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| <h1 class="page">Apache Felix Log</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The <a href="https://osgi.org/specification/osgi.cmpn/7.0.0/">OSGi Service Compendium specification</a> defines a general purpose <a href="https://osgi.org/specification/osgi.cmpn/7.0.0/service.log.html">Log Service</a> for the OSGi Platform. |
| It is a very simple specification that doesn’t provide all the functionality commonly available in enterprise-level logging tools, but its extensible service model can be used to build fairly sophisticated logging solutions.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The Log Service specification defines the following entities:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><em>`org.osgi.service.log.Logger`</em> - (<em>since 1.4</em>) interface that allows a bundle to log information, including a message, a level, an exception, and a <code>ServiceReference</code> object. |
| The formatting style using <code>{}</code> placeholders follows the <em>slf4j</em> approach. |
| A derivative of <code>Logger</code> called <code>FormatterLogger</code> uses the <code>java.util.Formatter</code> syntax.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.LoggerFactory`</em> - (<em>since 1.4</em>) service interface that allows a bundle to obtain a Logger. |
| A Logger is named and associated with a <code>Bundle</code> object.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.admin.LoggerContext`</em> - (<em>since 1.4</em>) interface that allows the configuration of effective logging levels for a Bundle. |
| The configuration can be set in Configuration Admin and via method calls.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.admin.LoggerAdmin`</em> - (<em>since 1.4</em>) interface for managing the configuration of log levels.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.LogService`</em> - <em>legacy</em> service interface that allows a bundle to log information, including a message, a level, an exception, a <code>ServiceReference</code> object, and a <code>Bundle</code> object. |
| The methods of this service are deprecated and it is recommended to use LoggerFactory and Loggers instead.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.LogReaderService`</em> - service interface that allows access to a list of recent <code>LogEntry</code> objects, and allows the registration of a <code>LogListener</code> object that receives <code>LogEntry</code> objects as they are created.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.LogEntry`</em> - interface defining a log entry.</p> |
| </li> |
| <li> |
| <p><em>`org.osgi.service.log.LogListener`</em> - interface defining a listener for log entries, which is notified about new log entries.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_accessing_loggers"><a class="anchor" href="#_accessing_loggers"></a>Accessing Loggers</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Loggers are obtained through the <code>LoggerFactory</code> service:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public class Activator implements BundleActivator |
| { |
| private volatile LoggerFactory loggerFactory; |
| |
| public void start(BundleContext context) throws Exception |
| { |
| ServiceReference ref = context.getServiceReference(LoggerFactory.class.getName()); |
| if (ref != null) |
| { |
| loggerFactory = (LoggerFactory) context.getService(ref); |
| } |
| } |
| |
| //..</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Elsewhere in the bundle you can then use the <code>LoggerFactory</code> to get a <code>Logger</code> for any class:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> Logger logger = loggerFactory.getLogger(Foo.class);</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Declarative Services (<em>since 1.4</em>) has a convenient integration which allows a component to obtain a logger specific to it’s class with little effort by using a reference who’s service type is <code>LoggerFactory</code> while the injection type is either <code>Logger</code> or <code>FormatterLogger</code>:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> @Reference(service = LoggerFactory.class) |
| private Logger logger;</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>Logger</code> interface defines 6 levels of logging to coincide with most other log APIs:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>AUDIT</p> |
| </li> |
| <li> |
| <p>ERROR</p> |
| </li> |
| <li> |
| <p>WARN</p> |
| </li> |
| <li> |
| <p>INFO</p> |
| </li> |
| <li> |
| <p>DEBUG</p> |
| </li> |
| <li> |
| <p>TRACE</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Each level has methods on the Logger interface appropriate to that level such as <code>.info(...)</code> and <code>.isInfoEnabled()</code>.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_configuring_log_levels"><a class="anchor" href="#_configuring_log_levels"></a>Configuring Log Levels</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Since 1.4 the <strong>Log Service Specification</strong> provides the ability to manage the log levels both programatically and through <strong>Configuration Admin</strong>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Programatic configuration is achieved through the <code>LoggerAdmin</code> service:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">ServiceReference ref = context.getServiceReference( |
| LoggerAdmin.class.getName()); |
| |
| if (ref != null) |
| { |
| LoggerAdmin loggerAdmin = (LoggerAdmin) context.getService(ref); |
| |
| // get the ROOT logger context |
| LoggerContext rootContext = loggerAdmin.getLoggerContext(null); |
| |
| Map<String, LogLevel> levels = rootContext.getLogLevels(); |
| // adjust the levels |
| rootContext.setLogLevels(levels); |
| |
| // get the levels for a bundle (felix scr in this case) |
| LoggerContext scrContext = loggerAdmin.getLoggerContext( |
| "org.apache.felix.scr"); |
| |
| // set all of scr to DEBUG mode |
| scrContext.setLogLevels( |
| Collections.singletonMap( |
| Logger.ROOT_LOGGER_NAME, LogLevel.DEBUG)); |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Likewise logging configuration can be handled through <strong>Configuration Admin</strong>. |
| Following the previous example of configuring <strong>Felix SCR</strong> for <code>DEBUG</code> mode:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>create a configuration object whose PID is <em>`org.osgi.service.log.admin|org.apache.felix.scr`</em></p> |
| </li> |
| <li> |
| <p>set the property <code>ROOT</code> in the configuration to <code>DEBUG</code> e.g. |
| <code>ROOT=DEBUG</code></p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_accessing_the_log_service_legacy"><a class="anchor" href="#_accessing_the_log_service_legacy"></a>Accessing the log service (legacy)</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>To access a <code>LogService</code> instance it is necessary to look it up in the OSGi service registry as demonstrated in the following code snippet:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public class Activator implements BundleActivator |
| { |
| public void start(BundleContext context) throws Exception |
| { |
| ServiceReference ref = context.getServiceReference(LogService.class.getName()); |
| if (ref != null) |
| { |
| LogService log = (LogService) context.getService(ref); |
| |
| // Use the log... |
| } |
| } |
| |
| //..</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>It is possible, and advisable, to use more sophisticated service acquisition mechanisms like a Service Tracker, Declarative Services or iPOJO.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_using_the_log_service_legacy"><a class="anchor" href="#_using_the_log_service_legacy"></a>Using the log service (legacy)</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The <code>LogService</code> interface provides four methods for logging:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public interface LogService |
| { |
| //.. |
| |
| // Log a message specifying a log level |
| public log(int level, java.lang.String message) |
| |
| // Log an exception |
| public log(int level, java.lang.String message, java.lang.Throwable exception) |
| |
| // Log a message specifying the ServiceReference that generated it |
| public log(ServiceReference sr, int level, java.lang.String message) |
| |
| // Log a message specifying the ServiceReference and exception |
| public log(ServiceReference sr, int level, java.lang.String message, java.lang.Throwable exception) |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Log levels are defined in the same interface:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>LogService.LOG_DEBUG</code></p> |
| </li> |
| <li> |
| <p><code>LogService.LOG_INFO</code></p> |
| </li> |
| <li> |
| <p><code>LogService.LOG_WARNING</code></p> |
| </li> |
| <li> |
| <p><code>LogService.LOG_ERROR</code></p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_retrieving_log_entries"><a class="anchor" href="#_retrieving_log_entries"></a>Retrieving log entries</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The <code>LogReaderService</code> provides a <code>getLog()</code> method to retrieve an <code>Enumeration</code> of the latest log entries. |
| The following code snippets demonstrates how to retrieve it from the service registry and use it:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> ServiceReference ref = context.getServiceReference(LogReaderService.class.getName()); |
| if (ref != null) |
| { |
| LogReaderService reader = (LogReaderService) context.getService(ref); |
| Enumeration<LogEntry> latestLogs = reader.getLog(); |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_creating_and_registering_a_loglistener"><a class="anchor" href="#_creating_and_registering_a_loglistener"></a>Creating and registering a <code>LogListener</code></h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Log Service specification doesn’t define any particular entity to store, display, or write log entries; |
| it’s up to the developer to implement this functionality or to choose an available implementation capable of doing that. |
| To create such a bundle, the first step is to create an implementation of the <code>LogListener</code> interface. |
| The following code shows a simple implementation that echoes the log message:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> public class LogWriter implements LogListener |
| { |
| // Invoked by the log service implementation for each log entry |
| public void logged(LogEntry entry) |
| { |
| System.out.println(entry.getMessage()); |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The only method to implement is <code>logged()</code> method, which is called every time a log entry is created in the associated logging service. |
| A <code>LogListener</code> implementation must be registered with the <code>LogReaderService</code> so it can start receiving log entries, as demonstrated in the following code snippet:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> ServiceReference ref = context.getServiceReference(LogReaderService.class.getName()); |
| if (ref != null) |
| { |
| LogReaderService reader = (LogReaderService) context.getService(ref); |
| reader.addLogListener(new LogWriter()); |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_setup_of_apache_felix_log_service"><a class="anchor" href="#_setup_of_apache_felix_log_service"></a>Setup of Apache Felix Log Service</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Apache Felix Log Service bundle doesn’t have any specific dependency on Felix, so it can run on any OSGi container. |
| For its configuration, it will use the following optional system properties:</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3333%;"> |
| <col style="width: 33.3334%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Property</th> |
| <th class="tableblock halign-left valign-top">Default</th> |
| <th class="tableblock halign-left valign-top">Description</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>org.apache.felix.log.maxSize</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">100</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">The maximum size of the log history. |
| A value of -1 means the log has no maximum size; |
| a value of 0 means that no historical information is maintained</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>org.apache.felix.log.storeDebug</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">false</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Determines whether or not debug messages will be stored in the history</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>org.osgi.service.log.admin.loglevel</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock"><code>WARN</code></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">The default log level of the root Logger Context</p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| </article> |
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