A simple interface to Java(tm)





(require-extension javahack)




This library provides a simple and convenient interface to Java. By running java as a sub-process and reading/writing s-expressions, no JNI and OS-specific hacks are needed which makes this approach more portable and robust.

On the Java side jscheme is used to parse expressions passed from CHICKEN and to convert results back into a Scheme-friendly format.

(jscheme.jar version 6.1 is included in this egg)

macro: (java SYMBOL)

Returns the value represented by SYMBOL which should designate a Java class, field, or method, using `Java Dot' notation:

Syntax Type of Member Example
"." at the end constructor ((java Font.) NAME STYLE SIZE)
"." at the beginning instance member ((java .setFont) COMP FONT)
"." at beginning and "$" at the end instance field (define (mycar x) ((java .first$) x))
(define (myset-car! x y) ((java .first$) x y))
"." only in the middle static member ((java Math.round) 123.456)
".class" suffix Java class (java Font.class)
"$" at the end static field (java Font.BOLD$)
(set! (java "U.useJavaSyntax$") #t)
"$" in the middle inner class (java java.awt.geom.Point2D$Double.class)
"$" at the beginning packageless class (java $ParseDemo.class)
"#" at the end allow private access ((java .name$#) ((java Symbol.#) "abc"))


  • Each evaluation of the java macro sends an expression to the java process and receives a result expression. Since most of the results will stay constant throughout the lifetime of the session (as they refer to classes, fields and methods), they are cached and subsequent evaluation of the same (java ...) expression will refer to the cached value instead. You can disable this caching (in case you are doing rather funky things) with thhe help of the java-enable-cache form (see below).

  • Setting static fields is supported through a SRFI-17 setter, as in the example above. Since the field-name is evaluated before being sent to the java-process, pass the field as a string.

  • Java objects which are returned from the Java-side can be safely used and are only garbage collected when no more references exist.

macro: (java-enable-cache FLAG)

Enables or disables caching of results returned by the java macro. FLAG should be either the symbol on or off. Caching is enabled by default.

procedure: (java-run #!key java jar debug options classpath)

Starts the java-VM as a subprocess, with any additional arguments customizing the JVM invocation. java specifies the jvm executable and defaults to java. jar gives the location of the jscheme jar file, options and classpath can be used to customize where the JVM should search for support classes and libraries, together with JVM specific options.

Passing #t for the debug parameter will print information about the message flow between CHICKEN and jscheme.

procedure: (java-stop)

Terminates the java process.

procedure: (java-send EXPR)

Send an expression to Java and returns whatever result is passed back.

procedure: (java-import STRING ...)

Imports Java packages. STRING should be a qualified package identifier, like "java.lang.*".

procedure: (java-object? X)

Returns #t if X is a raw Java object (that can not be meaningfully converted into a Scheme value).


(use javahack)

(java-run debug: #t)

(define s (java String.class))
(pp s)
(define s1 ((java String.) "hello!"))
(pp s1)
(do ((n 2 (sub1 n))) ((zero? n))
  (pp ((java .hashCode) s1)) )
(set! s #f)
(set! s1 #f)
Another example, a minimal SWT application. It assumes swt.jar and all necessary native libraries are in the path, or in the current directory:
(use javahack)

(java-run debug: #t options: '("-Djava.library.path=.") classpath: "swt.jar")

(java-import "org.eclipse.swt.*")
(java-import "org.eclipse.swt.widgets.*")
(java-import "*")
(java-import "org.eclipse.swt.layout.*")

((java Display.setAppName) "Hello")

(define disp ((java Display.)))
(define shell ((java Shell.) disp))

((java .setLayout) shell ((java FillLayout.) (java SWT.VERTICAL$)))
((java .setText) shell "Hello, world!")

(define label ((java Label.) shell (java SWT.CENTER$)))

((java .setText) label "Hello, world")
((java .setSize) shell 300 300)
((java .open) shell)

(do () (((java .isDisposed) shell))
  (unless ((java .readAndDispatch) disp) ((java .sleep) disp)) )

((java .dispose) disp)
Yet another example that performs a callback. The Java code performing the callback could look like this:
public class Callback
    public static Object invoke(jsint.Procedure proc, Object x) {
	return proc.apply(new jsint.Pair(x, jsint.Pair.EMPTY));
If you compile it with javac -classpath `chicken-setup -repository`/jscheme.jar you will have a classfile in the current directory that in combination with the following example code will call back to the CHICKEN side:
(use javahack)

(java-run debug: #t classpath: ".")

  ((java Callback.invoke)
    (lambda (x) 
      (print "ok: " x) 
    "something") )


The javahack extension is distributed under the modified
BSD license and Copyright (c) 2006 Felix L. Winkelmann

JScheme is
Copyright (c) 2002 Ken R. Anderson, Timothy J. Hickey, Peter Norvig

This software is provided 'as-is', without any express or
implied warranty.

In no event will the authors be held liable for any damages
arising from the use of this software.

Permission is granted to anyone to use this software for any
purpose, including commercial applications, and to alter it
and redistribute it freely, subject to the following

1. The origin of this software must not be misrepresented; you
   must not claim that you wrote the original software. If you
   use this software in a product, an acknowledgment in the
   product documentation would be appreciated but is not

2. Altered source versions must be plainly marked as such, and
   must not be misrepresented as being the original software.

3. This notice may not be removed or altered from any source