My notebook for a Stupid RDF Server

In this post, I'm writing my notes about the java package com.sun.net.httpserver.*. This package contains a lightweight HTTP server and I've tested it to create a simple-and-stupid RDF server.
The source code is available at:

StupidRDFServer.java

OK. First we need a 'Statement' class holding a RDF triple:

private static class Statement
{
/** subject of this statement */
private URI subject;
/** predicate of this statement */
private URI predicate;
/** value of this statement a String or a URI*/
private Object value;


boolean isLiteral()
{
return value.getClass()==String.class;
}

@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + predicate.hashCode();
result = prime * result + subject.hashCode();
result = prime * result + value.hashCode();
return result;
}

@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null) return false;
Statement other = (Statement) obj;
return subject.equals(other.subject) &&
predicate.equals(other.predicate) &&
value.equals(other.value)
;
}


@Override
public String toString() {
return "<"+subject+"> <"+predicate+"> "+(isLiteral()
?"\""+C.escape(String.class.cast(value))+"\""
:"<"+value+"> ."
);
}
}

The statements are just stored in a synchronized set. That is to say, we the server stops, all the statements are lost :-).

private Set<Statement> statements= Collections.synchronizedSet(new HashSet<Statement>());

The 'servlet' StupidRDFServer is a class that implementing com.sun.net.httpserver.HttpHandler, that is to say it must implement the method public void handle(HttpExchange http) throws IOException

The following code starts the server.

public static void main(String[] args)
{
try
{
HttpServer server = HttpServer.create(new InetSocketAddress(PORT), 0);
server.createContext(CONTEXT, new StupidRDFServer());
server.start();
}
catch(IOException err)
{
err.printStackTrace();
}
}

If the query is empty, the following form is returned to the client.

Add Statement

Subject 
Predicate 
Value Literal 
  

The 'Add Statements' button adds a statement in the list:

respHeader.add("Content-Type", "text/html");
http.sendResponseHeaders(200, 0);
boolean added=this.statements.add(stmt);
printForm(out,(added?"Statement Added":"Statement already in model"));

The 'Query N3' button prints the triples, using the parameters of the form as a filter:

respHeader.add("Content-Type", "text/plain");
http.sendResponseHeaders(200, 0);
synchronized(statements)
{
Iterator<Statement> iter= statements.iterator();
while(iter.hasNext())
{
Statement triple=iter.next();
if(
(stmt.subject==null || stmt.subject.equals(triple.subject)) &&
(stmt.predicate==null || stmt.predicate.equals(triple.predicate)) &&
(stmt.value==null || stmt.value.equals(triple.value))
)
{
out.println(triple);
}
}
}

The 'Query RDF' button prints the triples as RDF, using the parameters of the form as a filter. Example of output:

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Statement>
<rdf:subject rdf:resource="foaf%3Ame"/>
<rdf:predicate rdf:resource="foaf%3Aname"/>
<rdf:object>Pierre</rdf:object>
</rdf:Statement>
<rdf:Statement>
<rdf:subject rdf:resource="urn%3Aother"/>
<rdf:predicate rdf:resource="foaf%3Aknows"/>
<rdf:object rdf:resource="foaf%3Aknows"/>
</rdf:Statement>
</rdf:RDF>

That's it