Cassandra Java驱动程序获取Java类型的TypeCodec

Question

I have the following method with generic parameter:

public <K> Store<K> createStore(final Class<K> keyType);

which will just create a table in cassandra with the given type for me.

I want to check whether there is already a default implementation of a TypeCodec for the given keyType . If yes then I just want to use this codec otherwise I'll use a generic codec written by myself.

Cassandra's CodecRegistry class provides the methods:

public <T> TypeCodec<T> codecFor(DataType cqlType, Class<T> javaType);
public <T> TypeCodec<T> codecFor(DataType cqlType);
public <T> TypeCodec<T> codecFor(T value);
public <T> TypeCodec<T> codecFor(DataType cqlType, TypeToken<T> javaType);
public <T> TypeCodec<T> codecFor(DataType cqlType, T value);

The thing is, none of these methods fits for my use case. Almost all methods require DatatType cqlType parameter which I cannot provide because I did not find a way to map the my given keyType class literal to a possible matching Cassandra Datatype . (is this possible?)

The only one that comes close for what I want, is: public <T> TypeCodec<T> codecFor(T value); but it requires an instance of the the class and not a class literal. So what I want is:

public <T> TypeCodec<T> codecFor(Class<T> javaType);

which would give me just the default TypeCodec to use for the given type, ie if i pass in the type "java.lang.Integer" then the corresponding cassandra TypeCodec for Integers shoud be returned.

Any suggestions how to implement this behavior?

Answer 1

CodecRegistry class have two fields that store all TypeCodec

/**
 * List of Built in Codec
 */
private static final TypeCodec<?>[] BUILT_IN_CODECS;    

/**
 * The list of user-registered codecs.
 */
private final CopyOnWriteArrayList<TypeCodec<?>> codecs;

You can see that both are private. We can access them with java reflection

CodecRegistry codecRegistry = CodecRegistry.DEFAULT_INSTANCE;

Map<Class, TypeCodec> javaTypeCodecMap = new HashMap<>();

try {
    Field field = codecRegistry.getClass().getDeclaredField("BUILT_IN_CODECS");
    field.setAccessible(true);
    TypeCodec<?>[] builtInCodecs = (TypeCodec<?>[]) field.get(codecRegistry);
    for (TypeCodec<?> typeCodec : builtInCodecs) {
        javaTypeCodecMap.put(typeCodec.getJavaType().getRawType(), typeCodec);
    }
} catch (NoSuchFieldException | IllegalAccessException e) {
    e.printStackTrace();
}

try {
    Field field = codecRegistry.getClass().getDeclaredField("codecs");
    field.setAccessible(true);
    CopyOnWriteArrayList<TypeCodec<?>> codecs = (CopyOnWriteArrayList<TypeCodec<?>>) field.get(codecRegistry);
    for (TypeCodec<?> typeCodec : codecs) {
        javaTypeCodecMap.put(typeCodec.getJavaType().getRawType(), typeCodec);
    }
} catch (NoSuchFieldException | IllegalAccessException e) {
    e.printStackTrace();
}

//You have the map, now you can get TypeCodec from class name
System.out.println(javaTypeCodecMap.get(String.class));

Answer 2

The feature you need has been already requested but not implemented, see JAVA-1015 . Please vote for this ticket and explain why you would like to have it implemented.

In the meanwhile, I would suggest you to redesign your createStore method; your application probably knows – or at least should know – what CQL type to use for a given keyType . Not knowing which CQL type to use is generally a bad smell.