inline-java: Call any JVM function from Haskell

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.

Example

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

Requirements:

• the Bazel build tool, and
• the Nix package manager.

To build:

$ nix-shell --pure --run "bazel build //..."

To test:

$ nix-shell --pure --run "bazel test //..."

Building the safe interface

There is an experimental 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:

$ nix-shell --pure --run "bazel build //:inline-java --config ghc_9_0_1" shell-linear-types.nix

For examples of how to use the safe interface you can check the directory server and 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.

There is also a post which gives an overview of the safe interface.

Debugging

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

-fplugin-opt=Language.Java.Inline.Plugin:dump-java

If -ddump-to-file is in effect, the java code is dumped to <module>.dump-java instead.

Troubleshooting

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() |]
            `Language.Java.withLocalRef` Foreign.JNI.newGlobalRef
...
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 can be retrieved from the exception object with more JNI calls, e.g.

\(JVMException e) -> [java| { $e.printStackTrace(); } |]

or with JNI.Foreign.showException.

License

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.

Sponsors

inline-java is maintained by Tweag I/O.

Have questions? Need help? Tweet at @tweagio.