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<div class="main">
<h1><a name="LaunchingandEmbeddingApacheFelix-LaunchingandEmbeddingApacheFelix"></a>Launching and Embedding Apache Felix</h1>
<ul>
<li><a href="#LaunchingandEmbeddingApacheFelix-introduction" title="introduction on Launching and Embedding Apache Felix">Introduction</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-overview" title="overview on Launching and Embedding Apache Felix">API Overview</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-launching" title="launching on Launching and Embedding Apache Felix">Launching Felix</a>
<ul>
<li><a href="#LaunchingandEmbeddingApacheFelix-standardlauncher" title="standard-launcher on Launching and Embedding Apache Felix">Standard Felix Launcher</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-customlauncher" title="custom-launcher on Launching and Embedding Apache Felix">Custom Felix Launcher</a></li>
</ul>
</li>
<li><a href="#LaunchingandEmbeddingApacheFelix-embedding" title="embedding on Launching and Embedding Apache Felix">Embedding Felix</a>
<ul>
<li><a href="#LaunchingandEmbeddingApacheFelix-configproperty" title="config-property on Launching and Embedding Apache Felix">Embedded Execution Configuration Property</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-hostinteraction" title="host-interaction on Launching and Embedding Apache Felix">Host/Felix Interaction</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-hostservices" title="host-services on Launching and Embedding Apache Felix">Providing Host Application Services</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-hostserviceusage" title="host-service-usage on Launching and Embedding Apache Felix">Using Services Provided by Bundles</a>
<ul>
<li><a href="#LaunchingandEmbeddingApacheFelix-servicereflection" title="service-reflection on Launching and Embedding Apache Felix">Using Bundle Services via Reflection</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#LaunchingandEmbeddingApacheFelix-caveat" title="caveat on Launching and Embedding Apache Felix">Caveat</a></li>
<li><a href="#LaunchingandEmbeddingApacheFelix-feedback" title="feedback on Launching and Embedding Apache Felix">Feedback</a></li>
</ul>
<p><a name="LaunchingandEmbeddingApacheFelix-introduction"></a></p>
<h1><a name="LaunchingandEmbeddingApacheFelix-Introduction"></a>Introduction</h1>
<p>The Apache Felix OSGi framework is intended to be easily launchable
and embeddable. For example, Felix avoids the use of system properties
for configuration, since these are globals that can cause interference
if multiple framework instances are created in the same VM. Felix is
also implemented to multiplex singleton facilities, like the URL stream
handler factory. The goal is to make it possible to use Felix in as
many scenarios as possible; however, this is still just a goal. In
other words, this is a work in progress and if any issues arise, it
would be greatly appreciated if they are brought to the attention of
the Felix community. The next section provides a Felix API overview,
while the remainder of the document is divided into two sections, one
focusing on how to launch Felix and one focusing on how to embed Felix
into a host application.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-overview"></a></p>
<h1><a name="LaunchingandEmbeddingApacheFelix-APIOverview"></a>API Overview</h1>
<p>The Felix class that implements the OSGi framework is <tt>org.apache.felix.framework.Felix</tt> or just <tt>Felix</tt> for short. The OSGi specification defines a special bundle, called the <em><b>System Bundle</b></em>,
that represents the framework at run time and appears like any other
bundle in the list of installed bundles. To make this notion even more
intuitive, the <tt>Felix</tt> class implements the <tt>org.osgi.framework.Bundle</tt> interface, which is reiterated here:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> <span class="code-keyword">interface</span> Bundle
{
<span class="code-keyword">public</span> BundleContext getBundleContext();
<span class="code-keyword">public</span> <span class="code-object">long</span> getBundleId();
<span class="code-keyword">public</span> URL getEntry(<span class="code-object">String</span> name);
<span class="code-keyword">public</span> Enumeration getEntryPaths(<span class="code-object">String</span> path);
<span class="code-keyword">public</span> Enumeration findEntries(<span class="code-object">String</span> path, <span class="code-object">String</span> filePattern, <span class="code-object">boolean</span> recurse);
<span class="code-keyword">public</span> Dictionary getHeaders();
<span class="code-keyword">public</span> Dictionary getHeaders(<span class="code-object">String</span> locale);
<span class="code-keyword">public</span> <span class="code-object">long</span> getLastModified();
<span class="code-keyword">public</span> <span class="code-object">String</span> getLocation();
<span class="code-keyword">public</span> URL getResource(<span class="code-object">String</span> name);
<span class="code-keyword">public</span> Enumeration getResources(<span class="code-object">String</span> name) <span class="code-keyword">throws</span> IOException;
<span class="code-keyword">public</span> ServiceReference[] getRegisteredServices();
<span class="code-keyword">public</span> ServiceReference[] getServicesInUse();
<span class="code-keyword">public</span> <span class="code-object">int</span> getState();
<span class="code-keyword">public</span> <span class="code-object">String</span> getSymbolicName();
<span class="code-keyword">public</span> <span class="code-object">boolean</span> hasPermission(<span class="code-object">Object</span> obj);
<span class="code-keyword">public</span> <span class="code-object">Class</span> loadClass(<span class="code-object">String</span> name) <span class="code-keyword">throws</span> ClassNotFoundException;
<span class="code-keyword">public</span> void start() <span class="code-keyword">throws</span> BundleException;
<span class="code-keyword">public</span> void stop() <span class="code-keyword">throws</span> BundleException;
<span class="code-keyword">public</span> void uninstall() <span class="code-keyword">throws</span> BundleException;
<span class="code-keyword">public</span> void update() <span class="code-keyword">throws</span> BundleException;
<span class="code-keyword">public</span> void update(InputStream is) <span class="code-keyword">throws</span> BundleException;
}</pre>
</div></div>
<p>When you instantiate the <tt>Felix</tt> class, the resulting object is actually the System Bundle and can be cast to the <tt>Bundle</tt> interface. The <tt>start()</tt> method is used to start the framework instance, while the <tt>stop()</tt> method is used to asynchronously stop the framework instance. The <tt>Felix</tt> class also includes the following two additional public methods:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class Felix <span class="code-keyword">extends</span> AbstractBundle
{
<span class="code-keyword">public</span> Felix(Map configMutableMap, List activatorList);
<span class="code-keyword">public</span> void stopAndWait();
}</pre>
</div></div>
<p>The first method is the constructor used to instantiate framework
instances; the constructor accepts configuration properties and System
Bundle activators, which are both described in more detail later. The <tt>stopAndWait()</tt> method is a synchronous version of the <tt>stop()</tt> method, used to stop the framework and block the calling thread until the framework is completely stopped.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-launching"></a></p>
<h1><a name="LaunchingandEmbeddingApacheFelix-LaunchingFelix"></a>Launching Felix</h1>
<p>Launching Felix is fairly simple and involves only three steps:</p>
<ol>
<li>Defining some configuration properties.</li>
<li>Creating an instance of <tt>org.apache.felix.framework.Felix</tt> with the configuration properties.</li>
<li>Invoking the <tt>org.apache.felix.framework.Felix.start()</tt> method.</li>
</ol>
<p>The only configuration properties that are actually required to
start Felix are ones that tell it where/how to locate the bundle cache
profile directory where installed bundles will be cached. Felix' bundle
cache implementation allows you to configure the location where bundles
are cached using configuration properties. At a minimum, either a
bundle cache profile name or directory must be specified; see the <a href="http://felix.apache.org/site/apache-felix-bundle-cache.html" title="Apache Felix Bundle Cache">bundle cache document</a> for more detailed information on configuring the bundle cache.</p>
<p>Besides configuration properties for the bundle cache, it is usually necessary to set the <tt>org.osgi.framework.system.packages</tt> configuration property to export packages from the class path, such as the OSGi interface classes (e.g., <tt>org.osgi.framework</tt>) on which all bundles depend. If you are creating a launcher for Felix, then the <tt>felix.auto.start</tt> configuration property may also be used to automatically install and start various bundles; see the <a href="http://felix.apache.org/site/apache-felix-usage-documentation.html#ApacheFelixUsageDocumentation-configuringfelix" title="configuring-felix on Apache Felix Usage Documentation">usage document</a> for more information on configuring Felix and on the various configuration properties.</p>
<p>The remainder of this section describes how the standard Felix
launcher works as well as how to create a custom launcher for Felix.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-standardlauncher"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-StandardFelixLauncher"></a>Standard Felix Launcher</h2>
<p>The standard Felix launcher is very simple and is not intended to
solve every possible requirement; it is intended to work for most
standard situations. Most special launching requirements should be
resolved by creating a custom launcher. This section describes how the
standard launcher works. The following code represents the complete <tt>main()</tt> method of the standard launcher, each numbered comment will be described in more detail below:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> <span class="code-keyword">static</span> void main(<span class="code-object">String</span>[] argv) <span class="code-keyword">throws</span> Exception
{
<span class="code-comment">// (1) Load system properties.
</span> Main.loadSystemProperties();
<span class="code-comment">// (2) Read configuration properties.
</span> Properties configProps = Main.loadConfigProperties();
<span class="code-comment">// (3) Copy framework properties from the system properties.
</span> Main.copySystemProperties(configProps);
<span class="code-comment">// (4) See <span class="code-keyword">if</span> the profile name property was specified.
</span> <span class="code-object">String</span> profileName = configProps.getProperty(BundleCache.CACHE_PROFILE_PROP);
<span class="code-comment">// (4) See <span class="code-keyword">if</span> the profile directory property was specified.
</span> <span class="code-object">String</span> profileDirName = configProps.getProperty(BundleCache.CACHE_PROFILE_DIR_PROP);
<span class="code-comment">// Print welcome banner.
</span> <span class="code-object">System</span>.out.println(<span class="code-quote">"\nWelcome to Felix."</span>);
<span class="code-object">System</span>.out.println(<span class="code-quote">"=================\n"</span>);
<span class="code-comment">// (5) If no profile or profile directory is specified in the
</span> <span class="code-comment">// properties, then ask <span class="code-keyword">for</span> a profile name.
</span> <span class="code-keyword">if</span> ((profileName == <span class="code-keyword">null</span>) &amp;&amp; (profileDirName == <span class="code-keyword">null</span>))
{
<span class="code-object">System</span>.out.print(<span class="code-quote">"Enter profile name: "</span>);
BufferedReader in = <span class="code-keyword">new</span> BufferedReader(<span class="code-keyword">new</span> InputStreamReader(<span class="code-object">System</span>.in));
<span class="code-keyword">try</span>
{
profileName = in.readLine();
}
<span class="code-keyword">catch</span> (IOException ex)
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"Could not read input."</span>);
<span class="code-object">System</span>.exit(-1);
}
<span class="code-object">System</span>.out.println("");
<span class="code-keyword">if</span> (profileName.length() != 0)
{
configProps.setProperty(BundleCache.CACHE_PROFILE_PROP, profileName);
}
}
<span class="code-comment">// (6) A profile directory or name must be specified.
</span> <span class="code-keyword">if</span> ((profileDirName == <span class="code-keyword">null</span>) &amp;&amp; (profileName.length() == 0))
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"You must specify a profile name or directory."</span>);
<span class="code-object">System</span>.exit(-1);
}
<span class="code-keyword">try</span>
{
<span class="code-comment">// (7) Now create an instance of the framework.
</span> m_felix = <span class="code-keyword">new</span> Felix(<span class="code-keyword">new</span> StringMap(configProps, <span class="code-keyword">false</span>), <span class="code-keyword">null</span>);
m_felix.start();
}
<span class="code-keyword">catch</span> (Exception ex)
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"Could not create framework: "</span> + ex);
ex.printStackTrace();
<span class="code-object">System</span>.exit(-1);
}
}</pre>
</div></div>
<p>The general steps of the standard launcher are quite straightforward:</p>
<ol>
<li>Load any system properties specified in the <tt>system.properties</tt> file; this file is typically located in the <tt>conf/</tt> directory of the Felix installation directory, but it can be specified directly using the <tt>felix.system.properties</tt>
system property. This file is not needed to launch Felix and is
provided merely for convenience when system properties must be
specified. The file is a standard Java properties file, but it also
supports property substitution using <tt>${&lt;property-name</tt>} syntax. Property substitution can be nested; only system properties will be used for substitution.</li>
<li>Load any configuration properties specified in the <tt>config.properties</tt> file; this file is typically located in the <tt>conf/</tt> directory of the Felix installation directory, but it can be specified directly using the <tt>felix.config.properties</tt>
system property. This file is used to configure the Felix instance
created by the launcher. The file is a standard Java properties file,
but it also supports property substitution using "<tt>${&lt;property-name</tt>}"
syntax. Property substitution can be nested; configuration and system
properties will be used for substitution with configuration properties
having precedence.</li>
<li>For convenience, any configuration
properties that are set as system properties will be copied into the
set of configuration properties to provide an easy way to add to or
override configuration properties specified in the <tt>config.properties</tt> file.</li>
<li>Try to load the profile name or profile directory configuration properties. At least one of these must be specified so that the <a href="http://felix.apache.org/site/apache-felix-bundle-cache.html" title="Apache Felix Bundle Cache">bundle cache</a> knows where to save installed bundles.</li>
<li>If
either the profile name or profile directory configuration property has
not been specified, then ask the user to specify a profile name and add
it to the current set of configuration properties.</li>
<li>Error if there is no profile name or profile directory.</li>
<li>Create the Felix instance passing in the configuration properties and then call <tt>start()</tt>.</li>
</ol>
<p>The framework is not active until the <tt>start()</tt> method is called. If no shell bundles are specified in the <tt>config.properties</tt>
file or if there is difficulty locating the shell bundles that are
specified, then it will appear as if the framework is hung, but it is
actually running without any way to interact with it since the shell
bundles provide the only means of interaction.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-customlauncher"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-CustomFelixLauncher"></a>Custom Felix Launcher</h2>
<p>This section creates a bare-bones launcher to demonstrate the
minimum requirements for creating an interactive launcher for the Felix
framework. This example uses the standard Felix shell bundles for
interactivity, but any other bundles could be used instead. For
example, the shell service and telnet bundles could be used to launch
Felix and make it remotely accessible.</p>
<p>This example launcher project has the following directory structure:</p>
<div class="preformatted"><div class="preformattedContent">
<pre>launcher/
lib/
org.apache.felix.framework-1.0.0.jar
bundle/
org.apache.felix.shell-1.0.0.jar
org.apache.felix.shell.tui-1.0.0.jar
src/
example/
Main.java
</pre>
</div></div>
<p>The <tt>lib/</tt> directory contains the framework JAR file, which also contains the OSGi core interfaces. The <tt>bundle/</tt>
directory contains the shell service and textual shell interface
bundles that will be used for interacting with the framework instance.
Note: If you do not launch Felix with interactive bundles, it will
appear as if the framework instance is hung, but it is actually just
sitting there waiting for someone to tell it to do something. The <tt>src/example/</tt> directory contains the following <tt>Main.java</tt> file, which is a very simplistic Felix launcher.</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> example;
<span class="code-keyword">import</span> java.util.Map;
<span class="code-keyword">import</span> org.osgi.framework.Constants;
<span class="code-keyword">import</span> org.apache.felix.framework.Felix;
<span class="code-keyword">import</span> org.apache.felix.framework.cache.BundleCache;
<span class="code-keyword">import</span> org.apache.felix.framework.util.StringMap;
<span class="code-keyword">import</span> org.apache.felix.framework.util.FelixConstants;
<span class="code-keyword">public</span> class Main
{
<span class="code-keyword">private</span> <span class="code-keyword">static</span> Felix m_felix = <span class="code-keyword">null</span>;
<span class="code-keyword">public</span> <span class="code-keyword">static</span> void main(<span class="code-object">String</span>[] argv) <span class="code-keyword">throws</span> Exception
{
<span class="code-comment">// Print welcome banner.
</span> <span class="code-object">System</span>.out.println(<span class="code-quote">"\nWelcome to Felix."</span>);
<span class="code-object">System</span>.out.println(<span class="code-quote">"=================\n"</span>);
Map configMap = <span class="code-keyword">new</span> StringMap(<span class="code-keyword">false</span>);
configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
<span class="code-quote">"org.osgi.framework; version=1.3.0,"</span> +
<span class="code-quote">"org.osgi.service.packageadmin; version=1.2.0,"</span> +
<span class="code-quote">"org.osgi.service.startlevel; version=1.0.0,"</span> +
<span class="code-quote">"org.osgi.service.url; version=1.0.0"</span>);
configMap.put(FelixConstants.AUTO_START_PROP + <span class="code-quote">".1"</span>,
<span class="code-quote">"file:bundle/org.apache.felix.shell-1.0.0.jar "</span> +
<span class="code-quote">"file:bundle/org.apache.felix.shell.tui-1.0.0.jar"</span>);
configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, <span class="code-quote">"cache"</span>);
<span class="code-keyword">try</span>
{
<span class="code-comment">// Now create an instance of the framework.
</span> m_felix = <span class="code-keyword">new</span> Felix(configMap, <span class="code-keyword">null</span>);
m_felix.start();
}
<span class="code-keyword">catch</span> (Exception ex)
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"Could not create framework: "</span> + ex);
ex.printStackTrace();
<span class="code-object">System</span>.exit(-1);
}
}
}</pre>
</div></div>
<p>This launcher has all information hard coded in it, unlike the
default Felix launcher, which loads configuration properties from files
and performs variable substitution. This simple launcher provides a
good starting point if the features of the default launcher are not
necessary. For example, if you want to create a launcher that
automatically deletes the bundle cache directory each time it starts,
then it is quite easy to figure out how to do that with this simple
launcher.</p>
<p>By breaking down the above source code into small chunks, it is quite easy to see what is going on.</p>
<div class="code"><div class="codeContent">
<pre class="code-java">Map configMap = <span class="code-keyword">new</span> StringMap(<span class="code-keyword">false</span>);</pre>
</div></div>
<p>This simply creates a map to hold configuration properties where the keys are strings and lookups are case insensitive.</p>
<div class="code"><div class="codeContent">
<pre class="code-java">configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
<span class="code-quote">"org.osgi.framework; version=1.3.0,"</span> +
<span class="code-quote">"org.osgi.service.packageadmin; version=1.2.0,"</span> +
<span class="code-quote">"org.osgi.service.startlevel; version=1.0.0,"</span> +
<span class="code-quote">"org.osgi.service.url; version=1.0.0"</span>);</pre>
</div></div>
<p>This sets the <tt>Constants.FRAMEWORK_SYSTEMPACKAGES</tt> configuration property (string value "<tt>org.osgi.framework.system.packages</tt>"),
which specifies the class path packages the system bundle should
export; this is how classes on the class path are made available to
bundles. This example only exports the core OSGi API packages, but
other JRE packages could also be added, such as <tt>javax.swing</tt>.
For example, the default Felix launcher defines properties for all
packages in various JRE versions (e.g., 1.3.x, 1.4.x, 1.5.x) and
appends them to this property using property substitution.</p>
<div class="code"><div class="codeContent">
<pre class="code-java">configMap.put(FelixConstants.AUTO_START_PROP + <span class="code-quote">".1"</span>,
<span class="code-quote">"file:bundle/org.apache.felix.shell-1.0.0.jar "</span> +
<span class="code-quote">"file:bundle/org.apache.felix.shell.tui-1.0.0.jar"</span>);</pre>
</div></div>
<p>This sets the <tt>FelixConstants.AUTO_START_PROP</tt> configuration property (string value "<tt>felix.auto.start</tt>"),
which is a space-delimited list of bundle URLs that the framework will
automatically install and start when the framework starts. However,
this property key cannot be used as is; it must be appended with a "."
and then a number, where the number represents the start level for the
bundle when it is installed. In this particular example, ".1" is
appended to the property name, thus the two bundles will be installed
into start level one. This example uses relative <tt>file:</tt> URLs, which will load the bundles from the <tt>bundle/</tt>
directory assuming that the launcher is started from the root directory
of the launcher project. It is also possible to specify absolute URLs
or remote URLs.</p>
<div class="code"><div class="codeContent">
<pre class="code-java">configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, <span class="code-quote">"cache"</span>);</pre>
</div></div>
<p>This sets the last configuration property, <tt>BundleCache.CACHE_PROFILE_DIR_PROP</tt> (string value "<tt>felix.cache.profiledir</tt>"),
which is a string that specifies the precise directory to be used as
the bundle cache profile directory; the Felix bundle cache supports
other properties to configure its behavior, but those are not covered
here. In this example, the bundle cache profile directory is specified
as a relative directory called "<tt>cache</tt>". Assuming that the
launcher is executed from the root directory of the launcher project,
then the bundle cache profile directory will be created in the root
directory of the project.</p>
<div class="code"><div class="codeContent">
<pre class="code-java">m_felix = <span class="code-keyword">new</span> Felix(configMap, <span class="code-keyword">null</span>);
m_felix.start();</pre>
</div></div>
<p>The last steps create the framework instance and start it. The configuration property map is passed into the <tt>Felix</tt> constructor.</p>
<p>The following command compiles the launcher when run from the root directory of the launcher project:</p>
<div class="preformatted"><div class="preformattedContent">
<pre>javac -d . -classpath lib/org.apache.felix.framework-1.0.0.jar src/example/Main.java
</pre>
</div></div>
<p>After executing this command, an <tt>example/</tt> directory is
created in the current directory, which contains the generated class
file. The following command executes the simple launcher when run from
the root directory of the launcher project:</p>
<div class="preformatted"><div class="preformattedContent">
<pre>java -cp .:lib/org.apache.felix.framework-1.0.0.jar example.Main
</pre>
</div></div>
<p>After executing this command, a <tt>cache/</tt> directory is created that contains the installed bundles, which were installed from the <tt>bundle/</tt> directory.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-embedding"></a></p>
<h1><a name="LaunchingandEmbeddingApacheFelix-EmbeddingFelix"></a>Embedding Felix</h1>
<p>Embedding Felix into a host application is a simple way to provide a
sophisticated extensibility mechanism (i.e., plugin system) to the host
application. Embedding Felix is very similar to launching Felix as
described above, the main difference is that the host application
typically wants to interact with the framework instance and/or
installed bundles/services from the outside. This is fairly easy to
achieve with Felix, but there are some subtle issues to understand.
This section presents the mechanisms for embedding Felix into a host
application and the issues in doing so.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-configproperty"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-EmbeddedExecutionConfigurationProperty"></a>Embedded Execution Configuration Property</h2>
<p>When a Felix instance is embedded in a host application, the host
application must inform the Felix instance that it is embedded. To do
this, the host application sets the "<tt>felix.embedded.execution</tt>" configuration property to "<tt>true</tt>";
this can be accomplished in the same way that all configuration
properties are set, i.e., passing it into the Felix constructor via a
map. This property specifically controls whether or not the Felix
instance will shutdown the JVM (i.e., call <tt>System.exit()</tt> when
the framework is shutdown. When embedding Felix it is generally not
desirable for Felix to shutdown the JVM; therefore, by setting this
property to "<tt>true</tt>" it can be avoided.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-hostinteraction"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-Host/FelixInteraction"></a>Host/Felix Interaction</h2>
<p>In the section on <a href="#LaunchingandEmbeddingApacheFelix-launching" title="launching on Launching and Embedding Apache Felix">launching</a> Felix above, the <tt>Felix</tt>
constructor accepts two arguments, the first being the configuration
properties for the framework, the second being a list of bundle
activator instances. These bundle activator instances provide a
convenient way for host applications to interact with the Felix
framework.</p>
<p>Each bundle activator instance passed into the constructor
effectively becomes part of the System Bundle. This means that the
start()/stop() method of each bundle activator instance in the passed
in list gets invoked when the System Bundle's activator start()/stop()
method gets invoked. Consequently, each bundle activator instance will
be given the system bundle's <tt>BundleContext</tt> object so that they can interact with the framework externally. While it is possible to get the System Bundle's <tt>BundleContext</tt> object directly by calling <tt>Felix.getBundleContext()</tt>,
this is generally not as convenient since it requires that you monitor
when the System Bundle starts and/or stops. Consider following snippet
of a bundle activator:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HostActivator <span class="code-keyword">implements</span> BundleActivator
{
<span class="code-keyword">private</span> BundleContext m_context = <span class="code-keyword">null</span>;
<span class="code-keyword">public</span> void start(BundleContext context)
{
m_context = context;
}
<span class="code-keyword">public</span> void stop(BundleContext context)
{
m_context = <span class="code-keyword">null</span>;
}
<span class="code-keyword">public</span> Bundle[] getBundles()
{
<span class="code-keyword">if</span> (m_context != <span class="code-keyword">null</span>)
{
<span class="code-keyword">return</span> m_context.getBundles();
}
<span class="code-keyword">return</span> <span class="code-keyword">null</span>;
}
}</pre>
</div></div>
<p>Given the above bundle activator, it is now possible to embed Felix
into a host application and interact with it as the following snippet
illustrates:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">public</span> class HostApplication
{
<span class="code-keyword">private</span> HostActivator m_activator = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> Felix m_felix = <span class="code-keyword">null</span>;
<span class="code-keyword">public</span> HostApplication()
{
<span class="code-comment">// Create a <span class="code-keyword">case</span>-insensitive configuration property map.
</span> Map configMap = <span class="code-keyword">new</span> StringMap(<span class="code-keyword">false</span>);
<span class="code-comment">// Configure the Felix instance to be embedded.
</span> configMap.put(FelixConstants.EMBEDDED_EXECUTION_PROP, <span class="code-quote">"<span class="code-keyword">true</span>"</span>);
<span class="code-comment">// Add core OSGi packages to be exported from the class path
</span> <span class="code-comment">// via the system bundle.
</span> configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
<span class="code-quote">"org.osgi.framework; version=1.3.0,"</span> +
<span class="code-quote">"org.osgi.service.packageadmin; version=1.2.0,"</span> +
<span class="code-quote">"org.osgi.service.startlevel; version=1.0.0,"</span> +
<span class="code-quote">"org.osgi.service.url; version=1.0.0"</span>);
<span class="code-comment">// Explicitly specify the directory to use <span class="code-keyword">for</span> caching bundles.
</span> configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, <span class="code-quote">"cache"</span>);
<span class="code-keyword">try</span>
{
<span class="code-comment">// Create host activator;
</span> m_activator = <span class="code-keyword">new</span> HostActivator();
List list = <span class="code-keyword">new</span> ArrayList();
list.add(m_activator);
<span class="code-comment">// Now create an instance of the framework with
</span> <span class="code-comment">// our configuration properties and activator.
</span> m_felix = <span class="code-keyword">new</span> Felix(configMap, list);
<span class="code-comment">// Now start Felix instance.
</span> m_felix.start();
}
<span class="code-keyword">catch</span> (Exception ex)
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"Could not create framework: "</span> + ex);
ex.printStackTrace();
}
}
<span class="code-keyword">public</span> Bundle[] getInstalledBundles()
{
<span class="code-comment">// Use the system bundle activator to gain external
</span> <span class="code-comment">// access to the set of installed bundles.
</span> <span class="code-keyword">return</span> m_activator.getBundles();
}
<span class="code-keyword">public</span> void shutdownApplication()
{
<span class="code-comment">// Shut down the felix framework when stopping the
</span> <span class="code-comment">// host application.
</span> m_felix.shutdown();
}
}</pre>
</div></div>
<p>Notice how the <tt>HostApplication.getInstalledBundles()</tt> method
uses its activator instance to get access to the System Bundle's
context in order to interact with the embedded Felix framework
instance. This approach provides the foundation for all interaction
between the host application and the embedded framework instance.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-hostservices"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-ProvidingHostApplicationServices"></a>Providing Host Application Services</h2>
<p>Providing services from the host application to bundles inside the
embedded Felix framework instance follows the basic approach laid out
in <a href="#LaunchingandEmbeddingApacheFelix-hostinteraction" title="host-interaction on Launching and Embedding Apache Felix">above</a>.
The main complication for providing a host application service to
bundles is the fact that both the host application and the bundles must
be using the same class definitions for the service interface classes.
Since the host application cannot import classes from a bundle, this
means that the service interface classes <b>must</b> be accessible on
the class path, typically as part of the host application itself. The
host application then must export the service interface package via the
system bundle so that bundles installed into the embedded framework
instance can import it. This is achieved using the <tt>org.osgi.framework.system.packages</tt> configuration property previously presented.</p>
<p>Consider the follow simple property lookup service:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> host.service.lookup;
<span class="code-keyword">public</span> class Lookup
{
<span class="code-keyword">public</span> <span class="code-object">Object</span> lookup(<span class="code-object">String</span> name);
}</pre>
</div></div>
<p>This package is simply part of the host application, which is potentially packaged into a JAR file and started with the "<tt>java -jar</tt>"
command. Now consider the following host application bundle activator,
which will be used to register/unregister the property lookup service
when the embedded framework instance starts/stops:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> host.core;
<span class="code-keyword">import</span> java.util.Map;
<span class="code-keyword">import</span> org.osgi.framework.BundleActivator;
<span class="code-keyword">import</span> org.osgi.framework.BundleContext;
<span class="code-keyword">import</span> org.osgi.framework.ServiceRegistration;
<span class="code-keyword">import</span> host.service.lookup;
<span class="code-keyword">public</span> class HostActivator <span class="code-keyword">implements</span> BundleActivator
{
<span class="code-keyword">private</span> Map m_lookupMap = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> BundleContext m_context = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> ServiceRegistration m_registration = <span class="code-keyword">null</span>;
<span class="code-keyword">public</span> HostActivator(Map lookupMap)
{
<span class="code-comment">// Save a reference to the service's backing store.
</span> m_lookupMap = lookupMap;
}
<span class="code-keyword">public</span> void start(BundleContext context)
{
<span class="code-comment">// Save a reference to the bundle context.
</span> m_context = context;
<span class="code-comment">// Create a property lookup service implementation.
</span> Lookup lookup = <span class="code-keyword">new</span> Lookup() {
<span class="code-keyword">public</span> <span class="code-object">Object</span> lookup(<span class="code-object">String</span> name)
{
<span class="code-keyword">return</span> m_lookupMap.get(name);
}
};
<span class="code-comment">// Register the property lookup service and save
</span> <span class="code-comment">// the service registration.
</span> m_registration = m_context.registerService(
Lookup.class.getName(), lookup, <span class="code-keyword">null</span>);
}
<span class="code-keyword">public</span> void stop(BundleContext context)
{
<span class="code-comment">// Unregister the property lookup service.
</span> m_registration.unregister();
m_context = <span class="code-keyword">null</span>;
}
}</pre>
</div></div>
<p>Given the above host application bundle activator, the following
code snippet shows how the host application could create an embedded
version of the Felix framework and provide the property lookup service
to installed bundles:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> host.core;
<span class="code-keyword">import</span> java.util.List;
<span class="code-keyword">import</span> java.util.ArrayList;
<span class="code-keyword">import</span> java.util.Map;
<span class="code-keyword">import</span> java.util.HashMap;
<span class="code-keyword">import</span> host.service.lookup.Lookup;
<span class="code-keyword">import</span> org.apache.felix.framework.Felix;
<span class="code-keyword">import</span> org.apache.felix.framework.util.FelixConstants;
<span class="code-keyword">import</span> org.apache.felix.framework.util.StringMap;
<span class="code-keyword">import</span> org.apache.felix.framework.cache.BundleCache;
<span class="code-keyword">public</span> class HostApplication
{
<span class="code-keyword">private</span> HostActivator m_activator = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> Felix m_felix = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> Map m_lookupMap = <span class="code-keyword">new</span> HashMap();
<span class="code-keyword">public</span> HostApplication()
{
<span class="code-comment">// Initialize the map <span class="code-keyword">for</span> the property lookup service.
</span> m_lookupMap.put(<span class="code-quote">"name1"</span>, <span class="code-quote">"value1"</span>);
m_lookupMap.put(<span class="code-quote">"name2"</span>, <span class="code-quote">"value2"</span>);
m_lookupMap.put(<span class="code-quote">"name3"</span>, <span class="code-quote">"value3"</span>);
m_lookupMap.put(<span class="code-quote">"name4"</span>, <span class="code-quote">"value4"</span>);
<span class="code-comment">// Create a <span class="code-keyword">case</span>-insensitive configuration property map.
</span> Map configMap = <span class="code-keyword">new</span> StringMap(<span class="code-keyword">false</span>);
<span class="code-comment">// Configure the Felix instance to be embedded.
</span> configMap.put(FelixConstants.EMBEDDED_EXECUTION_PROP, <span class="code-quote">"<span class="code-keyword">true</span>"</span>);
<span class="code-comment">// Add the host provided service <span class="code-keyword">interface</span> <span class="code-keyword">package</span> and the core OSGi
</span> <span class="code-comment">// packages to be exported from the class path via the system bundle.
</span> configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
<span class="code-quote">"org.osgi.framework; version=1.3.0,"</span> +
<span class="code-quote">"org.osgi.service.packageadmin; version=1.2.0,"</span> +
<span class="code-quote">"org.osgi.service.startlevel; version=1.0.0,"</span> +
<span class="code-quote">"org.osgi.service.url; version=1.0.0,"</span> +
<span class="code-quote">"host.service.lookup; version=1.0.0"</span>);
<span class="code-comment">// Explicitly specify the directory to use <span class="code-keyword">for</span> caching bundles.
</span> configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, <span class="code-quote">"cache"</span>);
<span class="code-keyword">try</span>
{
<span class="code-comment">// Create host activator;
</span> m_activator = <span class="code-keyword">new</span> HostActivator(m_lookupMap);
List list = <span class="code-keyword">new</span> ArrayList();
list.add(m_activator);
<span class="code-comment">// Now create an instance of the framework with
</span> <span class="code-comment">// our configuration properties and activator.
</span> m_felix = <span class="code-keyword">new</span> Felix(configMap, list);
<span class="code-comment">// Now start Felix instance.
</span> m_felix.start();
}
<span class="code-keyword">catch</span> (Exception ex)
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"Could not create framework: "</span> + ex);
ex.printStackTrace();
}
}
<span class="code-keyword">public</span> void shutdownApplication()
{
<span class="code-comment">// Shut down the felix framework when stopping the
</span> <span class="code-comment">// host application.
</span> m_felix.shutdown();
}
}</pre>
</div></div>
<p>Rather than having the host application bundle activator register
the service, it is also possible for the the host application to simply
get the bundle context from the bundle activator and register the
service directly, but the presented approach is perhaps a little
cleaner since it allows the host application to register/unregister the
service when the system bundle starts/stops.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-hostserviceusage"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-UsingServicesProvidedbyBundles"></a>Using Services Provided by Bundles</h2>
<p>Using services provided by bundles follows the same general approach
of using a host application bundle activator. The main complication for
the host application using a service from a bundle is the fact that
both the host application and the bundle must be using the same class
definitions for the service interface classes. Since the host
application cannot import classes from a bundle, this means that the
service interface classes <b>must</b> be accessible on the class path,
typically as part of the host application itself. The host application
then must export the service interface package via the system bundle so
that bundles installed into the embedded framework instance can import
it. This is achieved using the <tt>org.osgi.framework.system.packages</tt> configuration property previously presented.</p>
<p>Consider the following simple command service interface for which
bundles provide implementations, such as might be used to create an
extensible interactive shell:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> host.service.command;
<span class="code-keyword">public</span> class Command
{
<span class="code-keyword">public</span> <span class="code-object">String</span> getName();
<span class="code-keyword">public</span> <span class="code-object">String</span> getDescription();
<span class="code-keyword">public</span> <span class="code-object">boolean</span> execute(<span class="code-object">String</span> commandline);
}</pre>
</div></div>
<p>This package is simply part of the host application, which is potentially packaged into a JAR file and started with the "<tt>java -jar</tt>"
command. Now consider the previously introduced host application bundle
activator below, which simply provides access to the system bundle
context:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> host.core;
<span class="code-keyword">import</span> org.osgi.framework.BundleActivator;
<span class="code-keyword">import</span> org.osgi.framework.BundleContext;
<span class="code-keyword">public</span> class HostActivator <span class="code-keyword">implements</span> BundleActivator
{
<span class="code-keyword">private</span> BundleContext m_context = <span class="code-keyword">null</span>;
<span class="code-keyword">public</span> void start(BundleContext context)
{
m_context = context;
}
<span class="code-keyword">public</span> void stop(BundleContext context)
{
m_context = <span class="code-keyword">null</span>;
}
<span class="code-keyword">public</span> BundleContext getContext()
{
<span class="code-keyword">return</span> m_context;
}
}</pre>
</div></div>
<p>With this bundle activator, the host application can command
services provided by bundles installed inside its embedded Felix
framework instance. The following code snippet illustrates one possible
approach:</p>
<div class="code"><div class="codeContent">
<pre class="code-java"><span class="code-keyword">package</span> host.core;
<span class="code-keyword">import</span> java.util.List;
<span class="code-keyword">import</span> java.util.ArrayList;
<span class="code-keyword">import</span> java.util.Map;
<span class="code-keyword">import</span> host.service.command.Command;
<span class="code-keyword">import</span> org.apache.felix.framework.Felix;
<span class="code-keyword">import</span> org.apache.felix.framework.util.FelixConstants;
<span class="code-keyword">import</span> org.apache.felix.framework.util.StringMap;
<span class="code-keyword">import</span> org.apache.felix.framework.cache.BundleCache;
<span class="code-keyword">import</span> org.osgi.util.tracker.ServiceTracker;
<span class="code-keyword">public</span> class HostApplication
{
<span class="code-keyword">private</span> HostActivator m_activator = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> Felix m_felix = <span class="code-keyword">null</span>;
<span class="code-keyword">private</span> ServiceTracker m_tracker = <span class="code-keyword">null</span>;
<span class="code-keyword">public</span> HostApplication()
{
<span class="code-comment">// Create a <span class="code-keyword">case</span>-insensitive configuration property map.
</span> Map configMap = <span class="code-keyword">new</span> StringMap(<span class="code-keyword">false</span>);
<span class="code-comment">// Configure the Felix instance to be embedded.
</span> configMap.put(FelixConstants.EMBEDDED_EXECUTION_PROP, <span class="code-quote">"<span class="code-keyword">true</span>"</span>);
<span class="code-comment">// Add the bundle provided service <span class="code-keyword">interface</span> <span class="code-keyword">package</span> and the core OSGi
</span> <span class="code-comment">// packages to be exported from the class path via the system bundle.
</span> configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
<span class="code-quote">"org.osgi.framework; version=1.3.0,"</span> +
<span class="code-quote">"org.osgi.service.packageadmin; version=1.2.0,"</span> +
<span class="code-quote">"org.osgi.service.startlevel; version=1.0.0,"</span> +
<span class="code-quote">"org.osgi.service.url; version=1.0.0,"</span> +
<span class="code-quote">"host.service.command; version=1.0.0"</span>);
<span class="code-comment">// Explicitly specify the directory to use <span class="code-keyword">for</span> caching bundles.
</span> configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, <span class="code-quote">"cache"</span>);
<span class="code-keyword">try</span>
{
<span class="code-comment">// Create host activator;
</span> m_activator = <span class="code-keyword">new</span> HostActivator(m_lookupMap);
List list = <span class="code-keyword">new</span> ArrayList();
list.add(m_activator);
<span class="code-comment">// Now create an instance of the framework with
</span> <span class="code-comment">// our configuration properties and activator.
</span> m_felix = <span class="code-keyword">new</span> Felix(configMap, list);
<span class="code-comment">// Now start Felix instance.
</span> m_felix.start();
}
<span class="code-keyword">catch</span> (Exception ex)
{
<span class="code-object">System</span>.err.println(<span class="code-quote">"Could not create framework: "</span> + ex);
ex.printStackTrace();
}
m_tracker = <span class="code-keyword">new</span> ServiceTracker(
m_activator.getContext(), Command.class.getName(), <span class="code-keyword">null</span>);
m_tracker.open();
}
<span class="code-keyword">public</span> <span class="code-object">boolean</span> execute(<span class="code-object">String</span> name, <span class="code-object">String</span> commandline)
{
<span class="code-comment">// See <span class="code-keyword">if</span> any of the currently tracked command services
</span> <span class="code-comment">// match the specified command name, <span class="code-keyword">if</span> so then execute it.
</span> <span class="code-object">Object</span>[] services = m_tracker.getServices();
<span class="code-keyword">for</span> (<span class="code-object">int</span> i = 0; (services != <span class="code-keyword">null</span>) &amp;&amp; (i &lt; services.length); i++)
{
<span class="code-keyword">try</span>
{
<span class="code-keyword">if</span> (((Command) services[i]).getName().equals(name))
{
<span class="code-keyword">return</span> ((Command) services[i]).execute(commandline);
}
}
<span class="code-keyword">catch</span> (Exception ex)
{
<span class="code-comment">// Since the services returned by the tracker could become
</span> <span class="code-comment">// invalid at any moment, we will <span class="code-keyword">catch</span> all exceptions, log
</span> <span class="code-comment">// a message, and then ignore faulty services.
</span> <span class="code-object">System</span>.err.println(ex);
}
}
<span class="code-keyword">return</span> <span class="code-keyword">false</span>;
}
<span class="code-keyword">public</span> void shutdownApplication()
{
<span class="code-comment">// Shut down the felix framework when stopping the
</span> <span class="code-comment">// host application.
</span> m_felix.shutdown();
}
}</pre>
</div></div>
<p>The above example is overly simplistic with respect to concurrency
issues and error conditions, but it demonstrates the overall approach
for using bundle-provided services from the host application. Note, to
compile the above code you will need to compile against the Felix
framework and Felix OSGi compendium JAR files, since the <tt>ServiceTracker</tt> classes are included in the compendium JAR file, not the framework JAR file.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-servicereflection"></a></p>
<h3><a name="LaunchingandEmbeddingApacheFelix-UsingBundleServicesviaReflection"></a>Using Bundle Services via Reflection</h3>
<p>It possible for the host application to use services provided by
bundles without having access to the service interface classes and thus
not needing to put the service interface classes on the class path. To
do this, the host application uses the same general approach to acquire
the system bundle context object, which it can use to look up service
objects. Using either an LDAP filter or the service interface class
name, the host application can retrieve the service object and then use
standard Java reflection to invoke methods on the service object.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-caveat"></a></p>
<h1><a name="LaunchingandEmbeddingApacheFelix-Caveat"></a>Caveat</h1>
<p>The code in this document has not been thoroughly tested or even
compiled and may be out of date with respect to the current Felix
source code. If you find errors please report them so the that they can
be corrected.</p>
<p><a name="LaunchingandEmbeddingApacheFelix-feedback"></a></p>
<h2><a name="LaunchingandEmbeddingApacheFelix-Feedback"></a>Feedback</h2>
<p>Subscribe to the Felix users mailing list by sending a message to <span class="nobr"><a href="mailto:users-subscribe@felix.apache.org" title="Send mail to users-subscribe@felix.apache.org" rel="nofollow">users-subscribe@felix.apache.org<sup><img class="rendericon" src="launching-and-embedding-apache-felix_files/mail_small.gif" alt="" align="absmiddle" border="0" height="12" width="13"></sup></a></span>; after subscribing, email questions or feedback to <span class="nobr"><a href="mailto:users@felix.apache.org" title="Send mail to users@felix.apache.org" rel="nofollow">users@felix.apache.org<sup><img class="rendericon" src="launching-and-embedding-apache-felix_files/mail_small.gif" alt="" align="absmiddle" border="0" height="12" width="13"></sup></a></span>.</p>
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