Convert between JSON and Protobuf

1. Overview

In this tutorial, we’ll demonstrate how to convert from JSON to Protobuf and from Protobuf to JSON.

Protobuf is a free and open-source cross-platform data format used to serialize structured data.

2. Maven Dependency

To start, let’s create a Spring Boot project by including the protobuf-java-util dependency:

<dependency>
    <groupId>com.google.protobuf</groupId>
    <artifactId>protobuf-java-util</artifactId>
    <version>3.21.5</version>
</dependency>

3. Convert JSON to Protobuf

We can convert JSON to a protobuf message by using JsonFormat. JsonFormat is a utility class to convert protobuf messages to/from JSON format. JsonFormat’s parser() creates a Parser, which uses the merge() method to parse JSON to protobuf message.

Let’s create a method that takes JSON and generates a protobuf message:

public static Message fromJson(String json) throws IOException {
    Builder structBuilder = Struct.newBuilder();
    JsonFormat.parser().ignoringUnknownFields().merge(json, structBuilder);
    return structBuilder.build();
}

Let’s use the following sample JSON:

{
    "boolean": true,
    "color": "gold",
    "object": {
      "a": "b",
      "c": "d"
    },
    "string": "Hello World"
}

Now, let’s write a simple test to validate the conversion from JSON to protobuf message:

@Test
public void givenJson_convertToProtobuf() throws IOException {
    Message protobuf = ProtobufUtil.fromJson(jsonStr);
    Assert.assertTrue(protobuf.toString().contains("key: \"boolean\""));
    Assert.assertTrue(protobuf.toString().contains("string_value: \"Hello World\""));
}

4. Convert Protobuf to JSON

*We can convert protobuf message to JSON by using JsonFormat‘s printer()* method, which accepts protobuf as a MessageOrBuilder:

public static String toJson(MessageOrBuilder messageOrBuilder) throws IOException {
    return JsonFormat.printer().print(messageOrBuilder);
}

Let’s write a simple test to validate the conversion from protobuf to JSON message:

@Test
public void givenProtobuf_convertToJson() throws IOException {
    Message protobuf = ProtobufUtil.fromJson(jsonStr);
    String json = ProtobufUtil.toJson(protobuf);
    Assert.assertTrue(json.contains("\"boolean\": true"));
    Assert.assertTrue(json.contains("\"string\": \"Hello World\""));
    Assert.assertTrue(json.contains("\"color\": \"gold\""));
}

5. Conclusion

In this article, we’ve demonstrated how we can convert JSON to protobuf and vice versa.

As always, all code samples used in this tutorial are available over on GitHub.