FromJsonInferSchema UDF from Mortar Pig 0.12 fork. Requires json.patch from https://issues.apache.org/jira/browse/PIG-1914

FromJsonInferSchema.java

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package org.apache.pig.piggybank.evaluation;

import java.io.IOException;
import java.util.Map;

import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.JsonParser;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.apache.pig.EvalFunc;
import org.apache.pig.PigWarning;
import org.apache.pig.ResourceSchema;
import org.apache.pig.ResourceSchema.ResourceFieldSchema;
import org.apache.pig.backend.executionengine.ExecException;
import org.apache.pig.data.DataType;
import org.apache.pig.data.Tuple;
import org.apache.pig.data.TupleFactory;
import org.apache.pig.impl.logicalLayer.schema.Schema;
import org.apache.pig.impl.util.Utils;

import org.apache.pig.piggybank.storage.JsonLoader;

public class FromJsonInferSchema extends EvalFunc<Map> {

    private static final Log log = LogFactory.getLog(FromJsonInferSchema.class);

    private JsonFactory jsonFactory;
    private TupleFactory tupleFactory;

    public FromJsonInferSchema() {
        jsonFactory = new JsonFactory();
        tupleFactory = TupleFactory.getInstance();
    }

    @Override
    public Map exec(Tuple input) throws IOException {
        if (input == null || input.size() == 0) {
            return null;
        }
        try {
            String jsonStr = (String) input.get(0);
            String schemaStr = "object: map[]";

            ResourceSchema schema = new ResourceSchema(Utils.getSchemaFromString(schemaStr));
            ResourceFieldSchema[] fields = schema.getFields();

            JsonParser p = jsonFactory.createJsonParser(jsonStr);

            Tuple t = tupleFactory.newTuple(1);
            try {
                p.nextToken(); // move to start of object
                t.set(0, JsonLoader.readField(jsonStr, p, fields[0]));
            } catch (JsonParseException jpe) {
                log.error("Error parsing input: " + jsonStr + ": " + jpe.toString());
            }

            p.close();
            return (Map) t.get(0);
        } catch (ExecException e) {
            warn("Error reading input: " + e.getMessage(), PigWarning.UDF_WARNING_1);
            return null;
        }
    }

    @Override
    public Schema outputSchema(Schema input) {
        return new Schema(
            new Schema.FieldSchema("object", DataType.MAP)
        );
    }
}