Launching and Embedding Apache Felix

[This document is based on Felix 1.0.3.]

• Introduction
• API Overview
• Launching Felix
  • Standard Felix Launcher
  • Custom Felix Launcher
• Embedding Felix
  • Embedded Execution Configuration Property
  • Host/Felix Interaction
  • Providing Host Application Services
  • Using Services Provided by Bundles
    • Using Bundle Services via Reflection
• Caveat
• Feedback

Introduction

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.

API Overview

The Felix class that implements the OSGi framework is org.apache.felix.framework.Felix or just Felix for short. The OSGi specification defines a special bundle, called the System Bundle, 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 Felix class implements the org.osgi.framework.Bundle interface, which is reiterated here:

public interface Bundle
{
    public BundleContext getBundleContext();
    public long getBundleId();
    public URL getEntry(String name);
    public Enumeration getEntryPaths(String path);
    public Enumeration findEntries(String path, String filePattern, boolean recurse);
    public Dictionary getHeaders();
    public Dictionary getHeaders(String locale);
    public long getLastModified();
    public String getLocation();
    public URL getResource(String name);
    public Enumeration getResources(String name) throws IOException;
    public ServiceReference[] getRegisteredServices();
    public ServiceReference[] getServicesInUse();
    public int getState();
    public String getSymbolicName();
    public boolean hasPermission(Object obj);
    public Class loadClass(String name) throws ClassNotFoundException;
    public void start() throws BundleException;
    public void stop() throws BundleException;
    public void uninstall() throws BundleException;
    public void update() throws BundleException;
    public void update(InputStream is) throws BundleException;
}

When you instantiate the Felix class, the resulting object is actually the System Bundle and can be cast to the Bundle interface. The start() method is used to start the framework instance, while the stop() method is used to asynchronously stop the framework instance. The Felix class also includes the following three additional public methods:

public class Felix extends FelixBundle
{
    public Felix(Map configMutableMap, List activatorList);
    public Felix(Logger logger, Map configMutableMap, List activatorList)
    public void stopAndWait();
}

The first two methods are constructors used to instantiate framework instances; the important parameters of the constructors are the configuration properties and System Bundle activators, which are both described in more detail later. The stopAndWait() method is a synchronous version of the stop() method, used to stop the framework and block the calling thread until the framework is completely stopped.

Launching Felix

Launching Felix is fairly simple and involves only three steps:

1. Defining some configuration properties.
2. Creating an instance of org.apache.felix.framework.Felix with the configuration properties.
3. Invoking the org.apache.felix.framework.Felix.start() method.

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 bundle cache document for more detailed information on configuring the bundle cache.

Besides configuration properties for the bundle cache, it is usually necessary to set the org.osgi.framework.system.packages configuration property to export packages from the class path, such as the OSGi interface classes (e.g., org.osgi.framework) on which all bundles depend. If you are creating a launcher for Felix, then often you want to automatically install and start bundles when you start the framework instance. The default Felix launcher defines reusable functionality to automatically install and/or start bundles upon framework startup; see the usage document for more information on configuring Felix and on the various configuration properties.

The remainder of this section describes how the standard Felix launcher works as well as how to create a custom launcher for Felix.

Standard Felix Launcher

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 main() method of the standard launcher, each numbered comment will be described in more detail below:

public static void main(String[] argv) throws Exception
{
    // (1) Load system properties.
    Main.loadSystemProperties();

    // (2) Read configuration properties.
    Properties configProps = Main.loadConfigProperties();

    // (3) Copy framework properties from the system properties.
    Main.copySystemProperties(configProps);

    // (4) See if the profile name property was specified.
    String profileName = configProps.getProperty(BundleCache.CACHE_PROFILE_PROP);

    // (4) See if the profile directory property was specified.
    String profileDirName = configProps.getProperty(BundleCache.CACHE_PROFILE_DIR_PROP);

    // Print welcome banner.
    System.out.println("\nWelcome to Felix.");
    System.out.println("=================\n");

    // (5) If no profile or profile directory is specified in the
    // properties, then ask for a profile name.
    if ((profileName == null) && (profileDirName == null))
    {
        System.out.print("Enter profile name: ");
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        try
        {
            profileName = in.readLine();
        }
        catch (IOException ex)
        {
            System.err.println("Could not read input.");
            System.exit(-1);
        }
        System.out.println("");
        if (profileName.length() != 0)
        {
            configProps.setProperty(BundleCache.CACHE_PROFILE_PROP, profileName);
        }
    }

    // (6) A profile directory or name must be specified.
    if ((profileDirName == null) && (profileName.length() == 0))
    {
        System.err.println("You must specify a profile name or directory.");
        System.exit(-1);
    }

    try
    {
        // (7) Create a list for custom framework activators and
        // add an instance of the auto-activator it for processing
        // auto-install and auto-start properties.
        List list = new ArrayList();
        list.add(new AutoActivator(configProps));
        // (8) Create a case-insensitive property map.
        Map configMap = new StringMap(configProps, false);
        // (9) Create an instance of the framework.
        m_felix = new Felix(configMap, list);
        m_felix.start();
    }
    catch (Exception ex)
    {
        System.err.println("Could not create framework: " + ex);
        ex.printStackTrace();
        System.exit(-1);
    }
}

The general steps of the standard launcher are quite straightforward:

1. Load any system properties specified in the system.properties file; this file is typically located in the conf/ directory of the Felix installation directory, but it can be specified directly using the felix.system.properties 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 ${<property-name} syntax. Property substitution can be nested; only system properties will be used for substitution.
2. Load any configuration properties specified in the config.properties file; this file is typically located in the conf/ directory of the Felix installation directory, but it can be specified directly using the felix.config.properties system property. This file is used to configure the Felix instance created by the launcher. The file is a standard Java properties file, but it also supports property substitution using "${<property-name}" syntax. Property substitution can be nested; configuration and system properties will be used for substitution with configuration properties having precedence.
3. 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 config.properties file.
4. Try to load the profile name or profile directory configuration properties. At least one of these must be specified so that the bundle cache knows where to save installed bundles.
5. 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.
6. Error if there is no profile name or profile directory.
7. Create a list to hold custom framework activators and add an instance of org.apache.felix.main.AutoActivator, which will process felix.auto.install and felix.auto.start configuration properties during framework startup to automatically install and/or start bundles; see the usage document for more information configuration properties.
8. Create a case-insensitive map to hold our configuration properties.
9. Create the Felix instance passing in the configuration properties and custom framework activators, then call start().

The framework is not active until the start() method is called. If no shell bundles are installed and started or if there is difficulty locating the shell bundles specified in the auto-start property, 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.

Custom Felix Launcher

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.

This example launcher project has the following directory structure:

launcher/
   lib/
      org.apache.felix.main-1.0.3.jar
   bundle/
      org.apache.felix.shell-1.0.0.jar
      org.apache.felix.shell.tui-1.0.0.jar
   src/
      example/
         Main.java

The lib/ directory contains Felix' main JAR file, which also contains the OSGi core interfaces. The main JAR file is used so that we can reuse the default launcher's auto-install/auto-start configuration property handling; if these capabilities are needed, then it would be possible to use the framework JAR file. The bundle/ 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 src/example/ directory contains the following Main.java file, which is a very simplistic Felix launcher.

package example;

import java.util.Map;
import org.osgi.framework.Constants;
import org.apache.felix.framework.Felix;
import org.apache.felix.framework.cache.BundleCache;
import org.apache.felix.framework.util.StringMap;
import org.apache.felix.framework.main.AutoActivator;

public class Main
{
    private static Felix m_felix = null;

    public static void main(String[] argv) throws Exception
    {
        // Print welcome banner.
        System.out.println("\nWelcome to Felix.");
        System.out.println("=================\n");

        Map configMap = new StringMap(false);
        configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
            "org.osgi.framework; version=1.3.0," +
            "org.osgi.service.packageadmin; version=1.2.0," +
            "org.osgi.service.startlevel; version=1.0.0," +
            "org.osgi.service.url; version=1.0.0");
        configMap.put(AutoActivator.AUTO_START_PROP + ".1",
            "file:bundle/org.apache.felix.shell-1.0.0.jar " +
            "file:bundle/org.apache.felix.shell.tui-1.0.0.jar");
        configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, "cache");

        try
        {
            List list = new ArrayList();
            list.add(new AutoActivator(configProps));
            Map configMap = new StringMap(configProps, false);
            m_felix = new Felix(configMap, list);
            m_felix.start();
        }
        catch (Exception ex)
        {
            System.err.println("Could not create framework: " + ex);
            ex.printStackTrace();
            System.exit(-1);
        }
    }
}

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.

By breaking down the above source code into small chunks, it is quite easy to see what is going on.

Map configMap = new StringMap(false);

This simply creates a map to hold configuration properties where the keys are strings and lookups are case insensitive.

configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
            "org.osgi.framework; version=1.3.0," +
            "org.osgi.service.packageadmin; version=1.2.0," +
            "org.osgi.service.startlevel; version=1.0.0," +
            "org.osgi.service.url; version=1.0.0");

This sets the Constants.FRAMEWORK_SYSTEMPACKAGES configuration property (string value "org.osgi.framework.system.packages"), 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 javax.swing. 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.

configMap.put(AutoActivator.AUTO_START_PROP + ".1",
            "file:bundle/org.apache.felix.shell-1.0.0.jar " +
            "file:bundle/org.apache.felix.shell.tui-1.0.0.jar");

This sets the AutoActivator.AUTO_START_PROP configuration property (string value "felix.auto.start"), 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 file: URLs, which will load the bundles from the bundle/ 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.

configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, "cache");

This sets the last configuration property, BundleCache.CACHE_PROFILE_DIR_PROP (string value "felix.cache.profiledir"), 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 "cache". 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.

List list = new ArrayList();
            list.add(new AutoActivator(configProps));

This above creates a list to hold custom framework activators and adds an instance of org.apache.felix.main.AutoActivator to it, which will process the auto-install and auto-start configuration properties during framework startup.

Map configMap = new StringMap(configProps, false);

This above creates a case-insensitive map from the configuration properties, since the OSGi specification says that properties are not case sensitive.

m_felix = new Felix(configMap, list);
            m_felix.start();

These last steps create the framework instance and start it. The configuration property map and custom framework activator list are passed into the Felix constructor.

The following command compiles the launcher when run from the root directory of the launcher project:

javac -d . -classpath lib/org.apache.felix.main-1.0.2.jar src/example/Main.java

After executing this command, an example/ 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:

java -cp .:lib/org.apache.felix.main-1.0.2.jar example.Main

After executing this command, a cache/ directory is created that contains the installed bundles, which were installed from the bundle/ directory.

Embedding Felix

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.

Embedded Execution Configuration Property

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 "felix.embedded.execution" configuration property to "true"; 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 System.exit()) 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 "true" it can be avoided.

Host/Felix Interaction

In the section on launching Felix above, the Felix 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.

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 BundleContext object so that they can interact with the framework externally. While it is possible to get the System Bundle's BundleContext object directly by calling Felix.getBundleContext(), 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:

public class HostActivator implements BundleActivator
{
    private BundleContext m_context = null;

    public void start(BundleContext context)
    {
        m_context = context;
    }

    public void stop(BundleContext context)
    {
        m_context = null;
    }

    public Bundle[] getBundles()
    {
        if (m_context != null)
        {
            return m_context.getBundles();
        }
        return null;
    }
}

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:

public class HostApplication
{
    private HostActivator m_activator = null;
    private Felix m_felix = null;

    public HostApplication()
    {
        // Create a case-insensitive configuration property map.
        Map configMap = new StringMap(false);
        // Configure the Felix instance to be embedded.
        configMap.put(FelixConstants.EMBEDDED_EXECUTION_PROP, "true");
        // Add core OSGi packages to be exported from the class path
        // via the system bundle.
        configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
            "org.osgi.framework; version=1.3.0," +
            "org.osgi.service.packageadmin; version=1.2.0," +
            "org.osgi.service.startlevel; version=1.0.0," +
            "org.osgi.service.url; version=1.0.0");
        // Explicitly specify the directory to use for caching bundles.
        configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, "cache");

        try
        {
            // Create host activator;
            m_activator = new HostActivator();
            List list = new ArrayList();
            list.add(m_activator);

            // Now create an instance of the framework with
            // our configuration properties and activator.
            m_felix = new Felix(configMap, list);

            // Now start Felix instance.
            m_felix.start();
        }
        catch (Exception ex)
        {
            System.err.println("Could not create framework: " + ex);
            ex.printStackTrace();
        }
    }

    public Bundle[] getInstalledBundles()
    {
        // Use the system bundle activator to gain external
        // access to the set of installed bundles.
        return m_activator.getBundles();
    }

    public void shutdownApplication()
    {
        // Shut down the felix framework when stopping the
        // host application.
        m_felix.shutdown();
    }
}

Notice how the HostApplication.getInstalledBundles() 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.

Providing Host Application Services

Providing services from the host application to bundles inside the embedded Felix framework instance follows the basic approach laid out in above. 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 must 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 org.osgi.framework.system.packages configuration property previously presented.

Consider the follow simple property lookup service:

package host.service.lookup;

public class Lookup
{
    public Object lookup(String name);
}

This package is simply part of the host application, which is potentially packaged into a JAR file and started with the "java -jar" 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:

package host.core;

import java.util.Map;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
import org.osgi.framework.ServiceRegistration;
import host.service.lookup;

public class HostActivator implements BundleActivator
{
    private Map m_lookupMap = null;
    private BundleContext m_context = null;
    private ServiceRegistration m_registration = null;

    public HostActivator(Map lookupMap)
    {
        // Save a reference to the service's backing store.
        m_lookupMap = lookupMap;
    }

    public void start(BundleContext context)
    {
        // Save a reference to the bundle context.
        m_context = context;
        // Create a property lookup service implementation.
        Lookup lookup = new Lookup() {
            public Object lookup(String name)
            {
                return m_lookupMap.get(name);
            }
        };
        // Register the property lookup service and save
        // the service registration.
        m_registration = m_context.registerService(
            Lookup.class.getName(), lookup, null);
    }

    public void stop(BundleContext context)
    {
        // Unregister the property lookup service.
        m_registration.unregister();
        m_context = null;
    }
}

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:

package host.core;

import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;
import host.service.lookup.Lookup;
import org.apache.felix.framework.Felix;
import org.apache.felix.framework.util.FelixConstants;
import org.apache.felix.framework.util.StringMap;
import org.apache.felix.framework.cache.BundleCache;

public class HostApplication
{
    private HostActivator m_activator = null;
    private Felix m_felix = null;
    private Map m_lookupMap = new HashMap();

    public HostApplication()
    {
        // Initialize the map for the property lookup service.
        m_lookupMap.put("name1", "value1");
        m_lookupMap.put("name2", "value2");
        m_lookupMap.put("name3", "value3");
        m_lookupMap.put("name4", "value4");

        // Create a case-insensitive configuration property map.
        Map configMap = new StringMap(false);
        // Configure the Felix instance to be embedded.
        configMap.put(FelixConstants.EMBEDDED_EXECUTION_PROP, "true");
        // Add the host provided service interface package and the core OSGi
        // packages to be exported from the class path via the system bundle.
        configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
            "org.osgi.framework; version=1.3.0," +
            "org.osgi.service.packageadmin; version=1.2.0," +
            "org.osgi.service.startlevel; version=1.0.0," +
            "org.osgi.service.url; version=1.0.0," +
            "host.service.lookup; version=1.0.0");
        // Explicitly specify the directory to use for caching bundles.
        configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, "cache");

        try
        {
            // Create host activator;
            m_activator = new HostActivator(m_lookupMap);
            List list = new ArrayList();
            list.add(m_activator);

            // Now create an instance of the framework with
            // our configuration properties and activator.
            m_felix = new Felix(configMap, list);

            // Now start Felix instance.
            m_felix.start();
        }
        catch (Exception ex)
        {
            System.err.println("Could not create framework: " + ex);
            ex.printStackTrace();
        }
    }

    public void shutdownApplication()
    {
        // Shut down the felix framework when stopping the
        // host application.
        m_felix.shutdown();
    }
}

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.

Using Services Provided by Bundles

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 must 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 org.osgi.framework.system.packages configuration property previously presented.

Consider the following simple command service interface for which bundles provide implementations, such as might be used to create an extensible interactive shell:

package host.service.command;

public class Command
{
    public String getName();
    public String getDescription();
    public boolean execute(String commandline);
}

This package is simply part of the host application, which is potentially packaged into a JAR file and started with the "java -jar" command. Now consider the previously introduced host application bundle activator below, which simply provides access to the system bundle context:

package host.core;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;

public class HostActivator implements BundleActivator
{
    private BundleContext m_context = null;

    public void start(BundleContext context)
    {
        m_context = context;
    }

    public void stop(BundleContext context)
    {
        m_context = null;
    }

    public BundleContext getContext()
    {
        return m_context;
    }
}

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:

package host.core;

import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import host.service.command.Command;
import org.apache.felix.framework.Felix;
import org.apache.felix.framework.util.FelixConstants;
import org.apache.felix.framework.util.StringMap;
import org.apache.felix.framework.cache.BundleCache;
import org.osgi.util.tracker.ServiceTracker;

public class HostApplication
{
    private HostActivator m_activator = null;
    private Felix m_felix = null;
    private ServiceTracker m_tracker = null;

    public HostApplication()
    {
        // Create a case-insensitive configuration property map.
        Map configMap = new StringMap(false);
        // Configure the Felix instance to be embedded.
        configMap.put(FelixConstants.EMBEDDED_EXECUTION_PROP, "true");
        // Add the bundle provided service interface package and the core OSGi
        // packages to be exported from the class path via the system bundle.
        configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES,
            "org.osgi.framework; version=1.3.0," +
            "org.osgi.service.packageadmin; version=1.2.0," +
            "org.osgi.service.startlevel; version=1.0.0," +
            "org.osgi.service.url; version=1.0.0," +
            "host.service.command; version=1.0.0");
        // Explicitly specify the directory to use for caching bundles.
        configMap.put(BundleCache.CACHE_PROFILE_DIR_PROP, "cache");

        try
        {
            // Create host activator;
            m_activator = new HostActivator(m_lookupMap);
            List list = new ArrayList();
            list.add(m_activator);

            // Now create an instance of the framework with
            // our configuration properties and activator.
            m_felix = new Felix(configMap, list);

            // Now start Felix instance.
            m_felix.start();
        }
        catch (Exception ex)
        {
            System.err.println("Could not create framework: " + ex);
            ex.printStackTrace();
        }

        m_tracker = new ServiceTracker(
            m_activator.getContext(), Command.class.getName(), null);
        m_tracker.open();
    }

    public boolean execute(String name, String commandline)
    {
        // See if any of the currently tracked command services
        // match the specified command name, if so then execute it.
        Object[] services = m_tracker.getServices();
        for (int i = 0; (services != null) && (i < services.length); i++)
        {
            try
            {
                if (((Command) services[i]).getName().equals(name))
                {
                    return ((Command) services[i]).execute(commandline);
                }
            }
            catch (Exception ex)
            {
                // Since the services returned by the tracker could become
                // invalid at any moment, we will catch all exceptions, log
                // a message, and then ignore faulty services.
                System.err.println(ex);
            }
        }
        return false;
    }

    public void shutdownApplication()
    {
        // Shut down the felix framework when stopping the
        // host application.
        m_felix.shutdown();
    }
}

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 ServiceTracker classes are included in the compendium JAR file, not the framework JAR file.

Using Bundle Services via Reflection

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.

Caveat

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.

Feedback

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