Fix NPE with rootLogger shorthand when no appenders are specified

Author: jhl221123

log4j-core-test/src/test/java/org/apache/logging/log4j/core/config/LoggerConfigTest.java

@@ -17,9 +17,11 @@
 package org.apache.logging.log4j.core.config;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
@@ -32,6 +34,7 @@
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.core.Appender;
 import org.apache.logging.log4j.core.Filter;
+import org.apache.logging.log4j.core.config.properties.PropertiesConfiguration;
 import org.apache.logging.log4j.core.impl.Log4jLogEvent.Builder;
 import org.apache.logging.log4j.message.SimpleMessage;
 import org.junit.jupiter.api.Test;
@@ -137,4 +140,18 @@ void testSingleFilterInvocation() {
         verify(appender, times(1)).append(any());
         verify(filter, times(1)).filter(any());
     }
+
+    @Test
+    void testLevelAndRefsWithoutAppenderRef() {
+        final Configuration configuration = mock(PropertiesConfiguration.class);
+        final LoggerConfig.Builder builder = LoggerConfig.newBuilder()
+                .withLoggerName(FQCN)
+                .withConfig(configuration)
+                .withLevelAndRefs(Level.INFO.name());
+
+        final LoggerConfig loggerConfig = builder.build();
+
+        assertNotNull(loggerConfig.getAppenderRefs());
+        assertTrue(loggerConfig.getAppenderRefs().isEmpty());
+    }
 }

log4j-core/src/main/java/org/apache/logging/log4j/core/config/LoggerConfig.java

@@ -1009,13 +1009,13 @@ protected static LevelAndRefs getLevelAndRefs(
                 }
                 final String[] parts = Strings.splitList(levelAndRefs);
                 result.level = Level.getLevel(parts[0]);
+                final List<AppenderRef> refList = new ArrayList<>();
                 if (parts.length > 1) {
-                    final List<AppenderRef> refList = new ArrayList<>();
                     Arrays.stream(parts)
                             .skip(1)
                             .forEach((ref) -> refList.add(AppenderRef.createAppenderRef(ref, null, null)));
-                    result.refs = refList;
                 }
+                result.refs = refList;
             } else {
                 LOGGER.warn("levelAndRefs are only allowed in a properties configuration. The value is ignored.");
                 result.level = level;