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| <article class="doc"> |
| <h1 class="page">Apache Felix Event Admin</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Event Admin Service Specification, part of the OSGi Compendium specification, defines a general inter-bundle communication mechanism. |
| The communication conforms to the popular publish/subscribe paradigm and can be performed in a synchronous or asysnchronous manner.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The main components in a publish/subscribe communication are:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><em>Event Publisher</em> - sends events or messages related to a specific <em>topic</em></p> |
| </li> |
| <li> |
| <p><em>Event Handler</em> (or <em>Subscriber</em>) - expresses interest in one or more topics and receives all the messages belonging to such topics.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Events are composed of two attributes:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>a <em>topic</em> defining the nature of the event; |
| topic names are usually arranged in a hierarchical namespace, where slashes are used to separate the levels (i.e. |
| "org/osgi/framework/FrameworkEvent/STARTED") and</p> |
| </li> |
| <li> |
| <p>a set of <em>properties</em> describing the event.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_creating_an_event_publisher"><a class="anchor" href="#_creating_an_event_publisher"></a>Creating an Event Publisher</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>An event publisher is a simple Java class that creates events and sends them using the <code>EventAdmin</code> service interface, for example:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>public void reportGenerated(Report report, BundleContext context) |
| { |
| ServiceReference ref = context.getServiceReference(EventAdmin.class.getName()); |
| if (ref != null) |
| { |
| EventAdmin eventAdmin = (EventAdmin) context.getService(ref);</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Dictionary properties = new Hashtable(); |
| properties.put("title", report.getTitle()); |
| properties.put("path" , report.getAbsolutePath()); |
| properties.put("time", System.currentTimeMillis());</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Event reportGeneratedEvent = new Event("com/acme/reportgenerator/GENERATED", properties);</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre> eventAdmin.sendEvent(reportGeneratedEvent); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>EventAdmin.sendEvent()</code> method sends the <code>Event</code> object synchronously. |
| To send it asynchronously, use the <code>EventAdmin.postEvent()</code> method, such as:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>//.. |
| Event reportGeneratedEvent = new Event("com/acme/reportgenerator/GENERATED", properties); |
| eventAdmin.postEvent(reportGeneratedEvent);</pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_creating_an_event_handler"><a class="anchor" href="#_creating_an_event_handler"></a>Creating an Event Handler</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Creating an event handler is as simple as implementing the <code>EventHandler</code> interface:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>public class ReportEventHandler implements EventHandler |
| { |
| public void handleEvent(Event event) |
| { |
| String reportTitle = (String) event.getProperty("title"); |
| String reportPath = (String) event.getProperty("path");</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre> sendReportByEmail(reportTitle, reportPath); |
| }</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>//..</pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_registering_an_event_handler"><a class="anchor" href="#_registering_an_event_handler"></a>Registering an Event Handler</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>To receive event notifications, the event handler must be registered as a OSGi service under the object class <code>org.osgi.service.event.EventHandler</code>. |
| When registering the service, a <code>String\[\]({{ refs..path }})</code> property named <code><strong>EVENT_TOPIC</strong></code> must be specified; |
| this property describes the list of topics in which the event handler is interested. |
| For example:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>String[] topics = new String[] { |
| "com/acme/reportgenerator/GENERATED" |
| };</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Dictionary props = new Hashtable(); |
| props.put(EventConstants.EVENT_TOPIC, topics); |
| bundleContext.registerService(EventHandler.class.getName(), new ReportEventHandler() , props);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>It is possible to use '*' as a wildcard in the final character of the EVENT_TOPIC, like in this example:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>String[] topics = new String[] { |
| "com/acme/reportgenerator/*" |
| };</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Dictionary props = new Hashtable(); |
| props.put(EventConstants.EVENT_TOPIC, topics); |
| bundleContext.registerService(EventHandler.class.getName(), new ReportEventHandler() , props);</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Finally, it is possible to specify an additional <code>EVENT_FILTER</code> property to filter event notifications; |
| the filter expression follows, the normal LDAP syntax:</p> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>String[] topics = new String[] { |
| "com/acme/reportgenerator/GENERATED" |
| };</pre> |
| </div> |
| </div> |
| <div class="literalblock"> |
| <div class="content"> |
| <pre>Dictionary props = new Hashtable(); |
| props.put(EventConstants.EVENT_TOPIC, topics); |
| props.put(EventConstants.EVENT_FILTER, "(title=samplereport)"); |
| bundleContext.registerService(EventHandler.class.getName(), new ReportEventHandler() , props);</pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_osgi_events"><a class="anchor" href="#_osgi_events"></a>OSGi Events</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Every OSGi framework sends (asynchronously) a number of events that can be captured by any bundle.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_framework_events"><a class="anchor" href="#_framework_events"></a>Framework Events</h3> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>org/osgi/framework/BundleEvent/STARTED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/ERROR</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/PACKAGES_REFRESHED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/STARTLEVEL_CHANGED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/WARNING</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/INFO</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_bundle_events"><a class="anchor" href="#_bundle_events"></a>Bundle Events</h3> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>org/osgi/framework/BundleEvent/INSTALLED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/STARTED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/STOPPED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/UPDATED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/UNINSTALLED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/RESOLVED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/BundleEvent/UNRESOLVED</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>The followig properties are always present in a bundle event object:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><em>bundle.id</em></p> |
| </li> |
| <li> |
| <p><em>bundle.symbolicName</em></p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_service_events"><a class="anchor" href="#_service_events"></a>Service Events</h3> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>org/osgi/framework/ServiceEvent/REGISTERED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/ServiceEvent/MODIFIED</p> |
| </li> |
| <li> |
| <p>org/osgi/framework/ServiceEvent/UNREGISTERING</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>The following properties are always present in a service event object:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><em>service</em></p> |
| </li> |
| <li> |
| <p><em>service.id</em></p> |
| </li> |
| <li> |
| <p><em>service.pid</em></p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Each of the aforementioned events actually contains a wider set of properties. |
| Check the OSGi Compendium specification, section 113.6, for a complete list.</p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_configuration"><a class="anchor" href="#_configuration"></a>Configuration</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Apache Felix Event Admin implementation is trying the deliver the events as fast as possible. |
| Events sent from different threads are sent in parallel. |
| Events from the same thread are sent in the order they are received (this is according to the spec). |
| A timeout can be configured which is used for event handlers. |
| If an event handler takes longer than the configured timeout to process an event, it is denied. |
| Once a handler is in a denylist, it doesn’t get sent any events anymore. |
| The Felix Event Admin can be configured either through framework properties or through the configuration admin using PID <code>org.apache.felix.eventadmin.impl.EventAdmin</code>. |
| This is a list of configuration properties:</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Thread Pool Size</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.ThreadPoolSize + <em>Default</em>: 20 + <em>Type</em>: Integer</p> |
| </div> |
| <div class="paragraph"> |
| <p>The size of the thread pool used for event delivery. |
| The default value is 20. |
| Increase in case of a large amount of events. |
| A value of less then 2 triggers the default value. |
| If the pool is exhausted, event delivery is blocked until a thread becomes available from the pool. |
| Each event is delivered in a thread from the pool unless the ignore timeouts is configured for the receiving event handler.</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Async/sync Thread Pool Ratio</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.AsyncToSyncThreadRatio + <em>Since</em>: Release 1.4.2 + <em>Default</em>: 0.5 + <em>Type</em>: Double</p> |
| </div> |
| <div class="paragraph"> |
| <p>The ratio of asynchronous to synchronous threads in the internal thread pool. |
| Ratio must be positive and may be adjusted to represent the distribution of post to send operations. |
| Applications with higher number of post operations should have a higher ratio.</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Timeout</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.Timeout + <em>Default</em>: 5000 + <em>Type</em>: Integer</p> |
| </div> |
| <div class="paragraph"> |
| <p>The deny-listing timeout in milliseconds. |
| The default value is 5000. |
| Increase or decrease at own discretion. |
| A value of less then 100 turns timeouts off. |
| Any other value is the time in milliseconds granted to each event handler before it gets denied.</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Require Topic</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.RequireTopic + <em>Default</em>: true + <em>Type</em>: Boolean</p> |
| </div> |
| <div class="paragraph"> |
| <p>Are event handlers required to be registered with a topic? |
| This is enabled by default. |
| The specification says that event handlers must register with a list of topics they are interested in. |
| Disabling this setting will enable that handlers without a topic are receiving all events (i.e., they are treated the same as with a topic=*).</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Ignore Timeouts</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.IgnoreTimeout + <em>Default</em>: empty + <em>Type</em>: String</p> |
| </div> |
| <div class="paragraph"> |
| <p>Configure event handlers to be called without a timeout. |
| If a timeout is configured by default all event handlers are called using the timeout. |
| For performance optimization it is possible to configure event handlers where the timeout handling is not used - this reduces the thread usage from the thread pools as the timout handling requires an additional thread to call the event handler. |
| However, the application should work without this configuration property. |
| It is a pure optimization! |
| The value is a list of strings. |
| If a string ends with a dot, all handlers in exactly this package are ignored. |
| If the string ends with a star, all handlers in this package and all subpackages are ignored. |
| If the string neither ends with a dot nor with a start, this is assumed to define an exact class name.</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Ignore Topics</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.IgnoreTopic + <em>Since</em>: Release 1.4.0 + <em>Default</em>: empty + <em>Type</em>: String</p> |
| </div> |
| <div class="paragraph"> |
| <p>For performance optimization it is possible to configure topics which are ignored by the event admin implementation. |
| In this case, a event is not delivered to registered event handlers. |
| The value is a list of strings (separated by comma). |
| " +If a single value ends with a dot, all topics in exactly this package are ignored. |
| If a single value ends with a star, all topics in this package and all sub packages are ignored. |
| If a single value neither ends with a dot nor with a start, this is assumed to define an exact topic. |
| A single star can be used to disable delivery completely.</p> |
| </div> |
| <hr> |
| <div class="paragraph"> |
| <p><strong>Log Level</strong></p> |
| </div> |
| <div class="paragraph"> |
| <p><em>Property</em>: org.apache.felix.eventadmin.LogLevel + <em>Note</em>: This property can’t be set through the OSGi configuration. |
| + <em>Default</em>: 2 (=WARNING) + <em>Type</em>: Integer</p> |
| </div> |
| <div class="paragraph"> |
| <p>This sets the log level used for messages outputted by the event admin implementation. |
| Valid values are 1 (=ERROR), 2 (=WARNING), 3 (=INFO), and 4 (=DEBUG). |
| The default is 2 and an invalid value sets the level to the default value.</p> |
| </div> |
| <hr> |
| </div> |
| </div> |
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