StderrLogService: allow overriding output streams

Now, the setPrintStreams method lets the caller define
which output stream is used for each log level.

This commit also adds a regression test verifying that
the log is written to output streams as expected.

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

Author: maarzt

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

@@ -33,6 +33,7 @@
 package org.scijava.log;
 
 import java.io.PrintStream;
+import java.util.function.Function;
 
 import org.scijava.Priority;
 import org.scijava.plugin.Plugin;
@@ -52,10 +53,16 @@
 @Plugin(type = Service.class, priority = Priority.LOW)
 public class StderrLogService extends AbstractLogService {
 
+	private Function<Integer, PrintStream> levelToStream =
+		level -> (level <= LogLevel.WARN) ? System.err : System.out;
+
+	public void setPrintStreams(Function<Integer, PrintStream> levelToStream) {
+		this.levelToStream = levelToStream;
+	}
+
 	@Override
-	public void notifyListeners(final LogMessage message) {
-		final PrintStream out = (message.level() <= LogLevel.WARN) ? System.err
-			: System.out;
+	public void notifyListeners(LogMessage message) {
+		final PrintStream out = levelToStream.apply(message.level());
 		out.print(message);
 	}
 }

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

@@ -35,19 +35,44 @@
 import static org.junit.Assert.assertTrue;
 import static org.scijava.log.LogLevel.WARN;
 
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
 import org.junit.Test;
 
 /**
  * Tests {@link StderrLogService}.
  * 
  * @author Johannes Schindelin
+ * @author Matthias Arzt
  */
 public class StderrLogServiceTest {
+
 	@Test
 	public void testDefaultLevel() {
 		final LogService log = new StderrLogService();
 		int level = log.getLevel();
 		assertTrue("default level (" + level + //
 			") is at least INFO(" + WARN + ")", level >= WARN);
 	}
+
+	@Test
+	public void testOutputToStream() {
+		// setup
+		final StderrLogService logService = new StderrLogService();
+		final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+		final PrintStream p = new PrintStream(outputStream);
+		logService.setPrintStreams(ignore -> p);
+
+		final String text1 = "Hello World!";
+		final String text2 = "foo bar";
+
+		// process
+		logService.warn(text1);
+		logService.subLogger("sub").error(text2);
+
+		// test
+		assertTrue(outputStream.toString().contains(text1));
+		assertTrue(outputStream.toString().contains(text2));
+	}
 }