A SWI-Prolog to Java interface
Author: Paul Singleton

# JPL 3.x Prolog API overview

- Introduction
- JPL types (Java types, as seen by Prolog)
- representation of Java values and references within Prolog
- representation of Java types within Prolog (1): structured notation
- representation of Java types within Prolog (2): descriptor notation
- representation of Java types within Prolog (3): classname notation
- creating instances of Java classes
- calling methods of Java objects or classes
- fetching field values of Java objects or classes
- setting field values of Java objects or classes
- a slightly longer example
- jpl_new/3
- jpl_call/4
- jpl_set/3
- jpl_get/3
- exceptions thrown by Java
- testing
- to do

## Introduction

This is an overview of an interface which allows SWI-Prolog 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 any Java classes which can be found at run time, and any objects which can be instantiated from them

- it is interoperable with JPL's Java API (which has evolved from Fred Dushin's JPL 1.0.1)

- it requires a Java 2 JVM and class libraries (although it doesn't depend on any Java 2-specific facilities, and originally was developed for use with both 1.1 and 1.2 JVMs, I haven't tested it with 1.1 recently, and don't support this)

- it exploits the Invocation API of the Java Native Interface: this is a mandatory feature of any compliant JVM (even the now defunct "Microsoft Virtual Machine" supported JNI, although they seemed to want to keep that a secret :-)

- it is implemented with a fair amount of Prolog code in one module (jpl.pl) (which I believe to be ISO Standard Prolog compliant and portable) and a SWI-Prolog-specific foreign library (jpl.dll for Windows), implemented in ANSI C but making a lot of use of the SWI-Prolog Foreign Language Interface

- the foreign-language part has so far been tested only under Windows NT4, but is believed to be readily portable to SWI-Prolog on other platforms

- as far as is 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:

- should be treated as opaque handles
- are canonical (two references are ==/2 equal if-and-only-if they refer to the same object within the JVM)
- are represented as structures containing a distinctive atom so as to exploit SWI-Prolog's atom 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 API is similar to that of XPCE: the four main interface calls are jsp_new, jsp_call, jsp_set and jsp_get (there is no jsp_free, since Java's garbage collection is extended transparently into Prolog)

- jsp_call 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, jsp_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 atomic JVM signatures

- the Prolog-calls-Java (mine) and Java-calls-Prolog (Fred's) parts of JPL are largely independent; mine concentrates on representing all Java data values and objects within Prolog, and supporting manipulation of objects; Fred's 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 a void value (which is distinct from all other JPL values and references)

- it 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 `@false`, `@true`, `@null` etc. in your source code; otherwise (and for portability) you'll have to write e.g. `@(true)` etc.

## JPL types (Java types, as seen by Prolog)

> 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 term is introduced, out of necessity, to refer jointly to values and refs)

> > 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 ref

> > > 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)

## representation of Java values and references within Prolog

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

> int, short and byte values are represented by corresponding Prolog integers

> long values are represented as Prolog integers if possible (32-bit in current SWI-Prolog), else as `jlong(Hi,Lo)` where `Hi` is an integer corresponding to the top32 bits of the long, and `Lo` similarly represents the lower 32 bits

> 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, but what the heck)

> string values (immutable instances of java.lang.String) are represented as Prolog 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 currently represented as `@(Tag)`, where Tag ia an atom whose name encodes a JNI global reference value; this may change, but won't affect Prolog programs which respect the opacity of references

## representation of Java types within Prolog (1): structured notation

The JPL 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 (as opposed to instances thereof) are represented. Predicates which pass these type representations include 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.

> void is represented as `void`

> null is represented as `null`

> the primitive types are represented as `boolean`, `char`, `byte`, `short`, `int`, `long`, `float`, `double`

> classes are represented as `class(`package_parts`,`classname_parts`)`

> > e.g. `class([java,util],['Date'])`

> array types are represented as `array(`type`)`

> > e.g. `array(boolean)`

> > e.g. `array(class([java,lang],['String'])
> > `

This structured notation for Java types is designed to be convenient for composition and decomposition by matching (unification).

## representation of Java types within Prolog (2): 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:

> `'Z'` denotes boolean

> `'B'` denotes byte

> `'C'` denotes char

> `'S'` denotes short

> `'I'` denotes int

> `'J'` denotes long

> `'F'` denotes float

> `'D'` denotes double

> `'Ljava/util/Date;'` (for example) denotes the Java class java.util.Date

> `'[`type`'` denotes an array of type

> `'(`argument_types`)`return_type`'` denotes the type of a method

## representation of Java types within Prolog (3): 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:

> `'java.util.Vector'` denotes the Java class `java.util.Vector`

> `'[B'` denotes an array of boolean

> `'[Ljava.lang.String;'` denotes an array of string

# Using the JPL 3.x Prolog API

## creating instances of Java classes

To create an instance of a Java class from within Prolog, call jpl_new(+Class,+Params,-Ref) 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'], F)`

which binds F to a new object reference, e.g.

> `@('J#0008272420')`

(not that the details of this structure are of any necessary concern to the Prolog programmer or to the applications she writes).
NB for convenience, this predicate is overloaded: Class can also be a class type in structured notation, e.g. `array(boolean)`.

## calling methods of Java objects or classes

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

> `jpl_call( +Ref, +Method, +Params, -Result)`

e.g.

> `jpl_call( F, setVisible, [@(true)], _)`

which calls the setVisible method of the object to which F 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 jsp_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)`.

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 +Ref argument represents a class, then the named static method of that class is called.

## fetching field values of Java objects or classes

The jpl_get/3 API predicate can retrieve the value of an instance field or 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.

More generally, jpl_get/3 has the following interface:

> `jpl_get( +Class_or_Object, +Field, -Datum)`

If the first argument represents a class, then a static field of that class with FieldName is accessed.

## setting field values of Java objects or classes

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], _),

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 code 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" (my phrase :-), 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 the relevant Appendix for fuller details of the API procedures.

Don't forget the possibility of writing and manipulating new Java classes to serve your Prolog applications: this interface is not designed to make Java programming redundant :-)

`jpl_new( +X, +Argz, -V) :-`

> `X` can be:
> - a suitable type
> - i.e. any `class(_,_)`, `array(_)` or primitive type (e.g. `byte` but not `void`)
> - an atomic classname
> - e.g. `'java.lang.String'`
> - e.g. `'Ljava.lang.String;'` (a redundant but legitimate form)
> - an atomic descriptor
> - e.g. `'[I'`
> - a class object
> - i.e. an object whose type is `class([java,lang],['Class'])`

> if `X` denotes a primitive type and `Argz` is castable to a value of that type, then `V` is that value (a pointless mode of operation, but somehow complete...)

> if `X` denotes an array type and `Argz` is a non-negative integer, then `V` is a new array of that many elements, initialised to the appropriate default value

> if `X` denotes an array type and `Argz` is a list of datums, each of which is (independently) castable to the array element type, then `V` is a new array of as many elements as `Argz` has members, initialised to the results of casting the respective members of `Argz`

> if `X` denotes a non-array object type and `Argz` is a list of datums, then `V` is the result of an invocation of that type's most specifically-typed constructor to whose respective parameters the members of `Argz` are assignable

`jpl_call( +X, +Method, +Args, -R) :-`

> `X` can be:

> > - a type, class object or classname (for static methods of the denoted class, or for static or instance methods of java.lang.Class)

> > - a class instance or array (for static or instance methods)

> `Method` can be:

> > - an atomic method name (if this name is ambiguous, as a result of method overloading, then it will be resolved by considering the types of Args, as far as they can be inferred)

> > - an integral method index (untested: for static overload resolution)

> > - a `methodID/1` structure (ditto)

> `Args` must be
> - a proper list (possibly empty) of ground arguments
> Finally, an attempt will be made to unify `R` with the returned result.

`jpl_set( +X, +Field, +V) :-`

> basically, sets the `Fspec`-th field of object `X` to value `V`

> `X` can be:

> - a class object, a classname, or an (object or array) type (for static fields, or java.lang.Class fields)

> - a class instance (for non-static fields)

> - an array (for indexed element or subrange assignment)

> - but not a string (no fields to retrieve)
> `Field` can be:
> - an atomic field name (overloading will be resolved dynamically, by considering the inferred type of V)

> - an integral field index (static resolution: not tried yet)

> - a `fieldID/1` (static resolution: not tried yet)

> - a variable (field names, or array indices, are generated)(?!)

> - an array index `I` (`X` must be an array object: `X[I]` is assigned `V`)

> - a pair `I-J` of integers (`J` can be a variable) (`X` must be an array object, `V` must be a list of values: `X[I-J]` will be assigned `V`)
> `V` must be ground (although one day we may pass variables to JPL?!)

`jpl_get( +X, +Field, -V) :-`

> `X` can be:
> - a class object, a classname, or an (object or array) type (for static fields, or java.lang.Class fields)

> - a class instance (for non-static fields)

> - an array (for the 'length' pseudo field, or for indexed element retrieval)

> - but not a String (clashes with classname; anyway, java.lang.String has no fields to retrieve)
> `Field` can be
> - an atomic field name

> - or an integral field index (these are a secret :-)

> - or a `fieldID/1` (not for general consumption :-)

> - or an integral array index (high-bound checking is done by JVM, maybe throwing an exception)

> - or a variable (field names, or array indices, are generated)

> - or a pair `I-J` of integers or variables (array subranges are generated) (relational or what?!)
> Immediately before jpl_get/4 returns, an attempt will be made to unify `V` with the internally computed result.

## exceptions thrown by Java

Uncaught exceptions thrown by the JVM in the course of handling a JPL 3.x Prolog API call are mapped onto Standard Prolog exceptions, e.g.

>     jpl_new( 'java.util.Date', [yesterday], D)

raises the Prolog exception

> `java_exception('java.lang.IllegalArgumentException', @'J#0008408972')`

because, as the exception suggests, yesterday is not a valid constructor argument.
Java exceptions are always returned as Prolog exceptions with this structure:

> `java_exception( classname, reference_to_exception_object)`

## testing

For a rudimentary test, run

>     ?- jpl_demo.

and wait patiently for some Swing windows to appear (but not too patiently, in case something is wrong...)

## to do

Apart from any bugs I don't know about, this interface is usable and useful as it stands. Nevertheless there are some things "to do" at some stage in the future, e.g.

- 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)