Teach default edn parser to produce values that can be serialized

[bpsm]

This addresses a use-case where Apache Spark configuration is stored as edn but needs to be distributed between machines via Java Serialization after parsing because that's the way Spark wants to work.

[bpsm]

Another option worth considering is explicitly wrapping the parsed edn value in an object which provides Java serialization by writing the edn text to the ObjectOutput stream. I expect this will be more compact than plain Java serialization in most cases.

package us.bpsm.edn.serialization;

import us.bpsm.edn.parser.Parser;
import us.bpsm.edn.parser.Parsers;
import us.bpsm.edn.printer.Printer;
import us.bpsm.edn.printer.Printers;
import us.bpsm.edn.protocols.Protocol;

import java.io.*;

@SuppressWarnings("unused")
public class SerializationWrapper implements Externalizable {
    private static final long MAGIC_V1 = 0x2eb534fa9a014827L;
    Object value;
    transient Protocol<Printer.Fn<?>> protocol;

    public SerializationWrapper() {
    }

    public SerializationWrapper(Object value) {
        this(value, Printers.defaultPrinterProtocol());
    }

    public SerializationWrapper(Object value, Protocol<Printer.Fn<?>> protocol) {
        this.value = value;
        this.protocol = protocol;
    }

    public Object getValue() {
        return value;
    }

    private Protocol<Printer.Fn<?>> getProtocol() {
        return protocol == null ? Printers.defaultPrinterProtocol() : protocol;
    }

    @Override
    public void writeExternal(final ObjectOutput out) throws IOException {
        out.writeLong(MAGIC_V1);
        out.writeUTF(Printers.printString(getProtocol(), value));
    }

    @Override
    public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
        long magic = in.readLong();
        if (magic == MAGIC_V1) {
            Parser parser = Parsers.newParser(Parsers.defaultConfiguration());
            this.value = parser.nextValue(Parsers.newParseable(in.readUTF()));
        } else {
            throw new InvalidObjectException("Unrecognized magic=" + magic);
        }
        if (this.value == Parser.END_OF_INPUT) {
            throw new InvalidObjectException("Unexpected end of edn data");
        }
    }
}

[chrisbetz]

:+1:

[bpsm]

Ok. I'll integrate this feature and include it in the next release. I don't intend to include the SerializationWrapper that I sketched above.

[bpsm]

Superseded by https://github.com/bpsm/edn-java/pull/44