Index: log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/Interpolation.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/Interpolation.java	(revision )
+++ log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/Interpolation.java	(revision )
@@ -0,0 +1,32 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache license, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the license for the specific language governing permissions and
+ * limitations under the license.
+ */
+package org.apache.logging.log4j.core.lookup;
+
+/**
+ * Helper class returning partial lookups.
+ */
+class Interpolation {
+    final StrLookup lookup;
+    final String prefix;
+    final String variableName;
+
+    public Interpolation(final StrLookup lookup, final String prefix, final String variableName) {
+        this.lookup = lookup;
+        this.prefix = prefix;
+        this.variableName = variableName;
+    }
+}
Index: log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/StrSubstitutor.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/StrSubstitutor.java	(revision 19ac9876f81629a318fad85ce5d795e059ba479b)
+++ log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/StrSubstitutor.java	(revision )
@@ -942,6 +942,8 @@
                                 pos += endMatchLen;
                                 final int endPos = pos;
 
+                                final Interpolation interpolation = interpolate(varNameExpr);
+                                varNameExpr = interpolation.variableName;
                                 String varName = varNameExpr;
                                 String varDefaultValue = null;
 
@@ -973,7 +975,11 @@
                                 priorVariables.add(varName);
 
                                 // resolve the variable
-                                String varValue = resolveVariable(event, varName, buf, startPos, endPos);
+                                final StrLookup lookup = interpolation.lookup;
+                                String varValue = null;
+                                if (lookup != null) {
+                                    varValue = lookup.lookup(event, varName);
+                                }
                                 if (varValue == null) {
                                     varValue = varDefaultValue;
                                 }
@@ -1025,33 +1031,23 @@
         throw new IllegalStateException(buf.toString());
     }
 
+    private Interpolation interpolate(final String varNameExpr) {
+        if (variableResolver instanceof Interpolator) {
+            return ((Interpolator) variableResolver).interpolate(varNameExpr);
+        }
+        return new Interpolation(null, null, varNameExpr);
+    }
+
     /**
-     * Internal method that resolves the value of a variable.
-     * <p>
-     * Most users of this class do not need to call this method. This method is
-     * called automatically by the substitution process.
-     * </p>
-     * <p>
-     * Writers of subclasses can override this method if they need to alter
-     * how each substitution occurs. The method is passed the variable's name
-     * and must return the corresponding value. This implementation uses the
-     * {@link #getVariableResolver()} with the variable's name as the key.
-     * </p>
+     * This method is never called and does nothing. (It only exists to preserve binary compatibility with earlier
+     * versions of this class.)
      *
-     * @param event The LogEvent, if there is one.
-     * @param variableName  the name of the variable, not null
-     * @param buf  the buffer where the substitution is occurring, not null
-     * @param startPos  the start position of the variable including the prefix, valid
-     * @param endPos  the end position of the variable including the suffix, valid
-     * @return the variable's value or <b>null</b> if the variable is unknown
+     * @deprecated with no replacement
      */
+    @Deprecated
     protected String resolveVariable(final LogEvent event, final String variableName, final StringBuilder buf,
-                                     final int startPos, final int endPos) {
+            final int startPos, final int endPos) {
-        final StrLookup resolver = getVariableResolver();
-        if (resolver == null) {
-            return null;
+        return null;
-        }
-        return resolver.lookup(event, variableName);
     }
 
     // Escape
Index: log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/Interpolator.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/Interpolator.java	(revision 19ac9876f81629a318fad85ce5d795e059ba479b)
+++ log4j-core/src/main/java/org/apache/logging/log4j/core/lookup/Interpolator.java	(revision )
@@ -210,4 +210,22 @@
         }
         return sb.toString();
     }
+
+    Interpolation interpolate(final String var) {
+        if (var == null) {
+            return null;
+        }
+
+        final int prefixPos = var.indexOf(PREFIX_SEPARATOR);
+        if (prefixPos >= 0) {
+            final String prefix = var.substring(0, prefixPos);
+            final String name = var.substring(prefixPos + 1);
+            final StrLookup lookup = lookups.get(prefix);
+            if (lookup instanceof ConfigurationAware) {
+                ((ConfigurationAware) lookup).setConfiguration(configuration);
+            }
+            return new Interpolation(lookup, prefix, name);
+        }
+        return new Interpolation(defaultLookup, null, var);
+    }
 }