Jena学习

Jena是一个用于构建语义网（Semantic Web）和Linked Data应用的Java开源框架，可处理RDF数据，详见：https://jena.apache.org/

Jena配置

在 https://jena.apache.org/download/index.cgi 下载Jena，目前是apache-jena-3.0.1.zip。
环境要求：Java8
在Eclipse中使用：右键点击项目 -> 属性/Properties -> Java创建路径/Java Build Path -> 库/Libraries -> 添加外部文件/Add Extenal JARs -> 添加解压apache-jena-3.0.1.zip后的lib目录下的jar文件。

RDF例子

参考https://jena.apache.org/tutorials/rdf_api.html

资源描述框架（Resource Description Framework，RDF）是描述资源的一项标准（W3C推荐标准）。
使用关于vcard的RDF：https://www.w3.org/TR/vcard-rdf/
一个vcard在RDF中如图：
{% image center https://jena.apache.org/tutorials/figures/fig2.png %}
资源JohnSmith在图中用椭圆表示，并被一个统一资源定位符(URI)所标识，在本例中是http://…/JohnSmith 。

创建RDF模型

import org.apache.jena.rdf.model.Model;
import org.apache.jena.rdf.model.ModelFactory;
import org.apache.jena.rdf.model.Resource;
import org.apache.jena.vocabulary.VCARD;

public class Tutorial extends Object {

    public static void main(String args[]) {
        // some definitions
        String personURI = "http://somewhere/JohnSmith";
        String givenName = "John";
        String familyName = "Smith";
        String fullName = givenName + " " + familyName;

        // create an empty model
        Model model = ModelFactory.createDefaultModel();

        // create the resource
        // and add the properties cascading style
        Resource johnSmith = model
                .createResource(personURI)
                .addProperty(VCARD.FN, fullName)
                .addProperty(
                        VCARD.N,
                        model.createResource()
                                .addProperty(VCARD.Given, givenName)
                                .addProperty(VCARD.Family, familyName));

    }
}

查询RDF模型

最通用的查询方法是Model.listStatements(Resource s, Property p, RDFNode o)。

import org.apache.jena.rdf.model.Property;
import org.apache.jena.rdf.model.RDFNode;
import org.apache.jena.rdf.model.Resource;
import org.apache.jena.rdf.model.Statement;
import org.apache.jena.rdf.model.StmtIterator;

// select all the resources with a VCARD.FN property
// whose value ends with "Smith"
StmtIterator iter = model.listStatements(new SimpleSelector(null,
        VCARD.FN, (RDFNode) null) {
    @Override
    public boolean selects(Statement s) {
        return s.getString().endsWith("Smith");
    }
});

// print out the predicate, subject and object of each statement
while (iter.hasNext()) {
    Statement stmt = iter.nextStatement(); // get next statement
    Resource subject = stmt.getSubject(); // get the subject
    Property predicate = stmt.getPredicate(); // get the predicate
    RDFNode object = stmt.getObject(); // get the object

    System.out.print(subject.toString());
    System.out.print(" " + predicate.toString() + " ");
    if (object instanceof Resource) {
        System.out.print(object.toString());
    } else {
        // object is a literal
        System.out.print(" \"" + object.toString() + "\"");
    }
    System.out.println(" .");
}

查询特定的资源时，可以使用model.getResource(String URI)和Model.listSubjectsWithProperty(Property p, RDFNode o)：

// retrieve the John Smith vcard resource from the model
Resource vcard = model.getResource(johnSmithURI);
// retrieve the value of the N property
Resource name = (Resource) vcard.getRequiredProperty(VCARD.N)
        .getObject();
// retrieve the given name property
String fullName = vcard.getRequiredProperty(VCARD.FN).getString();

import org.apache.jena.rdf.model.ResIterator;

// select all the resources with a VCARD.FN property
ResIterator iter = model.listResourcesWithProperty(VCARD.FN);

输出xml

使用model.write(System.out);即可输出xml到标准输出流中，使用不同的OutputStream参数或把标准输出流指向文件流即可输出xml文件。

<rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:vcard="http://www.w3.org/2001/vcard-rdf/3.0#">
  <rdf:Description rdf:about="http://somewhere/JohnSmith">
    <vcard:N rdf:parseType="Resource">
      <vcard:Family>Smithvcard:Family>
      <vcard:Given>Johnvcard:Given>
    vcard:N>
    <vcard:FN>John Smithvcard:FN>
  rdf:Description>
rdf:RDF>

此外也可以输出RDF/XML-ABBREV和N-TRIPLE形式的模型：

model.write(System.out, "RDF/XML-ABBREV");
model.write(System.out, "N-TRIPLE");

<rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:vcard="http://www.w3.org/2001/vcard-rdf/3.0#">
  <rdf:Description rdf:about="http://somewhere/JohnSmith">
    <vcard:N rdf:parseType="Resource">
      <vcard:Family>Smithvcard:Family>
      <vcard:Given>Johnvcard:Given>
    vcard:N>
    <vcard:FN>John Smithvcard:FN>
  rdf:Description>
rdf:RDF>

 .w3.org/2001/vcard-rdf/3.0#N> _:BX2D1e94da86X3A153bb598690X3AX2D7fff .
 .w3.org/2001/vcard-rdf/3.0#FN> "John Smith" .
_:BX2D1e94da86X3A153bb598690X3AX2D7fff .w3.org/2001/vcard-rdf/3.0#Family> "Smith" .
_:BX2D1e94da86X3A153bb598690X3AX2D7fff .w3.org/2001/vcard-rdf/3.0#Given> "John" .

读入xml

使用model.read(System.in, "");即从标准输入流中读入rdf模型，使用不同的InputStream参数或把标准输入流指向文件流即可读入xml文件。

InputStream in = (InputStream) FileManager.get().open( inputFileName );
if (in == null) {
    throw new IllegalArgumentException( "File: " + inputFileName + " not found");
}

// read the RDF/XML file
model.read(in, "");

参考

https://jena.apache.org/
https://github.com/apache/jena/blob/master/jena-core/src-examples/jena/examples/
http://domdong.blogspot.sg/2013/04/an-introduction-to-rdf-and-jena-rdf-api.html
http://www.ibm.com/developerworks/cn/java/j-jena/