methanol-jackson

Adapters for Jackson.

Installation

Gradle

implementation 'com.github.mizosoft.methanol:methanol-jackson:1.7.0'

Maven

<dependency>
  <groupId>com.github.mizosoft.methanol</groupId>
   <artifactId>methanol-jackson</artifactId>
   <version>1.7.0</version>
</dependency>

The adapters need to be registered as service providers so Methanol knows they're there. The way this is done depends on your project setup.

Module Path

Follow these steps if your project uses the Java module system.

1. Add this class to your module:

   public class JacksonJsonProviders {
     private static final ObjectMapper mapper = new ObjectMapper();
   
     public static class EncoderProvider {
       public static BodyAdapter.Encoder provider() {
         return JacksonAdapterFactory.createJsonEncoder(mapper);
       }
     }
   
     public static class DecoderProvider {
       public static BodyAdapter.Decoder provider() {
         return JacksonAdapterFactory.createJsonDecoder(mapper);
       }
     }
   }
   

2. Add the corresponding provider declarations in your module-info.java file.

   requires methanol.adapter.jackson;
   
   provides BodyAdapter.Encoder with JacksonJsonProviders.EncoderProvider;
   provides BodyAdapter.Decoder with JacksonJsonProviders.DecoderProvider;
   

Classpath

Registering adapters from the classpath requires declaring the implementation classes in provider-configuration files that are bundled with your JAR. You'll first need to implement delegating Encoder & Decoder that forward to the instances created by JacksonAdapterFactory. Extending from ForwardingEncoder & ForwardingDecoder makes this easier.

You can use Google's AutoService to generate the provider-configuration files automatically, so you won't bother writing them.

Using AutoService

First, install AutoService.

Gradle

implementation "com.google.auto.service:auto-service-annotations:$autoServiceVersion"
annotationProcessor "com.google.auto.service:auto-service:$autoServiceVersion"

Maven

<dependency>
  <groupId>com.google.auto.service</groupId>
  <artifactId>auto-service-annotations</artifactId>
  <version>${autoServiceVersion}</version>
</dependency>

Configure the annotation processor with the compiler plugin.

<plugin>
  <artifactId>maven-compiler-plugin</artifactId>
  <configuration>
    <annotationProcessorPaths>
      <path>
        <groupId>com.google.auto.service</groupId>
        <artifactId>auto-service</artifactId>
        <version>${autoServiceVersion}</version>
      </path>
    </annotationProcessorPaths>
  </configuration>
</plugin>

Next, add this class to your project:

public class JacksonJsonAdapters {
  private static final ObjectMapper mapper = new ObjectMapper();

  @AutoService(BodyAdapter.Encoder.class)
  public static class Encoder extends ForwardingEncoder {
    public Encoder() {
      super(JacksonAdapterFactory.createJsonEncoder(mapper));
    }
  }

  @AutoService(BodyAdapter.Decoder.class)
  public static class Decoder extends ForwardingDecoder {
    public Decoder() {
      super(JacksonAdapterFactory.createJsonDecoder(mapper));
    }
  }
}

Manual Configuration

You can also write the configuration files manually. First, add this class to your project:

public class JacksonJsonAdapters {
  private static final ObjectMapper mapper = new ObjectMapper();

  public static class Encoder extends ForwardingEncoder {
    public Encoder() {
      super(JacksonAdapterFactory.createJsonEncoder(mapper));
    }
  }

  public static class Decoder extends ForwardingDecoder {
    public Decoder() {
      super(JacksonAdapterFactory.createJsonDecoder(mapper));
    }
  }
}

Next, create two provider-configuration files in the resource directory: META-INF/services, one for the encoder and the other for the decoder. Each file must contain the fully qualified name of the implementation class.

Let's say the above class is in a package named com.example. You'll want to have one file for the encoder named:

META-INF/services/com.github.mizosoft.methanol.BodyAdapter$Encoder

and contains the following line:

com.example.JacksonJsonAdapters$Encoder

Similarly, the decoder's file is named:

META-INF/services/com.github.mizosoft.methanol.BodyAdapter$Decoder

and contains:

com.example.JacksonJsonAdapters$Decoder

Adapters for other formats

The Jackson adapter doesn't only support JSON. You can pair whatever ObjectMapper implementation with one or more MediaTypes to create adapters for any of the formats supported by Jackson. For instance, here's a provider for a XML adapter. You'll need to pull in jackson-dataformat-xml. You can install it as mentioned above.

public class JacksonXmlProviders {
  private static final ObjectMapper mapper = new XmlMapper();

  public static class EncoderProvider {
    public static BodyAdapter.Encoder provider() {
      return JacksonAdapterFactory.createEncoder(mapper, MediaType.TEXT_XML);
    }
  }

  public static class DecoderProvider {
    public static BodyAdapter.Decoder provider() {
      return JacksonAdapterFactory.createDecoder(mapper, MediaType.TEXT_XML);
    }
  }
}

For binary formats, you usually can't just plug in an ObjectMapper as a schema must be applied for each type. For this reason you can use a custom ObjectReaderFactory and/or ObjectWriterFactory. For instance, here's a provider for a Protocol Buffers adapter. You'll need to know what types are expected beforehand.

record Point(int x, int y) {}

public class JacksonProtobufProviders {
  private static final ObjectMapper mapper = new ProtobufMapper();

  /**
   * We'll store our schemas in a map. You can implement this in other ways, like loading the
   * protobuf files lazily when needed.
   */
  private static final Map<TypeRef<?>, ProtobufSchema> schemas;

  static {
    try {
      schemas = Map.of(
          TypeRef.from(Point.class),
          ProtobufSchemaLoader.std.parse(
              """
              message Point {
                required int32 x = 1;
                required int32 y = 2;
              }
              """));
    } catch (IOException e) {
      throw new ExceptionInInitializerError(e);
    }
  }

  public static class EncoderProvider {
    public static BodyAdapter.Encoder provider() {
      ObjectWriterFactory writerFactory = (mapper, type) -> mapper.writer(schemas.get(type));
      return JacksonAdapterFactory.createEncoder(
          mapper, writerFactory, MediaType.APPLICATION_X_PROTOBUF);
    }
  }

  public static class DecoderProvider {
    public static BodyAdapter.Decoder provider() {
      ObjectReaderFactory readerFactory =
          (mapper, type) -> mapper.readerFor(type.rawType()).with(schemas.get(type));
      return JacksonAdapterFactory.createDecoder(
          mapper, readerFactory, MediaType.APPLICATION_X_PROTOBUF);
    }
  }
}