Custom codecs

Quick overview

Define custom Java to CQL mappings.

  • implement the TypeCodec interface.
  • 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.

Writing codecs

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:

CqlSession session = CqlSession.builder()
    .addTypeCodecs(new CqlIntToStringCodec())

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 hack the driver internals to plug a custom registry
// implementation.
MutableCodecRegistry registry =
    (MutableCodecRegistry) session.getContext().getCodecRegistry();

registry.register(new CqlIntToStringCodec());

You can now use the new mapping 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

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

// cqlsh:ks> desc table test3;
// CREATE TABLE ks.test2 (k int PRIMARY KEY, v list<int>)...
ResultSet rs = session.execute("SELECT * FROM ks.test3 WHERE k = 1");
List<String> v ="v", String.class);

So far our examples have used a Java type with dedicated accessors in the driver: getString and setString. But you can also map your own Java types. For example, let’s assume you have a Price class, and have registered a codec that maps it to a particular CQL type. When reading or writing values, you need a way to tell the driver which Java type you want; this is done with the generic get and set methods with an extra type token arguments:

GenericType<Price> priceType = GenericType.of(Price.class);

// Reading
Price price = row.get("v", priceType);

// Writing
boundStatement.set("v", price, priceType);

Type tokens are instances of GenericType. They are immutable and thread-safe, you should store them as reusable constants. Generic Java types are fully supported, 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);

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

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();