Java Interop

Learn how to work with Java

Java Interop

Java Dependencies

Contents

Overview

If you are using Gradle, you can skip this section since have native support for Java dependencies.


Continue if you are directly using Etlas.

Adding Java Files

You can include Java-related files like .java, .class, .jar files in your project by adding their paths to the java-sources: field in the Cabal file.


In this example, we’ll take a look at including Java source files in our project and importing the defined methods into Eta for use.


Setup a project, just like Quick Start, with the following changes:


  1. Main.hs


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    import Java

foreign import java unsafe "@static eta.first.Utils.createFile"
  createFile :: String -> IO ()

-- Creates an empty file
main :: IO ()
main = createFile "HelloWorld.txt"

  2. Create a new folder called java and a file Utils.java with the following contents:


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    package eta.first;

import java.nio.file.Files;
import java.nio.file.Paths;

public class Utils {

  /* This helper method lets us avoid variadic arguments which
     are a bit cumbersome to work with in Eta. */

  public static void createFile(String path) {
    Files.createFile(Paths.get(path));
  }
}

    Your directory structure should look like this:


    eta-first
  -src
    -Main.hs
  -java
    -Utils.java
  -eta-first.cabal
  -Setup.hs

    Your directory structure may vary based on the options you chose in the etlas init step.


  3. Update eta-first.cabal, adding a java-sources: field:


    java-sources: java/Utils.java

    NOTE:


    You can add more Java-based files indented under the first entry with either relative or absolute paths. You can thus include arbitrary .jar files or even individual .class files that you need.


  4. That’s it! Run the example with etlas run.


Adding Maven Dependencies

You can check out the section on JVM Dependencies for more information on the syntax used in the example.


Example

In this example, we’ll be binding to the Unirest library.


Setup a project, just like Quick Start, with the following changes:


  1. Update eta-first.cabal, adding a maven-depends: field:


    maven-depends: com.mashape.unirest:unirest-java:1.4.9

  2. Main.hs


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    {-# LANGUAGE FlexibleContexts, DataKinds, TypeFamilies #-}

import Java

-- Imports from the Unirest API
data BaseRequest = BR @com.mashape.unirest.request.BaseRequest
  deriving Class

data GetRequest = GR @com.mashape.unirest.request.GetRequest
  deriving Class

type instance Inherits GetRequest = '[BaseRequest]

data HttpResponse a = HResp (@com.mashape.unirest.http.HttpResponse a)
  deriving Class

foreign import java unsafe "@static com.mashape.unirest.http.Unirest.shutdown"
  shutdownUnirest :: IO ()

foreign import java unsafe "@static com.mashape.unirest.http.Unirest.get"
  get :: String -> Java a GetRequest

foreign import java unsafe asString
  :: (a <: BaseRequest) => Java a (HttpResponse JString)

foreign import java unsafe getBody
  :: (a <: Object) => Java (HttpResponse a) a

-- Run a simple blocking GET request
main :: IO ()
main = do
  response <- java $ do
        get "https://jsonplaceholder.typicode.com/posts/1"
      >- asString
      >- getBody
  putStrLn $ read (show response)
  shutdownUnirest

  3. That’s it! Run the example with etlas run.


Next Module

In the next module, we will cover functions.

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