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Example
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2
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-
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Dictionary
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Service
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Bundle
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This
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example
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creates
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a
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bundle
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that
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implements
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an
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OSGi
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service.
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Implementing
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an
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OSGi
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service
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is
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a
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two-step
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process,
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first
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we
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must
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define
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the
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interface
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of
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the
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service
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and
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then
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we
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must
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define
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an
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implementation
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of
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the
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service
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interface.
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In
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this
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particular
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example,
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we
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will
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create
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a
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dictionary
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service
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that
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we
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can
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use
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to
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check
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if
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a
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word
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exists,
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which
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indicates
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if
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the
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word
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is
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spelled correctly
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or
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not.
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First,
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we
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will
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start
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by
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defining
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a
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simple
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dictionary
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service
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interface
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in
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a
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file
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called
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DictionaryService.java:
| Code Block |
|---|
/* * Apache Felix OSGi tutorial. **/ package tutorial.example2.service; /** * A simple service interface that defines a dictionary service. * A dictionary service simply verifies the existence of a word. **/ public interface DictionaryService { /** * Check for the existence of a word. * @param word the word to be checked. * @return true if the word is in the dictionary, * false otherwise. **/ public boolean checkWord(String word); } |
The
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service
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interface
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is
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quite
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simple,
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with
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only
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one
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method
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that
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needs
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to
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be
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implemented.
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Notice
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that
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we
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put
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the
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service
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interface
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in
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the
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package
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tutorial.example2.service,
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instead
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of
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just
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putting
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it
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in
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tutorial.example2.
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We
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did
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this
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because
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we
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need
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to
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share
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the
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interface
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definition
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with
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other
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bundles,
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therefore
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it
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is
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better
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to
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separate
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service
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interfaces
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that
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need
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to
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be
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shared
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from
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code
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that
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does
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not
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need
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to
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be
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shared.
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Such
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an
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approach
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ensures
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a
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strong
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separation
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between
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interface
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and
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implementation.
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In
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the
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following
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source
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code,
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the
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bundle
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uses
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its
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bundle
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context
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to
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register
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the
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dictionary
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service.
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We
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implement
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the
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dictionary
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service
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as
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an
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inner
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class
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of
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the
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bundle
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activator
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class,
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but
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we
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could
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have
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also
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put
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it
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in
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a
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separate
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file.
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The
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source
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code
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for
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our
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bundle
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is
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as
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follows
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in
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a
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file
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called
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Activator.java:
| Code Block |
|---|
/* * OSGiApache and Gravity Service Binder tutorial. * Copyright (c) 2003 Richard S. Hall * http://oscar-osgi.sourceforge.netFelix OSGi tutorial. **/ package tutorial.example2; import java.util.Properties; import org.osgi.framework.BundleActivator; import org.osgi.framework.BundleContext; import org.osgi.framework.ServiceListener; import org.osgi.framework.ServiceEvent; import tutorial.example2.service.DictionaryService; /** * This class implements a simple bundle that uses the bundle * context to register an English language dictionary service * with the OSGi framework. The dictionary service interface is * defined in a separate class file and is implemented by an * inner class. **/ public class Activator implements BundleActivator { /** * Implements BundleActivator.start(). Registers an * instance of a dictionary service using the bundle context; * attaches properties to the service that can be queried * when performing a service look-up. * @param context the framework context for the bundle. **/ public void start(BundleContext context) { Properties props = new Properties(); props.put("Language", "English"); context.registerService( DictionaryService.class.getName(), new DictionaryImpl(), props); } /** * Implements BundleActivator.stop(). Does nothing since * the framework will automatically unregister any registered services. * @param context the framework context for the bundle. **/ public void stop(BundleContext context) { // NOTE: The service is automatically unregistered. } /** * A private inner class that implements a dictionary service; * see DictionaryService for details of the service. **/ private static class DictionaryImpl implements DictionaryService { // The set of words contained in the dictionary. String[] m_dictionary = { "welcome", "to", "the", "osgi", "tutorial" }; /** * Implements DictionaryService.checkWord(). Determines * if the passed in word is contained in the dictionary. * @param word the word to be checked. * @return true if the word is in the dictionary, * false otherwise. **/ public boolean checkWord(String word) { word = word.toLowerCase(); // This is very inefficient for (int i = 0; i < m_dictionary.length; i++) { if (m_dictionary[i].equals(word)) { return true; } } return false; } } } |
Note
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that
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we
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do
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not
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need
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to
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unregister
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the
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service
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in
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the
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stop()
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method,
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because
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the
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OSGi
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framework
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will
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automatically
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do
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so
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for
...
us.
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The
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dictionary
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service
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that
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we
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have
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implemented
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is
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very
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simple;
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its
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dictionary
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is
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a
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static
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array
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of
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only
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five
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words,
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so
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this
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solution
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is
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not
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optimal
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and
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is
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only
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intended
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for
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educational
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purposes.
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We
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must
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create
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a
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manifest.mf
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file
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that
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contains
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the meta-data for our bundle;
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the
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manifest
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file
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contains
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the
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following:
| No Format |
|---|
Bundle-Activator: tutorial.example2.Activator Export-Package: tutorial.example2.service Bundle-Name: English dictionary Bundle-Description: A bundle that registers an English dictionary service Bundle-Vendor: Richard Hall Bundle-Version: 1.0.0 |
As
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before,
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much
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of the meta-data in the manifest file above is for human consumption. We specify which class is used to activate our bundle via the Bundle-Activator attribute (this is the class that implements the BundleActivator interface) and also specify that our bundle exports a shared package using the Export-Package attribute. The Export-Package attribute makes it possible for other bundles to import our dictionary service interface.
To compile our source code, we need the felix.jar file (found in Felix' bin directory) in our class path. We compile the source file using a command like:
| No Format |
|---|
the metadata in the manifest file above is for human consumption. We specify which class implements the Bundle-Activator interface and also specify that our bundle exports a shared package using the Export-Package attribute. This attribute makes it possible for other bundles to import our dictionary service interface. To compile our source code, we need to have the osgi.jar file (found in Oscar's lib directory) in our class path. We compile the source file using a command like: javac -d c:\classes *.java |
This
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command
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compiles
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all
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source
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files
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and
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outputs
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the
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generated
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classes
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into
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a
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subdirectory
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of
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the
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c:\classes
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directory;
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this
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subdirectory
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is
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tutorial\example2,
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named
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after
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the
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package
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we
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specified
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in
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the
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source
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file.
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For
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the
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above
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command
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to
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work,
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the
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c:\classes
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directory
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must
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exist.
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After
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compiling,
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we
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need
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to
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create
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a
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JAR
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file
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containing
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the
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generated
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package
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directories.
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We
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will
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also
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add
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our
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manifest
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file
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that
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contains
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the
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bundle's
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meta-data to the JAR file.
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To
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create
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the
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JAR
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file,
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we
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issue
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the
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command:
| No Format |
|---|
jar cfm example2.jar manifest.mf -C c:\classes tutorial\example2 |
This
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command
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creates
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a
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JAR
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file
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using
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the
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manifest
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file
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we
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created
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and
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includes
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all
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of
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the
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classes
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in
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the
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tutorial\example2
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directory
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inside
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of
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the
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c:\classes
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directory.
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Once
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the
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JAR
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file
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is
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created,
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we
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are
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ready
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to
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install
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and
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start
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the
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bundle.
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To
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run
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Felix,
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we
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follow
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the
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instructions
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described
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in
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usage.html.
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When
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we
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start
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Felix,
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it
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asks
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for
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a
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profile
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name,
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we
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will
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put
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all
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of
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our
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bundles
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in
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a
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profile
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named
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tutorial.
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After
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running
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Felix,
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we
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should
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make
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sure
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that
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the
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bundle
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from
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Example
...
1
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is
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active.
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We
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can
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use
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the
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Felix ps
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shell
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command
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to
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get
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a
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list
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of
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all
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bundles,
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their
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state,
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and
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their
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bundle
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identifier
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number.
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If
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the
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Example
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1
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bundle
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is
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not
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active,
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we
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should
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start
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the
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bundle
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using
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the
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start
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command
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and
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the
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bundle's
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identifier
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number
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that
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is
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displayed
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by
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the
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ps
...
command.
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Now
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we
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can
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install
...
and
...
start
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our
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dictionary
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service
...
bundle.
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Assuming
...
that
...
we
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created
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our
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bundle
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in
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the
...
directory
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c:\tutorial,
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we
...
can
...
install
...
and
...
start
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it
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in
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Felix'
...
shell
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using
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the
...
following
...
command:
| No Format |
|---|
start file:/c:/tutorial/example2.jar |
The
...
above
...
command
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installs
...
and
...
starts
...
the
...
bundle
...
in
...
a
...
single
...
step;
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it
...
is
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also
...
possible
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to
...
install
...
and
...
start
...
the
...
bundle
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in
...
two
...
steps
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by
...
using
...
the
...
Felix install
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and
...
start
...
shell
...
commands.
...
To
...
stop
...
the
...
bundle,
...
use
...
the
...
Felix stop
...
shell
...
command.
...
If
...
the
...
bundle
...
from
...
Example
...
1
...
is
...
still
...
active,
...
then
...
we
...
should
...
see
...
it
...
...
out
...
the
...
details
...
of
...
the
...
service
...
event
...
it
...
receives
...
when
...
our
...
new
...
bundle
...
registers
...
its
...
dictionary
...
service.
...
Using
...
the
...
Felix shell
...
ps
...
command
...
to
...
get
...
the
...
bundle
...
identifier
...
number
...
for
...
our
...
dictionary
...
service
...
bundle
...
and
...
we
...
can
...
stop
...
and
...
restart
...
it
...
at
...
will
...
using
...
the
...
stop
...
and
...
start
...
commands,
...
respectively.
...
Each
...
time
...
we
...
start
...
and
...
stop
...
our
...
dictionary
...
service
...
bundle,
...
we
...
should
...
see
...
the
...
details
...
of
...
the
...
associated
...
service
...
event
...
printed
...
from
...
the
...
bundle
...
from
...
Example
...
1.
...
In
...
Example
...
3,
...
we
...
will
...
create
...
a
...
client
...
for
...
our
...
dictionary
...
service.
...
To
...
exit
...
Felix,
...
we
...
use
...
the
...
shutdown
...
command.
...