JPL - Java API - Overview

The Class Hierarchy

The API consists of the following class hierarchy:

 +-- JPL
 +-- JPLException
 |    +-- PrologException
 +-- Query
 +-- Term
 |    +-- Atom
 |    +-- Compound
 |    +-- Float
 |    +-- Integer
 |    +-- Variable
 |    +-- JRef
 +-- Util
 +-- Version

org.jpl7.Term is an abstract class: only its subclasses can be instantiated.

Each instance of org.jpl7.Query contains a Term (denoting the goal which is to be proven), and much more besides.

Each instance of org.jpl7.Compound has a java.lang.String name and an array of Term arguments. For compatibility with SWI-Prolog version 7’s extension Compound terms with zero arguments, the argument array can be of zero length.

Initializing and Terminating Prolog

Typically, this is automatic.

JPL lazily initializes the Prolog VM, if necessary, when the first query is activated, using default initialization arguments (command line options). Before initialization takes place, these default values can be read, and altered.

public String[] getDefaultInitArgs();
public void setDefaultInitArgs(String[] args);

After initialization, the parameter values which were actually used can be read.

public String[] getActualInitArgs();

This method returns null if initialization has not occurred, and thus it can be used as a test. This allows Java library classes to employ JPL without placing any burden of initialization upon the applications which use them. It can also ensure that the Prolog VM is initialized only if and when it is needed.

Explicit initialization is supported:

public void init();
public void init(String args[]);

Java code which requires a Prolog VM to be initialized in a particular way can check whether initialization has already occurred: if not, it can specify parameters and force it to be attempted; if so, it can retrieve and check the initialisation parameters actually used, to determine whether the initialization meets its requirements.

JPL does not support reinitialization of a Prolog VM, but some command line options merely set flags, which can be altered later by calling set_prolog_flag/2 via a JPL query.

For details about the legal parameter values, see 2.4 Command Line Options in the SWI Prolog Reference Manual. Most users will rely on automatic initialization.

Creating Terms

The Term-based classes in the org.jpl7 package are a structured concrete syntax for Prolog terms: they are not references to actual terms within the Prolog engine; rather, they are a means for constructing queries which can be called within Prolog, and they are also a means for representing (and exploring) the results of such calls. In particular, instances of org.jpl7.Variable are never bound nor shared; they are merely tokens.

Term instances are never changed by any activity within the Prolog engine: indeed; it doesn’t know of their existence.

The Term class is abstract, so it cannot be directly instantiated; to create a Term, create an instance of one of its subclasses.


An org.jpl7.Atom instance represents a SWI Prolog text atom. To create an Atom, pass a (String) name to its constructor:

Atom a1 = new Atom("aristotle");
Atom a2 = new Atom("alexander");

As with Java strings, SWI Prolog text atoms represent arbitrarily long sequences of Unicode characters (including ASCII’s nul).

Two Atom instances with the same name are effectively identical. Feel free to reuse atoms when constructing compound terms.

The name of an atom need not be lower case: it can be any UCS string.

An atom’s name is retrieved with its name() method, e.g.

See Atom’s JavaDoc for details of how SWI Prolog version 7’s strings and blobs (including reserved symbols) are accommodated.


org.jpl7.Variable instances have identifying names, which must comply with conventional Prolog source text syntax.

Variable v1 = new Variable("X"); // a regular variable

Variable v2 = new Variable("_"); // an "anonymous" variable

Variable v3 = new Variable("_Y"); // a "dont-tell-me" variable, whose bindings we don't want to know

They are just tokens, and do not behave like Prolog variables.


A org.jpl7.Integer is a specialized org.jpl7.Term which holds a Java long value or a java.math.BigInteger object. This class corresponds to the Prolog integer type.

org.jpl7.Integer i = new org.jpl7.Integer(5);

Be careful to avoid confusion with java.lang.Integer, e.g. by qualifying the class name as in the example above.

The org.jpl7.Integer class has an intValue() accessor to obtain the int value of an instance, and also longValue(), floatValue() and doubleValue() (just like java.lang.Integer has).

If isBig() returns true, then the value is outside the range of a Java long, and is retrieved by bigValue().


A org.jpl7.Float is a specialized org.jpl7.Term which holds a Java double value. This class corresponds to the Prolog float type (64-bit ISO/IEC in SWI Prolog).

org.jpl7.Float f = new org.jpl7.Float(3.14159265);

As with integers, take care to avoid confusion between org.jpl7.Float and java.lang.Float.

The org.jpl7.Float class has a doubleValue() accessor to obtain the double value of an instance, and also a floatValue() accessor.

Compound Terms

A org.jpl7.Compound is a specialized org.jpl7.Term which contains a name and an array of org.jpl7.Term arguments, and can be constructed e.g.

Compound t1 = new Compound(
    new Term[] {
        new Atom("aristotle"),
        new Atom("alexander")

Note the use of Java’s anonymous array syntax

new Term[] {..., ...}

to specify any quantity (perhaps zero) of arguments.

In this example, the Java variable t1 refers to a Compound instance, which represents the Prolog term teacher_of(aristotle, alexander).

To obtain the (String) name of a Compound, use the name() accessor method.

public String name();

To obtain the arity of a Compound, use the arity() accessor method.

public int arity();

To obtain an array of a Compound’s arguments, use the args() accessor method.

public Term[] args();

To obtain the ith argument of a compound (numbered from 1), use the arg() accessor method (with an int parameter value between 1 and Arity inclusive).

public Term arg(int i);

To obtain the ith argument of a compound (numbered from 0), use the arg0() accessor method (with an int parameter value between 0 and Arity-1 inclusive).

public Term arg0(int i);

Creating Queries

A Query contains a Term, representing a Prolog goal:

Term goal = new Compound("teacher_of", new Term[] {new Atom("aristotle"), new Atom("alexander")});
Query q = new Query(goal);

The Query q in this example represents the Prolog query

?- teacher_of(aristotle, alexander).

org.jpl7.Query implements java.util.Iterator, allowing a query’s solutions to be retrieved one at a time.

Querying Prolog

To ask the Prolog engine a query, one first constructs a Query instance, as in the above example, and then uses the java.util.Iterator interface, which the Query class implements, to obtain solutions (where a “solution” is what is known in logic programming jargon as a substitution, which is a collection of bindings, each of which relates one of the variables within the query’s goal to a Term representation of the Prolog term to which the corresponding Prolog variable was bound by the proof).

public interface Iterator {
    public boolean hasNext();
    public Object next();

The hasNext() method can be used to determine whether a query has any (or any further) solutions. In the above example, the method call


returns true if the Prolog query teaches(aristotle, alexander) is provable, and false otherwise. In this example, the Prolog query is a ground term, so the “solution” to the query is merely a truth value, and is given by the hasNext() method.

Where a query contains variables, on the other hand, its execution yields a sequence of bindings of the variables’ names to Term instances. JPL uses a java.util.Map<String, Term> (implemented as a java.util.HashMap) to represent these bindings; the objects in the map are org.jpl7.Term instances, keyed (uniquely) by the String names of their associated variables.

For example, to print all of Aristotle’s pupils, i.e. all the bindings of X which satisfy teaches(aristotle,X), one could write

Query q = new Query("teaches", new Term[] {new Atom("aristotle"), new Variable("X")});
while (q.hasNext()) {
    Map binding =;
    Term t = (Term) binding.get("X");

or, more concisely

for (Map m : new Query("teaches", new Term[] {new Atom("aristotle"), new Variable("X")})) {

or, using a convenience constructor which builds the term from Prolog source text

for (Map m : new Query("teaches(aristotle,X)")) {

If a query’s goal contains no variables (i.e. it is ground), the will return an emnpty map for each solution.

If a query’s goal contains more than one occurrence of some (named) variable, then each solution will have only one binding for that name.

Obtaining one Solution

Often, you’ll want just the first solution to a query; org.jpl7.Query has a method for this:

public final Map<String, Term> oneSolution();

If the query has no solutions, this method returns null; otherwise, a non-null return indicates success. If the query is ground (i.e. contains no variables), the returned map will be empty (i.e. will contain no bindings).

Obtaining all Solutions

You may want all solutions to a query; org.jpl7.Query has a method for this:

public final Map<String, Term>[] allSolutions();

The returned array will contain all the query’s solutions, in the order they were obtained (as with Prolog’s findall/3, duplicates are not removed). If the query has no solutions, this method returns an empty array.

Ground Queries - Discovering whether a query has any solutions

Sometimes an application is interested only in whether a query is provable, but not in any details of its possible solutions; org.jpl7.Query has a method for this common special case:

public final boolean hasSolution();

This method is equivalent to calling oneSolution and asking whether the return value is non-null (i.e. whether the query succeeded).

Terminating Queries - Terminating Queries

Queries terminate automatically when hasNext() returns false (or when next() throws an exception), and once a query has terminated, its engine is returned to the pool for reuse (by any thread).

To terminate a query before all of its solutions have been exhausted, use its close() method:

public final void close();

This method stops a query, setting it back into a state where it can be restarted. Here is an example in which the first three solutions to the query are obtained:

Query query = // obtain Query somehow
for (int i = 0; i < 3 && query.hasNext(); ++i) {
    Map<String, Term> solution =;
    // process solution...

You may call close() on an inactive query without ill-effect, and you should always call close if you have not exhausted all solutions to a query, otherwise the associated Prolog engine will not be released.

If you are using the allSolutions(), hasSolution(), nSolutions(), or oneSolution() methods, you need not worry about closing the query; it is done automatically for you.

See Types of Queries: One shot vs Iterative (in Sebastian Sardina’s JPL Wiki) for further details and explanation.

Multi-Threaded Queries - Queries from multi-threaded applications

JPL maintains a finite pool of Prolog engines, one of which is allocated to a query when it is activated (i.e. when, one way or another, a solution is requested). A query’s engine is returned to the pool when it is closed (explicitly or automatically).

If no pool engine is available when a query is activated, the activation is blocked until an engine becomes available.

Each JVM thread can have at most one Prolog engine attached. A thread may nest (stack) two or more active queries, e.g. open and close a second query while a first is active, but it may not interleave the retrieval of solutions from two open queries.

Note that engines cannot communicate thread_local predicates or global variables. That means that you can only use these from Java within a single query. If a thread activates two consecutive queries, it may get two different engines.

Note also that, once a thread has activated a query, it cannot pass it to another thread: solutions of a query must be retrieved by the thread which activated it.

See Multi Threaded Queries (in Sebastian Sardina’s JPL Wiki) for further details and explanation.


JPL provides crude but adequate exception handling. The base class for all exceptions is org.jpl7.JPLException, which specialises java.lang.RuntimeException and hence is unchecked. Converting the exception to a java.lang.String should provide some descriptive information about the reason for the error. JPL’s only other exception class is org.jpl7.PrologException, which extends org.jpl7.JPLException.

A org.jpl7.PrologException is thrown either during execution of a Prolog built-in predicate or by an explicit call of throw/1 by application code.


Each Term type (together with the Query class) supports an implementation of toString() which returns a more-or-less familiar Prolog textual representation of the Term or Query.

In general, Term and Query instances are represented in the form (type data), where type is the name of the type (e.g., Atom, Compound, Tuple, etc.), and data is a representation of the contents of the Term. For example, if the Term is an Atom, the data is the Atom’s name. The arguments of Compounds are represented by comma-separated lists within square brackets (‘[’ ‘]’).

Viewing the structure of a term or query can be useful in determining whether an error lies on the Prolog or Java side of your JPL applications.

Version - Version information

To obtain the current version of JPL you are using, you may obtain a static reference to the org.jpl7.Version class by calling the org.jpl7.JPL#version static method. This will return a org.jpl7.Version structure, which has the following final fields:

package org.jpl7;
public class Version {
    public final int major;                // e.g. 7
    public final int minor;                // e.g. 4
    public final int patch;                // e.g. 0
    public final java.lang.String status;  // e.g. "alpha"

You may wish to use this class instance to obtain fine-grained information about the current JPL version, e.g.

if (JPL.version().major == 7) {

You may also call the version_string() static method of the org.jpl7.JPL class. This will return a String representation of the current JPL version.

The version string can be written to the standard output stream by running the main() method of the org.jpl7.JPL class.

linux% java org.jpl7.JPL
JPL 7.4.0-alpha


arg indexing

The Term[] args of a Compound are indexed (like all Java arrays) from zero, whereas in Prolog the args of a structure are conventionally numbered from one.

representing @(null)

there is no org.jpl7.JNull class: instead, use new JRef(null) to represent @(null) (which itself represents Java’s null). If you don’t know what this all means, don’t worry: it only affects those writing hybrid Java+Prolog programs which call each other nestedly.

all solutions of a Query with no solutions

Query.allSolutions() returns an empty array of Map<String, Term> if the query has no solutions (in 1.x versions it inconsistently returned null).

JPL - Java API - Overview