fix constant_keyword field type not working

Signed-off-by: kkewwei <kkewwei@163.com>

Author: kkewwei

CHANGELOG.md

@@ -55,6 +55,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix file cache initialization ([#14004](https://github.com/opensearch-project/OpenSearch/pull/14004))
 - Handle NPE in GetResult if "found" field is missing ([#14552](https://github.com/opensearch-project/OpenSearch/pull/14552))
 - Refactoring FilterPath.parse by using an iterative approach ([#14200](https://github.com/opensearch-project/OpenSearch/pull/14200))
+- Fix constant_keyword field type not working ([#14651](https://github.com/opensearch-project/OpenSearch/pull/14651))
 
 ### Security
 

server/src/main/java/org/opensearch/index/mapper/ConstantKeywordFieldMapper.java

@@ -68,11 +68,11 @@ private static ConstantKeywordFieldMapper toType(FieldMapper in) {
      */
     public static class Builder extends ParametrizedFieldMapper.Builder {
 
-        private final Parameter<String> value;
+        private final Parameter<String> value = Parameter.stringParam(valuePropertyName, false, m -> toType(m).value, null);
 
         public Builder(String name, String value) {
             super(name);
-            this.value = Parameter.stringParam(valuePropertyName, false, m -> toType(m).value, value);
+            this.value.setValue(value);
         }
 
         @Override

server/src/test/java/org/opensearch/index/mapper/ConstantKeywordFieldMapperTests.java

@@ -105,6 +105,14 @@ public void testMissingDefaultIndexMapper() throws Exception {
         assertThat(e.getMessage(), containsString("Field [field] is missing required parameter [value]"));
     }
 
+    public void testBuilderToXContent() throws IOException {
+        ConstantKeywordFieldMapper.Builder builder = new ConstantKeywordFieldMapper.Builder("name", "value1");
+        XContentBuilder xContentBuilder = JsonXContent.contentBuilder().startObject();
+        builder.toXContent(xContentBuilder, false);
+        xContentBuilder.endObject();
+        assertEquals("{\"value\":\"value1\"}", xContentBuilder.toString());
+    }
+
     private final SourceToParse source(CheckedConsumer<XContentBuilder, IOException> build) throws IOException {
         XContentBuilder builder = JsonXContent.contentBuilder().startObject();
         build.accept(builder);