Is there any way to convert a Map to a JSON representation using Jackson without writing to a file?

Pass your Map to ObjectMapper.writeValueAsString(Object value)

It’s more efficient than using StringWriter, according to the docs:

Method that can be used to serialize any Java value as a String. Functionally equivalent to calling writeValue(Writer,Object) with StringWriter and constructing String, but more efficient.

Example

import org.codehaus.jackson.map.ObjectMapper;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class Example {

    public static void main(String[] args) throws IOException {
        Map<String,String> map = new HashMap<>();
        map.put("key1","value1");
        map.put("key2","value2");

        String mapAsJson = new ObjectMapper().writeValueAsString(map);
        System.out.println(mapAsJson);
    }
}