the main protobuf compiler source code:

#include <google/protobuf/compiler/javanano/javanano_generator.h>
.... 

int main(int argc, char* argv[]) {

    google::protobuf::compiler::CommandLineInterface cli;
    cli.AllowPlugins("protoc-");
    ...
    // Proto2 JavaNano
    google::protobuf::compiler::javanano::JavaNanoGenerator javanano_generator;
    cli.RegisterGenerator("--javanano_out", &javanano_generator,
    "Generate Java source file nano runtime.");

    return cli.Run(argc, argv);
}

looking at protobuf's Readme

Nano Generator options

java_package -> <file-name>|<package-name>

java_outer_classname -> <file-name>|<package-name>

java_multiple_files -> true or false

java_nano_generate_has -> true or false [DEPRECATED]

optional_field_style -> default or accessors

enum_style -> c or java

To use nano protobufs outside of Android repo:

Link with the generated jar file <protobuf-root>java/target/protobuf-java-2.3.0-nano.jar.

Invoke with --javanano_out, e.g.:

./protoc '--javanano_out=java_package=src/proto/simple-data.proto|my_package,java_outer_classname=src/proto/simple-data.proto|OuterName:.' src/proto/simple-data.proto

To use nano protobufs within the Android repo:

Set 'LOCAL_PROTOC_OPTIMIZE_TYPE := nano' in your local .mk file. When building a Java library or an app (package) target, the build
system will add the Java nano runtime library to the
LOCAL_STATIC_JAVA_LIBRARIES variable, so you don't need to.

Set 'LOCAL_PROTO_JAVA_OUTPUT_PARAMS := ...' in your local .mk file for any command-line options you need. Use commas to join multiple
options. In the nano flavor only, whitespace surrounding the option
names and values are ignored, so you can use backslash-newline or
'+=' to structure your make files nicely.

The options will be applied to all proto files in LOCAL_SRC_FILES when you build a Java library or package. In case different options
are needed for different proto files, build separate Java libraries
and reference them in your main target. Note: you should make sure
that, for each separate target, all proto files imported from any
proto file in LOCAL_SRC_FILES are included in LOCAL_SRC_FILES. This
is because the generator has to assume that the imported files are
built using the same options, and will generate code that reference
the fields and enums from the imported files using the same code
style.

Hint: 'include $(CLEAR_VARS)' resets all LOCAL_ variables, including the two above.

Simple nano example from https://android.googlesource.com/platform/external/protobuf/+/master/src/google/protobuf/.

unittest_simple_nano.proto

package protobuf_unittest_import;

option java_package = "com.google.protobuf.nano";
// Explicit outer classname to suppress legacy info.
option java_outer_classname = "UnittestSimpleNano";

message SimpleMessageNano {
  message NestedMessage {
    optional int32 bb = 1;
  }

  enum NestedEnum {
    FOO = 1;
    BAR = 2;
    BAZ = 3;
  }

  optional int32 d = 1 [default = 123];
  optional NestedMessage nested_msg = 2;
  optional NestedEnum default_nested_enum = 3 [default = BAZ];
}

Command line

./protoc '--javanano_out=java_package=google/protobuf/unittest_simple_nano.proto|com.google.protobuf.nano,java_outer_classname=google/protobuf/unittest_simple_nano.proto|UnittestSimpleNano:target/generated-test-sources' google/protobuf/unittest_simple_nano.proto

Test extracted from NanoTest.java

  public void testSimpleMessageNano() throws Exception {
    SimpleMessageNano msg = new SimpleMessageNano();
    assertEquals(123, msg.d);
    assertEquals(null, msg.nestedMsg);
    assertEquals(SimpleMessageNano.BAZ, msg.defaultNestedEnum);

    msg.d = 456;
    assertEquals(456, msg.d);

    SimpleMessageNano.NestedMessage nestedMsg = new SimpleMessageNano.NestedMessage();
    nestedMsg.bb = 2;
    assertEquals(2, nestedMsg.bb);
    msg.nestedMsg = nestedMsg;
    assertEquals(2, msg.nestedMsg.bb);

    msg.defaultNestedEnum = SimpleMessageNano.BAR;
    assertEquals(SimpleMessageNano.BAR, msg.defaultNestedEnum);

    byte [] result = MessageNano.toByteArray(msg);
    int msgSerializedSize = msg.getSerializedSize();
    //System.out.printf("mss=%d result.length=%d\n", msgSerializedSize, result.length);
    assertTrue(msgSerializedSize == 9);
    assertEquals(result.length, msgSerializedSize);

    SimpleMessageNano newMsg = SimpleMessageNano.parseFrom(result);
    assertEquals(456, newMsg.d);
    assertEquals(2, msg.nestedMsg.bb);
    assertEquals(SimpleMessageNano.BAR, msg.defaultNestedEnum);
  }

There are a lot of test cases in the same class, and looking at the project pom you can find a maven-antrun-plugin configuration to generate that test resource classes

<!-- java nano -->
<exec executable="../src/protoc">
  <arg value="--javanano_out=java_package=google/protobuf/unittest_import_nano.proto|com.google.protobuf.nano,java_outer_classname=google/protobuf/unittest_import_nano.proto|UnittestImportNano:target/generated-test-sources" />
  <arg value="--proto_path=../src" />
  <arg value="--proto_path=src/test/java" />
  <arg value="../src/google/protobuf/unittest_nano.proto" />
  <arg value="../src/google/protobuf/unittest_simple_nano.proto" />
  <arg value="../src/google/protobuf/unittest_stringutf8_nano.proto" />
  <arg value="../src/google/protobuf/unittest_recursive_nano.proto" />
  <arg value="../src/google/protobuf/unittest_import_nano.proto" />
  <arg value="../src/google/protobuf/unittest_enum_multiplejava_nano.proto" />
  <arg value="../src/google/protobuf/unittest_multiple_nano.proto" />
</exec>

Hope this helps.