Allow subscribing to log events via LogListeners

Signed-off-by: Curtis Rueden <ctrueden@wisc.edu>

Author: maarzt

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

@@ -33,8 +33,10 @@
 package org.scijava.log;
 
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Properties;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 import org.scijava.service.AbstractService;
 
@@ -53,6 +55,8 @@ public abstract class AbstractLogService extends AbstractService implements
 
 	private final Map<String, Integer> classAndPackageLevels;
 
+	private final List<LogListener> listeners = new CopyOnWriteArrayList<>();
+
 	// -- constructor --
 
 	public AbstractLogService() {
@@ -68,6 +72,13 @@ public AbstractLogService(final Properties properties) {
 			LogService.LOG_LEVEL_PROPERTY + ":");
 	}
 
+	// -- AbstractLogService methods --
+
+	protected void notifyListeners(LogMessage message) {
+		for (LogListener listener : listeners)
+			listener.messageLogged(message);
+	}
+
 	// -- Logger methods --
 
 	@Override
@@ -87,6 +98,21 @@ public void setLevel(final String classOrPackageName, final int level) {
 		classAndPackageLevels.put(classOrPackageName, level);
 	}
 
+	@Override
+	public void alwaysLog(final int level, final Object msg, final Throwable t) {
+		notifyListeners(new LogMessage(level, msg, t));
+	}
+
+	@Override
+	public void addListener(final LogListener listener) {
+		listeners.add(listener);
+	}
+
+	@Override
+	public void removeListener(final LogListener listener) {
+		listeners.remove(listener);
+	}
+
 	// -- Deprecated --
 
 	/** @deprecated Use {@link LogLevel#prefix(int)} instead. */

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

@@ -0,0 +1,48 @@
+/*
+ * #%L
+ * SciJava Common shared library for SciJava software.
+ * %%
+ * Copyright (C) 2009 - 2017 Board of Regents of the University of
+ * Wisconsin-Madison, Broad Institute of MIT and Harvard, and Max Planck
+ * Institute of Molecular Cell Biology and Genetics.
+ * %%
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * #L%
+ */
+
+package org.scijava.log;
+
+/**
+ * Callback function used by {@link Logger}.
+ *
+ * @author Matthias Arzt
+ * @see LogMessage
+ */
+public interface LogListener {
+
+	/**
+	 * This method is normally called from many threads in parallel. It must be
+	 * implemented highly thread safe and must not use any kind of locks.
+	 */
+	void messageLogged(LogMessage message);
+
+}

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

@@ -184,4 +184,15 @@ default void log(final int level, final Object msg, final Throwable t) {
 
 	/** Returns the log level of this logger. see {@link LogLevel} */
 	int getLevel();
+
+	/**
+	 * {@link LogListener}s added with this method are notified of every message,
+	 * NB: Messages are only logged, if their level is lower than the logger's
+	 * level.
+	 *
+	 * @param listener
+	 */
+	void addListener(LogListener listener);
+
+	void removeListener(LogListener listener);
 }

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

@@ -53,8 +53,10 @@
 public class StderrLogService extends AbstractLogService {
 
 	@Override
-	public void alwaysLog(final int level, final Object msg, final Throwable t) {
-		final PrintStream out = (level <= LogLevel.WARN) ? System.err : System.out;
-		out.print(new LogMessage(level, msg, t));
+	public void notifyListeners(final LogMessage message) {
+		super.notifyListeners(message);
+		final PrintStream out = (message.level() <= LogLevel.WARN) ? System.err
+			: System.out;
+		out.print(message);
 	}
 }

src/test/java/org/scijava/log/LogServiceTest.java

@@ -165,6 +165,19 @@ public void testPackageLogLevel() {
 		assertEquals(LogLevel.TRACE, level);
 	}
 
+	@Test
+	public void testListener() {
+		// setup
+		TestableLogService logService = new TestableLogService();
+		TestLogListener listener = new TestLogListener();
+		String msg1 = "Hello World!";
+		// process
+		logService.addListener(listener);
+		logService.error(msg1);
+		// test
+		listener.hasLogged(m -> msg1.equals(m.text()));
+	}
+
 	// -- Helper classes --
 
 	private static class MyTestClass {

src/test/java/org/scijava/log/TestLogListener.java

@@ -0,0 +1,63 @@
+/*
+ * #%L
+ * SciJava Common shared library for SciJava software.
+ * %%
+ * Copyright (C) 2009 - 2017 Board of Regents of the University of
+ * Wisconsin-Madison, Broad Institute of MIT and Harvard, and Max Planck
+ * Institute of Molecular Cell Biology and Genetics.
+ * %%
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * #L%
+ */
+
+package org.scijava.log;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Predicate;
+
+/**
+ * TestLogListener is a {@ling LogListener} usable for testing. It stores all
+ * the LogMessages it receives and allows to test if any LogMessage fulfills a
+ * given predicate.
+ *
+ * @author Matthias Arzt
+ */
+
+public class TestLogListener implements LogListener {
+
+	List<LogMessage> messages = new ArrayList<>();
+
+	@Override
+	public void messageLogged(LogMessage message) {
+		messages.add(message);
+	}
+
+	public boolean hasLogged(Predicate<LogMessage> predicate) {
+		return messages.stream().anyMatch(predicate);
+	}
+
+	public void clear() {
+		messages.clear();
+	}
+
+}