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| <h1 class="page">Apache Felix Tutorial Example 3</h1> |
| <div class="paragraph"> |
| <p>Example 3 - Dictionary Client Bundle</p> |
| </div> |
| <div class="paragraph"> |
| <p>This example creates a bundle that is a client of the dictionary service implemented in Example 2. |
| In the following source code, our bundle uses its bundle context to query for a dictionary service. |
| Our client bundle uses the first dictionary service it finds and if none are found it simply prints a message saying so and stops. |
| Using an OSGi service is the same as using any Java interface, we simply need to cast it to the known dictionary service interface. |
| The source code for our bundle is as follows in a file called <code>Activator.java</code>:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>/* |
| * Apache Felix OSGi tutorial. |
| **/ |
| |
| package tutorial.example3; |
| |
| import java.io.BufferedReader; |
| import java.io.InputStreamReader; |
| import java.io.IOException; |
| |
| import org.osgi.framework.BundleActivator; |
| import org.osgi.framework.BundleContext; |
| import org.osgi.framework.ServiceReference; |
| |
| import tutorial.example2.service.DictionaryService; |
| |
| /** |
| * This class implements a bundle that uses a dictionary |
| * service to check for the proper spelling of a word by |
| * check for its existence in the dictionary. This bundle |
| * uses the first service that it finds and does not monitor |
| * the dynamic availability of the service (i.e., it does not |
| * listen for the arrival or departure of dictionary services). |
| * When starting this bundle, the thread calling the start() |
| * method is used to read words from standard input. You can |
| * stop checking words by entering an empty line, but to start |
| * checking words again you must stop and then restart the bundle. |
| **/ |
| public class Activator implements BundleActivator |
| { |
| /** |
| * Implements BundleActivator.start(). Queries for |
| * all available dictionary services. If none are found it |
| * simply prints a message and returns, otherwise it reads |
| * words from standard input and checks for their existence |
| * from the first dictionary that it finds. |
| * (NOTE: It is very bad practice to use the calling thread |
| * to perform a lengthy process like this; this is only done |
| * for the purpose of the tutorial.) |
| * @param context the framework context for the bundle. |
| **/ |
| public void start(BundleContext context) throws Exception |
| { |
| // Query for all service references matching any language. |
| ServiceReference[] refs = context.getServiceReferences( |
| DictionaryService.class.getName(), "(Language=*)"); |
| |
| if (refs != null) |
| { |
| try |
| { |
| System.out.println("Enter a blank line to exit."); |
| BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); |
| String word = ""; |
| |
| // Loop endlessly. |
| while (true) |
| { |
| // Ask the user to enter a word. |
| System.out.print("Enter word: "); |
| word = in.readLine(); |
| |
| // If the user entered a blank line, then |
| // exit the loop. |
| if (word.length() == 0) |
| { |
| break; |
| } |
| |
| // First, get a dictionary service and then check |
| // if the word is correct. |
| DictionaryService dictionary = |
| (DictionaryService) context.getService(refs[0]); |
| if (dictionary.checkWord(word)) |
| { |
| System.out.println("Correct."); |
| } |
| else |
| { |
| System.out.println("Incorrect."); |
| } |
| |
| // Unget the dictionary service. |
| context.ungetService(refs[0]); |
| } |
| } catch (IOException ex) { } |
| } |
| else |
| { |
| System.out.println("Couldn't find any dictionary service..."); |
| } |
| } |
| |
| /** |
| * Implements BundleActivator.stop(). Does nothing since |
| * the framework will automatically unget any used services. |
| * @param context the framework context for the bundle. |
| **/ |
| public void stop(BundleContext context) |
| { |
| // NOTE: The service is automatically released. |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Note that we do not need to unget or release the service in the <code>stop()</code> method, because the OSGi framework will automatically do so for us. |
| We must create a <code>manifest.mf</code> file that contains the meta-data for our bundle; |
| the manifest file contains the following:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Bundle-Name: Dictionary client |
| Bundle-Description: A bundle that uses the dictionary service if it finds it at startup |
| Bundle-Vendor: Apache Felix |
| Bundle-Version: 1.0.0 |
| Bundle-Activator: tutorial.example3.Activator |
| Import-Package: org.osgi.framework, |
| tutorial.example2.service</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>We specify the bundle’s activator class via the <code>Bundle-Activator</code> attribute and also specify that our class imports the OSGi core framework package and the dictionary service interface package via the <code>Import-Package</code> attribute. |
| The dictionary service interface package will be fulfilled by the bundle in Example 2, which exports this package. |
| The OSGi framework will automatically handle the details of resolving import packages. |
| (Note: Make sure your manifest file ends in a trailing carriage return or else the last line will be ignored.)</p> |
| </div> |
| <div class="paragraph"> |
| <p>To compile our source code, we need to have the <code>felix.jar</code> file (found in Felix' <code>bin</code> directory) and the <code>example2.jar</code> file in our class path. |
| We compile the source file using a command like:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>javac -d c:\classes *.java</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This command compiles all source files and outputs the generated classes into a subdirectory of the <code>c:\classes</code> directory; |
| this subdirectory is <code>tutorial\example3</code>, named after the package we specified in the source file. |
| For the above command to work, the <code>c:\classes</code> directory must exist. |
| After compiling, we need to create a JAR file containing the generated package directories. |
| We will also add our manifest file that contains the bundle’s meta-data to the JAR file. |
| To create the JAR file, we issue the command:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>jar cfm example3.jar manifest.mf -C c:\classes tutorial\example3</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>This command creates a JAR file using the manifest file we created and includes all of the classes in the <code>tutorial\example3</code> directory inside of the <code>c:\classes</code> directory. |
| Once the JAR file is created, we are ready to install and start the bundle.</p> |
| </div> |
| <div class="paragraph"> |
| <p>To run Felix, we follow the instructions described in usage.html. |
| When we start Felix, it asks for a profile name, we will put all of our bundles in a profile named <code>tutorial</code>. |
| After running Felix, we should check that all tutorial bundles are stopped, except for the English dictionary service bundle from Example 2. |
| We can use the Felix <code>lb</code> shell command to get a list of all bundles, their state, and their bundle identifier number. |
| If the Example 2 bundle is not active, we should start the bundle using the <code>start</code> command along with the bundle’s identifier number displayed by the <code>lb</code> command and <code>stop</code> any other unneeded tutorial bundles using the <code>stop</code> command. |
| (Note: Felix uses some bundles to provide its command shell, so do not stop these bundles.) Now we can install and start our dictionary client bundle. |
| Assuming that we created our bundle in the directory <code>c:\tutorial</code>, we can install and start it in Felix' shell using the following command:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>start file:/c:/tutorial/example3.jar</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The above command installs and starts the bundle in a single step; |
| it is also possible to install and start the bundle in two steps by using the Felix <code>install</code> and start shell commands. |
| When we start the bundle, it will use the shell thread to prompt us for words. |
| Enter one word at a time to check the words and enter a blank line to stop checking words. |
| To restart the bundle, we must use the Felix shell <code>lb</code> command to get the bundle identifier number for the bundle and first use the <code>stop</code> command to stop the bundle, then the <code>start</code> command to restart it. |
| To test the dictionary service, enter any of the words in the dictionary (e.g., "welcome", "to", "the", "OSGi", "tutorial") or any word not in the dictionary.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This example client is simple enough and, in fact, is too simple. |
| What would happen if the dictionary service were to unregister suddenly? |
| Our client would throw a null pointer exception when it tried to access the service object. |
| This dynamic service availability issue is a central tenent of the OSGi service model. |
| As a result, we must make our client more robust in dealing with such situations. |
| In Example 4, we explore a slightly more complicated dictionary client that dynamically monitors service availability.</p> |
| </div> |
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