JPL: A bidirectional Prolog/Java interface
Paul Singleton & Fred Dushin
Abstract
JPL is a library using the SWI-Prolog foreign interface and the Java jni interface providing a bidirectional interface between Java and Prolog that can be used to embed Prolog in Java as well as for embedding Java in Prolog. In both setups it provides a reentrant bidirectional interface.

This document is a reference for the Prolog API. The overall documentation is maintained on the GitHub Wiki

### 1 library(jpl): A Java interface for SWI Prolog 7.x

See also
  http://jpl7.org/

The `library(jpl)` provides a bidirectional interface to a Java Virtual Machine.

[det]jpl_new(+X, +Params, -V)
  X can be:

  - an atomic classname, e.g. `'java.lang.String'`
  - or an atomic descriptor, e.g. `'[I'` or `'Ljava.lang.String;'`
  - or a suitable type, i.e. any `class(_,_)` or `array(_)`, e.g. `class([java,util],['Date'])`

  If X is an object (non-array) type or descriptor and Params is a list of values or references, then V is the result of an invocation of that type's most specifically-typed constructor to whose respective formal parameters the actual Params are assignable (and assigned).

  If X is an array type or descriptor and Params is a list of values or references, each of which is (independently) assignable to the array element type, then V is a new array of as many elements as Params has members, initialised with the respective members of Params.

  If X is an array type or descriptor and Params is a non-negative integer N, then V is a new array of that type, with N elements, each initialised to Java's appropriate default value for the type.

  If V is literally `{Term}` then we attempt to convert a `new org.jpl7.Term` instance to a corresponding term; this is of little obvious use here, but is consistent with jpl_call/4 and jpl_get/3.

[det]jpl_call(+X, +MethodName:atom, +Params:list(datum), -Result:datum)
  X should be either

  - an object reference, e.g. `<jref>(1552320)` (for static or instance methods)
  - or a classname, e.g. `'java.util.Date'` (for static methods only)
  - or a descriptor, e.g. `'Ljava.util.Date;'` (for static methods only)
  - or type, e.g. `class([java,util],['Date'])` (for static methods only)

  MethodName should be a method name (as an atom) (may involve dynamic overload resolution based on inferred types of params)

  Params should be a proper list (perhaps empty) of suitable actual parameters for the named method.

  The class or object may have several methods with the given name; JPL will resolve (per call) to the most appropriate method based on the quantity and inferred types of Params. This resolution mimics the corresponding static resolution performed by Java compilers.

  Finally, an attempt will be made to unify Result with the method's returned value, or with `@(void)` (the compound term with name `@` and argument `void`) if it has none.

[det]jpl_get(+X, +Fspec, -V:datum)
  X can be

  - a classname
  - or a descriptor
  - or an (object or array) type (for static fields)
  - or a non-array object (for static and non-static fields)
  - or an array (for’length' pseudo field, or indexed element retrieval)

  Fspec can be

  - an atomic field name
  - or an integral array index (to get an element from an array)
  - or a pair I-J of integers (to get a subrange of an array).

  Finally, an attempt will be made to unify V with the retrieved value or object reference.

  Examples

      jpl_get('java.awt.Cursor', 'NE_RESIZE_CURSOR', Q).
      Q = 7.

      jpl_new(array(class([java,lang],['String'])), [for,while,do,if,then,else,try,catch,finally], A),
      jpl_get(A, 3-5, B).
      B = [if, then, else].

[det]jpl_set(+X, +Fspec, +V)
  sets the Fspec-th field of (class or object) X to value V iff it is assignable

  X can be

  - a class instance (for static or non-static fields)
  - or an array (for indexed element or subrange assignment)
  - or a classname, or a `class(_,_)` or `array(_)` type (for static fields)
  - but not a String (no fields to retrieve)

  Fspec can be

  - an atomic field name (overloading through shadowing has yet to be handled properly)
  - or an array index I (X must be an array object: V is assigned to X[I])
  - or a pair I-J of integers (X must be an array object, V must be a list of values: successive members of V are assigned to X[I..J])

  V must be a suitable value or object.

[det]jpl_get_default_jvm_opts(-Opts:list(atom))
  Returns (as a list of atoms) the options which will be passed to the JVM when it is initialised, e.g. `['-Xrs']`
[det]jpl_set_default_jvm_opts(+Opts:list(atom))
  Replaces the default JVM initialisation options with those supplied.
[semidet]jpl_get_actual_jvm_opts(-Opts:list(atom))
  Returns (as a list of atoms) the options with which the JVM was initialised.

  Fails silently if a JVM has not yet been started, and can thus be used to test for this.

jpl_pl_lib_version(-Version)
  Version is the fully qualified version identifier of the in-use Prolog component (`jpl.pl`) of JPL.

  It should exactly match the version identifiers of JPL's C (jpl.c) and Java (jpl.jar) components.

  Example

      ?- jpl_pl_lib_version(V).
      V = '7.6.1'.

jpl_c_lib_version(-Version)
  Version is the fully qualified version identifier of the in-use C component (jpl.c) of JPL.

  It should exactly match the version identifiers of JPL's Prolog (`jpl.pl`) and Java (jpl.jar) components.

  Example

      ?- jpl_c_lib_version(V).
      V = '7.4.0-alpha'.

jpl_class_to_classname(+Class:jref, -ClassName:entityName)
  Class is a reference to a class object.

  ClassName is its canonical (?) source-syntax (dotted) name, e.g. `'java.util.Date'`

  NB not used outside jni_junk and jpl_test (is this (still) true?)

  NB oughta use the available caches (but their indexing doesn't suit)

  `TODO` This shouldn't exist as we have jpl_class_to_entityname/2 ???

  The implementation actually just calls `Class.getName()` to get the entity name (dotted name)

jpl_class_to_type(+Class:jref, -Type:jpl_type)
  The Class is a reference to a (Java Universe) instance of `java.lang.Class`. The Type is the (Prolog Universe) JPL type term denoting the same type as does the instance of Class.

  NB should ensure that, if not found in cache, then cache is updated.

  Intriguingly, getParameterTypes returns class objects (undocumented AFAIK) with names’boolean',’byte' etc. and even’void' (?!)

jpl_classname_to_class(+EntityName:atom, -Class:jref)
  EntityName is the entity name to be mapped to a class reference.

  Class is a (canonical) reference to the corresponding class object.

  NB uses caches where the class has already been mapped once before.

jpl_entityname_to_type(+EntityName:atom, -Type:jpl_type)
  EntityName is the entity name (an atom) denoting a Java type, to be mapped to a JPL type. This is the string returned by `java.lang.Class.getName()`.

  Type is the JPL type (a ground term) denoting the same Java type as EntityName does.

  The Java type in question may be a reference type (class, abstract class, interface), and array type or a primitive, including "void".

  Examples:

      int                       int
      integer                   class([],[integer])
      void                      void
      char                      char
      double                    double
      [D                        array(double)
      [[I                       array(array(int))
      java.lang.String          class([java,lang],['String'])
      [Ljava.lang.String;       array(class([java,lang],['String']))
      [[Ljava.lang.String;      array(array(class([java, lang], ['String'])))
      [[[Ljava.util.Calendar;   array(array(array(class([java,util],['Calendar']))))
      foo.bar.Bling$Blong       class([foo,bar],['Bling','Blong'])

  NB uses caches where the class has already been mapped once before.

  See also
    https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/lang/Class.html#getName()
jpl_type_to_entityname(+Type:jpl_type, -EntityName:atom)
  This is the converse of jpl_entityname_to_type/2
jpl_classname_to_type(+EntityName:atom, -Type:jpl_type)
  This is a wrapper around jpl_entityname_to_type/2 to keep the old exported predicate alive. The name of this predicate does not fully reflect that it actually deals in entity names instead of just class names.

  Use jpl_entityname_to_type/2 in preference.

jpl_type_to_classname(+Type:jpl_type, -EntityName:atom)
  This is a wrapper around jpl_type_to_entityname/2 to keep the old exported predicate alive. The name of this predicate does not fully reflect that it actually deals in entity names instead of just class names.

  Use jpl_type_to_entityname/2 in preference.

jpl_datum_to_type(+Datum:datum, -Type:type)
  Datum must be a JPL representation of an instance of one (or more) Java types;

  Type is the unique most specialised type of which Datum denotes an instance;

  NB 3 is an instance of byte, char, short, int and long, of which byte and char are the joint, overlapping most specialised types, so this relates 3 to the pseudo subtype’char_byte';

  See also
    jpl_type_to_preferred_concrete_type/2 for converting inferred types to instantiable types
jpl_object_to_class(+Object:jref, -Class:jref)
  fails silently if Object is not a valid reference to a Java object

  Class is a (canonical) reference to the (canonical) class object which represents the class of Object

  NB what's the point of caching the type if we don't look there first?

jpl_object_to_type(+Object:jref, -Type:type)
  Object must be a proper JPL reference to a Java object (i.e. a class or array instance, but not null, void or String).

  Type is the JPL type of that object.

[nondet]jpl_primitive_type(-Type:atom)
  Type is an atomic JPL representation of one of Java's primitive types. N.B: `void` is not included.

      ?- setof(Type, jpl_primitive_type(Type), Types).
      Types = [boolean, byte, char, double, float, int, long, short].

jpl_ref_to_type(+Ref:jref, -Type:type)
  Ref must be a proper JPL reference (to an object, null or void).

  Type is its type.

jpl_type_to_class(+Type:jpl_type, -Class:jref)
  Type is the JPL type, a ground term designating a class or an array type.

  Incomplete types are now never cached (or otherwise passed around).

  jFindClass throws an exception if FCN can't be found.

jpl_is_class(@Term)
  True if Term is a JPL reference to an instance of `java.lang.Class`.
jpl_is_false(@Term)
  True if Term is `@(false)`, the JPL representation of the Java boolean value’false'.
jpl_is_null(@Term)
  True if Term is `@(null)`, the JPL representation of Java's’null' reference.
jpl_is_object(@Term)
  True if Term is a well-formed JPL object reference.

  NB this checks only syntax, not whether the object exists.

jpl_is_object_type(@Term)
  True if Term is an object (class or array) type, not e.g. a primitive, null or void.
jpl_is_ref(@Term)
  True if Term is a well-formed JPL reference, either to a Java object or to Java's notional but important’null' non-object.
jpl_is_true(@Term)
  True if Term is `@(true)`, the JPL representation of the Java boolean value’true'.
jpl_is_type(@Term)
  True if Term is a well-formed JPL type structure.
jpl_is_void(@Term)
  True if Term is `@(void)`, the JPL representation of the pseudo Java value’void' (which is returned by jpl_call/4 when invoked on void methods).

  NB you can try passing’void' back to Java, but it won't ever be interested.

[semidet]jpl_false(-X:datum)
  X is `@(false)`, the JPL representation of the Java boolean value’false'.
  See also
    jpl_is_false/1
[semidet]jpl_null(-X:datum)
  X is `@(null)`, the JPL representation of Java's’null' reference.
  See also
    jpl_is_null/1
[semidet]jpl_true(-X:datum)
  X is `@(true)`, the JPL representation of the Java boolean value’true'.
  See also
    jpl_is_true/1
[semidet]jpl_void(-X:datum)
  X is `@(void)`, the JPL representation of the pseudo Java value’void'.
  See also
    jpl_is_void/1
jpl_array_to_length(+Array:jref, -Length:integer)
  Array should be a JPL reference to a Java array of any type.

  Length is the length of that array. This is a utility predicate, defined thus:

      jpl_array_to_length(A, N) :-
          (   jpl_ref_to_type(A, array(_))
          ->  jGetArrayLength(A, N)
          ).

jpl_array_to_list(+Array:jref, -Elements:list(datum))
  Array should be a JPL reference to a Java array of any type.

  Elements is a Prolog list of JPL representations of the array's elements (values or references, as appropriate). This is a utility predicate, defined thus:

      jpl_array_to_list(A, Es) :-
          jpl_array_to_length(A, Len),
          (   Len > 0
          ->  LoBound is 0,
              HiBound is Len-1,
              jpl_get(A, LoBound-HiBound, Es)
          ;   Es = []
          ).

jpl_datums_to_array(+Datums:list(datum), -A:jref)
  A will be a JPL reference to a new Java array, whose base type is the most specific Java type of which each member of Datums is (directly or indirectly) an instance.

  NB this fails silently if

  - Datums is an empty list (no base type can be inferred)
  - Datums contains both a primitive value and an object (including array) reference (no common supertype)
jpl_enumeration_element(+Enumeration:jref, -Element:datum)
  Generates each Element from Enumeration.

  - if the element is a java.lang.String then Element will be an atom
  - if the element is null then Element will (oughta) be null
  - otherwise I reckon it has to be an object ref
jpl_enumeration_to_list(+Enumeration:jref, -Elements:list(datum))
  Enumeration should be a JPL reference to an object which implements the `Enumeration` interface.

  Elements is a Prolog list of JPL references to the enumerated objects. This is a utility predicate, defined thus:

      jpl_enumeration_to_list(Enumeration, Es) :-
          (   jpl_call(Enumeration, hasMoreElements, [], @(true))
          ->  jpl_call(Enumeration, nextElement, [], E),
              Es = [E|Es1],
              jpl_enumeration_to_list(Enumeration, Es1)
          ;   Es = []
          ).

[nondet]jpl_hashtable_pair(+HashTable:jref, -KeyValuePair:pair(datum,datum))
  Generates Key-Value pairs from the given HashTable.

  NB String is converted to atom but Integer is presumably returned as an object ref (i.e. as elsewhere, no auto unboxing);

  NB this is anachronistic: the Map interface is preferred.

jpl_iterator_element(+Iterator:jref, -Element:datum)
  Iterator should be a JPL reference to an object which implements the `java.util.Iterator` interface.

  Element is the JPL representation of the next element in the iteration. This is a utility predicate, defined thus:

      jpl_iterator_element(I, E) :-
          (   jpl_call(I, hasNext, [], @(true))
          ->  (   jpl_call(I, next, [], E)
              ;   jpl_iterator_element(I, E)
              )
          ).

jpl_list_to_array(+Datums:list(datum), -Array:jref)
  Datums should be a proper Prolog list of JPL datums (values or references).

  If Datums have a most specific common supertype, then Array is a JPL reference to a new Java array, whose base type is that common supertype, and whose respective elements are the Java values or objects represented by Datums.

[semidet]jpl_terms_to_array(+Terms:list(term), -Array:jref)
  Terms should be a proper Prolog list of arbitrary terms.

  Array is a JPL reference to a new Java array of `org.jpl7.Term`, whose elements represent the respective members of the list.

jpl_array_to_terms(+JRef:jref, -Terms:list(term))
  JRef should be a JPL reference to a Java array of org.jpl7.Term instances (or ots subtypes); Terms will be a list of the terms which the respective array elements represent.
[nondet]jpl_map_element(+Map:jref, -KeyValue:pair(datum,datum))
  Map must be a JPL Reference to an object which implements the `java.util.Map` interface

  This generates each Key-Value pair from the Map, e.g.

      ?- jpl_call('java.lang.System', getProperties, [], Map), jpl_map_element(Map, E).
      Map = @<jref>(0x20b5c38),
      E = 'java.runtime.name'-'Java(TM) SE Runtime Environment' ;
      Map = @<jref>(0x20b5c38),
      E = 'sun.boot.library.path'-'C:\\Program Files\\Java\\jre7\\bin'
      etc.

  This is a utility predicate, defined thus:

      jpl_map_element(Map, K-V) :-
          jpl_call(Map, entrySet, [], ES),
          jpl_set_element(ES, E),
          jpl_call(E, getKey, [], K),
          jpl_call(E, getValue, [], V).

[nondet]jpl_set_element(+Set:jref, -Element:datum)
  Set must be a JPL reference to an object which implements the `java.util.Set` interface.

  On backtracking, Element is bound to a JPL representation of each element of Set. This is a utility predicate, defined thus:

      jpl_set_element(S, E) :-
          jpl_call(S, iterator, [], I),
          jpl_iterator_element(I, E).

jpl_servlet_byref(+Config, +Request, +Response)
  This serves the byref servlet demo, exemplifying one tactic for implementing a servlet in Prolog by accepting the Request and Response objects as JPL references and accessing their members via JPL as required;
  See also
    jpl_servlet_byval/3
jpl_servlet_byval(+MultiMap, -ContentType:atom, -Body:atom)
  This exemplifies an alternative (to jpl_servlet_byref) tactic for implementing a servlet in Prolog; most Request fields are extracted in Java before this is called, and passed in as a multimap (a map, some of whose values are maps).
jpl_pl_syntax(-Syntax:atom)
  Unifies Syntax with’traditional' or’modern' according to the mode in which SWI Prolog 7.x was started

# Index

?
jpl_array_to_length/2
jpl_array_to_list/2
jpl_array_to_terms/2
jpl_c_lib_version/1
jpl_call/4
jpl_class_to_classname/2
jpl_class_to_type/2
jpl_classname_to_class/2
jpl_classname_to_type/2
jpl_datum_to_type/2
jpl_datums_to_array/2
jpl_entityname_to_type/2
jpl_enumeration_element/2
jpl_enumeration_to_list/2
jpl_false/1
jpl_get/3
jpl_get_actual_jvm_opts/1
jpl_get_default_jvm_opts/1
jpl_hashtable_pair/2
jpl_is_class/1
jpl_is_false/1
jpl_is_null/1
jpl_is_object/1
jpl_is_object_type/1
jpl_is_ref/1
jpl_is_true/1
jpl_is_type/1
jpl_is_void/1
jpl_iterator_element/2
jpl_list_to_array/2
jpl_map_element/2
jpl_new/3
jpl_null/1
jpl_object_to_class/2
jpl_object_to_type/2
jpl_pl_lib_version/1
jpl_pl_syntax/1
jpl_primitive_type/1
jpl_ref_to_type/2
jpl_servlet_byref/3
jpl_servlet_byval/3
jpl_set/3
jpl_set_default_jvm_opts/1
jpl_set_element/2
jpl_terms_to_array/2
jpl_true/1
jpl_type_to_class/2
jpl_type_to_classname/2
jpl_type_to_entityname/2
jpl_void/1