Repository: incubator-commonsrdf Updated Branches: refs/heads/master da91cb73a -> 567b775be
Introduction to RDF using Commons RDF SVG files are made using ttps://www.draw.io/ When saving, use File -> Export as -> SVG Zoom: 100% (x) Transparent background (x) Include a copy of the diagram Then re-insert these comments in the replaced SVG. Project: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/repo Commit: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/commit/567b775b Tree: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/tree/567b775b Diff: http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/diff/567b775b Branch: refs/heads/master Commit: 567b775be8eac714d557a2c3cc171895f7c08c6c Parents: da91cb7 Author: Stian Soiland-Reyes <st...@apache.org> Authored: Mon Nov 21 11:06:45 2016 +0000 Committer: Stian Soiland-Reyes <st...@apache.org> Committed: Mon Nov 21 11:15:46 2016 +0000 ---------------------------------------------------------------------- examples/src/example/IntroToRDF.java | 79 ++++++ examples/src/example/IntroToRDFTest.java | 30 ++ src/site/markdown/introduction.md | 383 ++++++++++++++++++++++++++ src/site/resources/images/rdf-01.svg | 30 ++ src/site/resources/images/rdf-02.svg | 31 +++ 5 files changed, 553 insertions(+) ---------------------------------------------------------------------- http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/examples/src/example/IntroToRDF.java ---------------------------------------------------------------------- diff --git a/examples/src/example/IntroToRDF.java b/examples/src/example/IntroToRDF.java new file mode 100644 index 0000000..0aa97f0 --- /dev/null +++ b/examples/src/example/IntroToRDF.java @@ -0,0 +1,79 @@ +/** + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you under the Apache License, Version 2.0 (the + * "License"); you may not use this file except in compliance + * with the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ +package example; + +import org.apache.commons.rdf.api.*; +import org.apache.commons.rdf.simple.SimpleRDF; + +public class IntroToRDF { + public static void main(String[] args) { + RDF rdf = new SimpleRDF(); + + IRI alice = rdf.createIRI("Alice"); + System.out.println(alice.ntriplesString()); + + IRI knows = rdf.createIRI("knows"); + IRI bob = rdf.createIRI("Bob"); + + Triple aliceKnowsBob = rdf.createTriple(alice, knows, bob); + System.out.println(aliceKnowsBob.getSubject().ntriplesString()); + + System.out.println(aliceKnowsBob); + + Graph graph = rdf.createGraph(); + graph.add(aliceKnowsBob); + + IRI charlie = rdf.createIRI("Charlie"); + + IRI plays = rdf.createIRI("plays"); + + IRI football = rdf.createIRI("Football"); + IRI tennis = rdf.createIRI("Tennis"); + + graph.add(alice, knows, charlie); + graph.add(alice, plays, tennis); + graph.add(bob, knows, charlie); + graph.add(bob, plays, football); + graph.add(charlie, plays, tennis); + + System.out.println("Who plays Tennis?"); + for (Triple triple : graph.iterate(null, plays, tennis)) { + System.out.println(triple.getSubject()); + System.out.println(plays.equals(triple.getPredicate())); + System.out.println(tennis.equals(triple.getObject())); + } + + System.out.println("Who does Alice know?"); + for (Triple triple : graph.iterate(alice, knows, null)) { + System.out.println(triple.getObject()); + } + + + System.out.println("Does Alice anyone that plays Football?"); + for (Triple triple : graph.iterate(alice, knows, null)) { + RDFTerm aliceFriend = triple.getObject(); + if (! (aliceFriend instanceof BlankNodeOrIRI)) { + continue; + } + if (graph.contains( (BlankNodeOrIRI)aliceFriend, plays, football)) { + System.out.println("Yes, " + aliceFriend); + } + } + + } +} http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/examples/src/example/IntroToRDFTest.java ---------------------------------------------------------------------- diff --git a/examples/src/example/IntroToRDFTest.java b/examples/src/example/IntroToRDFTest.java new file mode 100644 index 0000000..4399355 --- /dev/null +++ b/examples/src/example/IntroToRDFTest.java @@ -0,0 +1,30 @@ +/** + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you under the Apache License, Version 2.0 (the + * "License"); you may not use this file except in compliance + * with the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ +package example; + +import org.junit.Before; +import org.junit.Test; + +public class IntroToRDFTest { + + @Test + public void runIntroToRDF() { + IntroToRDF.main(new String[0]); + } + +} http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/src/site/markdown/introduction.md ---------------------------------------------------------------------- diff --git a/src/site/markdown/introduction.md b/src/site/markdown/introduction.md new file mode 100644 index 0000000..d494562 --- /dev/null +++ b/src/site/markdown/introduction.md @@ -0,0 +1,383 @@ +# Introduction to RDF using Commons RDF + +This page is a tutorial to introduce programming with the +[Resource Description Framework (RDF)](https://www.w3.org/TR/rdf11-concepts/) +using Java and [Apache Commons RDF](index.md). +If you already know RDF, you may instead jump ahead to the +[Commons RDF user guide](userguide.html). + +This is not meant as an extensive RDF tutorial, for that please consult the +[W3C RDF 1.1 Primer](https://www.w3.org/TR/rdf11-primer/). You may also like +the +[Apache Jena introduction to RDF](https://jena.apache.org/tutorials/rdf_api.html) +which uses the [Apache Jena](https://jena.apache.org/) implementation directly. + +This tutorial attempts to show the basic concepts of RDF and how you can +work with RDF programmatically using the +[Apache Commons RDF API](index.md) in a simple Java program. + + +## Getting started with Commons RDF + +This tutorial will assume you already know +a bit of [Java programming](http://docs.oracle.com/javase/tutorial/) +and that you use an _IDE_ like +[Eclipse](http://www.eclipse.org/) or +[Netbeans](https://netbeans.org/). Note that Commons RDF requires +Open JDK 8, [Java 8](https://www.java.com/) or equivalent. + +The Commons RDF JARs are [available from Maven Central](download.html). +While there are multiple [Commons RDF implementations](implementations.html), +this tutorial will use the built-in _simple_ implementation as it requires no +additional dependencies. + +First, create a new Java project for this tutorial, say `rdftutorial`. + +**Tip**: Check that your IDE project is using the **Java 8** syntax and compiler. + +We'll create the package name +`org.example`, but you can use whatever you prefer. Then create `RdfTutorial.java` +with a static `main()` method we can run: + +```java +package org.example; + +import org.apache.commons.rdf.api.*; +import org.apache.commons.rdf.simple.SimpleRDF; + +public class RdfTutorial { + public static void main(String[] args) { + // ... + } +} +``` + + +### Adding Commons RDF to the class path + +Above we added the `import` for the Commons RDF API, but the library +is not yet on your class path. + +**Note**: If you are already familiar with +_Maven_, then see instead +[how to use Commons RDF from Maven](userguide.html#Using_Commons_RDF_from_Maven) and add +the `commons-rdf-simple` dependency to your project. This will make it easier later +to share your project or to use newer versions of Commons RDF. + +This tutorial assumes a classic Java project with local `.jar` files (say in your project's `lib/` folder), so download and add to your project's class path: + +* [commons-rdf-api-0.3.0-incubating.jar](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-api/0.3.0-incubating/commons-rdf-api-0.3.0-incubating.jar) +([signature](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-api/0.3.0-incubating/commons-rdf-api-0.3.0-incubating.jar.asc)) +* [commons-rdf-simple-0.3.0-incubating.jar](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-simple/0.3.0-incubating/commons-rdf-simple-0.3.0-incubating.jar) ([signature](https://repo.maven.apache.org/maven2/org/apache/commons/commons-rdf-simple/0.3.0-incubating/commons-rdf-simple-0.3.0-incubating.jar.asc)) + +_Tip: If you prefer you can [verify the signatures](https://www.apache.org/info/verification.html) using the Commons RDF [KEYS](http://www.apache.org/dist/incubator/commonsrdf/KEYS)._ + +As there are [multiple Commons RDF implementations](implementations.html), +we have to say which one we want to use. Add to your `RdfTutorial` class: + +```java +RDF rdf = new SimpleRDF(); +``` + +If you have the classpath set up correctly, you should now +be able to compile `RdfTutorial` without warnings. + + +## RDF resources + +"The clue is in the name"; the _Resource Description Framework_ (RDF) is +for describing **resources**. But what is a resource? + +Anything can be a resource, it is just a concept we want to describe, +like computer _files_ (text document, image, database), +_physical things_ (person, place, cat), +_locations_ (city, point on a map), or more _abstract concepts_ (organization, +disease, theatre play). + +To know which concept we mean, in RDF the resource needs to either: + +* have a global _identifier_; we call this an **IRI** +* be used _indirectly_ in a statement; we call this a **blank node** +* be a _value_, we call this a **literal** + +In this tutorial we'll use the _IRI_ syntax `<identifier>` to indicate an identified resource, +the _blank node_ syntax `_:it` to indicate an indirectly referenced +resource, and the _literal_ syntax `"Hello"` to indicate a value. + +Don't worry about this syntax, RDF is a **model** with several +ways to represent it when saved to a file; the [Commons RDF API](apidocs/index.html?org/apache/commons/rdf/api/package-summary.html) directly +reflects the RDF model in a syntax-neutral way. + +Let's create our first identified resource, an `IRI` instance: + +```java +IRI alice = rdf.createIRI("Alice"); +System.out.println(alice.ntriplesString()); +``` + +This should print out: + +> `<Alice>` + + +**Note**: For simplicity this tutorial use _relative IRI references_ which +are not really global identifiers. While this is supported by +`SimpleRDF`, some implementations will require +_absolute IRIs_ like `<http://example.com/Alice>`. + +### Triples + + +To describe a resource in RDF we provide one or more statements, +which are called _triples_ of 3 resources +(_subject_, _predicate_, _object_): + +```turtle +<Alice> <knows> <Bob> . +``` + + + +This RDF statement is a relationship between the **subject** `<Alice>` +and the **object** `<Bob>`, not dissimilar from the subject and direct object +of the similar English sentence _"Alice knows Bob"_. + +What kind of relationship? Well, that is +identified with the **predicate** `<knows>`. +The relationship is _directional_, from the subject to the object; +although _Alice knows Bob_, we don't know if Bob really knows Alice! +In RDF the predicate is also called a _property_ as it is +describing the subject. + +You may have noticed that properties are also resources - +to understand +the kind of relationship we also need a description of it's concept. +More about this later! + +Let's try to create the above statement +in Commons RDF; first we'll create the remaining +resources `<knows>` and `<Bob>`: + +```java +IRI knows = rdf.createIRI("knows"); +IRI bob = rdf.createIRI("Bob"); +``` + +Note that the Java variable names `alice`, `knows` and `bob` +are not important to Commons RDF, +we could as well have called these `a`, `k`, `b`, +but to not confuse yourself it's good to keep the variable names +somewhat related to the captured identifiers. + +Next we'll create a `Triple`: + +```java +Triple aliceKnowsBob = rdf.createTriple(alice, knows, bob); +``` + +We can access `.getSubject()`, `.getPredicate()` and `.getObject()` from a `Triple`: + +```java +System.out.println(aliceKnowsBob.getSubject().ntriplesString()); +System.out.println(aliceKnowsBob.getPredicate().ntriplesString()); +System.out.println(aliceKnowsBob.getObject().ntriplesString()); +``` + +> `<Alice>` <br> +> `<knows>` <br> +> `<Bob>` + +_**Tip**: Instances from `SimpleRDF` can be printed directly, as +`System.out` would use their `.toString()`, +but for consistent behaviour across implementations we use `.ntriplesString()` above._ + +With `SimpleRDF` we can also print the `Triple` for debugging: + +```java +System.out.println(aliceKnowsBob); +``` + +> `<Alice> <knows> <Bob> .` + + +### Graph + +By using the same identified resources in multiple triples, you can +create a _graph_. For instance, this graph shows multiple relations +of `<knows>` and `<plays>`: + +```turtle +<Alice> <knows> <Bob> . +<Alice> <knows> <Charlie> . +<Alice> <plays> <Tennis> . +<Bob> <knows> <Charlie> . +<Bob> <plays> <Football> . +<Charlie> <plays> <Tennis> . +``` + +The power of a graph as a data structure is that you don't have to decide a +hierarchy. The statements of an RDF graph can be listed in any order, and so +we should not consider the `<Alice>` resource as anything more special +than `<Bob>` or `<Tennis>`. + + + +It is therefore possible to _query_ the graph, such as _"Who plays Tennis?_ or +_"Who does Alice know?"_, but also more complex, like +_"Does Alice anyone that plays Football?"_. + +Let's try that now using Commons RDF. To keep the triples we'll need a `Graph`: + +```java +Graph graph = rdf.createGraph(); +``` + +We already have the first triple, so we'll `.add()` it to the `graph`: + +```java +graph.add(aliceKnowsBob); +``` + +Before adding the remaining statements we need a few more resources: + +```java +IRI charlie = rdf.createIRI("Charlie"); + +IRI plays = rdf.createIRI("plays"); + +IRI football = rdf.createIRI("Football"); +IRI tennis = rdf.createIRI("Tennis"); +``` + +Now we use the `graph.add(subj,pred,obj)` shorthand which creates the +`Triple` instances and add them to the graph. + +```java +graph.add(alice, knows, charlie); +graph.add(alice, plays, tennis); +graph.add(bob, knows, charlie); +graph.add(bob, plays, football); +graph.add(charlie, plays, tennis); +``` + +Next we'll ask the graph those questions using `.iterate(s,p,o)` and +`null` as the wildcard. + +```java +System.out.println("Who plays Tennis?"); +for (Triple triple : graph.iterate(null, plays, tennis)) { + System.out.println(triple.getSubject()); +} +``` + +> `Who plays Tennis?` <br> +> `<Alice>` <br> +> `<Charlie>` + + +Notice how we only print out the `.getSubject()` (our wildcard), if you +check `.getPredicate()` or `.getObject()` you will find they are equal to +`plays` and `tennis`: + +```java +System.out.println("Who plays Tennis?"); +for (Triple triple : graph.iterate(null, plays, tennis)) { + System.out.println(triple.getSubject()); + System.out.println(plays.equals(triple.getPredicate())); + System.out.println(tennis.equals(triple.getObject())); +} +``` + +We can query with wildcards in any positions, for instance for +the _object_: + +```java +System.out.println("Who does Alice know?"); +for (Triple triple : graph.iterate(alice, knows, null)) { + System.out.println(triple.getObject()); +} +``` + +> `Who does Alice know?` <br> +> `<Bob>` <br> +> `<Charlie>` + +Let's try to look up which of those friends play football: + +```java +System.out.println("Does Alice anyone that plays Football?"); +for (Triple triple : graph.iterate(alice, knows, null)) { + RDFTerm aliceFriend = triple.getObject(); + if (graph.contains(aliceFriend, plays, football)) { + System.out.println("Yes, " + aliceFriend); + } +} +``` + +You will get a compiler error: + +> `RDFTerm` cannot be converted to `BlankNodeOrIRI` + +This is because in an RDF triple, not all kind of resources can be used in all +positions, and the kind of resource in Commons RDF is indicated by the +interfaces: + +* [IRI](apidocs/org/apache/commons/rdf/api/IRI.html) (identified) +* [BlankNode](apidocs/org/apache/commons/rdf/api/BlankNode.html) (indirect) +* [Literal](apidocs/org/apache/commons/rdf/api/Literal.html) (value) + +Look at the method signature of [graph.contains(s,p,o)](http://commonsrdf.incubator.apache.org/apidocs/org/apache/commons/rdf/api/Graph.html#contains-org.apache.commons.rdf.api.BlankNodeOrIRI-org.apache.commons.rdf.api.IRI-org.apache.commons.rdf.api.RDFTerm-): + +```java +boolean contains(BlankNodeOrIRI subject, + IRI predicate, + RDFTerm object) +``` + +In short, for any RDF triple: + +* The **subject** must be a [BlankNodeOrIRI](apidocs/org/apache/commons/rdf/api/BlankNodeOrIRI.html), that is either a `BlankNode` or `IRI` +* The **predicate** must be a [IRI](apidocs/org/apache/commons/rdf/api/IRI.html) (so we can look up what it means) +* The **object** must be a [RDFTerm](apidocs/org/apache/commons/rdf/api/RDFTerm.html), that is either a `BlankNode`, `IRI` or `Literal` + +As we are retrieving triples from the graph, the `triple.getObject()` is only known +to be an RDFTerm if we use it as a Java variable - there could in theory be triples +in the graph with `Literal` and `BlankNode` objects: + +``` +<Alice> <knows> "Santa Claus". +<Alice> <knows> _:someone. +``` + + +In this case we could have done a naive casting like `(IRI)aliceFriend`; we +inserted her `IRI`-represented friends right before, but this is a toy +example - there's no need to use RDF if you already know the answer! + +So unless you know for sure in your graph that `<knows>` is never used with a +literal value as object, this would not be safe. So we'll do an `instanceof` +check (skipping any literals) and cast to `BlankNodeOrIRI`: + + +```java +System.out.println("Does Alice anyone that plays Football?"); +for (Triple triple : graph.iterate(alice, knows, null)) { + RDFTerm aliceFriend = triple.getObject(); + if (! (aliceFriend instanceof BlankNodeOrIRI)) { + continue; + } + if (graph.contains( (BlankNodeOrIRI)aliceFriend, plays, football)) { + System.out.println("Yes, " + aliceFriend); + } +} +``` + +> `Does Alice anyone that plays Football?` +> `Yes, <Bob>` + +## Literal values + +We talked briefly about literals above as a way to represent values in RDF. +What is a value? In a way you could a value is when we no longer want to +stay in graph land and just want to use primitive types like `long`, +`int` or `String`. http://git-wip-us.apache.org/repos/asf/incubator-commonsrdf/blob/567b775b/src/site/resources/images/rdf-01.svg ---------------------------------------------------------------------- diff --git a/src/site/resources/images/rdf-01.svg b/src/site/resources/images/rdf-01.svg new file mode 100644 index 0000000..19056b3 --- /dev/null +++ b/src/site/resources/images/rdf-01.svg @@ -0,0 +1,30 @@ +<?xml version="1.0" encoding="UTF-8"?> +<!-- + + Licensed to the Apache Software Foundation (ASF) under one + or more contributor license agreements. See the NOTICE file + distributed with this work for additional information + regarding copyright ownership. The ASF licenses this file + to you under the Apache License, Version 2.0 (the + "License"); you may not use this file except in compliance + with the License. You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + + Unless required by applicable law or agreed to in writing, software + distributed under the License is distributed on an "AS IS" BASIS, + WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + See the License for the specific language governing permissions and + limitations under the License. + +--> +<!-- +To edit, use https://www.draw.io/ +When saving, use +File -> Export as -> SVG + Zoom: 100% + (x) Transparent background + (x) Include a copy of the diagram +--> +<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd";> +<svg xmlns="http://www.w3.org/2000/svg"; xmlns:xlink="http://www.w3.org/1999/xlink"; width="425px" height="136px" version="1.1" content="<mxfile userAgent="Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Ubuntu Chromium/53.0.2785.143 Chrome/53.0.2785.143 Safari/537.36" version="6.0.1.5" editor="www.draw.io" type="google"><diagram>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 ru0OFkQTu8EfbswO2aQrowDqzv5dqQrtaqPVCz+geTEeRSO/XXGVGv4P5fCtMM8uAzB2dz6S8XLMOGNXxd8QZdnp6axWK80QwtYC6cu46dXu7WokzsWp0Y+GVYZRfioBwYdDRHE/RHNm9DpsPEeBoslAZbhAzU1PAoFa6O7ArLIG9RsOjHceOteIjC/Zw6A5QVeBbZ5vKsroSsN9Ke3rdLnlQj8y/SLUvBavdLE3sDLErID1KPFGD+dane1roHl5jwuFSNN9C7jSp88UyGAlnaucwa6KjzXeonsiD/VNLurb/+r78V2Pvi/IvuaSGrHu19cNsndhgiOZq+Wy5uZIGW/VwuhTC/+ghcu9LiJBTwjB5MKL+pGkx1dIBw6g539+c737myv8wG8umHbf6o0Yul88ZPYX</diagram></mxfile>"><defs><linearGradient x1="0%" y1="0%" x2="0%" y2="100%" id="mx-gradient-ffebf7-1-ffabcf-1-s-0"><stop offset="0%" style="stop-color:#FFEBF7"/><stop offset="100%" style="stop-color:#FFABCF"/></linearGradient></defs><g transform="translate(0.5,0.5)"><ellipse cx="361" cy="91" rx="60" ry="40" fill="#000000" stroke="#000000" transform="translate(2,3)" opacity="0.25"/><ellipse cx="361" cy="91" rx="60" ry="40" fill="url(#mx-gradient-ffebf7-1-ffabcf-1-s-0)" stroke="#000000" pointer-events="none"/><g transform="translate(341.5,84.5)"><switch><foreignObject style="overflow:visible;" pointer-events="all" width="38" height="13" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility";><div xmlns="http://www.w3.org/1999/xhtml"; 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