Wednesday, August 26, 2015

Getting Started with Simple Apache Abdera Server

Apache Abdera provides an implementation of the IETF Atom Syndication Format and Atom Publishing Protocol standards (RFC's 4287 and 5023). In this blog we're going to walk through how to build an Atom Publishing Protocol service server sample using Abdera's concepts of Providers and Collection Adapters. If you remember your AtomPub basics, you'll recall that AtomPub services are organized in a hierarchical way such that we have:

  • Services - A grouping of Workspaces
  • Workspace - A grouping of Collections
  • Collections - An Atom Feed which contains Atom entries. Entries can be created, deleted, updated, etc through the HTTP methods and the mapping that AtomPub defines.

Let's start to build a simple Abdera server sample for our Testing. So create a simple java project in your IDE and add below dependencies in your pom file to provide server function and abdera libraries.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.atomfeed.connector</groupId>
  <artifactId>ATOMPubServer</artifactId>
  <version>1.0.0</version>
  <packaging>jar</packaging>

  <name>ATOMPubServer</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>org.mortbay.jetty</groupId>
      <artifactId>jetty-maven-plugin</artifactId>
      <version>7.4.5.v20110725</version>
    </dependency>
    <dependency>
      <groupId>org.apache.abdera</groupId>
      <artifactId>abdera-server</artifactId>
      <version>1.1.2</version>
    </dependency>
  </dependencies>
</project>


Let's begin our code with the setting up the URL Space since we are using jetty server for our sample we need to initialize the sever and service url for our service which we are creating.



In this code we're setting up a Provider which contains an Atom workspace which contains our Atom collection.
When we initialize our CollectionAdapter we call setHref. Abdera uses this to determine the URL space

App.java 


package com.atomfeed.connector.server;

import org.apache.abdera.protocol.server.Provider;
import org.apache.abdera.protocol.server.impl.DefaultProvider;
import org.apache.abdera.protocol.server.impl.SimpleWorkspaceInfo;
import org.apache.abdera.protocol.server.servlet.AbderaServlet;
import org.eclipse.jetty.server.Server;
import org.eclipse.jetty.servlet.ServletContextHandler;
import org.eclipse.jetty.servlet.ServletHolder;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class App {

    public static void main(String... args) throws Exception {
        int port = 9002;
        try {
            port = args.length > 0 ? Integer.parseInt(args[0]) : 9002;
        } catch (Exception e) {
        }
        Server server = new Server(port);
        ServletContextHandler context = new ServletContextHandler(server, "/", ServletContextHandler.SESSIONS);
        ServletHolder servletHolder = new ServletHolder(new EmployeeProviderServlet());
        context.addServlet(servletHolder, "/*");
        server.start();
        server.join();
    }

    public static final class EmployeeProviderServlet extends AbderaServlet {
        @Override
        protected Provider createProvider() {
            EmployeeCollectionAdapter ca = new EmployeeCollectionAdapter();
            ca.setHref("FeedConnector");

            SimpleWorkspaceInfo wi = new SimpleWorkspaceInfo();
            wi.setTitle("Feed Update");
            wi.addCollection(ca);

            DefaultProvider provider = new DefaultProvider("/");
            provider.addWorkspace(wi);

            provider.init(getAbdera(), null);
            return provider;
        }

        @Override
        protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

            super.service(request, response);
        }

    }
}


Abdera comes with a class called the AbstractEntityCollectionAdapter. This class provides a simple "fill in the blanks" approach so that you can easily map your concepts to an AtomPub collection. The idea is that you have an entity class which represents each individual entry. For instance, if you were writing a blog, you would have a BlogEntry class which backs the entry. Or in our case of an employee directory, it would be an Employee class. Abdera will then provide some template methods which you fill in to give Abdera the necessary metadata.
Our Atom Collection is going to be a simple store of employees which can be added to, deleted from, updated, etc. To get started, we must first build our Employee class:
package com.atomfeed.connector.server;

import java.util.Date;


public class Employee {
    private int id;
    private String Content;
    private String title;
    private Date updated;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getContent() {
        return Content;
    }

    public void setContent(String Content) {
        this.Content = Content;
    }

    public Date getUpdated() {
        return updated;
    }

    public void setUpdated(Date updated) {
        this.updated = updated;
    }

    public void settitle(String title) {
        this.title = title;
    }

    public String gettitle() {

        return title;
    }
}

The ID is going to be used as a unique identifier for our Employee so that even if the employee's name changes, we can tie it to a previous atom entry and track the changes. The updated date will be used for our Atom feed, so that consumers know if there were changes and when they occurred. when we check the last updated date this is the information we are going to check to receive the last updated news feeds.

The first thing we'll do is provide a hashmap to store our Employees in. Typically this will be a database or some other type of backend store. To keep things simple, we're going to store employees in a Map according to their employee id.
Next up, we need to implement some methods which provide some basic metadata about our collection such as the feed author, the feed ID, and the feed title:
The ID in getId(RequestContext) must be a unique identifier for your feed. The idea is that even if your feed location changes, or someone is redistributing your feed, the feed reader can use this ID to identify that two feeds are the exact same. For information on how to construct feed IDs and why they matter, its recommend that you use this article on how to create feed ids.
Walking through this we have:
  • getId: this ID will uniquely identify your entry. It follows the same rules as the feed ID.
  • getTitle: The title of this entry.
  • getUpdated: The time this entry was last updated.
  • getAuthors: the author of this entry. It is ok to leave this empty provided that there is an author supplied for the feed.
  • getContent: The actual content of this entry that will be displayed in the feed. In this case it is just the employee name. You can return a Content or String object from this method. Using the Content object allows you to have control over whether or not the content is rendered as HTML. Alternatively you could provider a summary by overriding getSummary. You must have at least one of the getContent or getSummary methods not return null.

Mapping Entries to Resources

The AbstractEntityCollectionAdapter maps individual entries via a "resource name." You must provide a way to create a resource name for your entry and also provide a way to look up an entry from your resource name. In this case, our entry is an Employee.

When we do this we must ensure that we won't have conflicts. Which means we don't want to use the employee name as the unique resource name as there may be conflicts. However we don't necessarily want to use just the employee ID either as the URL isn't very friendly. So we'll create a hybrid of the form: "EMPLOYEE_ID-EMPLOYEE_NAME".


 Note the use of the sanitizer. This will take any invalid characters for a URL and either strip or replace them from the string.
The POST method corresponds to creating an Employee in the database. Here we're:
  • Creating a new Employee object
  • Creating a new id for it
  • Setting the name of the Employee from the Content. In a more advanced case, you woudl want to store the Employee information in an XML structure or an HTML microformat.
  • Store the Employee in our HashMap.

The putEntry method is similar. In this case we're just updating the employee name.

Finally, we have the deleteEntry method which gives us the resource name and allows us to remove it from the Map.

package com.atomfeed.connector.server;

import java.util.Arrays;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;

import org.apache.abdera.Abdera;
import org.apache.abdera.factory.Factory;
import org.apache.abdera.i18n.iri.IRI;
import org.apache.abdera.model.Content;
import org.apache.abdera.model.Person;
import org.apache.abdera.protocol.server.RequestContext;
import org.apache.abdera.protocol.server.context.ResponseContextException;
import org.apache.abdera.protocol.server.impl.AbstractEntityCollectionAdapter;


public class EmployeeCollectionAdapter extends AbstractEntityCollectionAdapter<Employee> {
    private static final String ID_PREFIX = "tag:wso2.com,2015,feed:entry:";

    private AtomicInteger nextId = new AtomicInteger(1000);
    private Map<Integer, Employee> employees = new HashMap<Integer, Employee>();
    private Factory factory = new Abdera().getFactory();

    @Override
    public String getId(RequestContext request) {
        return "tag:wso2.com,2015,feed:entry:";
    }

    public String getTitle(RequestContext request) {
        return "Feed Connector feeds";
    }

    @Override
    public String getAuthor(RequestContext request) {
        return "WSO2 Inc";
    }

    @Override
    public Iterable<Employee> getEntries(RequestContext request) {
        return employees.values();
    }

    @Override
    public Employee getEntry(String resourceName, RequestContext request) throws ResponseContextException {
        Integer id = getIdFromResourceName(resourceName);
        return employees.get(id);
    }

    private Integer getIdFromResourceName(String resourceName) throws ResponseContextException {
        int idx = resourceName.indexOf("-");
        if (idx == -1) {
            throw new ResponseContextException(404);
        }
        return new Integer(resourceName.substring(0, idx));
    }

    @Override
    public String getName(Employee entry) {
        return entry.getId() + "-" + entry.gettitle().replaceAll(" ", "_");
    }

    @Override
    public String getId(Employee entry) {
        return ID_PREFIX + entry.getId();
    }

    @Override
    public String getTitle(Employee entry) {
        return entry.gettitle();
    }

    @Override
    public Date getUpdated(Employee entry) {
        return entry.getUpdated();
    }

    @Override
    public List<Person> getAuthors(Employee entry, RequestContext request) throws ResponseContextException {
        Person author = request.getAbdera().getFactory().newAuthor();
        author.setName("WOS2 Inc");
        return Arrays.asList(author);
    }

    @Override
    public Object getContent(Employee entry, RequestContext request) {
        Content content = factory.newContent(Content.Type.TEXT);
        content.setText(entry.gettitle());
        return content;
    }

    @Override
    public Employee postEntry(String title,
                              IRI id,
                              String summary,
                              Date updated,
                              List<Person> authors,
                              Content content,
                              RequestContext request) throws ResponseContextException {
        Employee employee = new Employee();
        employee.setContent(content.getText().trim());
        employee.setId(nextId.getAndIncrement());
        employee.settitle(title);
        employee.setUpdated(updated);
        employees.put(employee.getId(), employee);

        return employee;
    }

    @Override
    public void putEntry(Employee employee,
                         String title,
                         Date updated,
                         List<Person> authors,
                         String summary,
                         Content content,
                         RequestContext request) throws ResponseContextException {
        employee.settitle(content.getText().trim());
    }

    @Override
    public void deleteEntry(String resourceName, RequestContext request) throws ResponseContextException {
        Integer id = getIdFromResourceName(resourceName);
        employees.remove(id);
    }
}


After complete  the development just run the application as java application you will see the console output as


2015-10-19 11:33:16.825:INFO::jetty-7.4.5.v20110725
2015-10-19 11:33:16.862:INFO::started o.e.j.s.ServletContextHandler{/,null}
2015-10-19 11:33:16.985:INFO::Started SelectChannelConnector@0.0.0.0:9002 STARTING



Later we can check the the output for this server after build a client for this sever now you can download full source code and can contribute for the further development of this Source Code

References
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