test default variable values in appender-ref ref attribute

Signed-off-by: ceki <ceki@qos.ch>

Author: ceki

logback-classic/src/test/input/joran/refViaDefaultSubstitution.xml

@@ -0,0 +1,29 @@
+
+<!--
+  ~ Logback: the reliable, generic, fast and flexible logging framework.
+  ~  Copyright (C) 1999-2026, QOS.ch. All rights reserved.
+  ~
+  ~ This program and the accompanying materials are dual-licensed under
+  ~ either the terms of the Eclipse Public License v1.0 as published by
+  ~ the Eclipse Foundation
+  ~
+  ~     or (per the licensee's choosing)
+  ~
+  ~ under the terms of the GNU Lesser General Public License version 2.1
+  ~ as published by the Free Software Foundation.
+  -->
+
+<!DOCTYPE configuration>
+
+<configuration debug="false">
+
+    <appender name="A" class="ch.qos.logback.core.read.ListAppender"/>
+    <appender name="NOP" class="ch.qos.logback.core.helpers.NOPAppender"/>
+
+    <root level="DEBUG">
+        <appender-ref ref="${NONEXISTENT:-NOP}"/>
+        <appender-ref ref="A"/>
+
+    </root>
+
+</configuration>

logback-classic/src/test/java/ch/qos/logback/classic/joran/JoranConfiguratorTest.java

@@ -25,6 +25,7 @@
 import ch.qos.logback.core.ConsoleAppender;
 import ch.qos.logback.core.CoreConstants;
 import ch.qos.logback.core.encoder.LayoutWrappingEncoder;
+import ch.qos.logback.core.helpers.NOPAppender;
 import ch.qos.logback.core.joran.action.ParamAction;
 import ch.qos.logback.core.joran.spi.ActionException;
 import ch.qos.logback.core.joran.spi.JoranException;
@@ -205,7 +206,7 @@ public void appenderRefSettingBySystemPropertyDefault() throws JoranException {
     public void refToUndefinedAppender() throws JoranException {
 
         configure(ClassicTestConstants.JORAN_INPUT_PREFIX + "refToUndefinedAppender.xml");
-        StatusPrinter.print(loggerContext);
+        //StatusPrinter.print(loggerContext);
         final Logger logger = loggerContext.getLogger("ch.qos.logback.classic.joran");
         final ListAppender<ILoggingEvent> listAppender = (ListAppender<ILoggingEvent>) root.getAppender("A");
         assertNotNull(listAppender);
@@ -216,7 +217,27 @@ public void refToUndefinedAppender() throws JoranException {
         assertEquals(1, listAppender.list.size());
 
         checker.assertContainsMatch(Status.WARN, "Appender named \\[NON_EXISTENT_APPENDER\\] could not be found. Skipping attachment to Logger\\[ROOT\\]");
+    }
+
+    @Test
+    public void refViaDefaultSubstitution() throws JoranException {
+
+        configure(ClassicTestConstants.JORAN_INPUT_PREFIX + "refViaDefaultSubstitution.xml");
+        StatusPrinter.print(loggerContext);
+        final Logger logger = loggerContext.getLogger("ch.qos.logback.classic.joran");
+        final ListAppender<ILoggingEvent> listAppender = (ListAppender<ILoggingEvent>) root.getAppender("A");
+        final NOPAppender<ILoggingEvent> nopAppender = (NOPAppender) root.getAppender("NOP");
 
+        assertNotNull(listAppender);
+        assertNotNull(nopAppender);
+
+        assertEquals(0, listAppender.list.size());
+        final String msg = "hello world";
+        logger.info(msg);
+
+        assertEquals(1, listAppender.list.size());
+
+        checker.assertIsWarningOrErrorFree();
     }
 
 

logback-core/src/main/java/ch/qos/logback/core/model/processor/AppenderDeclarationAnalyser.java

@@ -47,8 +47,8 @@ protected Class<AppenderModel> getSupportedModelClass() {
     @Override
     public void handle(ModelInterpretationContext mic, Model model) throws ModelHandlerException {
         AppenderModel appenderModel = (AppenderModel) model;
+        String appenderName = mic.subst(appenderModel.getName());
 
-        String appenderName = appenderModel.getName();
         addAppenderDeclaration(mic, appenderName);
     }
 

logback-core/src/main/java/ch/qos/logback/core/model/processor/ModelInterpretationContext.java

@@ -159,6 +159,23 @@ public BeanDescriptionCache getBeanDescriptionCache() {
         return beanDescriptionCache;
     }
 
+    /**
+     * Performs variable substitution on the provided {@code ref} string.
+     *
+     * <p>Value substitution will follow the order</p>
+     * <ol>
+     * <li>properties defined in this {@link ModelInterpretationContext}</li>
+     * <li>properties defined in the {@link Context context} of this {@link ModelInterpretationContext}</li>
+     * <li>System properties</li>
+     * <li>Environment variables</li>
+     * </ol>
+     *
+     * <p>If value substitution occurs it will be output as a status message, unless marked confidential, that is,
+     * if {@code ref} contains the case-insensitive strings PASSWORD, SECRET or CONFIDENTIAL.</p>
+     *
+     * @param ref the string that may contain variables to be substituted; can be {@code null}
+     * @return the string with substitutions applied if applicable; may return {@code null} if {@code ref} is {@code null}
+     */
     public String subst(String ref)  {
 
         String substituted = variableSubstitutionsHelper.subst(ref);

logback-core/src/main/java/ch/qos/logback/core/util/OptionHelper.java

@@ -115,12 +115,12 @@ public static String substVars(String input, PropertyContainer pc0, PropertyCont
 
     /**
      * Try to lookup the property in the following order:
-     * <ul>
-     * <li>pc1 (usually the local property container)</li>
+     * <ol>
+     * <li>pc1 (the local property container, usually the {@link ch.qos.logback.core.model.processor.ModelInterpretationContext ModelInterpretationContext})</li>
      * <li>pc2 (usually the {@link Context context})</li>
      * <li>System properties</li>
      * <li>Environment variables</li>
-     * </ul>
+     * </ol>
      *
      * @param key the property key
      * @param pc1 the first property container to search