Polishing.

src/main/java/org/springframework/data/elasticsearch/repository/query/ElasticsearchStringQuery.java

@@ -42,6 +42,7 @@ public class ElasticsearchStringQuery extends AbstractElasticsearchRepositoryQue
 	public ElasticsearchStringQuery(ElasticsearchQueryMethod queryMethod, ElasticsearchOperations elasticsearchOperations,
 			String queryString, QueryMethodEvaluationContextProvider evaluationContextProvider) {
 		super(queryMethod, elasticsearchOperations);
+
 		Assert.notNull(queryString, "Query cannot be empty");
 		Assert.notNull(evaluationContextProvider, "ExpressionEvaluationContextProvider must not be null");
 
@@ -65,11 +66,11 @@ public class ElasticsearchStringQuery extends AbstractElasticsearchRepositoryQue
 	}
 
 	protected BaseQuery createQuery(ElasticsearchParametersParameterAccessor parameterAccessor) {
-		ConversionService conversionService = elasticsearchOperations.getElasticsearchConverter().getConversionService();
-		String queryString = new StringQueryUtil(conversionService)
-				.replacePlaceholders(this.queryString, parameterAccessor);
 
-		QueryStringSpELEvaluator evaluator = new QueryStringSpELEvaluator(queryString, parameterAccessor, queryMethod,
+		ConversionService conversionService = elasticsearchOperations.getElasticsearchConverter().getConversionService();
+		var replacedString = new StringQueryUtil(conversionService).replacePlaceholders(this.queryString,
+				parameterAccessor);
+		var evaluator = new QueryStringSpELEvaluator(replacedString, parameterAccessor, queryMethod,
 				evaluationContextProvider, conversionService);
 		var query = new StringQuery(evaluator.evaluate());
 		query.addSort(parameterAccessor.getSort());

src/main/java/org/springframework/data/elasticsearch/repository/query/ReactiveElasticsearchStringQuery.java

@@ -23,6 +23,7 @@ import org.springframework.data.elasticsearch.repository.support.StringQueryUtil
 import org.springframework.data.elasticsearch.repository.support.spel.QueryStringSpELEvaluator;
 import org.springframework.data.repository.query.QueryMethodEvaluationContextProvider;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
+import org.springframework.util.Assert;
 
 /**
  * @author Christoph Strobl
@@ -45,8 +46,11 @@ public class ReactiveElasticsearchStringQuery extends AbstractReactiveElasticsea
 	public ReactiveElasticsearchStringQuery(String query, ReactiveElasticsearchQueryMethod queryMethod,
 			ReactiveElasticsearchOperations operations, SpelExpressionParser expressionParser,
 			QueryMethodEvaluationContextProvider evaluationContextProvider) {
-
 		super(queryMethod, operations);
+
+		Assert.notNull(query, "query must not be null");
+		Assert.notNull(evaluationContextProvider, "evaluationContextProvider must not be null");
+
 		this.query = query;
 		this.evaluationContextProvider = evaluationContextProvider;
 	}

src/main/java/org/springframework/data/elasticsearch/repository/support/StringQueryUtil.java

@@ -22,6 +22,7 @@ import org.springframework.core.convert.ConversionService;
 import org.springframework.data.elasticsearch.core.convert.ConversionException;
 import org.springframework.data.elasticsearch.repository.support.value.ElasticsearchQueryValueConversionService;
 import org.springframework.data.repository.query.ParameterAccessor;
+import org.springframework.util.Assert;
 import org.springframework.util.NumberUtils;
 
 /**
@@ -36,6 +37,9 @@ final public class StringQueryUtil {
 	private final ConversionService conversionService;
 
 	public StringQueryUtil(ConversionService conversionService) {
+
+		Assert.notNull(conversionService, "conversionService must not be null");
+
 		this.conversionService = ElasticsearchQueryValueConversionService.getInstance(conversionService);
 	}
 
@@ -43,8 +47,8 @@ final public class StringQueryUtil {
 
 		Matcher matcher = PARAMETER_PLACEHOLDER.matcher(input);
 		String result = input;
-		while (matcher.find()) {
 
+		while (matcher.find()) {
 			String placeholder = Pattern.quote(matcher.group()) + "(?!\\d+)";
 			int index = NumberUtils.parseNumber(matcher.group(1), Integer.class);
 			String replacement = Matcher.quoteReplacement(getParameterWithIndex(accessor, index, input));
@@ -59,7 +63,8 @@ final public class StringQueryUtil {
 	private String getParameterWithIndex(ParameterAccessor accessor, int index, String input) {
 
 		Object parameter = accessor.getBindableValue(index);
-		String value =  conversionService.convert(parameter, String.class);
+		String value = conversionService.convert(parameter, String.class);
+
 		if (value == null) {
 			throw new ConversionException(String.format(
 					"Parameter value can't be null for placeholder at index '%s' in query '%s' when querying elasticsearch",

src/main/java/org/springframework/data/elasticsearch/repository/support/spel/QueryStringSpELEvaluator.java

@@ -21,8 +21,8 @@ import java.util.concurrent.ConcurrentHashMap;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.data.elasticsearch.core.convert.ConversionException;
 import org.springframework.data.elasticsearch.repository.query.ElasticsearchParametersParameterAccessor;
-import org.springframework.data.elasticsearch.repository.support.value.ElasticsearchQueryValueConversionService;
 import org.springframework.data.elasticsearch.repository.support.value.ElasticsearchCollectionValueToStringConverter;
+import org.springframework.data.elasticsearch.repository.support.value.ElasticsearchQueryValueConversionService;
 import org.springframework.data.elasticsearch.repository.support.value.ElasticsearchStringValueToStringConverter;
 import org.springframework.data.repository.query.QueryMethod;
 import org.springframework.data.repository.query.QueryMethodEvaluationContextProvider;
@@ -59,6 +59,13 @@ public class QueryStringSpELEvaluator {
 	public QueryStringSpELEvaluator(String queryString, ElasticsearchParametersParameterAccessor parameterAccessor,
 			QueryMethod queryMethod, QueryMethodEvaluationContextProvider evaluationContextProvider,
 			ConversionService conversionService) {
+
+		Assert.notNull(queryString, "queryString must not be null");
+		Assert.notNull(parameterAccessor, "parameterAccessor must not be null");
+		Assert.notNull(queryMethod, "queryMethod must not be null");
+		Assert.notNull(evaluationContextProvider, "evaluationContextProvider must not be null");
+		Assert.notNull(conversionService, "conversionService must not be null");
+
 		this.queryString = queryString;
 		this.parameterAccessor = parameterAccessor;
 		this.queryMethod = queryMethod;
@@ -74,9 +81,11 @@ public class QueryStringSpELEvaluator {
 	 */
 	public String evaluate() {
 		Expression expr = getQueryExpression(queryString);
+
 		if (expr != null) {
 			EvaluationContext context = evaluationContextProvider.getEvaluationContext(parameterAccessor.getParameters(),
 					parameterAccessor.getValues());
+
 			if (context instanceof StandardEvaluationContext standardEvaluationContext) {
 				standardEvaluationContext.setTypeConverter(elasticsearchSpELTypeConverter);
 			}
@@ -97,12 +106,14 @@ public class QueryStringSpELEvaluator {
 	 */
 	private String parseExpressions(Expression rootExpr, EvaluationContext context) {
 		StringBuilder parsed = new StringBuilder();
+
 		if (rootExpr instanceof LiteralExpression literalExpression) {
 			// get the string literal directly
 			parsed.append(literalExpression.getExpressionString());
 		} else if (rootExpr instanceof SpelExpression spelExpression) {
 			// evaluate the value
 			String value = spelExpression.getValue(context, String.class);
+
 			if (value == null) {
 				throw new ConversionException(String.format(
 						"Parameter value can't be null for SpEL expression '%s' in method '%s' when querying elasticsearch",

src/main/java/org/springframework/data/elasticsearch/repository/support/value/ElasticsearchCollectionValueToStringConverter.java

@@ -70,14 +70,18 @@ public class ElasticsearchCollectionValueToStringConverter implements GenericCon
 	@Override
 	@Nullable
 	public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
+
 		if (source == null) {
 			return "[]";
 		}
 		Collection<?> sourceCollection = (Collection<?>) source;
+
 		if (sourceCollection.isEmpty()) {
 			return "[]";
 		}
+
 		StringJoiner sb = new StringJoiner(DELIMITER, "[", "]");
+
 		for (Object sourceElement : sourceCollection) {
 			// ignore the null value in collection
 			if (Objects.isNull(sourceElement)) {
@@ -86,8 +90,9 @@ public class ElasticsearchCollectionValueToStringConverter implements GenericCon
 
 			Object targetElement = this.conversionService.convert(
 					sourceElement, sourceType.elementTypeDescriptor(sourceElement), targetType);
+
 			if (sourceElement instanceof String) {
-				sb.add("\"" + targetElement + "\"");
+				sb.add("\"" + targetElement + '"');
 			} else {
 				sb.add(String.valueOf(targetElement));
 			}

src/main/java/org/springframework/data/elasticsearch/repository/support/value/ElasticsearchQueryValueConversionService.java

@@ -27,7 +27,8 @@ import org.springframework.util.Assert;
 
 /**
  * A {@link ConversionService} using custom converters to handle query values in elasticsearch query. If the value to be
- * converted beyond the scope of custom converters, it'll delegate to the {@link #delegate delegated conversion service}.
+ * converted beyond the scope of custom converters, it'll delegate to the {@link #delegate delegated conversion
+ * service}.
  * <p>
  * This is a better solution for converting query values in elasticsearch query, because it has all the capability the
  * {@link #delegate delegated conversion service} has, especially for user-registered {@link Converter}s.
@@ -44,7 +45,9 @@ public class ElasticsearchQueryValueConversionService implements ConversionServi
 	private final ConversionService delegate;
 
 	private ElasticsearchQueryValueConversionService(ConversionService delegate) {
+
 		Assert.notNull(delegate, "delegated ConversionService must not be null");
+
 		this.delegate = delegate;
 
 		// register elasticsearch custom type converters for conversion service
@@ -85,6 +88,7 @@ public class ElasticsearchQueryValueConversionService implements ConversionServi
 	@Override
 	@Nullable
 	public Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType) {
+
 		if (valueConversionService.canConvert(sourceType, targetType)) {
 			return valueConversionService.convert(source, sourceType, targetType);
 		} else {

src/main/java/org/springframework/data/elasticsearch/repository/support/value/ElasticsearchStringValueToStringConverter.java

@@ -40,10 +40,7 @@ public class ElasticsearchStringValueToStringConverter implements GenericConvert
 	@Override
 	@Nullable
 	public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
-		if (source == null) {
+		return source != null ? escape(source) : null;
-			return null;
-		}
-		return escape(source);
 	}
 
 	private String escape(@Nullable Object source) {

src/test/java/org/springframework/data/elasticsearch/repositories/custommethod/CustomMethodRepositoryIntegrationTests.java

@@ -2109,8 +2109,8 @@ public abstract class CustomMethodRepositoryIntegrationTests {
 		SearchHits<SampleEntity> queryByType(String type);
 
 		/**
-		 * The parameter is annotated with {@link Nullable} deliberately to test that our placeholder parameter will
+		 * The parameter is annotated with {@link Nullable} deliberately to test that our placeholder parameter will not
-		 * not accept a null parameter as query value.
+		 * accept a null parameter as query value.
 		 */
 		@Query("{\"bool\": {\"must\": [{\"term\": {\"type\": \"?0\"}}]}}")
 		@Highlight(fields = { @HighlightField(name = "type") })
@@ -2444,8 +2444,7 @@ public abstract class CustomMethodRepositoryIntegrationTests {
 	}
 
 	static class SampleProperty {
-		@Nullable
+		@Nullable private String first;
-		private String first;
 		@Nullable private String last;
 
 		SampleProperty(@Nullable String first, @Nullable String last) {