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| <h1>API Separation</h1> |
| <p>When selecting a logging library, some care must be taken in order to ensure |
| that multiple different logging libraries are properly accounted for. For |
| example, library code that you depend on may use slf4j, while other libraries |
| may simply use java.util.logging. All of these can be routed to the log4j |
| core in order to be logged.</p> |
| <p>If however you want to use a different logging implementation (such as logback), |
| it is possible to route messages from the Log4j API to logback, ensuring that |
| your application is not tied to a specific logging framework.</p> |
| <p>A typical class using the Log4j2 API looks like the following:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-java">import org.apache.logging.log4j.LogManager; |
| import org.apache.logging.log4j.Logger; |
| |
| public class Log4j2Test { |
| private static final Logger logger = LogManager.getLogger(); |
| |
| public Log4j2Test(){ |
| logger.info( "Hello World!" ); |
| } |
| } |
| </code></pre></div> |
| <p>In order to use the API portion of Log4j2, we only need to provide a single |
| dependency, log4j-api. Using Maven, you would add the following to your |
| dependencies:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-xml"><dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-api</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| </code></pre></div><section> |
| <h2><a name="Using_Log4j2_API_and_Core"></a>Using Log4j2 API and Core</h2> |
| <p>Using the Log4j2 API and the implementation (Core) together means that log messages will be routed |
| through the Log4j2 Core. The Log4j2 core implementation is responsible for the |
| following (note: this is not an exhaustive list):</p> |
| <ul> |
| |
| <li>Configuration of the system (via an XML file for example)</li> |
| <li>Routing messages to Appenders</li> |
| <li>Opening files and other resources for logging (e.g. network sockets)</li> |
| </ul> |
| <p>The <a href="configuration.html">configuration</a> page in the manual describes |
| the configuration format supported by the Log4j2 core implementation.</p> |
| <p>To use both the API and the core implementation, you would add the following to your |
| dependencies (assuming that you are using Maven):</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-xml"><dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-api</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| <dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-core</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| </code></pre></div> |
| <p>Note that having two different versions of log4j-api and log4j-core on your |
| classpath is not guaranteed to work correctly (e.g., log4j-api version 2.15 and |
| log4j-core version 2.17 are not guaranteed to work correctly together).</p></section><section> |
| <h2><a name="Using_Log4j2_API_with_Logback"></a>Using Log4j2 API with Logback</h2> |
| <p>Since the Log4j2 API is generic, we can use it to send messages via SLF4J |
| and then have Logback do the actual logging of the messages. This means |
| that you can write your code tied to the Log4j2 API, but users of your |
| code do not need to use the Log4j2 core if they are already using Logback.</p> |
| <p>To switch to using Logback as your logging backend, you will need to add the following to your |
| dependencies (assuming that you are using Maven):</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-xml"><dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-api</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| <dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-to-slf4j</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| <dependency> |
| <groupId>ch.qos.logback</groupId> |
| <artifactId>logback-classic</artifactId> |
| <version>1.2.10</version> |
| </dependency> |
| </code></pre></div></section><section> |
| <h2><a name="Using_Log4j2_as_an_SLF4J_Implementation"></a>Using Log4j2 as an SLF4J Implementation</h2> |
| <p>If you don't want to depend on the Log4j2 API and instead want to use SLF4J, |
| that is possible as well. Assuming that our code looks like the following:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-java">import org.slf4j.Logger; |
| import org.slf4j.LoggerFactory; |
| |
| public class Log4j2Test { |
| |
| private static final Logger logger = LoggerFactory.getLogger(Log4j2Test.class); |
| |
| public Log4j2Test(){ |
| logger.info( "Hello World!" ); |
| } |
| } |
| </code></pre></div> |
| <p>We can then route the messages to Log4j2 using the log4j-slf4j-impl like the following:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-xml"><dependency> |
| <groupId>org.slf4j</groupId> |
| <artifactId>slf4j-api</artifactId> |
| <version>1.7.32</version> |
| </dependency> |
| <dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-slf4j-impl</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| <dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-core</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| </code></pre></div> |
| <p>Note that if we were using SLF4J 1.8 instead of 1.7, that requires us to use |
| log4j-slf4j18-impl instead of log4j-slf4j-impl.</p></section><section> |
| <h2><a name="Using_Log4j2_with_JUL"></a>Using Log4j2 with JUL</h2> |
| <p>It is also possible to route messages that are logged using java.util.logging |
| to Log4j2. Assuming that the code looks like the following:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-java">import java.util.logging.Logger; |
| |
| public class Log4j2Test { |
| |
| private static final Logger logger = Logger.getLogger(Log4j2Test.class.getName()); |
| |
| public Log4j2Test() { |
| logger.info("Hello World!"); |
| } |
| } |
| </code></pre></div> |
| <p>We can then also route these messages to the Log4j2 core by adding in the JUL bridge, |
| and setting <code>-Djava.util.logging.manager=org.apache.logging.log4j.jul.LogManager</code> on the JVM (see the documentation on |
| <a href="../log4j-jul/index.html">the JUL adapter</a> for |
| more information as to how this works).</p> |
| <p>In order to route these messages to Log4j2, your dependencies would look like the following:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-xml"><dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-jul</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| <dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-core</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| </code></pre></div></section><section> |
| <h2><a name="Using_Log4j2_as_a_backend_for_Log4j1"></a>Using Log4j2 as a backend for Log4j1</h2> |
| <p>Some software may still depend on Log4j1, and in some cases it may be infeasible |
| to modify this software to migrate it to Log4j2.</p> |
| <p>However, it may be possible to start using Log4j2 without modifying the application.</p> |
| <p>Assuming that our code looks like the following:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-java">import org.apache.log4j.Logger; |
| |
| public class Log4j2Test { |
| |
| private static final Logger logger = Logger.getLogger(Log4j2Test.class); |
| |
| public Log4j2Test(){ |
| logger.info( "Hello World!" ); |
| } |
| } |
| </code></pre></div> |
| <p>we can then quickly and easily configure these messages to use Log4j2 |
| as the logging implementation by depending on the <code>log4j-1.2-api</code> bridge, like so:</p> |
| |
| <div class="source"><pre class="prettyprint"><code class="language-xml"><dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-1.2-api</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| <dependency> |
| <groupId>org.apache.logging.log4j</groupId> |
| <artifactId>log4j-core</artifactId> |
| <version>2.17.0</version> |
| </dependency> |
| </code></pre></div> |
| <p>There are some limitations to this, but it is expected to work for the majority of common cases. |
| See the <a href="migration.html">manual page on migration</a> for more information on this feature.</p> |
| <h1>Conclusion</h1> |
| <p>With the API separation that Log4j2 provides, it is possible to use multiple |
| logging APIs and implementations in the same project. This allows for greater |
| flexibility, ensuring that you are not tied to a single API or implementation.</p></section> |
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