Improve DefaultLogService to use stdout and stderr

When logging critical messages (WARN and ERROR levels), use stderr.
When logging non-critical messages (e.g., INFO level), use stdout.

This change will be very useful for differentiating between different
log levels in a simpler way; in particular, ImageJ can send both stdout
and stderr to a Console window, coloring stderr messages in red and
setting the Console to visible when they occur, but not displaying the
Console by default when only stdout messages occur (because many ImageJ
plugins spam stdout with lots of debugging output).

Author: ctrueden

src/main/java/org/scijava/log/AbstractLogService.java

@@ -31,6 +31,7 @@
 
 package org.scijava.log;
 
+import java.io.PrintStream;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Properties;
@@ -51,22 +52,6 @@ public abstract class AbstractLogService extends AbstractService implements
 	private final Map<String, Integer> classAndPackageLevels =
 		new HashMap<String, Integer>();
 
-	// -- abstract methods --
-
-	/**
-	 * Displays a message.
-	 *
-	 * @param msg the message to display.
-	 */
-	protected abstract void log(final String msg);
-
-	/**
-	 * Displays an exception.
-	 *
-	 * @param t the exception to display.
-	 */
-	protected abstract void log(final Throwable t);
-
 	// -- constructor --
 
 	public AbstractLogService() {
@@ -98,15 +83,30 @@ public AbstractLogService() {
 
 	// -- Internal methods --
 
-	protected void log(final int level, final Object msg, final Throwable t) {
-		if (msg != null || t == null) {
-			log(level, msg);
-		}
-		if (t != null) log(t);
-	}
+	/**
+	 * Displays a message and/or exception at the given logging level.
+	 *
+	 * @param level The logging level of the information.
+	 * @param msg The message to display.
+	 * @param t The exception to display.
+	 */
+	protected abstract void log(final int level, final Object msg,
+		final Throwable t);
 
-	protected void log(final int level, final Object msg) {
-		log(getPrefix(level) + msg);
+	/**
+	 * Displays a message and/or exception at the given logging level, using the
+	 * specific output stream.
+	 *
+	 * @param stream The output stream to which the information should be sent.
+	 * @param level The logging level of the information.
+	 * @param msg The message to display.
+	 * @param t The exception to display.
+	 */
+	protected void log(final PrintStream stream, final int level,
+		final Object msg, final Throwable t)
+	{
+		if (msg != null || t == null) stream.println(getPrefix(level) + msg);
+		if (t != null) t.printStackTrace(stream);
 	}
 
 	// -- LogService methods --

src/main/java/org/scijava/log/DefaultLogService.java

@@ -36,32 +36,23 @@
 import org.scijava.service.Service;
 
 /**
- * Implementation of {@link LogService} using the standard error stream.
+ * Default implementation of {@link LogService}.
+ * <p>
+ * It uses the standard output and error streams, logging critical messages (
+ * {@link LogService#WARN} and {@link LogService#ERROR} levels) to
+ * {@code stderr}, and non-critical messages ({@link LogService#INFO},
+ * {@link LogService#DEBUG} and {@link LogService#TRACE} levels) to
+ * {@code stdout}.
+ * </p>
  * 
- * @author Johannes Schindelin
  * @author Curtis Rueden
  */
 @Plugin(type = Service.class, priority = Priority.LOW_PRIORITY)
 public class DefaultLogService extends AbstractLogService {
 
-	/**
-	 * Prints a message to stderr.
-	 * 
-	 * @param message the message
-	 */
 	@Override
-	protected void log(final String message) {
-		System.err.println(message);
-	}
-
-	/**
-	 * Prints an exception to stderr.
-	 * 
-	 * @param t the exception
-	 */
-	@Override
-	protected void log(final Throwable t) {
-		t.printStackTrace();
+	protected void log(final int level, final Object msg, final Throwable t) {
+		log(level > WARN ? System.out : System.err, level, msg, t);
 	}
 
 }