Add default values for variables in the configuration file

tinylog-api/src/main/java/org/tinylog/configuration/Configuration.java

@@ -259,14 +259,24 @@ private static String resolve(final String value, final Resolver resolver) {
 				return value;
 			}
 
-			String data = resolver.resolve(name);
-			if (data == null) {
-				InternalLogger.log(Level.WARN, "'" + name + "' could not be found in " + resolver.getName());
+			String[] colonSplittedName = name.split(":", -1);
+			if (colonSplittedName.length > 2) {
+				InternalLogger.log(Level.WARN, "Multiple default values found: " + value);
 				return value;
-			} else {
-				builder.append(data);
 			}
 
+			String key = colonSplittedName[0];
+			String defaultValue = colonSplittedName.length == 2 ? colonSplittedName[1] : null;
+			String data = resolver.resolve(key);
+			if (data == null) {
+				if (defaultValue == null) {
+					InternalLogger.log(Level.WARN, "'" + key + "' could not be found in " + resolver.getName());
+					return value;
+				}
+				data = defaultValue;
+			}
+			builder.append(data);
+
 			position = end + 1;
 		}
 

tinylog-api/src/test/java/org/tinylog/configuration/ConfigurationTest.java

@@ -322,6 +322,56 @@ public void nonExistentVariable() throws Exception {
 		assertThat(systemStream.consumeErrorOutput()).contains("WARN").containsOnlyOnce("my.invalid.variable");
 	}
 
+	/**
+	 * Verifies that environment variables will be resolved and the given default value is ignored.
+	 *
+	 * @throws Exception
+	 *             Failed creating temporary file or invoking private method {@link Configuration#load()}
+	 */
+	@Test
+	public void existingVariableWithUnusedDefaultValue() throws Exception {
+		loadProperties(FileSystem.createTemporaryFile("test = ${PATH:/a/default/path}"));
+		assertThat(Configuration.get("test")).isEqualTo(System.getenv("PATH"));
+	}
+
+	/**
+	 * Verifies that a given empty string is used as default value, if a variable exists neither as system property nor
+	 * as environment variable..
+	 *
+	 * @throws Exception
+	 *             Failed creating temporary file or invoking private method {@link Configuration#load()}
+	 */
+	@Test
+	public void nonExistingVariableWithEmptyDefaultValue() throws Exception {
+		loadProperties(FileSystem.createTemporaryFile("test = ${my.invalid.variable:}"));
+		assertThat(Configuration.get("test")).isEqualTo("");
+	}
+
+	/**
+	 * Verifies that a given default value is used, if a variable exists neither as system property nor
+	 * as environment variable..
+	 *
+	 * @throws Exception
+	 *             Failed creating temporary file or invoking private method {@link Configuration#load()}
+	 */
+	@Test
+	public void nonExistingVariableWithDefaultValue() throws Exception {
+		loadProperties(FileSystem.createTemporaryFile("test = ${my.invalid.variable:a_default_value}"));
+		assertThat(Configuration.get("test")).isEqualTo("a_default_value");
+	}
+
+	/**
+	 * Verifies that an accurate warning message will be output, if multiple default values were given.
+	 *
+	 * @throws Exception
+	 *             Failed creating temporary file or invoking private method {@link Configuration#load()}
+	 */
+	@Test
+	public void nonExistingVariableMultipleDefaultValues() throws Exception {
+		loadProperties(FileSystem.createTemporaryFile("test = ${PATH:a_default_value:an_other_default_value}"));
+		assertThat(systemStream.consumeErrorOutput()).contains("WARN").containsOnlyOnce("PATH");
+	}
+
 	/**
 	 * Verifies that a new property can be added.
 	 *