gls_docs/src/main/java/ch/psi/daq/rest/ResponseStreamWriter.java

/**
 * 
 */
package ch.psi.daq.rest;

import java.io.IOException;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import javax.servlet.ServletResponse;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;

import ch.psi.daq.domain.cassandra.DataEvent;
import ch.psi.daq.rest.queries.AbstractQuery;

import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
import com.fasterxml.jackson.databind.ser.impl.SimpleBeanPropertyFilter;
import com.fasterxml.jackson.databind.ser.impl.SimpleFilterProvider;

/**
 * Takes a Java 8 stream and writes it to the output stream provided by the {@link ServletResponse}
 * of the current request.
 */
public class ResponseStreamWriter {

   private static final Logger logger = LoggerFactory.getLogger(ResponseStreamWriter.class);

   @Autowired
   private JsonFactory jsonFactory;

   @Autowired
   private ObjectMapper mapper;

   /**
    * Responding with the the contents of the stream by writing into the output stream of the
    * {@link ServletResponse}.
    * 
    * @param stream {@link Stream} instance of {@link DataEvent}s
    * @param query concrete instance of {@link AbstractQuery}
    * @param response {@link ServletResponse} instance given by the current HTTP request
    * @throws IOException thrown if writing to the output stream fails
    */
   public void respond(Stream<DataEvent> stream, AbstractQuery query, ServletResponse response) throws IOException {

      Set<String> includedFields = query.getFields();

      if (query.getAggregations() != null) {
         includedFields.addAll(query.getAggregations()
               .stream()
               .map(a -> {
                  return a.getType().toString();
               })
               .collect(Collectors.toSet()));
      }

      ObjectWriter writer = configureWriter(includedFields);

      respondInternal(stream, response, writer);
   }

   /**
    * Configures the writer dynamically by including the fields which should be included in the
    * response.
    * 
    * @param includedFields set of strings which correspond to the getter method names of the
    *        classes registered as a mixed-in
    * @return the configured writer that includes the specified fields
    */
   private ObjectWriter configureWriter(Set<String> includedFields) {
      SimpleFilterProvider propertyFilter = new SimpleFilterProvider();
      propertyFilter.addFilter("namedPropertyFilter", SimpleBeanPropertyFilter.filterOutAllExcept(includedFields));
      // only write the properties not excluded in the filter
      ObjectWriter writer = mapper.writer(propertyFilter);
      return writer;
   }

   /**
    * Writes the Java stream into the output stream.
    * 
    * @param stream {@link Stream} instance of {@link DataEvent}s
    * @param response {@link ServletResponse} instance given by the current HTTP request
    * @param writer configured writer that includes the fields the end user wants to see
    * @throws IOException thrown if writing to the output stream fails
    */
   private void respondInternal(Stream<DataEvent> stream, ServletResponse response, ObjectWriter writer)
         throws IOException {

      JsonGenerator generator = jsonFactory.createGenerator(response.getOutputStream(), JsonEncoding.UTF8);
      generator.writeStartArray();
      stream.forEach(ds -> {
         try {
            logger.trace("Writing value for: {}", ds);
            // use the writer created just before
            writer.writeValue(generator, ds);
         } catch (Exception e) {
            logger.error("", e);
         }
      });
      generator.writeEndArray();
      generator.flush();
      generator.close();
   }

}