an API between SWI-Prolog and the Java Virtual Machine - hosted on GitHub

Prolog API - Overview

The Prolog API comprises Prolog library predicates which support:

  • creating instances (objects) of Java classes (built-in and user-defined);
  • calling methods of Java objects (and static methods of classes), perhaps returning values or object references; and
  • getting and setting the values of fields of Java objects and classes.


JPL’s Prolog API is an interface which allows SWI Prolog 7.x programs to dynamically create and manipulate Java objects.

Here are some significant features of the interface and its implementation:

  • it is completely dynamic: no precompilation is required to manipulate public Java classes which can be found at run time, and methods or fields of objects which can be instantiated from them
  • it is interoperable with the JPL 7.4 Java API (which evolved from Fred Dushin’s JPL 1.0.1)
  • it requires SWI Prolog 7.4+ and a recent Java SE Runtime Environment
  • it exploits the Invocation API of the Java Native Interface (both are mandatory features of any compliant JVM)
  • it is implemented as a single SWI Prolog library module (, a compiled ANSI C foreign library (jpl.dll/.so/.dylib for Windows/Linux/MacOS), and a Java class library (jpl.jar)
  • wherever feasible, Java data values and object references are represented within Prolog canonically and without loss of information (minor exceptions: Java float and double values are both converted to Prolog float values; Java byte, char, short, int and long values are all converted to Prolog integer values; the type distinctions which are lost are normally of no significance)
  • references within Prolog to Java objects:
  • are opaque handles (details follow)
  • are canonical - two references are equal by ==/2 if-and-only-if they refer to the same object within the JVM
  • cooperate with SWI Prolog’s garbage collection: when an object reference is garbage-collected in Prolog, the JVM garbage collector is informed, so there is sound and complete overall garbage collection of Java objects within the combined Prolog+Java system
  • Java class methods can be called by name: JPL invisibly fetches (and caches) essential details of method invocation, exploiting Java Reflection facilities
  • the Prolog API is similar to that of XPCE: the four main interface calls are jpl_new/3, jpl_call/4, jpl_set/3 and jpl_get/3 (there is no jpl_free, since Java’s garbage collection is extended transparently into Prolog)
  • jpl_call/4 resolves overloaded methods automatically and dynamically, inferring the types of the call’s actual parameters, and identifying the most specific of the applicable method implementations (similarly, jpl_new resolves overloaded constructors)
  • Prolog code which uses the API calls is responsible for passing suitably-typed values and references, since the JNI doesn’t perform complete dynamic type-checking, and nor currently does JPL (although the overloaded method resolution mechanism could probably be adapted to do this)
  • Prolog code can reason about the types of Java data values, object references, fields and methods: JPL supports a canonical representation of all Java types as structured terms (e.g. array(array(byte))) and also as JVM signatures (text atoms) in descriptor and classname syntax (details follow)
  • the Prolog and Java APIs of JPL are largely independent; the Prolog API concentrates on representing all Java data values and objects within Prolog, and supporting manipulation of classes and objects;the Java API concentrates on representing any Prolog term within Java, and supporting the calling of goals within Prolog and the retrieving of results back into Java
  • when called from Prolog, void methods return an invented @(void) value (which is distinct from all other JPL values and references); this simplifies and regularises the API (all methods return a typed value)
  • JPL uses @/1 to construct representations of certain Java values; if @/1 is defined as a prefix operator (as used by XPCE), then you can write @true, @false, @null or @void in your source code; otherwise (and for portability, and recommended) you should write @(true), @(false), @(null) or @(void).

JPL types

All Java values and object references which are passed between Prolog engines and Java VMs via JPL’s Prolog API are seen as instances of types within this simplified JPL type system:

  • a datum (this jargon is introduced, out of necessity, to refer to the union of values and references)
    • is a value (values are copied between Prolog and the JVM)
      • is a boolean
      • or a char
      • or a long, int, short or byte
      • or a double or float
      • or a string (an instance of java.lang.String)
      • or a void (an artificial value returned by calls to Java void methods)
    • or a reference
      • is null
      • or an object (held within the JVM, and represented in Prolog by a canonical reference)
        • is an array
        • or a class instance (other than of java.lang.String)

Java values and references

Instances of JPL types are represented within Prolog as follows:

  • boolean has two values, represented by @(true) and @(false)
  • char values are represented by corresponding Prolog integers in the range 0..65,535
  • byte values are represented by corresponding Prolog integers in the range -128..127
  • short values are represented by corresponding Prolog integers in the range -32,768..32,767
  • int values are represented by corresponding Prolog integers in the range -2147483648..2147483647
  • long values are represented as Prolog integers in the range 9,223,372,036,854,775,808..9,223,372,036,854,775,807
  • double and float values are represented as Prolog floats (which are equivalent to Java doubles) (there may be minor rounding, normalisation or loss-of-precision issues when a Java float is widened to a Prolog float then narrowed back again)
  • string values (immutable instances of java.lang.String) are represented as Prolog text atoms (in UTF-8 encoding)
  • null has only one value, represented as @(null)
  • void has only one value, represented as @(void)
  • array and class instance references are represented (since 7.4) as blobs of type jref, portrayed e.g. <jref>(0x12345678) but (like stream handles) with no source syntax acceptable to read/1.

Java types: structured notation

The Prolog API allows Prolog applications to inspect, manipulate, and reason about the types of Java values, references, methods etc., and this section describes how these types themselves are represented. Predicates which pass these type representations include (the clue is in the name):


The pseudo-type void is represented by this atom:


The pseudo-type null is represented by this atom:


The primitive types are represented by these atoms:


class types are represented as class(package_parts,classname_parts) e.g.


array types are represented as array(type) e.g.


This structured notation for Java types is a term-encoding, designed to be convenient for composition and decomposition by unification.

Java types: descriptor notation

The descriptor notation for Java types is one of two textual notations employed by the JVM and the Java class libraries; JPL (necessarily) supports both (and supports conversion between all three representations).

Examples of descriptor notation:

  • 'Z' denotes boolean
  • 'B' denotes byte
  • 'C' denotes char
  • 'S' denotes short
  • 'I' denotes int
  • 'J' denotes long
  • 'F' denotes float
  • 'D' denotes double
  • '[type' denotes an array of type
  • 'Ljava/util/Date;' denotes the Java class java.util.Date
  • '(argument_types)return_type' denotes the type of a method

Java types: classname notation

The classname notation for Java types is the other textual notation employed by the JVM and the Java class libraries. It is a (seemingly unnecessary) variation on the descriptor notation, used by a few JNI routines. It has the slight advantage that, in the case of simple class types only, it resembles the Java source text notation for classes. This representation is supported only because certain JNI functions use it; it is used within JPL’s implementation of jpl_call/4 etc. You may encounter this notation when tracing JPL activity, but otherwise you need not know about it.

Examples of classname notation:

  • 'java.util.Vector' denotes the Java class java.util.Vector
  • '[B' denotes an array of boolean
  • '[Ljava.lang.String;' denotes an array of string

Creating Java objects

To create an instance of a Java class from within Prolog, call jpl_new(+Class, +Params, -JRef) with a classname, a list of actual parameters for the constructor, and a variable to be bound to the new reference, e.g.

jpl_new('javax.swing.JFrame', ['frame with dialog'], JRef)

which binds JRef to a new object reference, e.g.<jref>(0x12345678).

NB for convenience, this predicate is overloaded: Class can also be a class type in structured notation, e.g. array(boolean).

Calling Java methods

The object reference generated by the jpl_new/3 call (above) can be passed to other JPL API predicates such as:

jpl_call(+JRef, +Method, +Params, -Result)


jpl_call(JRef, setVisible, [@(true)], _)

which calls the setVisible() method of the object to which JRef refers, effectively passing it the Java value true.

(This call should display the new JFrame in the top left corner of the desktop.)

Note the anonymous variable passed as the fourth argument to jpl_call/4. A variable in this position receives the result of the method call: either a value or a reference.

Since SetVisible() is a void method, the call returns the (artificial) reference @(void), which can be ignored.

Some may prefer to code this call thus:

jpl_call(F, setVisible, [@(true)], @(void))

which documents the programmer’s understanding that this is a void method, and fails if it isn’t.

If the JRef argument represents a class, then the named static method of that class is called.

Fetching Java field values

The jpl_get/3 API predicate has the following interface:

jpl_get(+Class_or_Object, +Field, -Datum)

and can retrieve the value of an instance field e.g.

jpl_new('java.util.GregorianCalendar', [], JRef),
jpl_get(JRef, time, Ms)

or of a static field, e.g.

jpl_get('java.awt.Color', pink, Pink)

which binds the Prolog variable Pink to a reference to the predefined java.awt.Color “constant” which is held in the static final .pink field of the java.awt.Color class.

Setting Java field values

Object and class fields can be set (i.e. have values or references assigned to them) by the jpl_set/3 API procedure, which has the following interface:

jpl_set(+Class_or_Object, +Field, +Datum)

where Datum must be a value or reference of a type suitable for assignment to the named field of the class or object.

A slightly longer example

This code fragment

    (   current_prolog_flag(N, V),
        term_to_atom(V, Va),
        jpl_new('[Ljava.lang.String;', [N,Va], Ar)
jpl_new('[[Ljava.lang.String;', Ars, Ac),
jpl_datums_to_array([name,value], Ah),
jpl_new('javax.swing.JFrame', ['current_prolog_flag'], F),
jpl_call(F, getContentPane, [], CP),
jpl_new('javax.swing.JTable', [Ac,Ah], T),
jpl_new('javax.swing.JScrollPane', [T], SP),
jpl_call(CP, add, [SP,'Center'], _),
jpl_call(F, setSize, [600,400], _),
jpl_call(F, setVisible, [@(true)], _).

builds an array of arrays of strings containing the names and values of the current SWI-Prolog “flags”, and displays it in a JTable within a ScrollPane within a JFrame:

screendump of current prolog flags in a JTable within a ScrollPane within a JFrame{width=”524” height=”269”}

In addition to JPL API calls, this example calls jpl_datums_to_array/2, a utility which converts any list of valid representations of Java values (or objects) into a new Java array, whose base type is the most specialised type of which all list members are instances, and which is defined thus:

jpl_datums_to_array(Ds, A) :-
    jpl_datums_to_most_specific_common_ancestor_type(Ds, T),
    jpl_new(array(T), Ds, A).

Having found the “most specific common ancestor type”, a new array of this type is created, whose elements are initialised to the successive members of the list of datums.

This illustrates another mode of operation of jpl_new/3:

jpl_new(+ArrayType, +InitialValues, -ArrayRef)

See Prolog API - Reference for fuller details of the API procedures.

Don’t overlook the possibility and advantages of writing custom Java classes to serve your Prolog applications: this interface is not designed to make Java programming redundant.

Exceptions thrown by Java

Uncaught exceptions thrown by the JVM while handling a Prolog API call are mapped onto error(_,_) structures, e.g.

?- catch(jpl_new('java.util.Date',[yesterday],_), E, true).
E = error(java_exception((0x1026D40)), 'java.lang.IllegalArgumentException').

because, as the exception suggests, yesterday is not a valid constructor argument.

Java exceptions are always returned as Prolog exceptions with this structure:

error(java_exception(reference_to_exception_object), exception_classname)

Naming static nested classes

Although, in Java source code, static nested classes are named with the dot notation, e.g.


the JVM’s Java Native Interface (JNI), which JPL uses, requires a special notation, hence

jpl_new('java.awt.geom.Path2D$Float', [], P).

which you may have noticed in the name of the corresponding class file:


Unfortunately it is far from trivial to support the Java naming in JPL. Thanks go to Timo Baumann and Sebastian Godelet for their input on this issue.

Instantiating a non-static member class

If you need to create an instance of a non-static member class, you must explicitly pass (a reference to) an instance of its parent class as an additional (first) argument of any of its constructors. Java source syntax hides this, but reflection shows that every constructor (implicit or explicit) has this extra argument.

public class Parent {
    public class Child {
jpl_new('Parent', [], P),
jpl_new('Parent$Child', [P], C).

Enums are static nested classes


public class Foo {
    public enum Bar {
        UP, DOWN
    public static String legend(Bar x) {
        switch(x) {
            case UP:
                return "upwards";
            case DOWN:
                return "downwards";
                return "?";

we can do

1 ?- jpl_get('Foo$Bar', 'UP', X).
X = @('J#00000000000056425292').


1 ?- jpl_get('Foo$Bar', 'UP', X), jpl_call('Foo', legend, [X], S).
X = @('J#00000000000056425292'),
S = upwards.

where X is bound to the nominated enum constant (actually an object reference), then passed as an arg to legend/1

Calling parameterless methods

You must pass an empty parameter list when calling Java methods which take no parameters, e.g.

jpl_call('java.lang.System', gc, [], _)

There is (deliberately) no jpl_call/3 convenience predicate which defaults parameters to [] (see below).

Calling void methods

You must accept an @(void) result when calling void Java methods, e.g. either

jpl_call('java.lang.System', gc, [], @(void))

which explicitly matches the expected result, or

jpl_call('java.lang.System', gc, [], _)

which uses an anonymous variable to ignore the result.

There is (deliberately) no jpl_call/3 convenience predicate which conceals the return value of void methods (see above).

Exiting the JVM

Note that

jpl_call('java.lang.System', exit, [0], _)

will terminate the SWI-Prolog process (into which the JVM library has been dynamically loaded) without calling any at_halt/1 or PL_at_halt() hooks, or performing system cleanup actions. Beware also of calling Java classes which might call java.lang.System#exit().

To do

Here are a few longer-term (and tricky) aims:

  • support non-virtual method calls (i.e. explicitly call a method of some ancestor class despite there being an overriding method (i.e. of the same name etc.) in a “nearer” class). I believe this is a fairly arcane Java feature, but it is needed for completeness; I want to accommodate it without complicating the syntax of regular method calls.
  • map the JVM’s vprintf() messages onto something in SWI-Prolog (the user_error stream?)
  • catch the JVM’s abort and exit events, and handle them appropriately (e.g. stop a Java abort from killing the SWI-Prolog process)
  • propagate SWI-Prolog’s ABORT action into the JVM as appropriate, e.g. to interrupt a pending JPL call
  • reduce the (extravagant) overheads of each JPL call (without compromising functionality or safety)