A very minimalistic configuration showing how zalando-incubator/json-fields can be used with a Spring Boot application

JsonFieldsApplication.java

package org.zalando.demo.boot.jsonfields;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.google.common.base.Supplier;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
import org.zalando.guild.api.json.fields.jackson.JsonFieldsModule;
import org.zalando.guild.api.json.fields.jackson.ThreadLocalContextProvider;
import org.zalando.guild.api.json.fields.java.expression.ParserFramework;
import org.zalando.guild.api.json.fields.java.model.FieldPredicate;
import org.zalando.guild.api.json.fields.java.model.FieldPredicates;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;
import java.util.List;

@SpringBootApplication
public class JsonFieldsApplication {

  @Configuration
  static final class WebMvcConfiguration extends WebMvcConfigurerAdapter {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
      converters.add(mappingJackson2HttpMessageConverter());
    }

    @Bean
    public FilterRegistrationBean fieldsFilter() {
      FilterRegistrationBean registration = new FilterRegistrationBean();
      registration.setFilter(new FieldsFilter());
      registration.addUrlPatterns("/*");
      registration.setName("fieldsFilter");
      return registration;
    }

    private MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
      MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter();
      ObjectMapper objectMapper = new Jackson2ObjectMapperBuilder()
          .modules(
              JsonFieldsModule.createJsonFieldsModule(
                  FieldsFilter.FIELD_PREDICATE_SUPPLIER,
                  ThreadLocalContextProvider.getInstance()))
          .build();
      jsonConverter.setObjectMapper(objectMapper);
      return jsonConverter;
    }

    /**
     * A plain Servlet Filter that will parse the configured parameter from the incoming request and
     * expose it afterwards via a {@link ThreadLocal} variable.
     */
    static final class FieldsFilter implements Filter {

      /**
       * {@link ThreadLocal} based implementation for a {@link Supplier<FieldPredicate>} as
       * needed by
       * the <i>json-fields</i> library
       */
      public static final Supplier<FieldPredicate> FIELD_PREDICATE_SUPPLIER = new Supplier<FieldPredicate>() {

        @Override
        public FieldPredicate get() {
          return FIELD_PREDICATE.get();
        }
      };

      /**
       * Default filter which will not alter the original reponse
       */
      private static final FieldPredicate DEFAULT_FIELDS_PREDICATE = FieldPredicates.alwaysTrue();

      /**
       * {@link ThreadLocal} variable which holds the parsed filter expression
       */
      private static final ThreadLocal<FieldPredicate> FIELD_PREDICATE = new InheritableThreadLocal<FieldPredicate>() {
        @Override
        protected FieldPredicate initialValue() {
          return DEFAULT_FIELDS_PREDICATE;
        }
      };

      private final String parameter;

      public FieldsFilter() {
        this("fields");
      }

      public FieldsFilter(String parameter) {
        this.parameter = parameter;
      }

      @Override
      public void init(FilterConfig filterConfig) throws ServletException {
      }

      @Override
      public void doFilter(
          ServletRequest servletRequest,
          ServletResponse servletResponse,
          FilterChain filterChain) throws IOException, ServletException {

        final String fields = servletRequest.getParameter(parameter);

        if (fields == null || fields.isEmpty()) {
          FIELD_PREDICATE.set(DEFAULT_FIELDS_PREDICATE);
        } else {
          try {
            final FieldPredicate fieldsExpression = ParserFramework.parseFieldsExpression(fields);
            FIELD_PREDICATE.set(fieldsExpression);
          } catch (RuntimeException ignore) {
            FIELD_PREDICATE.set(DEFAULT_FIELDS_PREDICATE);
          }
        }

        filterChain.doFilter(servletRequest, servletResponse);
      }

      @Override
      public void destroy() {
      }
    }
  }
}