The Result Object

The Result object represents the result of a component’s execution. It is used for cases in which the component not only needs to return a value to be set on the message payload but also will specify message attributes and/or a mime type.

The class has two generic types, one for the type of the value that goes into the payload and another for the type of the returned message attributes.

For example, consider an oversimplified read operation from the File connector. This operation sets the payload to an InputStream with the file’s content. It also sets the attributes to a FileAttributes object that contains information, such as file name, size, timestamp, permissions, and so on.

public Result<InputStream, FileAttributes> read(String path) { (1)
  InputStream content = getContent(path);
  FileAttributes attributes = getAttributes(path);

  return Result.<InputStream, FileAttributes>builder() (2)
        .output(content) (3)
        .attributes(attributes) (4)
        .build(); (5)
}

1	The operation returns a `Result<InputStream, FileAttributes>`. This tells the runtime that the operation returns an `InputStream` as payload and `FileAttributes` as attributes.
2	Once it obtains the result values, it uses `Result.builder()` to create a new instance. Note that this is strongly typed, so you need to specify the generics when creating the builder.
3	The `output(Object)` method in the builder is used to set the payload.
4	The `attributes(Object)` is used to set the attributes.
5	The example calls the `build()` method and returns the created `Result`.

Setting the Mime Type

The Result object can be used to set the output mimeType. Building on the previous example, the operation here makes a best guess about the file’s mimeType based on its file extension:

public Result<InputStream, FileAttributes> read(String path) {
  InputStream content = getContent(path);
  FileAttributes attributes = getAttributes(path);

  MediaType guessedMediaType = MediaType.ANY;
  if (path.endsWith(".json")) {
    guessedMediaType = MediaType.APPLICATION_JSON;
  } else if (path.endsWith("*.xml")) {
    guessedMediaType = MediaType.APPLICATION_XML;
  } else if (path.endsWith("*.bin")) {
    guessedMediaType = MediaType.BINARY;
  }

  return Result.<InputStream, FileAttributes>builder()
        .output(content)
        .attributes(attributes)
        .mediaType(guessedMediaType)
        .build();
}

In the example above, the MediaType class is not the @MediaType annotation but Mule API’s org.mule.runtime.api.metadata.MediaType.

Void Attributes

For a use case where you want to set the payload and the mimeType but do not want to set attributes, you use the <Void> type, for example:

public Result<InputStream, Void> read(String path) {
  InputStream content = getContent(path);

  return Result.<InputStream, Void>builder()
        .output(content)
        .mediaType(bestGuessMediaType(path))
        .build();
}