Support log4j 2.x.

This is an RC dependency. We can update that when log4j 2.0 is release, but that shouldn't be
necessary.

pom.xml

@@ -55,6 +55,13 @@
       <scope>compile</scope>
       <optional>true</optional>
     </dependency>
+    <dependency>
+      <groupId>org.apache.logging.log4j</groupId>
+      <artifactId>log4j-api</artifactId>
+      <version>2.0-rc1</version>
+      <scope>compile</scope>
+      <optional>true</optional>
+    </dependency>
     <dependency>
       <groupId>javax.mail</groupId>
       <artifactId>mail</artifactId>

src/main/java/com/samskivert/util/Log4J2Logger.java

@@ -0,0 +1,56 @@
+//
+// samskivert library - useful routines for java programs
+// Copyright (C) 2001-2012 Michael Bayne, et al.
+// http://github.com/samskivert/samskivert/blob/master/COPYING
+
+package com.samskivert.util;
+
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.LogManager;
+
+/**
+ * A factory for creating log4j2 logger implementations.
+ */
+public class Log4J2Logger implements Logger.Factory
+{
+    // from interface Logger.Factory
+    public void init ()
+    {
+    }
+
+    // from interface Logger.Factory
+    public Logger getLogger (String name)
+    {
+        return new Impl(LogManager.getLogger(name));
+    }
+
+    // from interface Logger.Factory
+    public Logger getLogger (Class<?> clazz)
+    {
+        return new Impl(LogManager.getLogger(clazz));
+    }
+
+    protected static class Impl extends Logger
+    {
+        public Impl (org.apache.logging.log4j.Logger impl)
+        {
+            _impl = impl;
+        }
+
+        @Override // from Logger
+        protected boolean shouldLog (int levIdx)
+        {
+            return _impl.isEnabled(LEVELS[levIdx]);
+        }
+
+        @Override // from Logger
+        protected void doLog (int levIdx, String formatted, Throwable throwable)
+        {
+            _impl.log(LEVELS[levIdx], formatted, throwable);
+        }
+
+        protected final org.apache.logging.log4j.Logger _impl;
+        protected static final Level[] LEVELS = {
+            Level.DEBUG, Level.INFO, Level.WARN, Level.ERROR };
+    }
+}

src/main/java/com/samskivert/util/Logger.java

@@ -178,6 +178,17 @@ public abstract class Logger
             System.err.println("Unable to instantiate Log4JLogger: " + t);
         }
 
+        // create and a log4j2 logger if the log4j2 configuration system property is set
+        try {
+            if (factory == null && System.getProperty("log4j.configurationFile") != null) {
+                factory = (Factory)Class.forName("com.samskivert.util.Log4J2Logger").newInstance();
+            }
+        } catch (SecurityException se) {
+            // in a sandbox, no biggie
+        } catch (Throwable t) {
+            System.err.println("Unable to instantiate Log4J2Logger: " + t);
+        }
+
         // lastly, fall back to the Java logging system
         if (factory == null) {
             factory = new JDK14Logger();