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Apr 20, 2016

inline-java: Call any JVM function from Haskell

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The Haskell standard includes a native foreign function interface (FFI). Using it can be a bit involved and only C support is implemented in GHC. inline-java lets you call any JVM function directly, from Haskell, without the need to write your own foreign import declarations using the FFI. In the style of inline-c for C and inline-r for calling R, inline-java lets you name any function to call inline in your code. It is implemented on top of the jni and jvm packages using a GHC Core plugin to orchestrate compilation and loading of the inlined Java snippets.


Graphical Hello World using Java Swing:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.Text (Text)
import Language.Java
import Language.Java.Inline

main :: IO ()
main = withJVM [] $ do
    message <- reflect ("Hello World!" :: Text)
    [java| {
      javax.swing.JOptionPane.showMessageDialog(null, $message);
      } |]

Building it


  • the Stack build tool;
  • either, the Nix package manager,
  • or, OpenJDK installed from your distro.

To build:

$ stack build

You can optionally get Stack to download a JDK in a local sandbox (using Nix) for good build results reproducibility. This is the recommended way to build inline-java. Alternatively, you'll need it installed through your OS distribution's package manager for the next steps (and you'll need to tell Stack how to find the JVM header files and shared libraries).

To use Nix, set the following in your ~/.stack/config.yaml (or pass --nix to all Stack commands, see the Stack manual for more):

  enable: true

Building the safe interface

There is [an experimental interface][safe-interface] which catches common memory management mistakes at compile time. This interface currently needs a fork of GHC which supports the LinearTypes language extension. Both the GHC fork and the safe interface can be built with:

$ stack --nix --stack-yaml stack-linear.yaml build inline-java

For examples of how to use the safe interface you can check the directory server or the wizzardo-http benchmark.

Further reading

Check the tutorial on how to use inline-java. If you want to know more about how it is implemented, look at our post on the plugin implementation.


The generated java output can be dumped to stderr by passing to GHC


If -ddump-to-file is in effect (as when using stack), the java code is dumped to <module>.dump-java instead.


Build-time error package or class Blah does not exist

inline-java is going to invoke the javac compiler, and any classes used in java quotations need to be reachable via the CLASSPATH environment variable. For instance,

CLASSPATH=/path/to/my.jar:/some/other/path ghc --make program.hs

Run-time error ThreadNotAttached

Haskell threads need to be attached to the JVM before making JNI calls. Foreign.JNI.withJVM attaches the calling thread, and other threads can be attached with Foreign.JNI.runInAttachedThread. When the JVM calls into Haskell, the thread is already attached.

Run-time error ThreadNotBound

JNI calls need to be done from bound threads. The thread invoking the main function of a program is bound. Threads created with forkOS are bound. In other threads, Control.Concurrent.runInBoundThread can be used to run a computation in a bound thread.

Run-time error java.lang.NoClassDefFoundError

Classes might not be found at runtime if they are not in a folder or jar listed in the parameter -Djava.class.path=<classpath> passed to withJVM.

withJVM ["-Djava.class.path=/path/to/my.jar:/some/other/path"] $ do

Additionally, classes might not be found if a thread other than the one calling main is trying to use them. One solution is to have the thread calling main load all the classes in advance. Then the classes will be available in the JVM for other threads that need them. Calling Language.Java.Inline.loadJavaWrappers will have the effect of loading all classes needed for java quotations, which will suffice in many cases.

Another option is to set the context class loader of other threads, so they earn the ability to load classes on their own. This might work when the thread was attached to the JVM via the JNI, and the context class loader is just null.

loader <- [java| Thread.currentThread().getContextClassLoader() |]
forkOS $ runInAttachedThread $ do
  [java| Thread.currentThread().setContextClassLoader($loader) |]

Run-time error JVMException

Any java exception that goes from Java to Haskell will be wrapped as a value of type JVMException with a reference to the Java object representing the exception. The message and the stack trace of the exception are printed to stderr when the exception is rethrown on the Haskell side, but they can be retrieved from the exception object with more JNI calls as well.


Copyright (c) 2015-2016 EURL Tweag.

All rights reserved.

inline-java is free software, and may be redistributed under the terms specified in the LICENSE file.


         Tweag I/O              LeapYear

inline-java is maintained by Tweag I/O.

Have questions? Need help? Tweet at @tweagio.

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