package cz.metacentrum.perun.ldapc.beans;

import java.util.Collection;
import java.util.Map;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import cz.metacentrum.perun.core.api.Attribute;
import cz.metacentrum.perun.ldapc.model.AttributeValueTransformer;

public class JsonValueTransformer extends ValueTransformerBase implements AttributeValueTransformer {

	private final static Logger log = LoggerFactory.getLogger(JsonValueTransformer.class);

	protected static ObjectMapper mapper = new ObjectMapper();

	@Override
	public String getValue(String value, Attribute attr) {
		try {
			return mapper.writeValueAsString(value);
		} catch (JsonProcessingException e) {
			log.warn("Error converting {} to JSON", value, e);
			return null;
		}
	}

	@Override
	public String getValue(Collection<String> value, Attribute attr) {
		ObjectMapper mapper = new ObjectMapper();
		try {
			return mapper.writeValueAsString(value);
		} catch (JsonProcessingException e) {
			log.warn("Error converting {} to JSON", value, e);
			return null;
		}
	}

	@Override
	public String getValue(Map<String, String> value, Attribute attr) {
		ObjectMapper mapper = new ObjectMapper();
		try {
			return mapper.writeValueAsString(value);
		} catch (JsonProcessingException e) {
			log.warn("Error converting {} to JSON", value, e);
			return null;
		}
	}

	@Override
	public Boolean isMassTransformationPreferred() {
		return true;
	}

	@Override
	public Boolean isReduce() {
		return true;
	}

}

package cz.metacentrum.perun.ldapc.beans;

import java.util.Collection;
import java.util.Map;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import cz.metacentrum.perun.core.api.Attribute;
import cz.metacentrum.perun.ldapc.model.AttributeValueTransformer;

public class JsonValueTransformer extends ValueTransformerBase implements AttributeValueTransformer {

	private final static Logger log = LoggerFactory.getLogger(JsonValueTransformer.class);

	protected static ObjectMapper mapper = new ObjectMapper();

	@Override
	public String getValue(String value, Attribute attr) {
		try {
			return mapper.writeValueAsString(value);
		} catch (JsonProcessingException e) {
			log.warn("Error converting {} to JSON", value, e);
			return null;
		}
	}

	@Override
	public String getValue(Collection<String> value, Attribute attr) {
		try {
			return mapper.writeValueAsString(value);
		} catch (JsonProcessingException e) {
			log.warn("Error converting {} to JSON", value, e);
			return null;
		}
	}

	@Override
	public String getValue(Map<String, String> value, Attribute attr) {
		try {
			return mapper.writeValueAsString(value);
		} catch (JsonProcessingException e) {
			log.warn("Error converting {} to JSON", value, e);
			return null;
		}
	}

	@Override
	public Boolean isMassTransformationPreferred() {
		return true;
	}

	@Override
	public Boolean isReduce() {
		return true;
	}

}

Use static mapper in all methods of JsonValueTransformer