added Slf4jConfigurationFactory to detect actual slf4j logging

implementation and load appropriate Slf4jConfiguration implementation
from META-INF/maven/slf4j-configuration.properties

maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java

@@ -40,9 +40,9 @@ import org.apache.maven.Maven;
 import org.apache.maven.cli.event.DefaultEventSpyContext;
 import org.apache.maven.cli.event.ExecutionEventLogger;
 import org.apache.maven.cli.logging.Slf4jConfiguration;
+import org.apache.maven.cli.logging.Slf4jConfigurationFactory;
 import org.apache.maven.cli.logging.Slf4jLoggerManager;
 import org.apache.maven.cli.logging.Slf4jStdoutLogger;
-import org.apache.maven.cli.logging.impl.Slf4jSimpleConfiguration;
 import org.apache.maven.cli.transfer.ConsoleMavenTransferListener;
 import org.apache.maven.cli.transfer.QuietMavenTransferListener;
 import org.apache.maven.cli.transfer.Slf4jMavenTransferListener;
@@ -133,8 +133,6 @@ public class MavenCli
 
     private DefaultSecDispatcher dispatcher;
 
-    private Slf4jConfiguration slf4jConfiguration = new Slf4jSimpleConfiguration();
-
     public MavenCli()
     {
         this( null );
@@ -307,6 +305,9 @@ public class MavenCli
         cliRequest.quiet = !cliRequest.debug && cliRequest.commandLine.hasOption( CLIManager.QUIET );
         cliRequest.showErrors = cliRequest.debug || cliRequest.commandLine.hasOption( CLIManager.ERRORS );
 
+        slf4jLoggerFactory = LoggerFactory.getILoggerFactory();
+        Slf4jConfiguration slf4jConfiguration = Slf4jConfigurationFactory.getConfiguration( slf4jLoggerFactory );
+
         if ( cliRequest.debug )
         {
             cliRequest.request.setLoggingLevel( MavenExecutionRequest.LOGGING_LEVEL_DEBUG );
@@ -346,7 +347,6 @@ public class MavenCli
         }
 
         plexusLoggerManager = new Slf4jLoggerManager();
-        slf4jLoggerFactory = LoggerFactory.getILoggerFactory();
         slf4jLogger = slf4jLoggerFactory.getLogger( this.getClass().getName() );
     }
 

maven-embedder/src/main/java/org/apache/maven/cli/logging/BaseSlf4jConfiguration.java

@@ -29,10 +29,10 @@ import org.slf4j.LoggerFactory;
  * 
  * @author Hervé Boutemy
  */
-public class AbstractSlf4jConfiguration
+public class BaseSlf4jConfiguration
     implements Slf4jConfiguration
 {
-    private final Logger logger = LoggerFactory.getLogger( AbstractSlf4jConfiguration.class );
+    private final Logger logger = LoggerFactory.getLogger( BaseSlf4jConfiguration.class );
 
     public void setRootLoggerLevel( Level level )
     {

maven-embedder/src/main/java/org/apache/maven/cli/logging/Slf4jConfigurationFactory.java

@@ -0,0 +1,81 @@
+package org.apache.maven.cli.logging;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import java.io.IOException;
+import java.net.URL;
+import java.util.Enumeration;
+import java.util.Properties;
+
+import org.codehaus.plexus.util.PropertyUtils;
+import org.slf4j.ILoggerFactory;
+
+/**
+ * Slf4jConfiguration factory, loading implementations from <code>META-INF/maven/slf4j-configuration.properties</code>
+ * configuration files in class loader.
+ *
+ * @author Hervé Boutemy
+ */
+public class Slf4jConfigurationFactory
+{
+    public static final String RESOURCE = "META-INF/maven/slf4j-configuration.properties";
+
+    public static Slf4jConfiguration getConfiguration( ILoggerFactory loggerFactory )
+    {
+        try
+        {
+            Enumeration<URL> resources = Slf4jConfigurationFactory.class.getClassLoader().getResources( RESOURCE );
+
+            String key = loggerFactory.getClass().getCanonicalName();
+
+            while ( resources.hasMoreElements() )
+            {
+                URL resource = resources.nextElement();
+
+                Properties conf = PropertyUtils.loadProperties( resource.openStream() );
+
+                String impl = conf.getProperty( key );
+
+                if ( impl != null )
+                {
+                    return (Slf4jConfiguration) Class.forName( impl ).newInstance();
+                }
+            }
+        }
+        catch ( IOException e )
+        {
+            e.printStackTrace();
+        }
+        catch ( InstantiationException e )
+        {
+            e.printStackTrace();
+        }
+        catch ( IllegalAccessException e )
+        {
+            e.printStackTrace();
+        }
+        catch ( ClassNotFoundException e )
+        {
+            e.printStackTrace();
+        }
+
+        return new BaseSlf4jConfiguration();
+    }
+}

maven-embedder/src/main/java/org/apache/maven/cli/logging/impl/Slf4jSimpleConfiguration.java

@@ -21,7 +21,7 @@ package org.apache.maven.cli.logging.impl;
 
 import java.io.File;
 
-import org.apache.maven.cli.logging.AbstractSlf4jConfiguration;
+import org.apache.maven.cli.logging.BaseSlf4jConfiguration;
 
 /**
  * Configuration for slf4j-simple.
@@ -29,7 +29,7 @@ import org.apache.maven.cli.logging.AbstractSlf4jConfiguration;
  * @author Hervé Boutemy
  */
 public class Slf4jSimpleConfiguration
-    extends AbstractSlf4jConfiguration
+    extends BaseSlf4jConfiguration
 {
     @Override
     public void setRootLoggerLevel( Level level )

maven-embedder/src/main/resources/META-INF/maven/slf4j-configuration.properties

@@ -0,0 +1,3 @@
+# key = Slf4j effective logger factory implementation
+# value = corresponding o.a.m.cli.logging.Slf4jConfiguration class
+org.slf4j.impl.SimpleLoggerFactory org.apache.maven.cli.logging.impl.Slf4jSimpleConfiguration