Escape backslash in StringQuery.

Original Pull Request
Closes #2326

(cherry picked from commit 03ecc48)

Author: sothawo

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

@@ -46,7 +46,11 @@ public String replacePlaceholders(String input, ParameterAccessor accessor) {
 
 			String placeholder = Pattern.quote(matcher.group()) + "(?!\\d+)";
 			int index = NumberUtils.parseNumber(matcher.group(1), Integer.class);
-			result = result.replaceAll(placeholder, Matcher.quoteReplacement(getParameterWithIndex(accessor, index)));
+			String replacement = Matcher.quoteReplacement(getParameterWithIndex(accessor, index));
+			result = result.replaceAll(placeholder, replacement);
+			// need to escape backslashes that are not escapes for quotes so that they are sent as double-backslashes
+			// to Elasticsearch
+			result = result.replaceAll("\\\\([^\"'])", "\\\\\\\\$1");
 		}
 		return result;
 	}
@@ -56,7 +60,6 @@ private String getParameterWithIndex(ParameterAccessor accessor, int index) {
 		Object parameter = accessor.getBindableValue(index);
 		String parameterValue = "null";
 
-		// noinspection ConstantConditions
 		if (parameter != null) {
 
 			parameterValue = convert(parameter);

src/test/java/org/springframework/data/elasticsearch/repository/query/ElasticsearchStringQueryUnitTests.java

@@ -129,6 +129,18 @@ void shouldEscapeStringsInCollectionsQueryParameters() throws Exception {
 				"{ 'bool' : { 'must' : { 'terms' : { 'name' : [\"hello \\\"Stranger\\\"\",\"Another string\"] } } } }");
 	}
 
+	@Test // #2326
+	@DisplayName("should escape backslashes in collection query parameters")
+	void shouldEscapeBackslashesInCollectionQueryParameters() throws NoSuchMethodException {
+
+		final List<String> parameters = Arrays.asList("param\\1", "param\\2");
+		List<String> params = new ArrayList<>(parameters);
+		org.springframework.data.elasticsearch.core.query.Query query = createQuery("findByNameIn", params);
+
+		assertThat(query).isInstanceOf(StringQuery.class);
+		assertThat(((StringQuery) query).getSource()).isEqualTo(
+			"{ 'bool' : { 'must' : { 'terms' : { 'name' : [\"param\\\\1\",\"param\\\\2\"] } } } }");
+	}
 	private org.springframework.data.elasticsearch.core.query.Query createQuery(String methodName, Object... args)
 			throws NoSuchMethodException {