Custom codecs

Quick overview

Define custom Java to CQL mappings.

  • implement the TypeCodec interface, or use one of the alternative codecs in ExtraTypeCodecs.
  • registering a codec:

    MutableCodecRegistry registry =
        (MutableCodecRegistry) session.getContext().getCodecRegistry();    
  • using a codec:

    • if already registered: row.get("columnName", MyCustomType.class)
    • otherwise: row.get("columnName", myCodec)

Out of the box, the driver comes with default CQL to Java mappings. For example, if you read a CQL text column, it is mapped to its natural counterpart java.lang.String:

// cqlsh:ks> desc table test;
// CREATE TABLE ks.test (k int PRIMARY KEY, v text)...
ResultSet rs = session.execute("SELECT * FROM ks.test WHERE k = 1");
String v ="v");

Sometimes you might want to use different mappings, for example:

  • read a text column as a Java enum;
  • map an address UDT to a custom Address class in your application;
  • manipulate CQL collections as arrays in performance-intensive applications.

Custom codecs allow you to define those dedicated mappings, and plug them into your session.

Using alternative codecs provided by the driver

The first thing you can do is use one of the many alternative codecs shipped with the driver. They are exposed on the ExtraTypeCodecs class. In this section we are going to introduce these codecs, then you will see how to register and use them in the next sections.

Mapping CQL blobs to Java arrays

The driver default is TypeCodecs.BLOB, which maps CQL blob to Java’s java.nio.ByteBuffer. Check out our CQL blob example to understand how to manipulate the ByteBuffer API correctly.

If the ByteBuffer API is too cumbersome for you, an alternative is to use ExtraTypeCodecs.BLOB_TO_ARRAY which maps CQL blobs to Java’s byte[].

Mapping CQL lists to Java arrays

By default, the driver maps CQL list to Java’s java.util.List. If you prefer to deal with arrays, the driver offers the following codecs:

  1. For primitive types:

    Codec CQL type Java type
    ExtraTypeCodecs.BOOLEAN_LIST_TO_ARRAY list<boolean> boolean[]
    ExtraTypeCodecs.BYTE_LIST_TO_ARRAY list<tinyint> byte[]
    ExtraTypeCodecs.SHORT_LIST_TO_ARRAY list<smallint> short[]
    ExtraTypeCodecs.INT_LIST_TO_ARRAY list<int> int[]
    ExtraTypeCodecs.LONG_LIST_TO_ARRAY list<bigint> long[]
    ExtraTypeCodecs.FLOAT_LIST_TO_ARRAY list<float> float[]
    ExtraTypeCodecs.DOUBLE_LIST_TO_ARRAY list<double> double[]
  2. For other types, you should use ExtraTypeCodecs.listToArrayOf(TypeCodec); for example, to map CQL list<text> to String[]:

    TypeCodec<String[]> stringArrayCodec = ExtraTypeCodecs.listToArrayOf(TypeCodecs.TEXT);

Mapping CQL timestamps to Java “instant” types

By default, the driver maps CQL timestamp to Java’s java.time.Instant (using TypeCodecs.TIMESTAMP). This is the most natural mapping, since neither type contains any time zone information: they just represent absolute points in time.

The driver also provides codecs to map to a Java long representing the number of milliseconds since the epoch (this is the raw form return by Instant.toEpochMilli, and also how Cassandra stores the value internally).

In either case, you can pick the time zone that the codec will use for its format() and parse() methods. Note that this is only relevant for these two methods (follow the links for more explanations on how the driver uses them); for regular encoding and decoding, like setting a value on a bound statement or reading a column from a row, the time zone does not matter.

Codec CQL type Java type Time zone used by format() and parse()
TypeCodecs.TIMESTAMP timestamp Instant System default
ExtraTypeCodecs.TIMESTAMP_UTC timestamp Instant UTC
ExtraTypeCodecs.timestampAt(ZoneId) timestamp Instant User-provided
ExtraTypeCodecs.TIMESTAMP_MILLIS_SYSTEM timestamp long System default
ExtraTypeCodecs.TIMESTAMP_MILLIS_UTC timestamp long UTC
ExtraTypeCodecs.timestampMillisAt(ZoneId) timestamp long User-provided

For example, given the schema:

CREATE TABLE example (k int PRIMARY KEY, ts timestamp);
INSERT INTO example(k, ts) VALUES (1, 0);

When reading column ts, all Instant codecs return Instant.ofEpochMilli(0). But if asked to format it, they behave differently:

  • ExtraTypeCodecs.TIMESTAMP_UTC returns '1970-01-01T00:00:00.000Z'
  • ExtraTypeCodecs.timestampAt(ZoneId.of("Europe/Paris") returns '1970-01-01T01:00:00.000+01:00'

Mapping CQL timestamps to ZonedDateTime

If your application works with one single, pre-determined time zone, then you probably would like the driver to map timestamp to java.time.ZonedDateTime with a fixed zone. Use one of the following codecs:

Codec CQL type Java type Time zone used by all codec operations
ExtraTypeCodecs.ZONED_TIMESTAMP_SYSTEM timestamp ZonedDateTime System default
ExtraTypeCodecs.ZONED_TIMESTAMP_UTC timestamp ZonedDateTime UTC
ExtraTypeCodecs.zonedTimestampAt(ZoneId) timestamp ZonedDateTime User-provided

This time, the zone matters for all codec operations, including encoding and decoding. For example, given the schema:

CREATE TABLE example (k int PRIMARY KEY, ts timestamp);
INSERT INTO example(k, ts) VALUES (1, 0);

When reading column ts:

  • ExtraTypeCodecs.ZONED_TIMESTAMP_UTC returns the same value as ZonedDateTime.parse("1970-01-01T00:00Z")
  • ExtraTypeCodecs.zonedTimestampAt(ZoneId.of("Europe/Paris")) returns the same value as ZonedDateTime.parse("1970-01-01T01:00+01:00[Europe/Paris]")

These are two distinct ZonedDateTime instances: although they represent the same absolute point in time, they do not compare as equal.

Mapping CQL timestamps to LocalDateTime

If your application works with one single, pre-determined time zone, but only exposes local date-times, then you probably would like the driver to map timestamps to java.time.LocalDateTime obtained from a fixed zone. Use one of the following codecs:

Codec CQL type Java type Time zone used by all codec operations
ExtraTypeCodecs.LOCAL_TIMESTAMP_SYSTEM timestamp LocalDateTime System default
ExtraTypeCodecs.LOCAL_TIMESTAMP_UTC timestamp LocalDateTime UTC
ExtraTypeCodecs.localTimestampAt(ZoneId) timestamp LocalDateTime User-provided

Again, the zone matters for all codec operations, including encoding and decoding. For example, given the schema:

CREATE TABLE example (k int PRIMARY KEY, ts timestamp);
INSERT INTO example(k, ts) VALUES (1, 0);

When reading column ts:

  • ExtraTypeCodecs.LOCAL_TIMESTAMP_UTC returns LocalDateTime.of(1970, 1, 1, 0, 0)
  • ExtraTypeCodecs.localTimestampAt(ZoneId.of("Europe/Paris")) returns LocalDateTime.of(1970, 1, 1, 1, 0)

Storing the time zone in Cassandra

If your application needs to remember the time zone that each date was entered with, you need to store it in the database. We suggest using a tuple<timestamp, text>, where the second component holds the zone id.

If you follow this guideline, then you can use ExtraTypeCodecs.ZONED_TIMESTAMP_PERSISTED to map the CQL tuple to java.time.ZonedDateTime.

For example, given the schema:

CREATE TABLE example(k int PRIMARY KEY, zts tuple<timestamp, text>);
INSERT INTO example (k, zts) VALUES (1, (0, 'Z'));
INSERT INTO example (k, zts) VALUES (2, (-3600000, 'Europe/Paris'));

When reading column zts, ExtraTypeCodecs.ZONED_TIMESTAMP_PERSISTED returns:

  • ZonedDateTime.parse("1970-01-01T00:00Z") for the first row
  • ZonedDateTime.parse("1970-01-01T00:00+01:00[Europe/Paris]") for the second row

Each value is read back in the time zone that it was written with. But note that you can still compare rows on a absolute timeline with the timestamp component of the tuple.

Mapping to Optional instead of null

If you prefer to deal with java.util.Optional in your application instead of nulls, then you can use ExtraTypeCodecs.optionalOf(TypeCodec):

TypeCodec<Optional<UUID>> optionalUuidCodec = ExtraTypeCodecs.optionalOf(TypeCodecs.UUID);

Note that because the CQL native protocol does not distinguish empty collections from null collection references, this codec will also map empty collections to Optional.empty().

Mapping Java Enums

Java Enums can be mapped to CQL in two ways:

  1. By name: ExtraTypeCodecs.enumNamesOf(Class) will create a codec for a given Enum class that maps its constants to their programmatic names. The corresponding CQL column must be of type text. Note that this codec relies on the enum constant names; it is therefore vital that enum names never change.
  2. By ordinal: ExtraTypeCodecs.enumOrdinalsOf(Class) will create a codec for a given Enum class that maps its constants to their ordinal value. The corresponding CQL column must be of type int.

    We strongly recommend against this approach. It is provided for compatibility with driver 3, but relying on ordinals is a bad practice: any reordering of the enum constants, or insertion of a new constant before the end, will change the ordinals. The codec won’t fail, but it will insert different codes and corrupt your data.

    If you really want to use integer codes for storage efficiency, implement an explicit mapping (for example with a toCode() method on your enum type). It is then fairly straightforward to implement a codec with MappingCodec, using TypeCodecs#INT as the “inner” codec.

For example, assuming the following enum:

public enum WeekDay {

You can define codecs for it the following ways:

// MONDAY will be persisted as "MONDAY", TUESDAY as "TUESDAY", etc.
TypeCodec<String> weekDaysByNameCodec = ExtraTypeCodecs.enumNamesOf(WeekDay.class);

// MONDAY will be persisted as 0, TUESDAY as 1, etc.
TypeCodec<Integer> weekDaysByNameCodec = ExtraTypeCodecs.enumOrdinalsOf(WeekDay.class);

Mapping Json

The driver provides out-of-the-box support for mapping Java objects to CQL text using the popular Jackson library. The method ExtraTypeCodecs.json(Class) will create a codec for a given Java class that maps instances of that class to Json strings, using a newly-allocated, default ObjectMapper. It is also possible to pass a custom ObjectMapper instance using ExtraTypeCodecs.json(Class, ObjectMapper) instead.

Writing codecs

If none of the driver built-in codecs above suits you, it is also possible to roll your own.

To write a custom codec, implement the TypeCodec interface. Here is an example that maps a CQL int to a Java string containing its textual representation:

public class CqlIntToStringCodec implements TypeCodec<String> {

  public GenericType<String> getJavaType() {
    return GenericType.STRING;

  public DataType getCqlType() {
    return DataTypes.INT;

  public ByteBuffer encode(String value, ProtocolVersion protocolVersion) {
    if (value == null) {
      return null;
    } else {
      int intValue = Integer.parseInt(value);
      return TypeCodecs.INT.encode(intValue, protocolVersion);

  public String decode(ByteBuffer bytes, ProtocolVersion protocolVersion) {
    Integer intValue = TypeCodecs.INT.decode(bytes, protocolVersion);
    return intValue.toString();

  public String format(String value) {
    int intValue = Integer.parseInt(value);
    return TypeCodecs.INT.format(intValue);

  public String parse(String value) {
    Integer intValue = TypeCodecs.INT.parse(value);
    return intValue == null ? null : intValue.toString();

Admittedly, this is a trivial – and maybe not very realistic – example, but it illustrates a few important points:

  • which methods to override. Refer to the TypeCodec javadocs for additional information about each of them;
  • how to piggyback on a built-in codec, in this case TypeCodecs.INT. Very often, this is the best approach to keep the code simple. If you want to handle the binary encoding yourself (maybe to squeeze the last bit of performance), study the driver’s built-in codec implementations.

Using codecs

Once you have your codec, register it when building your session. The following example registers CqlIntToStringCodec along with a few driver-supplied alternative codecs:

class Price {
  ... // a custom POJO that will be serialized as JSON

CqlSession session =
      new CqlIntToStringCodec(),                  // user-created codec
      ExtraTypeCodecs.ZONED_TIMESTAMP_PERSISTED,  // tuple<timestamp,text> <-> ZonedDateTime
      ExtraTypeCodecs.BLOB_TO_ARRAY,              // blob <-> byte[]
      ExtraTypeCodecs.arrayOf(TypeCodecs.TEXT),   // list<text> <-> String[]
      ExtraTypeCodecs.enumNamesOf(WeekDay.class), // text <-> WeekDay
      ExtraTypeCodecs.json(Price.class),          // text <-> MyJsonPojo
      ExtraTypeCodecs.optionalOf(TypeCodecs.UUID) // uuid <-> Optional<UUID>

You may also add codecs to an existing session at runtime:

// The cast is required for backward compatibility reasons (registry mutability was introduced in
// 4.3.0). It is safe as long as you didn't write a custom registry implementation.
MutableCodecRegistry registry =
    (MutableCodecRegistry) session.getContext().getCodecRegistry();

registry.register(new CqlIntToStringCodec());

You can now use the new mappings in your code:

// cqlsh:ks> desc table test2;
// CREATE TABLE ks.test2 (k int PRIMARY KEY, v int)...
ResultSet rs = session.execute("SELECT * FROM ks.test2 WHERE k = 1");
String v ="v"); // read a CQL int as a java.lang.String

PreparedStatement ps = session.prepare("INSERT INTO ks.test2 (k, v) VALUES (?, ?)");
        .setInt("k", 2)
        .setString("v", "12") // write a java.lang.String as a CQL int

In the above example, the driver will look up in the codec registry a codec for CQL int and Java String, and will transparently pick CqlIntToStringCodec for that.

So far our examples have used a Java type with dedicated accessors in the driver: getString and setString. But sometimes you won’t find suitable accessor methods; for example, there is no accessor for ZonedDateTime or for Optional<UUID>, and yet we registered codecs for these types.

When you want to retrieve such objects, you need a way to tell the driver which Java type you want. You do so by using one of the generic get and set methods:

// Assuming that ExtraTypeCodecs.ZONED_TIMESTAMP_PERSISTED was registered
// Assuming that ExtraTypeCodecs.BLOB_TO_ARRAY was registered
// Assuming that ExtraTypeCodecs.arrayOf(TypeCodecs.TEXT) was registered

// Reading
ZonedDateTime v1 = row.get("v1", ZonedDateTime.class); // assuming column is of type timestamp
byte[] v2        = row.get("v2", byte[].class);        // assuming column is of type blob
String[] v3      = row.get("v3", String[].class);      // assuming column is of type list<text>

// Writing
boundStatement.set("v1", v1, ZonedDateTime.class);
boundStatement.set("v2", v2, byte[].class);
boundStatement.set("v3", v3, String[].class);

This is also valid for arbitrary Java types. This is particularly useful when dealing with Enums and JSON mappings, for example our WeekDay and Price types:

// Assuming that TypeCodecs.enumNamesOf(WeekDay.class) was registered
// Assuming that TypeCodecs.json(Price.class) was registered

// Reading
WeekDay v1 = row.get("v1", WeekDay.class); // assuming column is of type text
Price v2   = row.get("v2", Price.class);   // assuming column is of type text

// Writing
boundStatement.set("v1", v1, WeekDay.class);
boundStatement.set("v2", v2, Price.class);

Note that, because the underlying CQL type is text you can still retrieve the column’s contents as a plain string:

// Reading
String enumName = row.getString("v1");
String priceJson = row.getString("v2");

// Writing
boundStatement.setString("v1", enumName);
boundStatement.setString("v2", priceJson);

And finally, for Optional<UUID>, you will need the get and set methods with an extra type token argument, because Optional<UUID> is a parameterized type:

// Assuming that TypeCodecs.optionalOf(TypeCodecs.UUID) was registered

// Reading
Optional<UUID> opt = row.get("v", GenericType.optionalOf(UUID.class));

// Writing
boundStatement.set("v", opt, GenericType.optionalOf(UUID.class));

Type tokens are instances of GenericType. They are immutable and thread-safe, you should store them as reusable constants. The GenericType class itself has constants and factory methods to help creating GenericType objects for common types. If you don’t see the type you are looking for, a type token for any Java type can be created using the following pattern:

// Notice the '{}': this is an anonymous inner class
GenericType<Foo<Bar>> fooBarType = new GenericType<Foo<Bar>>(){};

Foo<Bar> v = row.get("v", fooBarType);

Custom codecs are used not only for their base type, but also recursively in collections, tuples and UDTs. For example, once your Json codec for the Price class is registered, you can also read a CQL list<text> as a Java List<Price>:

// Assuming that TypeCodecs.json(Price.class) was registered
// Assuming that each element of the list<text> column is a valid Json string

// Reading
List<Price> prices1 = row.getList("v", Price.class);
// alternative method using the generic get method with type token argument:
List<Price> prices2 = row.get("v", GenericType.listOf(Price.class));

// Writing
boundStatement.setList("v", prices1, Price.class);
// alternative method using the generic set method with type token argument:
boundStatement.set("v", prices2, GenericType.listOf(Price.class));

Whenever you read or write a value, the driver tries all the built-in mappings first, followed by custom codecs. If two codecs can process the same mapping, the one that was registered first is used. Note that this means that built-in mappings can’t be overridden.

In rare cases, you might have a codec registered in your application, but have a legitimate reason to use a different mapping in one particular place. In that case, you can pass a codec instance to get / set instead of a type token:

TypeCodec<String> defaultCodec = new CqlIntToStringCodec();
TypeCodec<String> specialCodec = ...; // a different implementation

CqlSession session =

String s1 = row.getString("anIntColumn");         // int -> String, will decode with defaultCodec
String s2 = row.get("anIntColumn", specialCodec); // int -> String, will decode with specialCodec

By doing so, you bypass the codec registry completely and instruct the driver to use the given codec. Note that it is your responsibility to ensure that the codec can handle the underlying CQL type (this cannot be enforced at compile-time).

Creating custom Java-to-CQL mappings with MappingCodec

The above example, CqlIntToStringCodec, could be rewritten to leverage MappingCodec, an abstract class that ships with the driver. This class has been designed for situations where we want to represent a CQL type with a different Java type than the Java type natively supported by the driver, and the conversion between the former and the latter is straightforward.

All you have to do is extend MappingCodec and implement two methods that perform the conversion between the supported Java type – or “inner” type – and the target Java type – or “outer” type:

public class CqlIntToStringCodec extends MappingCodec<Integer, String> {

  public CqlIntToStringCodec() {
    super(TypeCodecs.INT, GenericType.STRING);

  protected String innerToOuter(@Nullable Integer value) {
    return value == null ? null : value.toString();

  protected Integer outerToInner(@Nullable String value) {
    return value == null ? null : Integer.parseInt(value);

This technique is especially useful when mapping user-defined types to Java objects. For example, let’s assume the following user-defined type:

CREATE TYPE coordinates (x int, y int);

And let’s suppose that we want to map it to the following Java class:

public class Coordinates {
  public final int x;
  public final int y;
  public Coordinates(int x, int y) { this.x = x; this.y = y; }

All you have to do is create a MappingCodec subclass that piggybacks on an existing TypeCodec<UdtValue> for the above user-defined type:

public class CoordinatesCodec extends MappingCodec<UdtValue, Coordinates> {

  public CoordinatesCodec(@NonNull TypeCodec<UdtValue> innerCodec) {
    super(innerCodec, GenericType.of(Coordinates.class));

  @NonNull @Override public UserDefinedType getCqlType() {
    return (UserDefinedType) super.getCqlType();

  @Nullable @Override protected Coordinates innerToOuter(@Nullable UdtValue value) {
    return value == null ? null : new Coordinates(value.getInt("x"), value.getInt("y"));

  @Nullable @Override protected UdtValue outerToInner(@Nullable Coordinates value) {
    return value == null ? null : getCqlType().newValue().setInt("x", value.x).setInt("y", value.y);

Then the new mapping codec could be registered as follows:

CqlSession session = ...
CodecRegistry codecRegistry = session.getContext().getCodecRegistry();
// The target user-defined type
UserDefinedType coordinatesUdt =
        .flatMap(ks -> ks.getUserDefinedType("coordinates"))
// The "inner" codec that handles the conversions from CQL from/to UdtValue
TypeCodec<UdtValue> innerCodec = codecRegistry.codecFor(coordinatesUdt);
// The mapping codec that will handle the conversions from/to UdtValue and Coordinates
CoordinatesCodec coordinatesCodec = new CoordinatesCodec(innerCodec);
// Register the new codec
((MutableCodecRegistry) codecRegistry).register(coordinatesCodec);

…and used just like explained above:

BoundStatement stmt = ...;
stmt.set("coordinates", new Coordinates(10,20), Coordinates.class);

Row row = ...;
Coordinates coordinates = row.get("coordinates", Coordinates.class);

Note: if you need even more advanced mapping capabilities, consider adopting the driver’s object mapping framework.

Subtype polymorphism

Suppose the following class hierarchy:

class Animal {}
class Cat extends Animal {}

By default, a codec will accept to serialize any object that extends or implements its declared Java type: a codec such as AnimalCodec extends TypeCodec<Animal> will accept Cat instances as well.

This allows a codec to handle interfaces and superclasses in a generic way, regardless of the actual implementation being used by client code; for example, the driver has a built-in codec that handles List instances, and this codec is capable of serializing any concrete List implementation.

But this has one caveat: when setting or retrieving values with get() and set(), you must pass the exact Java type the codec handles:

BoundStatement bs = ...
bs.set(0, new Cat(), Animal.class); // works
bs.set(0, new Cat(),    Cat.class); // throws CodecNotFoundException

Row row = ...
Animal animal = row.get(0, Animal.class); // works
Cat    cat    = row.get(0,    Cat.class); // throws CodecNotFoundException

The codec registry

The driver stores all codecs (built-in and custom) in an internal CodecRegistry:

CodecRegistry getCodecRegistry = session.getContext().getCodecRegistry();

// Get the custom codec we registered earlier:
TypeCodec<String> cqlIntToString = codecRegistry.codecFor(DataTypes.INT, GenericType.STRING);

If all you’re doing is executing requests and reading responses, you probably won’t ever need to access the registry directly. But it’s useful if you do some kind of generic processing, for example printing out an arbitrary row when the schema is not known at compile time:

private static String formatRow(Row row) {
  StringBuilder result = new StringBuilder();
  for (int i = 0; i < row.size(); i++) {
    String name = row.getColumnDefinitions().get(i).getName().asCql(true);
    Object value = row.getObject(i);
    DataType cqlType = row.getType(i);

    // Find the best codec to format this CQL type: 
    TypeCodec<Object> codec = row.codecRegistry().codecFor(cqlType);

    if (i != 0) {
      result.append(", ");
    result.append(name).append(" = ").append(codec.format(value));
  return result.toString();