[feat] (re-invent) log4j as a logging backend

README.md

@@ -288,12 +288,12 @@ logging system, configure `jruby.rack.logging` as follows:
 - `servlet_context` (default): Sends log messages to the servlet context.
 - `stdout`: Sends log messages to the standard output stream `System.out`.
 - `slf4j`: Sends log messages to SLF4J. SLF4J configuration is left up to you,
-  please refer to http://www.slf4j.org/docs.html .
-- `log4j`: Sends log messages to log4J. Again, Log4J configuration is
-  left up to you, consult http://logging.apache.org/log4j/ .
+  please refer to https://www.slf4j.org/manual.html .
+- `log4j`: Sends log messages through Log4j. Only Log4j 2.x is supported, for 
+- configuration please consult https://logging.apache.org/log4j/2.x/index.html .
 - `commons_logging`: Routes logs to commons-logging. You still need to configure
-  an underlying logging implementation with JCL. We recommend using the logger
-  library wrapper directly if possible, see http://commons.apache.org/logging/ .
+  an underlying logging implementation with JCL. 
+  We recommend rather using the logger library wrapper directly when possible.
 - `jul`: Directs log messages via Java's core logging facilities (util.logging).
 
 For those loggers that require a specific named logger, set it with the

pom.xml

@@ -23,6 +23,7 @@
     <jruby.maven.plugins.version>3.0.6</jruby.maven.plugins.version>
     <gem.home>${project.build.directory}/rubygems</gem.home>
     <slf4j.version>2.0.17</slf4j.version>
+    <log4j.version>2.22.1</log4j.version>
     <spring.version>5.3.39</spring.version>
   </properties>
 
@@ -128,6 +129,18 @@
       <version>${slf4j.version}</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api</artifactId>
+      <version>${log4j.version}</version>
+      <scope>provided</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-core</artifactId>
+      <version>${log4j.version}</version>
+      <scope>test</scope>
+    </dependency>
     <dependency>
       <groupId>org.springframework</groupId>
       <artifactId>spring-web</artifactId>

src/main/java/org/jruby/rack/DefaultRackConfig.java

@@ -414,6 +414,7 @@ private static Map<String,String> getLoggerTypes() {
         final Map<String,String> loggerTypes = new HashMap<>(8);
         loggerTypes.put("commons_logging", "org.jruby.rack.logging.CommonsLoggingLogger");
         loggerTypes.put("clogging", "org.jruby.rack.logging.CommonsLoggingLogger");
+        loggerTypes.put("log4j", "org.jruby.rack.logging.Log4jLogger");
         loggerTypes.put("slf4j", "org.jruby.rack.logging.Slf4jLogger");
         loggerTypes.put("jul", "org.jruby.rack.logging.JulLogger");
         return loggerTypes;

src/main/java/org/jruby/rack/logging/Log4jLogger.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2025 Karol Bucek LTD.
+ * This source code is available under the MIT license.
+ * See the file LICENSE.txt for details.
+ */
+package org.jruby.rack.logging;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.jruby.rack.RackLogger;
+
+public class Log4jLogger extends RackLogger.Base {
+
+    private Logger logger;
+
+    public Log4jLogger() {
+        this(LogManager.ROOT_LOGGER_NAME);
+    }
+
+    public Log4jLogger(String loggerName) {
+        setLoggerName(loggerName);
+    }
+
+    public Logger getLogger() {
+        return logger;
+    }
+
+    public void setLogger(Logger logger) {
+        this.logger = logger;
+    }
+
+    public void setLoggerName(String loggerName) {
+        logger = LogManager.getLogger(loggerName);
+    }
+
+    @Override
+    public boolean isEnabled(Level level) {
+        if ( level == null ) return logger.isInfoEnabled();
+        switch ( level ) {
+            case DEBUG: return logger.isDebugEnabled();
+            case INFO:  return logger.isInfoEnabled();
+            case WARN:  return logger.isWarnEnabled();
+            case ERROR: return logger.isErrorEnabled();
+            case FATAL: return logger.isFatalEnabled();
+        }
+        return logger.isTraceEnabled();
+    }
+
+    @Override
+    public void log(Level level, CharSequence message) {
+        if ( level == null ) { logger.info(message); return; }
+        switch ( level ) {
+            case DEBUG: logger.debug(message); break;
+            case INFO:  logger.info(message);  break;
+            case WARN:  logger.warn(message);  break;
+            case ERROR: logger.error(message); break;
+            case FATAL: logger.fatal(message); break;
+        }
+    }
+
+    @Override
+    public void log(Level level, CharSequence message, Throwable ex) {
+        if ( level == null ) { logger.error(message, ex); return; }
+        switch ( level ) {
+            case DEBUG: logger.debug(message, ex); break;
+            case INFO:  logger.info(message, ex);  break;
+            case WARN:  logger.warn(message, ex);  break;
+            case ERROR: logger.error(message, ex); break;
+            case FATAL: logger.fatal(message, ex); break;
+        }
+    }
+
+    @Override
+    public Level getLevel() {
+        if ( logger.isDebugEnabled() ) return Level.DEBUG;
+        if ( logger.isInfoEnabled() )  return Level.INFO ;
+        if ( logger.isWarnEnabled() )  return Level.WARN ;
+        if ( logger.isErrorEnabled() ) return Level.ERROR;
+        if ( logger.isFatalEnabled() ) return Level.FATAL;
+        return null;
+    }
+
+}

src/spec/ruby/rack/config_spec.rb

@@ -41,6 +41,11 @@
       logger.should be_a(org.jruby.rack.logging.Slf4jLogger)
     end
 
+    it "constructs a log4j logger from the context init param" do
+      @servlet_context.should_receive(:getInitParameter).with("jruby.rack.logging").and_return "log4j"
+      logger.should be_a(org.jruby.rack.logging.Log4jLogger)
+    end
+
     it "constructs a commons logging logger from system properties" do
       java.lang.System.setProperty("jruby.rack.logging", "commons_logging")
       logger.should be_a(org.jruby.rack.logging.CommonsLoggingLogger)
@@ -50,13 +55,19 @@
       @servlet_context.should_receive(:getInitParameter).with("jruby.rack.logging.name").and_return "/myapp"
       @servlet_context.should_receive(:getInitParameter).with("jruby.rack.logging").and_return "JUL"
       logger.should be_a(org.jruby.rack.logging.JulLogger)
-      logger.logger.name.should == '/myapp'
+      logger.getLogger.name.should == '/myapp'
     end
 
     it "constructs a slf4j logger with default logger name" do
       java.lang.System.setProperty("jruby.rack.logging", "slf4j")
       logger.should be_a(org.jruby.rack.logging.Slf4jLogger)
-      logger.logger.name.should == 'jruby.rack'
+      logger.getLogger.name.should == 'jruby.rack'
+    end
+
+    it "constructs a log4j logger with default logger name" do
+      java.lang.System.setProperty("jruby.rack.logging", "log4j")
+      logger.should be_a(org.jruby.rack.logging.Log4jLogger)
+      logger.getLogger.name.should == 'jruby.rack'
     end
 
     it "constructs a logger from the context init params over system properties" do