Protocol Buffers: How to parse a .proto file in Java

Question

I am trying to dynamically parse a given .proto file in Java to decode a Protobuf-encoded binary.

I have the following parsing method, in which the "proto" string contains the content of the .proto file:

public static Descriptors.FileDescriptor parseProto (String proto) throws InvalidProtocolBufferException, Descriptors.DescriptorValidationException {
        DescriptorProtos.FileDescriptorProto descriptorProto = DescriptorProtos.FileDescriptorProto.parseFrom(proto.getBytes());
        return Descriptors.FileDescriptor.buildFrom(descriptorProto, null);
}

Though, on execution the previous method throws an exception with the message "Protocol message tag had invalid wire type.". I use the example .proto file from Google so I guess it is valid: https://github.com/google/protobuf/blob/master/examples/addressbook.proto

Here is the stack trace:

15:43:24.707 [pool-1-thread-1] ERROR com.github.whiver.nifi.processor.ProtobufDecoderProcessor - ProtobufDecoderProcessor[id=42c8ab94-2d8a-491b-bd99-b4451d127ae0] Protocol message tag had invalid wire type.
com.google.protobuf.InvalidProtocolBufferException$InvalidWireTypeException: Protocol message tag had invalid wire type.
    at com.google.protobuf.InvalidProtocolBufferException.invalidWireType(InvalidProtocolBufferException.java:115)
    at com.google.protobuf.UnknownFieldSet$Builder.mergeFieldFrom(UnknownFieldSet.java:551)
    at com.google.protobuf.GeneratedMessageV3.parseUnknownField(GeneratedMessageV3.java:293)
    at com.google.protobuf.DescriptorProtos$FileDescriptorSet.<init>(DescriptorProtos.java:88)
    at com.google.protobuf.DescriptorProtos$FileDescriptorSet.<init>(DescriptorProtos.java:53)
    at com.google.protobuf.DescriptorProtos$FileDescriptorSet$1.parsePartialFrom(DescriptorProtos.java:773)
    at com.google.protobuf.DescriptorProtos$FileDescriptorSet$1.parsePartialFrom(DescriptorProtos.java:768)
    at com.google.protobuf.AbstractParser.parsePartialFrom(AbstractParser.java:163)
    at com.google.protobuf.AbstractParser.parseFrom(AbstractParser.java:197)
    at com.google.protobuf.AbstractParser.parseFrom(AbstractParser.java:209)
    at com.google.protobuf.AbstractParser.parseFrom(AbstractParser.java:214)
    at com.google.protobuf.AbstractParser.parseFrom(AbstractParser.java:49)
    at com.google.protobuf.DescriptorProtos$FileDescriptorSet.parseFrom(DescriptorProtos.java:260)
    at com.github.whiver.nifi.parser.SchemaParser.parseProto(SchemaParser.java:9)
    at com.github.whiver.nifi.processor.ProtobufDecoderProcessor.lambda$onTrigger$0(ProtobufDecoderProcessor.java:103)
    at org.apache.nifi.util.MockProcessSession.write(MockProcessSession.java:895)
    at org.apache.nifi.util.MockProcessSession.write(MockProcessSession.java:62)
    at com.github.whiver.nifi.processor.ProtobufDecoderProcessor.onTrigger(ProtobufDecoderProcessor.java:100)
    at org.apache.nifi.processor.AbstractProcessor.onTrigger(AbstractProcessor.java:27)
    at org.apache.nifi.util.StandardProcessorTestRunner$RunProcessor.call(StandardProcessorTestRunner.java:251)
    at org.apache.nifi.util.StandardProcessorTestRunner$RunProcessor.call(StandardProcessorTestRunner.java:245)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)

Any idea? Thank you!

Answer 1

It looks like you're trying to use FileDescriptorSet.parseFrom to populate a FileDescriptorSet. This will only work if the bytes you're providing are the binary protobuf contents - which is to say: a compiled schema. You can get a compiled schema by using the protoc command-line-tool with the --descriptor_set_out option. What you're actually passing it right now is the text bytes that make up the text schema, which is not what parseFrom expects.

Without a compiled schema, you would need a runtime .proto parser. I'm not aware of one for Java; protobuf-net includes one (protobuf-net.Reflection), but that is C#/.NET. Without an available runtime .proto parser, you'd need to shell-execute protoc instead.

Answer 2

Drawing from the other answers, here's a snippet of working Kotlin code from a library I'm developing. https://github.com/asarkar/okgrpc

private fun lookupProtos(
    protoPaths: List<String>,
    protoFile: String,
    tempDir: Path,
    resolved: MutableSet<String>
): List<DescriptorProtos.FileDescriptorProto> {
    val schema = generateSchema(protoPaths, protoFile, tempDir)
    return schema.fileList
        .filter { resolved.add(it.name) }
        .flatMap { fd ->
            fd.dependencyList
                .filterNot(resolved::contains)
                .flatMap { lookupProtos(protoPaths, it, tempDir, resolved) } + fd
        }
}

private fun generateSchema(
    protoPaths: List<String>,
    protoFile: String,
    tempDir: Path
): DescriptorProtos.FileDescriptorSet {
    val outFile = Files.createTempFile(tempDir, null, null)
    val stderr = ByteArrayOutputStream()
    val exitCode = Protoc.runProtoc(
        (protoPaths.map { "--proto_path=$it" } + listOf("--descriptor_set_out=$outFile", protoFile)).toTypedArray(),
        DevNull,
        stderr
    )
    if (exitCode != 0) {
        throw IllegalStateException("Failed to generate schema for: $protoFile")
    }
    return Files.newInputStream(outFile).use { DescriptorProtos.FileDescriptorSet.parseFrom(it) }
}

The idea is to use os72/protoc-jar to write out a compiled schema/file descriptor. Then use FileDescriptorSet.parseFrom to read that file, and recurse on its dependencies.