javanano is only available from v3.0.0-alpha-2. You can download its sources here: https://github.com/google/protobuf/releases

@xfxyjwf I still fail to compile (getting the same message), using the same command. I've tried running the command on all possible files:
protobuf-java-3.0.0-alpha-2.zip
protobuf-javanano-3.0.0-alpha-2.zip
Source code (zip)

I've used the latest compiler:
protoc-3.0.0-alpha-2-win32.zip

Can you please provide a step by step explanation?
Will it create all needed files? for example the "BackupProtos.java" file?

Is that a single dash in the option, i.e., -javanano_out ? Try it with two dashes: --javanano_out.

@cfallin Can you please point me to the correct file? so that we could continue in the right direction?

As @xfxyjwf said above, you need 3.0.0-alpha-2. The windows compiler binary should be fine, unless I'm mistaken. Basically, I'm just pointing out a typo (two dashes instead of one).

@cfallin But which file? I've found 3 possible candidates...

Use this to get protoc: protoc-3.0.0-alpha-2-win32.zip

Do exactly the same thing you did before, except for these changes we have suggested: (i) use 3.0.0-alpha-2 instead of the old version you had, and (ii) fix the dash typo.

@cfallin I meant, which file should I download and perform the compilation command on? What you've provided is the compiler. For Android, which of those should I work on:

• protobuf-java-3.0.0-alpha-2.zip
• protobuf-javanano-3.0.0-alpha-2.zip
• Source code (zip)

Anyway, I've tried doing it for each of those files, and now I get this error for all of them:

protos/backup.proto: No such file or directory

So I created a folder ".../protos/backup.proto" , and then I got this error (again, for all of those files):

protos/backup.proto: Permission denied

I used this command this time:

protoc --javanano_out=src/ -I protos protos/backup.proto

I'm afraid I don't understand what you're trying to do. It doesn't make sense to "perform the [protoc] compilation command" on the Protocol Buffers source code distribution. protoc is a compiler that takes your .proto source code (i.e., a .proto file that you have personally written) and generates code that implements the proto types.

If you had literally invoked protoc with a .zip file as input, you would have gotten parse errors. So you must be extracting the .zip and picking a .proto file from our source code. You shouldn't need to do this if your only goal is to use protocol buffers.

Other parts of your last message also don't make sense. Why do you create a folder with the name backup.proto? .proto files are supposed to be files, not folders. This might explain the "Permission denied" error you are getting (I don't know how Windows behaves when you try to open a folder as a plain file).

Could you explain from first principles what you are trying to do?

Please read the first post. I don't perform the command on zipped file. I do it on the extracted one, as the instructions say.
Maybe I didn't understand it correctly? Maybe I should have done it on the "Launcher3" cloned folder?

I've tried now to run the command on the cloned "Launcher3" project, and this is what I got:

[libprotobuf WARNING google/protobuf/compiler/parser.cc:471] No syntax specified
 for the proto file. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to
specify a syntax version. (Defaulted to proto2 syntax.)

OK, making progress. It seems you've been able to invoke protoc on a .proto file now.

Do you see the output files in the output directory you specified? If so, protoc is working correctly (and any further issues should go to this "Launcher3" project). If not, let us know and we'll diagnose further.

I assume the input is the "protos/backup.proto" file, and the output is inside the "src" folder, but all I can see there is a single file : "...src\com\android\launcher3\backup\BackupProtos.java" .

Shouldn't there be more files? Or at least a jar file?
By importing the project, I see a lot of classes that are missing. I might be wrong, but I think all start with "com.google.protobuf.nano.*" . For example:
com.google.protobuf.nano.MessageNano

protoc is a code generator -- its output is Java. It is not a full build system, so it does not produce JAR files.

The com.google.protobuf.* classes are in the runtime. These are what you will find in those other files you mentioned earlier (you want protobuf-javanano-3.0.0-alpha-2.zip).

It sounds like everything is working as intended. Please let us know if there are any other issues.

After downloading "protobuf-java-3.0.0-alpha-2.zip" from here:
https://github.com/google/protobuf/releases

I got all source code errors figured out, so I think you've solved everything. Thank you.
Maybe I should post a tutorial about how to import the launcher, so that others would see how to do it. I could also put it into a repository.

How can i use this:
protoc –javanano_out=src/ -I protos protos/backup.proto
to generate the "BackupProtos.java" file?
Is there a tutorial to introduce the step?

@yuchuangu85 see:
https://developers.google.com/protocol-buffers/docs/javatutorial

This tutorial talks about Java but it's mostly the same for javanano regarding how to compile proto files.

@xfxyjwf :
Thank you! I have generated the "BackupProtos.java" file.