fix: treat null parameters as empty in CsvParameterLayout

log4j-core-test/src/test/java/org/apache/logging/log4j/core/layout/CsvParameterLayoutTest.java

@@ -29,11 +29,14 @@
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.core.Appender;
+import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.Logger;
+import org.apache.logging.log4j.core.impl.Log4jLogEvent;
 import org.apache.logging.log4j.core.test.appender.ListAppender;
 import org.apache.logging.log4j.core.test.categories.Layouts;
 import org.apache.logging.log4j.core.test.junit.LoggerContextRule;
 import org.apache.logging.log4j.message.ObjectArrayMessage;
+import org.apache.logging.log4j.message.SimpleMessage;
 import org.apache.logging.log4j.test.junit.ThreadContextRule;
 import org.junit.Assert;
 import org.junit.Rule;
@@ -170,6 +173,19 @@ public void testLayoutTab() throws Exception {
         testLayoutNormalApi(root, CsvParameterLayout.createLayout(CSVFormat.TDF), true);
     }
 
+    @Test
+    public void testNullParametersProduceEmptyRecord() {
+        // SimpleMessage#getParameters() returns null; must not NPE (GH-4243)
+        final AbstractCsvLayout layout = CsvParameterLayout.createDefaultLayout();
+        final LogEvent event = Log4jLogEvent.newBuilder()
+                .setLoggerName("test")
+                .setLevel(Level.INFO)
+                .setMessage(new SimpleMessage("plain text without parameters"))
+                .build();
+        final String result = layout.toSerializable(event);
+        Assert.assertEquals(layout.getFormat().getRecordSeparator(), result);
+    }
+
     @Test
     public void testLogJsonArgument() throws InterruptedException {
         final ListAppender appender = init.getAppender("List");

log4j-core/src/main/java/org/apache/logging/log4j/core/layout/CsvParameterLayout.java

@@ -30,6 +30,7 @@
 import org.apache.logging.log4j.core.config.plugins.PluginFactory;
 import org.apache.logging.log4j.message.Message;
 import org.apache.logging.log4j.status.StatusLogger;
+import org.apache.logging.log4j.util.Constants;
 
 /**
  * A Comma-Separated Value (CSV) layout to log event parameters.
@@ -95,7 +96,7 @@ public String toSerializable(final LogEvent event) {
         final Object[] parameters = message.getParameters();
         final StringBuilder buffer = getStringBuilder();
         try {
-            getFormat().printRecord(buffer, parameters);
+            getFormat().printRecord(buffer, parameters == null ? Constants.EMPTY_OBJECT_ARRAY : parameters);
             return buffer.toString();
         } catch (final IOException e) {
             StatusLogger.getLogger().error(message, e);

src/changelog/.2.x.x/4243_fix_CsvParameterLayout_null_parameters.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns="https://logging.apache.org/xml/ns"
+       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xsi:schemaLocation="
+           https://logging.apache.org/xml/ns
+           https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="4243" link="https://github.com/apache/logging-log4j2/issues/4243"/>
+  <description format="asciidoc">
+    Fix `NullPointerException` in `CsvParameterLayout` when a log event has no parameters (for example `SimpleMessage`).
+  </description>
+</entry>