GH-2277 - Don't log warnings on literal null parameters.

This changes the log level to `debug` for literal null parameters and
also improves the wording around it.

In addition, a TODO has been removed:

- It doesn't make sense to move the conversion of RANGE etc. to the
  conversion service, as those will be converted to maps etc. which are
  subject to further conversion anyway

This fixes #2277.

Author: michael-simons

src/main/java/org/springframework/data/neo4j/repository/query/CypherQueryCreator.java

@@ -259,6 +259,7 @@ protected QueryFragmentsAndParameters complete(@Nullable Condition condition, So
 		QueryFragments queryFragments = createQueryFragments(condition, sort);
 
 		Map<String, Object> convertedParameters = this.boundedParameters.stream()
+				.peek(p -> Neo4jQuerySupport.logParameterIfNull(p.nameOrIndex, p.value))
 				.collect(Collectors.toMap(p -> p.nameOrIndex, p -> parameterConversion.apply(p.value, p.conversionOverride)));
 		return new QueryFragmentsAndParameters(nodeDescription, queryFragments, convertedParameters);
 	}

src/main/java/org/springframework/data/neo4j/repository/query/Neo4jQuerySupport.java

@@ -24,6 +24,7 @@
 import java.util.Map;
 import java.util.function.BiFunction;
 import java.util.function.Function;
+import java.util.function.Supplier;
 
 import org.apache.commons.logging.LogFactory;
 import org.neo4j.driver.Value;
@@ -115,6 +116,21 @@ protected final List<String> getInputProperties(final ResultProcessor resultProc
 		return returnedType.isProjecting() ? returnedType.getInputProperties() : Collections.emptyList();
 	}
 
+	static void logParameterIfNull(String name, Object value) {
+
+		if (value != null || !REPOSITORY_QUERY_LOG.isDebugEnabled()) {
+			return;
+		}
+
+		Supplier<CharSequence> messageSupplier = () -> {
+			String pointer = name == null || name.trim().isEmpty() ? "An unknown parameter" : "$" + name;
+			return String.format("%s points to a literal `null` value during a comparison. " +
+						  "The comparisons will always resolve to false and probably lead to an empty result.",
+					pointer);
+		};
+		REPOSITORY_QUERY_LOG.debug(messageSupplier);
+	}
+
 	/**
 	 * Converts parameter as needed by the query generated, which is not covered by standard conversion services.
 	 *
@@ -135,17 +151,8 @@ final Object convertParameter(Object parameter) {
 	final Object convertParameter(Object parameter, @Nullable Function<Object, Value> conversionOverride) {
 
 		if (parameter == null) {
-			// According to https://neo4j.com/docs/cypher-manual/current/syntax/working-with-null/#cypher-null-intro
-			// it does not make any sense to continue if a `null` value gets into a comparison
-			// but we just warn the users and do not throw an exception on `null`.
-			REPOSITORY_QUERY_LOG.warn("Do not use `null` as a property value for comparison."
-					+ " It will always be false and return an empty result.");
-
 			return Values.NULL;
-		}
-
-		// Maybe move all of those into Neo4jConverter at some point.
-		if (parameter instanceof Range) {
+		} else if (parameter instanceof Range) {
 			return convertRange((Range) parameter);
 		} else if (parameter instanceof Distance) {
 			return calculateDistanceInMeter((Distance) parameter);

src/main/java/org/springframework/data/neo4j/repository/query/ReactiveStringBasedNeo4jQuery.java

@@ -150,25 +150,25 @@ Map<String, Object> bindParameters(Neo4jParameterAccessor parameterAccessor) {
 
 		// Values from the parameter accessor can only get converted after evaluation
 		for (Map.Entry<String, Object> evaluatedParam : spelEvaluator.evaluate(parameterAccessor.getValues()).entrySet()) {
-			Object value;
-
-			if (evaluatedParam.getValue() instanceof Neo4jSpelSupport.LiteralReplacement) {
-				value = evaluatedParam.getValue();
-			} else {
+			Object value = evaluatedParam.getValue();
+			if (!(evaluatedParam.getValue() instanceof Neo4jSpelSupport.LiteralReplacement)) {
+				Neo4jQuerySupport.logParameterIfNull(evaluatedParam.getKey(), value);
 				value = super.convertParameter(evaluatedParam.getValue());
 			}
 			resolvedParameters.put(evaluatedParam.getKey(), value);
 		}
 
 		formalParameters.stream().filter(Parameter::isBindable).forEach(parameter -> {
 
-			int parameterIndex = parameter.getIndex();
-			Object parameterValue = super.convertParameter(parameterAccessor.getBindableValue(parameterIndex));
+			int index = parameter.getIndex();
+			Object value = parameterAccessor.getBindableValue(index);
+			Neo4jQuerySupport.logParameterIfNull(parameter.getName().orElseGet(() -> Integer.toString(index)), value);
+			Object convertedValue = super.convertParameter(value);
 
 			// Add the parameter under its name when possible
-			parameter.getName().ifPresent(parameterName -> resolvedParameters.put(parameterName, parameterValue));
+			parameter.getName().ifPresent(parameterName -> resolvedParameters.put(parameterName, convertedValue));
 			// Always add under its index.
-			resolvedParameters.put(Integer.toString(parameterIndex), parameterValue);
+			resolvedParameters.put(Integer.toString(index), convertedValue);
 		});
 
 		return resolvedParameters;

src/main/java/org/springframework/data/neo4j/repository/query/StringBasedNeo4jQuery.java

@@ -200,24 +200,25 @@ Map<String, Object> bindParameters(Neo4jParameterAccessor parameterAccessor, boo
 
 		// Values from the parameter accessor can only get converted after evaluation
 		for (Entry<String, Object> evaluatedParam : spelEvaluator.evaluate(parameterAccessor.getValues()).entrySet()) {
-			Object value;
-			if (evaluatedParam.getValue() instanceof LiteralReplacement) {
-				value = evaluatedParam.getValue();
-			} else {
+			Object value = evaluatedParam.getValue();
+			if (!(evaluatedParam.getValue() instanceof LiteralReplacement)) {
+				Neo4jQuerySupport.logParameterIfNull(evaluatedParam.getKey(), value);
 				value = super.convertParameter(evaluatedParam.getValue());
 			}
 			resolvedParameters.put(evaluatedParam.getKey(), value);
 		}
 
 		formalParameters.getBindableParameters().forEach(parameter -> {
 
-			int parameterIndex = parameter.getIndex();
-			Object parameterValue = super.convertParameter(parameterAccessor.getBindableValue(parameterIndex));
+			int index = parameter.getIndex();
+			Object value = parameterAccessor.getBindableValue(index);
+			Neo4jQuerySupport.logParameterIfNull(parameter.getName().orElseGet(() -> Integer.toString(index)), value);
+			Object convertedValue = super.convertParameter(value);
 
 			// Add the parameter under its name when possible
-			parameter.getName().ifPresent(parameterName -> resolvedParameters.put(parameterName, parameterValue));
+			parameter.getName().ifPresent(parameterName -> resolvedParameters.put(parameterName, convertedValue));
 			// Always add under its index.
-			resolvedParameters.put(Integer.toString(parameterIndex), parameterValue);
+			resolvedParameters.put(Integer.toString(index), convertedValue);
 		});
 
 		if (formalParameters.hasPageableParameter() && includePageableParameter) {