Reduce the memory usage of inline ResultMap

[vonlinee]

In my project, most of select statement use resultType instead of resultMap, like below:

<select id="selectAuthors"  parameterType="int" resultType="org.apache.ibatis.domain.blog.Author">
  select * from author where id = #{id}
</select>

every single inline resultMap holds 6 empty unmodifiable collection , see ResultMap.java. i think it is unnecessary.

public class ResultMap {
  ....
  private List<ResultMapping> resultMappings;
  private List<ResultMapping> idResultMappings;
  private List<ResultMapping> constructorResultMappings;
  private List<ResultMapping> propertyResultMappings;
  private Set<String> mappedColumns;
  private Set<String> mappedProperties;
  .....
}

before this change, the memory usage of single resultmap is shown below:

with this optimization, it will be like below:

[coveralls]

coverage: 87.396% (+0.01%) from 87.382%
when pulling 7cdf925 on vonlinee:inline-resultmap-optimization
into 2299e6d on mybatis:master.

[hazendaz]

@vonlinee

The screenshots only show the size of reference fields on the owning object, which will not change regardless of whether the field points to a new collection, an empty singleton, or null.

To demonstrate actual memory savings, can you provide either:

a heap dump comparison showing reduced ArrayList / HashSet instance counts or retained size, or

JOL output illustrating eliminated per-instance collection objects for inline ResultMaps?

Without that, the PR doesn’t yet substantiate the claimed memory reduction, and it also introduces immutability assumptions that should be explicitly justified.

@harawata What are your thoughts here?

[vonlinee]

@hazendaz
Hi, appreciated for the reply. i wrote a test demo shown below: it will create 200 inline resultMaps by method createInlineResultMap1 or createInlineResultMap2.

import org.apache.ibatis.mapping.ResultMap;
import org.apache.ibatis.session.Configuration;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.function.Supplier;

public class InlineResultMapMemoryUsageDemo {

  static final Configuration configuration = new Configuration();

  public static ResultMap createInlineResultMap1() {
    return new ResultMap.Builder(configuration, "selectAuthors-Inline", Map.class,
      new ArrayList<>(), null).build();
  }

  public static ResultMap createInlineResultMap2() {
    return ResultMap.inline(configuration, "selectAuthors-Inline", Map.class);
  }

  public static List<ResultMap> createInlineResultMaps(int count, Supplier<ResultMap> supplier) {
    final ArrayList<ResultMap> resultMaps = new ArrayList<>();
    for (int i = 0; i < count; i++) {
      resultMaps.add(supplier.get());
    }
    return resultMaps;
  }

  public static void main(String[] args) throws InterruptedException {
    final int resultMapCount = 200;
    List<ResultMap> resultMaps = createInlineResultMaps(resultMapCount, InlineResultMapMemoryUsageDemo::createInlineResultMap1);
    // List<ResultMap> resultMaps = createInlineResultMaps(resultMapCount, InlineResultMapMemoryUsageDemo::createInlineResultMap2);
    TimeUnit.MINUTES.sleep(30); // to dump heap
  }
}

i run this demo with the two methods respectively, then dump the heap as file heapdump-1.hprof (createInlineResultMap1) and heapdump-2.hprof (createInlineResultMap2), then i compare heapdump-1.hprof with file heapdump-2.hprof using visualvm, and i get the result shown in the screenshot below:

please tell me if there is something wrong with my steps.

[vonlinee]

and it also introduces immutability assumptions that should be explicitly justified.

i think the immutability of these collections has partly already existed after the ResultMap is built with method org.apache.ibatis.mapping.ResultMap.Builder#build, except mappedProperties.

// lock down collections
resultMap.resultMappings = Collections.unmodifiableList(resultMap.resultMappings);
resultMap.idResultMappings = Collections.unmodifiableList(resultMap.idResultMappings);
resultMap.constructorResultMappings = Collections.unmodifiableList(resultMap.constructorResultMappings);
resultMap.propertyResultMappings = Collections.unmodifiableList(resultMap.propertyResultMappings);
resultMap.mappedColumns = Collections.unmodifiableSet(resultMap.mappedColumns);