MyBatis性能优化 - 访问映射器

Question

我在一个获取超过 2M 行的项目中使用 MyBatis。

我有一个简单的问题，关于 MyBatis 是如何工作的。每当我需要 mapper 的操作时，MyBatis 是读取 XML 文件并提取查询语句吗？还是将 mappers 放入内存中，然后 MyBatis 直接访问它们？

这很重要，因为访问和读取 XML 文件可能会对我们期望的性能值产生影响。

谢谢你的帮助。 敬礼

- sexta13

Answer 1

简单来说，MyBatis在第一次从配置XML文件构建SqlSessionFactory时解析XML文件。在此之后，所有属性、映射器和设置都存储在内存中。 说明： 如文档所述，您可以直接在Java中设置SqlSessionFactory，而无需使用XML，方法如下（请参见最后一行）：

DataSource dataSource = BlogDataSourceFactory.getBlogDataSource();
TransactionFactory transactionFactory = new JdbcTransactionFactory();
Environment environment = new Environment("development", transactionFactory, dataSource);
Configuration configuration = new Configuration(environment);
configuration.addMapper(BlogMapper.class);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);

实际上，当你从XML构建SqlSessionFactory时，你将会写出以下类似代码:

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

如果你追踪SqlSessionFactoryBuilder的源代码，

  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
      return build(parser.parse());
 ...

parse() 方法返回一个 Configuration 对象，该对象保存了您在 XML 文件中提供的所有信息。

public class Configuration {

  protected Environment environment;

  protected boolean safeRowBoundsEnabled = false;
  protected boolean safeResultHandlerEnabled = true;
  protected boolean mapUnderscoreToCamelCase = false;
  protected boolean aggressiveLazyLoading = true;
  protected boolean multipleResultSetsEnabled = true;
  protected boolean useGeneratedKeys = false;
  protected boolean useColumnLabel = true;
  protected boolean cacheEnabled = true;
  protected boolean callSettersOnNulls = false;
  protected String logPrefix;
  protected Class <? extends Log> logImpl;
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  protected Set<String> lazyLoadTriggerMethods = new HashSet<String>(Arrays.asList(new String[] { "equals", "clone", "hashCode", "toString" }));
  protected Integer defaultStatementTimeout;
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;

  protected Properties variables = new Properties();
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();
  protected MapperRegistry mapperRegistry = new MapperRegistry(this);

  protected boolean lazyLoadingEnabled = false;
  protected ProxyFactory proxyFactory;

  protected String databaseId;
  /**
   * Configuration factory class.
   * Used to create Configuration for loading deserialized unread properties.
   *
   * @see <a href='https://code.google.com/p/mybatis/issues/detail?id=300'>Issue 300</a> (google code)
   */
  protected Class<?> configurationFactory;

  protected final InterceptorChain interceptorChain = new InterceptorChain();
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry();
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();

  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection");
  protected final Map<String, Cache> caches = new StrictMap<Cache>("Caches collection");
  protected final Map<String, ResultMap> resultMaps = new StrictMap<ResultMap>("Result Maps collection");
  protected final Map<String, ParameterMap> parameterMaps = new StrictMap<ParameterMap>("Parameter Maps collection");
  protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<KeyGenerator>("Key Generators collection");

  protected final Set<String> loadedResources = new HashSet<String>();
  protected final Map<String, XNode> sqlFragments = new StrictMap<XNode>("XML fragments parsed from previous mappers");

  protected final Collection<XMLStatementBuilder> incompleteStatements = new LinkedList<XMLStatementBuilder>();
  protected final Collection<CacheRefResolver> incompleteCacheRefs = new LinkedList<CacheRefResolver>();
  protected final Collection<ResultMapResolver> incompleteResultMaps = new LinkedList<ResultMapResolver>();
  protected final Collection<MethodResolver> incompleteMethods = new LinkedList<MethodResolver>();
...

- Yigitalp Ertem

---

非常感谢您的解释。正是我所需要的。 - undefined

Answer 2

别担心这个。

在设置完成后，MyBatis会将mapper xml解析为一个本地变量（第一次读取xml文件时）。

现在，当你调用mapper.add()、sqlSession.selectOne()或其他方法时，它会首先从mappedStatements中获取参数/结果映射/resultType，而不会再次读取xml。

此外，MyBatis还缓存了mapper代理方法。就像这样（第一次调用时创建代理方法实例）。

final String resource = "org/apache/ibatis/builder/MapperConfig.xml";
final Reader reader = Resources.getResourceAsReader(resource);
manager = SqlSessionManager.newInstance(reader);
AuthorMapper mapper = manager.getMapper(AuthorMapper.class);
Author expected = new Author(500, "cbegin", "******", "[e-mail]", "Something...", null);
mapper.insertAuthor(expected);

（如何获取映射器）

public class MapperRegistry {
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
        throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
        return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
        throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
}
}

而且，在MyBatis 3.x+中，你可以使用查询缓存。按照以下步骤操作：

<configuration>
  <settings>
    <setting name="cacheEnabled" value="true" />
  <settings>
</configuration>

- BobJiang