Omit empty JSON field #cf Log4j2 without custom fields

When there are no custom fields defined in the log4j configuration,
we never need to generate the #cf field in any log messages. The
converter can be removed in this case entirely.

Author: KarstenSchnitter

cf-java-logging-support-log4j2/src/main/java/com/sap/hcp/cf/log4j2/layout/LayoutPatternBuilder.java

@@ -56,6 +56,9 @@ public LayoutPatternBuilder addContextProperties(List<String> exclusions) {
 	}
 
 	public LayoutPatternBuilder addCustomFields(List<String> mdcKeyNames) {
+		if (mdcKeyNames == null || mdcKeyNames.isEmpty()) {
+			return this;
+		}
 		sb.append("\"").append(Fields.CUSTOM_FIELDS).append("\":");
 		sb.append("{%").append(CustomFieldsConverter.WORD);
 		appendParameters(mdcKeyNames);

cf-java-logging-support-log4j2/src/test/java/com/sap/hcp/cf/log4j2/layout/LayoutPatternBuilderTest.java

@@ -2,11 +2,12 @@
 
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.isEmptyString;
-import static org.junit.Assert.assertThat;
 
 import java.util.Arrays;
+import java.util.Collections;
 
 import org.hamcrest.FeatureMatcher;
 import org.hamcrest.Matcher;
@@ -54,6 +55,20 @@ public void customFields() throws Exception {
 		assertThat(pattern, specificPart(is(",\"#cf\":{%cf{this key}{that key}}")));
 	}
 
+	@Test
+	public void emptyCustomFields() throws Exception {
+		String pattern = new LayoutPatternBuilder().addCustomFields(Collections.emptyList()).build();
+
+		assertThat(pattern, specificPart(is("")));
+	}
+
+	@Test
+	public void nullCustomFields() throws Exception {
+		String pattern = new LayoutPatternBuilder().addCustomFields(null).build();
+
+		assertThat(pattern, specificPart(is("")));
+	}
+
 	@Test
 	public void stacktrace() throws Exception {
 		String pattern = new LayoutPatternBuilder().addStacktraces().build();