Apache Felix Framework Launching and Embedding

Wiki Markup_\[This document describes framework launching introduced in Felix Framework 2.0.0 and is incompatible with older versions of the Felix framework.\]_and continuing with the latest releases; it is incompatible with older versions of the Felix framework.]

• Introduction
• OSGi Launching and Embedding
• Introduction
• API Overview
  • Creating and Configuring the Framework Instance
  • Starting the Framework Instance
  • Stopping the Framework Instance
• Launching Felix
  • Standard Felix Launcher
  • Custom Felix Launcher
• Embedding Felix
  • Host/Felix Interaction
  • Providing Host Application Services
  • Using Services Provided by Bundles
    • Using Bundle Services via Reflection
    • Other Approaches
• Caveat
• Feedback

...

Introduction

The Apache Felix framework Framework is intended to be easily launchable and embeddable. For example, the Felix framework implementation avoids the use of system properties for configuration, since these are globals and can cause interference if multiple framework instances are created in the same VM. Felix The framework also tries 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 possiblethe framework in a variety of scenarios; 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 overviewan overview of the standard OSGi launching and embedding API for frameworks, 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.

OSGi Launching and Embedding API Overview

The Felix framework is implemented by the org.apache.felix.framework.Felix class or just Felix for short. As part of the R4.2 OSGi specification, the launching and embedding API of the OSGi framework has been standardized. The approach is to have the framework implement the org.osgi.framework.launch.Framework interface, which extends the org.osgi.framework.Bundle interface. These interfaces provide the necessary means to launch and manage framework instances. The Bundle interface is defined as:

...

public interface Framework extends Bundle
{
    void init();
    FrameworkEvent waitForStop(long timeout);
}

To actually construct a framework instance, the R4.2 specification defines the FrameworkFactory interface:

public interface FrameworkFactory
{
    Framework newFramework(Map configMapconfig);
}

The framework factory can be used to create configured framework instances. It is obtained following the standard META-INF/services approach.

...

You use the framework factory to construct and configure a framework instance (or by directly instantiating the Felix class). The configuration map may contain the following OSGi standard properties:

• org.osgi.framework.system.packages - specifies a list of packages the system bundle should export from the environment; if this is not set, then the framework uses a reasonable default fault.
• org.osgi.framework.system.packages.extra - specifies a list of additional packages the system bundle should export from the environment that are appended to the packages specified in org.osgi.framework.system.packages; there is no default value for this property.
• org.osgi.framework.bootdelegation - specifies a list of packages that should be made implicitly available to all bundles from the environment (i.e., no need to import them); there is no default value for this property and its use should be avoided.
• org.osgi.framework.storage - specifies the path to a directory, which will be created if it does not exist, to use for bundle cache storage; the default value for this property is "felix-cache" in the current working directory.
• org.osgi.framework.storage.clean - specifies whether the bundle cache should be flushed; the default value for this property is "none", but it can be changed to "onFirstInit" to flush the bundle cache when the framework is initialized.
• org.osgi.framework.startlevel.beginning - specifies the start level the framework enters upon startup; the default value for this property is 1.

Felix also has the following, non-standard configuration properties:

• felix.cache.rootdir - specifies which directory should be used to calculate absolute paths when relative paths are used for the org.osgi.framework.storage property; the default value for this property is the current working directory.
• felix.systembundle.activators - specifies a List of BundleActivator instances that are started/stopped when the System Bundle is started/stopped; the specified instances will receive the System Bundle's BundleContext when invoked.
• felix.log.logger - specifies an instance of org.apache.felix.framework.util.Logger that the framework uses as its default logger.
• felix.log.level - specifies an integer String whose value indicates the degree of logging reported by the framework; the default value is "1" and "0" turns off logging completely, otherwise log levels match those specified in the OSGi Log Service (i.e., 1 = error, 2 = warning, 3 = information, and 4 = debug).
• felix.startlevel.bundle - specifies the start level for newly installed bundles; the default value is 1.
• felix.bootdelegation.implicit - specifies whether or not the framework should try to guess when to boot delegate when external code tries to load classes or resources; the default value is "true".
• framework.service.urlhandlers - specifies whether or not to activate the URL Handlers service for the framework instance; the default value is "true", which results in the URL.setURLStreamHandlerFactory() and URLConnection.setContentHandlerFactory() being called.

a framework instance (or by directly instantiating the Felix class). The configuration map may contain any of the framework configuration properties listed in the Apache Felix Framework Configuration Properties document, not the launcher configuration properties. The configuration map is copied and the keys are treated as case insensitive. You are not able to change the framework's configuration after construction. If you need a different configuration, you must create a new framework instance.

...

The init() method is necessary since the framework does not have a BundleContext when it is first created, so a transition to the Bundle.STARTING state is required to acquire its context (via Bundle.getBundleContext()) for performing various tasks, such as installing bundles. Note that the Felix framework also provides the felix.systembundle.activators property that serves a similar purpose, but is not standard. After the init() method completes, the follow actions have been performed:

...

In reality, the first step is optional, since all properties will have reasonable defaults, but if you are creating a launcher you will generally want to more than that, such as automatically installing and starting 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 the Felix framework 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 Framework Launcher

The standard Felix framework 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[] args) throws Exception
{
    // (1) Check for command line arguments and verify usage.
    String bundleDir = null;
    String cacheDir = null;
    boolean expectBundleDir = false;
    for (int i = 0; i < args.length; i++)
    {
        if (args[i].equals(BUNDLE_DIR_SWITCH))
        {
            expectBundleDir = true;
        }
        else if (expectBundleDir)
        {
            bundleDir = args[i];
            expectBundleDir = false;
        }
        else
        {
            cacheDir = args[i];
        }
    }

    if ((args.length > 3) || (expectBundleDir && bundleDir == null))
    {
        System.out.println("Usage: [-b <bundle-deploy-dir>] [<bundle-cache-dir>]");
        System.exit(0);
    }

    // (2) Load system properties.
    Main.loadSystemProperties();

    // (3) Read configuration properties.
    Properties configProps = Main.loadConfigProperties();
    if (configProps == null)
    {
        System.err.println("No " + CONFIG_PROPERTIES_FILE_VALUE + " found.");
        configProps = new Properties();
    }

    // (4) Copy framework properties from the system properties.
    Main.copySystemProperties(configProps);
        
    // (5) Use the specified auto-deploy directory over default.
    if (bundleDir != null)
    {
        configProps.setProperty(AutoProcessor.AUTO_DEPLOY_DIR_PROPERY, bundleDir);
    }

    // (6) Use the specified bundle cache directory over default.
    if (cacheDir != null)
    {
        configProps.setProperty(Constants.FRAMEWORK_STORAGE, cacheDir);
    }

    // (7) Add a shutdown hook to clean stop the framework.
    String enableHook = configProps.getProperty(SHUTDOWN_HOOK_PROP);
    if ((enableHook == null) || !enableHook.equalsIgnoreCase("false"))
    {
        Runtime.getRuntime().addShutdownHook(new Thread("Felix Shutdown Hook") {
            public void run()
            {
                try
                {
                    if (m_fwk != null)
                    {
                        m_fwk.stop();
                        m_fwk.waitForStop(0);
                    }
                }
                catch (Exception ex)
                {
                    System.err.println("Error stopping framework: " + ex);
             + ex);
    }
            }
        });
    }

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

    try
    {
        // (8) Create an instance and initialize the framework.
        FrameworkFactory factory = getFrameworkFactory();
        m_fwk = factory.newFramework(configProps);
        m_fwk.init();
        // (9) Use the system bundle context to process the auto-deploy
        // and auto-install/auto-start properties.
        AutoProcessor.process(configProps, m_fwk.getBundleContext());
        // (10) Start the framework.
        m_fwk.start();
        // (11) Wait for framework to stop to exit the VM.
        m_fwk.waitForStop(0);
        System.exit(0);
    }
    catch (Exception ex)
    {
        System.err.println("Could not create framework: " + ex);
        ex.printStackTrace();
        System.exit(-10);
    }
}

The general steps of the standard launcher are quite straightforward:

1. The launcher supports setting the auto-deploy directory (with the -b switch) and setting the bundle cache path with a single argument, so check for this and issue a usage message it there are more than one arguments.
2. 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 ${<property-name} syntax. Property substitution can be nested; only system properties will be used for substitution.
3. Load any configuration properties specified in the config.properties file; this file is typically located in the conf/ directory of the Felix installation directory, but it can be specified directly using the felix.config.properties system property. This file is used to configure the Felix framework instance created by the launcher. The file is a standard Java properties file, but it also supports property substitution using "${<property-name>}" syntax. Property substitution can be nested; configuration and system properties will be used for substitution with configuration properties having precedence.
4. For convenience, any configuration properties that are set as system properties are copied into the set of configuration properties. This provide an easy way to add to or override configuration properties specified in the config.properties file, since the Felix instance will never look at system properties for configuration.
5. If the -b switch was used to specify an auto-deploy directory, then use that to set the value of felix.auto.deploy.dir.
6. If a single command-line argument is specified, then use that to set the value of org.osgi.framework.storage; relative paths are relative to the current directory unless the felix.cache.rootdir property is set.
7. Add a shutdown hook to cleanly stop the framework, unless the hook is disabled.
8. Create a framework instance using the FrameworkFactory passing in the configuration properties, then initialize the factory instance; see the custom launcher example below to see how the META-INF/services FrameworkFactory is obtained.
9. Use org.apache.felix.main.AutoProcessor, which will automatically deploy any bundles in the auto-deploy directory as well as bundles specified in the 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 and bundle auto-deploy.
10. Invoke waitForStop() to wait for the framework to stop to force the VM to exit; this is necessary because the framework never calls System.exit() and some libraries (e.g., Swing) create threads that will not allow the VM to exit.

...

Custom

...

Framework 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 Gogo 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-23.0.0.jar
   bundle/
      org.apache.felix.shell-1.4.gogo.command-0.6.0.jar
      org.apache.felix.gogo.runtime-0.6.0.jar
      org.apache.felix.gogo.shell.tui-10.46.0.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 not needed, then it would be possible to use the framework JAR file instead of the main 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 the framework 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 framework launcher.

package example;

import java.io.*;
import org.osgi.framework.launch.*;
import org.apache.felix.main.AutoProcessor;

public class Main
{
    private static Framework m_fwk = null;

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

        try
        {
            m_fwk = getFrameworkFactory().newFramework(null);
            m_fwk.init();
            AutoProcessor.process(null, m_fwk.getBundleContext());
            m_fwk.start();
            m_fwk.waitForStop(0);
            System.exit(0);
        }
        catch (Exception ex)
        {
            System.err.println("Could not create framework: " + ex);
            ex.printStackTrace();
            System.exit(-1);
        }
    }

    private static FrameworkFactory getFrameworkFactory() throws Exception
    {
        java.net.URL url = Main.class.getClassLoader().getResource(
            "META-INF/services/org.osgi.framework.launch.FrameworkFactory");
        if (url != null)
        {
            BufferedReader br = new BufferedReader(new InputStreamReader(url.openStream()));
            try
            {
                for (String s = br.readLine(); s != null; s = br.readLine())
                {
                    s = s.trim();
                    // Try to load first non-empty, non-commented line.
                    if ((s.length() > 0) && (s.charAt(0) != '#'))
                    {
                        return (FrameworkFactory) Class.forName(s).newInstance();
                    }
                }
            }
            finally
            {
                if (br != null) br.close();
            }
        }

        throw new Exception("Could not find framework factory.");
    }
}

...

The AutorProcessor will automatically deploy bundles in the auto-deploy directory and any referenced from the auto-install/start properties. Since we are using an empty configuration, the auto-deploy directory is the bundle directory in the current directory and there are no auto properties. Therefore, in this case, the two shell bundles will be installed.

            m_fwk.start();
            m_fwk.waitForStop(0);
            System.exit(0);

These final steps start the framework and cause the launching application thread to wait for the framework to stop and when it does the launching thread calls System.exit() to make sure the VM actually exits.

...

javac -d . -classpath lib/org.apache.felix.main-23.0.0.jar src/example/Main.java

...

java -cp .:lib/org.apache.felix.main-23.0.0.jar example.Main

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

Embedding the Felix Framework

Embedding the Felix framework into a host application is a simple way to provide a sophisticated extensibility mechanism (i.e., a plugin system) to the host application. Embedding the Felix framework is very similar to launching Felix it 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.

...

In the section on launching Felix the framework above, the Felix class accepts a configuration property called felix.systembundle.activators, which is a list of bundle activator instances. These bundle activator instances provide a convenient way for host applications to interact with the Felix framework. The ability offered by these activators can also be accomplished by invoking

...

...

Each activator instance passed into the constructor effectively becomes part of the System Bundlesystem bundle. This means that the start()/stop() methods of each activator instance in the list gets invoked when the System Bundlesystem bundle's activator start()/stop() methods gets invoked, respectively. Each activator instance will be given the System Bundlesystem bundle's BundleContext object so that they can interact with the framework. Consider following snippet of a bundle activator:

...

Given the above bundle activator, it is now possible to embed the Felix framework 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 configuration property map.
        Map configMapconfig = new HashMap();
        // Create host activator;
        m_activator = new HostActivator();
        List list = new ArrayList();
        list.add(m_activator);
        configMap.put(FelixConstants.SYSTEMBUNDLE_ACTIVATORS_PROP, list);

        try
        {
            // Now create an instance of the framework with
            // our configuration properties.
            m_felix = new Felix(configMapconfig);
            // 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.stop();
        m_felix.waitForStop(0);
    }
}

Notice how the HostApplication.getInstalledBundles() method uses its activator instance to get access to the System Bundlesystem 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.

...

package host.service.lookup;

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

...

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.osgi.framework.Constants;

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 configuration property map.
        Map configMap = new HashMap();
        // Export the host provided service interface package.
        configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES_EXTRA,
            "host.service.lookup; version=1.0.0");
        // Create host activator;
        m_activator = new HostActivator(m_lookupMap);
        List list = new ArrayList();
        list.add(m_activator);
        configMap.put(FelixConstants.SYSTEMBUNDLE_ACTIVATORS_PROP, list);

        try
        {
            // Now create an instance of the framework with
            // our configuration properties.
            m_felix = new Felix(configMap);
            // 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.stop();
        m_felix.waitForStop(0);
    }
}

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.

...

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.cache.BundleCache;
import org.osgi.framework.Constants;
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 configuration property map.
        Map configMap = new HashMap();
        // Export the host provided service interface package.
        configMap.put(Constants.FRAMEWORK_SYSTEMPACKAGES_EXTRA,
            "host.service.command; version=1.0.0");
        // Create host activator;
        m_activator = new HostActivator();
        List list = new ArrayList();
        list.add(m_activator);
        configMap.put(FelixConstants.SYSTEMBUNDLE_ACTIVATORS_PROP, list);

        try
        {
            // Now create an instance of the framework with
            // our configuration properties.
            m_felix = new Felix(configMap);
            // 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.stop();
        m_felix.waitForStop(0);
    }
}

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.

...