ConsoleService: add API for stdout/stderr output

Now, interested parties can add themselves as OutputListeners, and they
will receive notification when output occurs to stdout and stderr.

One cool thing is that the notifications are limited to output produced
by threads _of this SciJava Context_. This is achieved via the new
ThreadService#getThreadContext(Thread) method, which checks the calling
thread to see whether its Context matches this one.

Actually, this commit does not _quite_ finish adding working support for
this feature. One more piece is needed -- see the next commit.

Author: ctrueden

src/main/java/org/scijava/console/ConsoleService.java

@@ -40,7 +40,8 @@
  * Interface for service that manages interaction with the console.
  * <p>
  * In particular, this is the service that defines how command line arguments
- * are handled.
+ * are handled. It also provides an extension mechanism for {@code stdout} and
+ * {@code stderr} logging.
  * </p>
  *
  * @author Curtis Rueden
@@ -55,4 +56,13 @@ public interface ConsoleService extends
 	 */
 	void processArgs(String... args);
 
+	/** Adds a listener for output sent to {@code stdout} or {@code stderr}. */
+	void addOutputListener(OutputListener l);
+
+	/** Removes a listener for output sent to {@code stdout} or {@code stderr}. */
+	void removeOutputListener(OutputListener l);
+
+	/** Notifies listeners of output sent to {@code stdout} or {@code stderr}. */
+	void notifyListeners(OutputEvent event);
+
 }

src/main/java/org/scijava/console/DefaultConsoleService.java

@@ -31,6 +31,8 @@
 
 package org.scijava.console;
 
+import java.io.PrintStream;
+import java.util.ArrayList;
 import java.util.LinkedList;
 
 import org.scijava.log.LogService;
@@ -53,6 +55,11 @@ public class DefaultConsoleService extends
 	@Parameter
 	private LogService log;
 
+	private MultiPrintStream sysout, syserr;
+
+	/** List of listeners for {@code stdout} and {@code stderr} output. */
+	private ArrayList<OutputListener> listeners;
+
 	// -- ConsoleService methods --
 
 	@Override
@@ -76,6 +83,32 @@ public void processArgs(final String... args) {
 		}
 	}
 
+	@Override
+	public void addOutputListener(final OutputListener l) {
+		if (listeners == null) initListeners();
+		synchronized (listeners) {
+			listeners.add(l);
+		}
+	}
+
+	@Override
+	public void removeOutputListener(final OutputListener l) {
+		if (listeners == null) initListeners();
+		synchronized (listeners) {
+			listeners.remove(l);
+		}
+	}
+
+	@Override
+	public void notifyListeners(final OutputEvent event) {
+		if (listeners == null) initListeners();
+		synchronized (listeners) {
+			for (final OutputListener l : listeners) {
+				l.outputOccurred(event);
+			}
+		}
+	}
+
 	// -- PTService methods --
 
 	@Override
@@ -91,4 +124,25 @@ public Class<LinkedList<String>> getType() {
 		return (Class) LinkedList.class;
 	}
 
+	// -- Helper methods - lazy initialization --
+
+	/** Initializes {@link #listeners} and related data structures. */
+	private synchronized void initListeners() {
+		if (listeners != null) return; // already initialized
+
+		sysout = multiPrintStream(System.out);
+		if (System.out != sysout) System.setOut(sysout);
+		syserr = multiPrintStream(System.err);
+		if (System.err != syserr) System.setErr(syserr);
+
+		listeners = new ArrayList<OutputListener>();
+	}
+
+	// -- Helper methods --
+
+	private MultiPrintStream multiPrintStream(final PrintStream ps) {
+		if (ps instanceof MultiPrintStream) return (MultiPrintStream) ps;
+		return new MultiPrintStream(ps);
+	}
+
 }