Java Generics Map with values limited by key's type parameter

Question

How can I declare a map variable using Generics, that has Class of X as Key, and the related Serializer of X as Value.

as example I want to have a Map that can contains:

K, V
BigDecimal.class, new BigDecimalSerializer()
LocalDate.class, new LocalDateSerializer()
Date.class, new DateSerializer()

Serializer example:

public class BigDecimalSerializer extends JsonSerializer<BigDecimal> {

    @Override
    public void serialize(BigDecimal value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeString(FormatUtils.format(value));
    }

}

I tried Map<Class<?>, ? extends JsonSerializer<?>>

@Getter
@AllArgsConstructor
public class CsvWriterConfig {
    
    private final Map<Class<?>, ? extends JsonSerializer<?>> serializers;
    private final char separator;
    
}

but adding them to SimpleModule

SimpleModule module = new SimpleModule();
serializers.forEach(module::addSerializer);

gives me an error:

The type SimpleModule does not define addSerializer(Class<capture#5-of ?>, capture#4-of ? extends JsonSerializer<?>) that is applicable here

The SimpleModule addSerializer is:

public <T> SimpleModule addSerializer(Class<? extends T> type, JsonSerializer<T> ser)
{
    _checkNotNull(type, "type to register serializer for");
    _checkNotNull(ser, "serializer");
    if (_serializers == null) {
        _serializers = new SimpleSerializers();
    }
    _serializers.addSerializer(type, ser);
    return this;
}

gneginskiy · Accepted Answer · 2023-02-12 14:33:21Z

Currently, there's no good solution for it .

one possible approach is to use raw types (and delete generics whatsoever)

another would be to add a method to add serializers to the module, and cast types within the method:

    @SuppressWarnings("unchecked")
    private <T> void addSerializers(SimpleModule module) {
        this.serializers.forEach((type, serializer) ->
                module.addSerializer((Class<? extends T>) type, (JsonSerializer<T>) serializer));
    }

btw, the code above is taken from spring-web sources

The third approach would be to create a wrapper for a Map with accessors methods, which will ensure that no incorrect type-serializer pairs were added. e.g.:

        SerializersWrapper serializersWrapper = new SerializersWrapper();
        SimpleModule module = new SimpleModule();
        serializersWrapper.getAll().forEach(module::addSerializer);

and the wrapper code is


import com.fasterxml.jackson.databind.JsonSerializer;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class SerializersWrapper {
    private final Map<Class,JsonSerializer> serializers = new HashMap<>();

    public <T> void addSerializer(Class<T> type, JsonSerializer<T> serializer) {
        serializers.put(type, serializer);
    }

    public <T> JsonSerializer<T>  getSerializer(Class<T> type){
        return serializers.get(type);
    }

    public <T> Map<Class<T>,JsonSerializer<T>> getAll(){
        return (Map<Class<T>, JsonSerializer<T>>) (Map) Collections.unmodifiableMap(serializers);
    }
}

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