One of the simplest configurations is a repository that just stores RDF data in main memory without applying any inferencing or whatsoever. This is also by far the fastest type of repository that can be used. The following code creates and initialize a non-inferencing main-memory repository:

import org.openrdf.sail.memory.MemoryStore;
import org.openrdf.repository.Repository;
import org.openrdf.repository.sail.SailRepository;


...

Repository myRepository = new SailRepository(new MemoryStore());
myRepository.initialize();

The constructor of the SailRepository class accepts any object of type Sail, so we simply pass it a new main-memory store object (which is, of course, a Sail implementation). Following this, the repository needs to be initialized to prepare the Sail(s) that it operates on, which includes operations such as restoring previously stored data, setting up connections to a relational database, etc.

The repository that is created by the above code is volatile: its contents are lost when the object is garbage collected or when the program is shut down. This is fine for cases where, for example, the repository is used as a means for manipulating an RDF model in memory.

Different types of Sail objects take parameters in their constructor that change their behaviour. The MemoryStore for example takes a file parameter that specifies a data file for persisent storage. If specified, the MemoryStore will write its contents to this file so that it can restore it when it is initialized in a future session:

File dataFile = new File("c:\temp\myRepository.dat");
Repository myRepository = new SailRepository( new MemoryStore(dataFile) );
myRepository.initialize();

As you can see, we can fine-tune the configuration of our repository by passing parameters to the constructor of the Sail object. Some Sail types may offer additional configuration methods, all of which need to be called before the repository is initialized. The MemoryStore currently has one such method: setSyncDelay(long), which can be used to control the strategy that is used for writing to the data file, e.g.:

File dataFile = new File("c:\temp\myRepository.dat");
MemoryStore memStore = new MemoryStore(dataFile);
memStore.setSyncDelay(1000L);

Repository myRepository = new SailRepository(memStore);
myRepository.initialize();

As we have seen, we can create Repository objects for any kind of back-end store by passing them a reference to the appropriate Sail object. We can pass any stack of Sails this way, allowing all kinds of different repository configurations to be created quite easily. For example, to stack an RDF Schema inferencer on top of a memory store, we simply create a repository like so:

import org.openrdf.sail.memory.MemoryStore;
import org.openrdf.sail.inferencer.MemoryStoreRDFSInferencer;
import org.openrdf.repository.Repository;

...

Repository myRepository = new SailRepository(
                          new MemoryStoreRDFSInferencer(
                          new MemoryStore()));
myRepository.initialize();

Each layer in the Sail stack is created by a constructor that takes the underlying Sail as a parameter. Finally, we create the SailRepository object as a functional wrapper around the Sail stack.

Working with remote repositories is just as easy as working with local ones. We can simply use a different Repository object, the HTTPRepository, instead of the SailRepository class.

A requirement is of course that there is a Sesame 2 server running on some remote system, which is accessible over HTTP. For example, suppose that at http://example.org/sesame2/ a Sesame server is running, which has a repository with the identification 'example-db'. We can access this repository in our code as follows:

import org.openrdf.repository.Repository;
import org.openrdf.repository.http.HTTPRepository;

...

String sesameServer = "http://example.org/sesame2";
String repositoryID = "example-db";

Repository myRepository = new HTTPRepository(sesameServer, repositoryID);
myRepository.initialize();

The Repository API offers various methods for adding data to a repository. Data can be added by specifying the location of a file that contains RDF data, and statements can be added individually or in collections.

We perform operations on a repository by requesting a RepositoryConnection from the repository. On this RepositoryConnection object we can the various operations, such as query evaluation, getting, adding, or removing statements, etc.

The following example code adds two files, one local and one available through HTTP, to a repository:

import org.openrdf.OpenRDFException;
import org.openrdf.rio.RDFFormat;
import org.openrdf.repository.Repository;
import org.openrdf.repository.RepositoryConnection;
import java.io.File;
import java.net.URL;

...

File file = new File("/path/to/example.rdf");
String baseURI = "http://example.org/example/local";

try {
   RepositoryConnection con = myRepository.getConnection();
   con.add(file, baseURI, RDFFormat.RDFXML);

   URL url = new URL("http://example.org/example/remote");
   con.add(url, url.toString(), RDFFormat.RDFXML);
   
   con.close();
}
catch (OpenRDFException e) { // handle exception }
catch (java.io.IOEXception e) { // handle io exception }	

More information on other available methods can be found in the javadoc reference of the RepositoryConnection interface.

The Repository API has a number of methods for creating and evaluating queries. Two types of queries are distinguished: tuple queries and graph queries. The query types differ in the type of results that they produce.

The result of a tuple query is a set of tuples (or variable bindings), where each tuple represents a solution of a query. This type of query is commonly used to get specific values (URIs, blank nodes, literals) from the stored RDF data.

The result of Graph queries is an RDF graph (or set of statements). This type of query is very useful for extracting sub-graphs from the stored RDF data, which can then be queried further, serialized to an RDF document, etc.

Note: Sesame 2 currently supports two query languages: SeRQL and SPARQL. The former is explained in Chapter 5, The SeRQL query language (revision 2.0), the specification for the latter is available online.

import java.util.List;
import org.openrdf.OpenRDFException;
import org.openrdf.repository.RepositoryConnection;
import org.openrdf.query.TupleQueryResult;
import org.openrdf.query.BindingSet;

...

try {
  RepositoryConnection con = myRepository.getConnection();
  String query = "SELECT x, y FROM {x} p {y}";
  TupleQueryResult result = null;
  try {
     result = con.prepareTupleQuery(QueryLanguage.SERQL, query).evaluate();
     .... // do something with the result
  }
  finally {
    result.close();
	 con.close();
  }
}
catch (OpenRDFException e) { // handle exception}

while (result.hasNext()) {
  BindingSet bindingSet = result.next();			  
  Value valueOfX = bindingSet.getValue("x");
  Value valueOfY = bindingSet.getValue("y");

  // do something interesting with the values here...
}

List<String> bindingNames = result.getBindingNames();
while (result.hasNext()) {
  BindingSet bindingSet = result.next();   
  Value firstValue = bindingSet.getValue(bindingNames.get(0)); 
  Value secondValue = bindingSet.getValue(bindingNames.get(1));

  // do something interesting with the values here...
}

while (result.hasNext()) {
  BindingSet bindingSet = result.next();			  
  Value firstValue = bindingSet.getValue(0); 
  Value secondValue = bindingSet.getValue(1);

  // do something interesting with the values here...
}

import org.openrdf.query.resultio.sparqlxml.SPARQLResultsXMLWriter;

...

FileOutputStream out = new FileOutputStream("/path/to/result.srx");
try {
    SPARQLResultsXMLWriter sparqlWriter = new SPARQLResultsXMLWriter();
    sparqlWriter.setOutputStream(out);

    RepositoryConnection con = myRepository.getConnection();
    
    con.prepareTupleQuery(QueryLanguage.SERQL,
        "SELECT * FROM {x} p {y}").evaluate(sparqlWriter);

    con.close();
}
finally {
    out.close();
}

import org.openrdf.query.GraphQueryResult;

GraphQueryResult graphResult = con.prepareGraphQuery(
      QueryLanguage.SERQL, "CONSTRUCT * FROM {x} p {y}").evaluate();

while (graphResult.hasNext()) {
      Statement st = graphResult.next();
      // ... do something with the resulting statement here.
}

import org.openrdf.rio.turtle.TurtleWriter;

...

TurtleWriter turtleWriter = new TurtleWriter(System.out);

con.prepareGraphQuery(QueryLanguage.SERQL,
       "CONSTRUCT * FROM {x} p {y}").evaluate(turtleWriter);
  con.close();

    RepositoryConnection con = myRepository.getConnection();

    // first, prepare a query that retrieves all names of persons
    TupleQuery nameQuery = con.prepareTupleQuery(QueryLanguage.SERQL,
       "SELECT name FROM {person} ex:name {name}");

    // then, we prepare another query that retrieves all e-mails of
    // persons:
    TupleQuery mailQuery = con.prepareTupleQuery(QueryLanguage.SERQL,
       "SELECT mail FROM {person} ex:mail {mail}; ex:name {name}");

    // we evaluate the first query to get all names
    TupleQueryResult nameResult = nameQuery.evaluate();
    try {    
      // now we loop over all names in this result, and for each name we
      // retrieve a corresponding mail address.
      while (nameResult.hasNext()) {
        BindingSet bindingSet = nameResult.next();
        Value name = bindingSet.get("name");

        // now for each name we retrieve the matching mailbox, by setting
        // the binding for the variable 'name' to the retrieved value:
        mailQuery.addBinding("name", name);

        TupleQueryResult mailResult = mailQuery.evaluate();

        // the mailResult now contains the mail for one particular person
        try {
          ....
        }
        finally {
          // after we are done, close the result
          mailResult.close();
        }
      }
    } 
    finally {
      nameResult.close();
    }
    con.close();

    ValueFactory factory = myRepository.getValueFactory();

    // In this example, we specify the keyword string. Of course, this
    // could just as easily be obtained by user input, or by reading from
    // a file, or...
    String keyword = "foobar";
    
    // We prepare a query that retrieves all documents for a keyword.
    // Notice that in this query the 'keyword' variable is not bound to
    // any specific value yet.
    TupleQuery keywordQuery = con.prepareTupleQuery(QueryLanguage.SERQL,
       "SELECT document FROM {document} ex:keyword {keyword}");

    // then we set the binding to a literal representation of our keyword.
    // Evaluation of the query object will now effectively be the same as
    // if we had specified the query as follows:
    //   SELECT document FROM {document} ex:keyword {"foobar"}
    keywordQuery.addBinding("keyword", factory.createLiteral(keyword));

    // we then evaluate the prepared query and can process the result.   
    TupleQueryResult keywordQueryResult = keywordQuery.evaluate();

The RepositoryConnection can also be used for adding, retrieving, removing or otherwise manipulating individual statements, or sets of statements.

To be able to add new statements, we can use a ValueFactory to create the Values out of which the statements consist. For example, we want to add a few statements about two resources, Alice and Bob:

import org.openrdf.model.vocabulary.RDF;
import org.openrdf.model.vocabulary.RDFS;
...

ValueFactory f = myRepository.getValueFactory();

// create some resources and literals to make statements out of
URI alice = f.createURI("http://example.org/people/alice");
URI bob = f.createURI("http://example.org/people/bob");
URI name = f.createURI("http://example.org/ontology/name");
URI person = f.createURI("http://example.org/ontology/Person");
Literal bobsName = f.createLiteral("Bob");
Literal alicesName = f.createLiteral("Alice");

try {
  RepositoryConnection con = myRepository.getConnection();

  // alice is a person
  con.add(alice, RDF.TYPE, person);
  // alice's name is "Alice"
  con.add(alice, name, alicesName);
  
  // bob is a person
  con.add(bob, RDF.TYPE, person);
  // bob's name is "Bob"
  con.add(bob, name, bobsName);
}
catch (OpenRDFException e) { // handle exception}

Of course, it will not always be necessary to use a ValueFactory to create URIs. In practice, you will find that you quite often retrieve existing URIs from the repository (for example, by evaluating a query) and then reusing those values to add new statements.

As you can see in the above code, for the default RDF and RDF Schema properties (such as 'rdf:type' and 'rdfs:subClassOf') it is not necessary to create new URI objects. Instead, you can import the vocabulary classes org.openrdf.model.vocabulary.RDF and RDFS which provide you static references to the vocabulary primitives.

Retrieving statements works in a very similar way. One way of retrieving statements we have already seen actually: we can get a GraphQueryResult containing statements by evaluating a graph query. However, we can also use direct method calls to retrieve (sets of) statements. For example, to retrieve all statements about Alice, we could do:

RepositoryResult<Statement> statements = 
                  con.getStatements(alice, null, null, true);

The additional boolean parameter at the end (set to 'true' in this example) indicates wether inferred triples should be included in the result. Of course, this parameter only makes a difference if your repository uses an inferencer.

The RepositoryResult is an iterator-like object that that lazily retrieves each matching statement from the repository when its next() method is called. Note that, like is the case with QueryResult objects, iterating over a RepositoryResult may result in exceptions which you should catch to make sure that the RepositoryResult is always properly closed after use:

RepositoryResult<Statement> statements = con.getStatements(alice, null, null, true);

try {
  while (statements.hasNext()) {
     Statement st = statements.next();

     ... // do something with the statement
  }
}
finally {
  statements.close(); // make sure the result object is closed properly
  }

In the above method invocation, we see four parameters being passed. The first three represent the subject, predicate and object of the RDF statements which should be retrieved. A null value indicates a wildcard, so the above method call retrieves all statements which have as their subject Alice, and have any kind of predicate and object. The fourth parameter indicates whether or not inferred statements should be included or not.

Removing statements again works in a very similar fashion. Suppose we want to retract the statement that the name of Alice is "Alice"):

con.remove(alice, name, alicesName);

Or, if we want to erase all statements about Alice completely, we can do:

con.remove(alice, null, null);

Most of these examples have been on the level of individual statements. However, the Repository API offers several methods that work with Collections of statements, allowing more batch-like update operations.

For example, in the following bit of code, we first retrieve all statements about Alice, put them in a Collection and then remove them:

import info.aduna.commons.collections.util.iterations.Iterations;

// retrieve all statements about Alice and put them in a list.
RepositoryResult<Statement> statements = 
   con.getStatements(alice, null, null, true));
List<Statement> aboutAlice = Iterators.addAll(statements, new
	  ArrayList<Statement>());
// then, remove them from the repository.
con.remove(aboutAlice);

As you can see, the info.aduna.commons.collections.iterators.Iterators class provides a convenient method that takes an Iteration (of which RepositoryResult is a subclass) and a Collection as input, and returns the Collection with the contents of the iterator added to it. It also automatically closes the Iteration for you.

In the above code, you first retrieve all statements, put them in a list, and then remove them. Although this works fine, it can be done in an easier fashion, by simply supplying the resulting object directly:

con.remove(con.getStatements(alice, null, null, true));

The RepositoryConnection interface has several variations of add, retrieve and remove operations. See the Javadoc API documentation for a full overview of the options.

Sesame 2 supports the notion of context, which you can think of as a way to group sets of statements together through a single group identifier (this identifier can be a blank node or a URI).

A very typical way to use context is tracking provenance of the statements in a repository, that is, which file these statements originate from. For example, consider an application where you add RDF data from different files to a repository, and then one of those files is updated. You would then like to replace the data from that single file in the repository, and to be able to do this you need a way to figure out which statements need to be removed. The context mechanism gives you a way to do that.

In the following example, we add an RDF document from the Web to our repository, in a context. In the example, we make the context identifier equal to the Web location of the file being uploaded.

String location = "http://example.org/example/example.rdf";
String baseURI = location;
URL url = new URL(location);
URI context = f.createURI(location);

con.add(url, baseURI, RDFFormat.RDFXML, context);

We can now use the context mechanism to specifically address these statements in the repository for retrieve and remove operations:

// get all statements in the context
RepositoryResult<Statement> result = 
        con.getStatements(null, null, null, true, context);

while (result.hasNext()) {
  Statement st = result.next();        
  ... // do something interesting with the result
}
result.close();

// export all statements in the context to System.out, in RDF/XML format
RDFHandler rdfxmlWriter = new RDFXMLWriter(System.out);		
con.export(context, rdfxmlWriter);

// remove all statements in the context from the repository
con.clear(context);

In most methods in the Repository API, the context parameter is a vararg, meaning that you can specify an arbitrary number (zero, one, or more) of context identifiers. This way, you can very flexibly combine different contexts together. For example, we can very easily retrieve statements that appear in either 'context1' or 'context2'.

In the following example we add information about Bob and Alice again, but this time each has their own context. We also create a new property called 'creator' that has as its value the name of the person who is the creator a particular context. The knowledge about creators of contexts we do not add to any particular context, however:

URI context1 = f.createURI("http://example.org/context1");
URI context2 = f.createURI("http://example.org/context2");
URI creator = f.createURI("http://example.org/ontology/creator");

// add stuff about Alice to context1
con.add(alice, RDF.TYPE, person, context1);
con.add(alice, name, alicesName, context1);

// Alice is the creator of context1
con.add(context1, creator, alicesName);

// add stuff about Bob to context2
con.add(bob, RDF.TYPE, person, context2);
con.add(bob, name, bobsName, context2);

// Bob is the creator of context2
con.add(context2, creator, bobsName);

Once we have this information in our repository, we can retrieve all statements about either Alice or Bob by using the context vararg:

// get all statements in either context1 or context2.
RepositoryResult<Statement> result = 
  con.getStatements(null, null, null, true, context1, context2);

You should observe that the above RepositoryResult will not contain the information that context1 was created by Alice and context2 by Bob. This is because those statements were added without any context, thus they do not appear in context1 or context2, themselves.

To explicitly retrieve statements that do not have an associated context, we do the following:

// get all statements that do not have an associated context
RepositoryResult<Statement> result = 
  con.getStatements(null, null, null, true, (Resource)null);

This will give us only the statements about the creators of the contexts, because those are the only statements that do not have an associated context. Note that we have to explicitly cast the null argument to Resource, because otherwise it is ambiguous whether we are specifying a single value or an entire array that is null (a vararg is internally treated as an array). Simply invoking getStatements(s, p, o, true, null) without an explicit cast will result in an IllegalArgumentException.

We can also get everything that either has no context or is in context1:

// get all statements that do not have an associated context, or that 
// are in context1      
RepositoryResult<Statement> result = 
  con.getStatements(null, null, null, true, (Resource)null, context1);

So as you can see, you can freely combine contexts in this fashion.

Important:

getStatements(null, null, null, true);

is not the same as:

getStatements(null, null, null, true, (Resource)null);

The former (without any context id parameter) retrieves all statements in the repository, ignoring any context information. The latter, however, only retrieves statements that explicitly do not have any associated context.

So far, we have shown individual operations on repositories: adding statements, removing them, etc. By default, a RepositoryConnection runs in autoCommit mode, meaning that each operation on a RepositoryConnection is immediately sent to the store and committed.

The RepositoryConnection interface supports a full transactional mechanism that allows one to group modification operations together and treat them as a single update: before the transaction is committed, none of the operations in the transaction has taken effect, and after, they all take effect. If something goes wrong at any point during a transaction, it can be rolled back so that the state of the repository is the same as before the transaction started. Bundling update operations in a single transaction often also improves update performance compared to multiple smaller transactions.

We can achieve this behaviour by switching off the RepositoryConnection's autoCommit mode. In the following example, we use a non-autocommit connection to bundle two file addition operations in a single transaction:

File inputFile1 = new File("/path/to/example1.rdf");
String baseURI1 = "http://example.org/example1/";

File inputFile2 = new File("/path/to/example2.rdf");
String baseURI2 = "http://example.org/example2/";

RepositoryConnection con = null;
try {
    con = myRepository.getConnection();
    con.setAutoCommit(false);
	
    // add the first file
    con.add(inputFile1, baseURI1, RDFFormat.RDFXML);

    // add the second file
    con.add(inputFile2, baseURI2, RDFFormat.RDFXML);

    // if everything went as planned, we can commit the result
    con.commit();
}
catch (RepositoryException e) {
    // something went wrong during the transaction, so we roll it back
    con.rollback();
} 
finally {
    // whatever happens, we want to close the connection when we are done.
    con.close();
}

In the above example, we use a transaction to add two files to the repository. Only if both files can be successfully added will the repository change. If one of the files can not be added (for example because it can not be read), then the entire transaction is cancelled and none of the files is added to the repository.