fix(client-v2): reject null in non-nullable Enum with clear error

serializeEnumData dereferenced the value before the null check, so
inserting null into a non-nullable Enum8/Enum16 column failed the
RowBinary serialization path with a confusing NullPointerException.
Add a null guard that throws IllegalArgumentException naming the
column, consistent with how other unsupported enum values are
rejected. Nullable enum columns are unaffected.

Fixes: #2931

Author: polyglotAI-bot

CHANGELOG.md

@@ -4,6 +4,12 @@
 
 ### Bug Fixes 
 
+- **[client-v2]** Fixed a `NullPointerException` when serializing a `null` value into a non-nullable
+  `Enum8`/`Enum16` column. `SerializerUtils.serializeEnumData` dereferenced the value before the `null`
+  check, so a `null` in a non-nullable enum column failed the RowBinary insert path with a confusing NPE
+  instead of a clear error. It now throws `IllegalArgumentException` naming the column, consistent with the
+  existing `IllegalArgumentException` for other unsupported enum values. Nullable enum columns are unaffected.
+  (https://github.com/ClickHouse/clickhouse-java/issues/2931)
 - **[client-v2]** Fixed POJO insert error classification so transport write failures such as java.net.SocketException:
   Broken pipe (Write failed) are now surfaced as transfer/network errors instead of being wrapped as
   DataSerializationException. This only changes the exception type reported for request-body transport failures during

client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/SerializerUtils.java

@@ -706,6 +706,10 @@ private static void serializeTime64(OutputStream stream, Object value) throws IO
     }
 
     public static void serializeEnumData(OutputStream stream, ClickHouseColumn column, Object value) throws IOException {
+        if (value == null) {
+            throw new IllegalArgumentException("Cannot insert null into non-nullable column " + column.getColumnName()
+                    + " of type " + column.getOriginalTypeName());
+        }
         int enumValue = -1;
         if (value instanceof String) {
             enumValue = column.getEnumConstants().value((String) value);

client-v2/src/test/java/com/clickhouse/client/api/data_formats/internal/SerializerUtilsTest.java

@@ -143,6 +143,39 @@ public void testGeometrySerializationRejectsMalformedList() {
                         ClickHouseColumn.of("v", "Geometry")));
     }
 
+    @Test(dataProvider = "nonNullableEnumTypes")
+    public void testNullIntoNonNullableEnumThrowsIllegalArgument(String typeName) {
+        ClickHouseColumn column = ClickHouseColumn.of("v", typeName);
+
+        IllegalArgumentException ex = Assert.expectThrows(IllegalArgumentException.class,
+                () -> SerializerUtils.serializeData(new ByteArrayOutputStream(), null, column));
+        Assert.assertTrue(ex.getMessage().contains("Cannot insert null into non-nullable column"),
+                "Unexpected message: " + ex.getMessage());
+    }
+
+    @DataProvider(name = "nonNullableEnumTypes")
+    private Object[][] nonNullableEnumTypes() {
+        return new Object[][] {
+                {"Enum8('B' = 1, 'S' = 2)"},
+                {"Enum16('B' = 1, 'S' = 2)"},
+        };
+    }
+
+    @Test
+    public void testEnumSerializationUnaffectedByNullGuard() throws Exception {
+        // A Nullable(Enum) with null still takes the early null-marker path and never reaches
+        // enum serialization, so a single null-marker byte is written.
+        ByteArrayOutputStream nullableOut = new ByteArrayOutputStream();
+        SerializerUtils.serializeData(nullableOut, null,
+                ClickHouseColumn.of("v", "Nullable(Enum8('B' = 1, 'S' = 2))"));
+        Assert.assertEquals(nullableOut.toByteArray(), new byte[] {1});
+
+        // A present value in a non-nullable Enum column still serializes to its mapped numeric value.
+        ByteArrayOutputStream valueOut = new ByteArrayOutputStream();
+        SerializerUtils.serializeData(valueOut, "S", ClickHouseColumn.of("v", "Enum8('B' = 1, 'S' = 2)"));
+        Assert.assertEquals(valueOut.toByteArray(), new byte[] {2});
+    }
+
     @Test(dataProvider = "nestedNullableData")
     public void testNestedNullableRoundTrip(String typeName, Object value) throws Exception {
         ClickHouseColumn column = ClickHouseColumn.of("v", typeName);