JPL - Prolog API - Overview
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 (
jpl.pl), a compiled ANSI C foreign library (
jpl.dll/.so/.dylibfor Windows/Linux/MacOS), and a Java class library (
- 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
==/2if-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_get/3(there is no
jpl_free, since Java’s garbage collection is extended transparently into Prolog)
jpl_call/4resolves 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
@/1to construct representations of certain Java values; if
@/1is defined as a prefix operator (as used by XPCE), then you can write
@voidin your source code; otherwise (and for portability, and recommended) you should write
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)
- is a value (values are copied between Prolog and the JVM)
Java values and references
Instances of JPL types are represented within Prolog as follows:
- boolean has two values, represented by
- 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
- void has only one value, represented as
- 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
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):
jpl_class_to_type/2 jpl_classname_to_type/2 jpl_datum_to_type/2 jpl_is_object_type/1 jpl_is_type/1 jpl_object_to_type/2 jpl_primitive_type/1 jpl_ref_to_type/2 jpl_type_to_class/2 jpl_type_to_classname/2
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:
boolean char byte short int long float double
class types are represented as
array types are represented as
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:
'[type'denotes an array of type
'Ljava/util/Date;'denotes the Java class
'(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)
JRef to a new object reference, e.g.
NB for convenience, this predicate is overloaded:
Class can also be a class type in structured notation, e.g.
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.
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.
JRef argument represents a class, then the named static method of that class is called.
Fetching Java field values
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
findall( Ar, ( current_prolog_flag(N, V), term_to_atom(V, Va), jpl_new('[Ljava.lang.String;', [N,Va], Ar) ), Ars ), 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:
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) :- ground(Ds), 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(+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:
Here are a few things to watch out for.
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).
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)