Mybatis整体流程及关键组件

整体流程

mybatis主流程如下图所示，

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• SqlSesison：面向用户的对外API
• Configuration：描述Mybatis的主配置信息
• Executor：SQL执行器，MyBatis中对数据库所有的增删改查操作都是由Executor组件完成的
• StatementHandler：StatementHandler封装了对JDBC Statement对象的操作，比如为Statement对象设置参数，调用Statement接口提供的方法与数据库交互
• ParameterHandler：MyBatis框架使用的Statement类型为CallableStatement和PreparedStatement时，ParameterHandler用于为Statement对象参数占位符设置值
• ResultSetHandler：ResultSetHandler封装了对JDBC中的ResultSet对象操作，当执行SQL类型为SELECT语句时，ResultSetHandler用于将查询结果转换成Java对象
• TypeHandler：TypeHandler是MyBatis中的类型处理器，用于处理Java类型与JDBC类型之间的映射。

核心组件

Executor

在Mybatis中对数据库的操作都是通过Executor进行操作，Executor核心类图如下所示：

• SimpleExecutor是基础实现，能够进行基础的增删改查操作。
• ResueExecutor对JDBC中的Statement对象做了缓存，当执行相同的SQL语句时，直接从缓存中取出Statement对象进行复用，避免了频繁创建和销毁Statement对象，从而提升系统性能
• BatchExecutor则会对调用同一个Mapper执行的update、insert和delete操作，调用Statement对象的批量操作功能
• 当开启二级缓存时，CachingExecutor对SimpleExecutor、ResueExecutor、BatchExecutor进行装饰，为查询操作增加二级缓存功能

StatementHandler

以org.apache.ibatis.executor.SimpleExecutor#doUpdate方法为例，通过构建StatementHandler对象调用StatementHandler的update方法进行实际的处理逻辑，下面看下StatementHandler的相关实现。

@Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

StatementHandler的实现类图如下所示：

• SimpleStatementHandler继承至BaseStatementHandler，封装了对JDBC Statement对象的操作
• PreparedStatementHandler封装了对JDBC PreparedStatement对象的操作
• CallableStatementHandler则封装了对JDBC CallableStatement对象的操作
• RoutingStatementHandler会根据Mapper配置中的statementType属性（取值为STATEMENT、PREPARED或CALLABLE）创建对应的StatementHandler实现

ParameterHandler

Mybatis是如何实现占位符的替换？org.apache.ibatis.executor.statement.PreparedStatementHandler#parameterize实现参数的替换，具体逻辑如下：

@Override
  public void parameterize(Statement statement) throws SQLException {
    parameterHandler.setParameters((PreparedStatement) statement);
  }

setParameters方法如下：

 @Override
  public void setParameters(PreparedStatement ps) {
    ErrorContext.instance().activity("setting parameters").object(mappedStatement.getParameterMap().getId());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings != null) {
      //获取所有参数的映射信息  
      for (int i = 0; i < parameterMappings.size(); i++) {
        ParameterMapping parameterMapping = parameterMappings.get(i);
        if (parameterMapping.getMode() != ParameterMode.OUT) {
          Object value;
          String propertyName = parameterMapping.getProperty();
          //根据参数属性名称获取参数值
          if (boundSql.hasAdditionalParameter(propertyName)) { // issue #448 ask first for additional params
            value = boundSql.getAdditionalParameter(propertyName);
          } else if (parameterObject == null) {
            value = null;
          } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
            value = parameterObject;
          } else {
            MetaObject metaObject = configuration.newMetaObject(parameterObject);
            value = metaObject.getValue(propertyName);
          }
          //获取参数对应的TypeHandler
          TypeHandler typeHandler = parameterMapping.getTypeHandler();
          JdbcType jdbcType = parameterMapping.getJdbcType();
          if (value == null && jdbcType == null) {
            jdbcType = configuration.getJdbcTypeForNull();
          }
          try {
            //调用TypeHandler的setParameter方法为占位符设置值  
            typeHandler.setParameter(ps, i + 1, value, jdbcType);
          } catch (TypeException | SQLException e) {
            throw new TypeException("Could not set parameters for mapping: " + parameterMapping + ". Cause: " + e, e);
          }
        }
      }
    }
  }

核心逻辑：获取对应的TypeHandler然后调用setParameter方法设置参数值。

TypeHandler

TypeHandler类图如下：

public interface TypeHandler<T> {

  // PreparedStatement对象参数的占位符设置值
  void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;

  /**
   * Gets the result.
   *
   * @param rs
   *          the rs
   * @param columnName
   *          Column name, when configuration <code>useColumnLabel</code> is <code>false</code>
   * @return the result
   * @throws SQLException
   *           the SQL exception
   */
  //从ResultSet获取值
  T getResult(ResultSet rs, String columnName) throws SQLException;
  //从ResultSet获取值
  T getResult(ResultSet rs, int columnIndex) throws SQLException;

  //从存储过程结果获取值
  T getResult(CallableStatement cs, int columnIndex) throws SQLException;

}

MyBatis通过TypeHandlerRegistry建立JDBC类型、Java类型与TypeHandler之间的映射关系,

 public TypeHandlerRegistry(Configuration configuration) {
    this.unknownTypeHandler = new UnknownTypeHandler(configuration);

    register(Boolean.class, new BooleanTypeHandler());
    register(boolean.class, new BooleanTypeHandler());
    register(JdbcType.BOOLEAN, new BooleanTypeHandler());
    register(JdbcType.BIT, new BooleanTypeHandler());

    register(Byte.class, new ByteTypeHandler());
    register(byte.class, new ByteTypeHandler());
    register(JdbcType.TINYINT, new ByteTypeHandler());

    register(Short.class, new ShortTypeHandler());
    register(short.class, new ShortTypeHandler());
    register(JdbcType.SMALLINT, new ShortTypeHandler());

    register(Integer.class, new IntegerTypeHandler());
    register(int.class, new IntegerTypeHandler());
    register(JdbcType.INTEGER, new IntegerTypeHandler());

    register(Long.class, new LongTypeHandler());
    register(long.class, new LongTypeHandler());

    register(Float.class, new FloatTypeHandler());
    register(float.class, new FloatTypeHandler());
    register(JdbcType.FLOAT, new FloatTypeHandler());

    register(Double.class, new DoubleTypeHandler());
    register(double.class, new DoubleTypeHandler());
    register(JdbcType.DOUBLE, new DoubleTypeHandler());

    register(Reader.class, new ClobReaderTypeHandler());
    register(String.class, new StringTypeHandler());
    register(String.class, JdbcType.CHAR, new StringTypeHandler());
    register(String.class, JdbcType.CLOB, new ClobTypeHandler());
    register(String.class, JdbcType.VARCHAR, new StringTypeHandler());
    register(String.class, JdbcType.LONGVARCHAR, new StringTypeHandler());
    register(String.class, JdbcType.NVARCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCLOB, new NClobTypeHandler());
    register(JdbcType.CHAR, new StringTypeHandler());
    register(JdbcType.VARCHAR, new StringTypeHandler());
    register(JdbcType.CLOB, new ClobTypeHandler());
    register(JdbcType.LONGVARCHAR, new StringTypeHandler());
    register(JdbcType.NVARCHAR, new NStringTypeHandler());
    register(JdbcType.NCHAR, new NStringTypeHandler());
    register(JdbcType.NCLOB, new NClobTypeHandler());

    register(Object.class, JdbcType.ARRAY, new ArrayTypeHandler());
    register(JdbcType.ARRAY, new ArrayTypeHandler());

    register(BigInteger.class, new BigIntegerTypeHandler());
    register(JdbcType.BIGINT, new LongTypeHandler());

    register(BigDecimal.class, new BigDecimalTypeHandler());
    register(JdbcType.REAL, new BigDecimalTypeHandler());
    register(JdbcType.DECIMAL, new BigDecimalTypeHandler());
    register(JdbcType.NUMERIC, new BigDecimalTypeHandler());

    register(InputStream.class, new BlobInputStreamTypeHandler());
    register(Byte[].class, new ByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.BLOB, new BlobByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.LONGVARBINARY, new BlobByteObjectArrayTypeHandler());
    register(byte[].class, new ByteArrayTypeHandler());
    register(byte[].class, JdbcType.BLOB, new BlobTypeHandler());
    register(byte[].class, JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.BLOB, new BlobTypeHandler());

    register(Object.class, unknownTypeHandler);
    register(Object.class, JdbcType.OTHER, unknownTypeHandler);
    register(JdbcType.OTHER, unknownTypeHandler);

    register(Date.class, new DateTypeHandler());
    register(Date.class, JdbcType.DATE, new DateOnlyTypeHandler());
    register(Date.class, JdbcType.TIME, new TimeOnlyTypeHandler());
    register(JdbcType.TIMESTAMP, new DateTypeHandler());
    register(JdbcType.DATE, new DateOnlyTypeHandler());
    register(JdbcType.TIME, new TimeOnlyTypeHandler());

    register(java.sql.Date.class, new SqlDateTypeHandler());
    register(java.sql.Time.class, new SqlTimeTypeHandler());
    register(java.sql.Timestamp.class, new SqlTimestampTypeHandler());

    register(String.class, JdbcType.SQLXML, new SqlxmlTypeHandler());

    register(Instant.class, new InstantTypeHandler());
    register(LocalDateTime.class, new LocalDateTimeTypeHandler());
    register(LocalDate.class, new LocalDateTypeHandler());
    register(LocalTime.class, new LocalTimeTypeHandler());
    register(OffsetDateTime.class, new OffsetDateTimeTypeHandler());
    register(OffsetTime.class, new OffsetTimeTypeHandler());
    register(ZonedDateTime.class, new ZonedDateTimeTypeHandler());
    register(Month.class, new MonthTypeHandler());
    register(Year.class, new YearTypeHandler());
    register(YearMonth.class, new YearMonthTypeHandler());
    register(JapaneseDate.class, new JapaneseDateTypeHandler());

    // issue #273
    register(Character.class, new CharacterTypeHandler());
    register(char.class, new CharacterTypeHandler());
  }

ResultSetHandler

ResultSetHandler用于在StatementHandler对象执行完查询操作或存储过程后，对结果集或存储过程的执行结果进行处理。ResultSetHandler接口定义如下：

public interface ResultSetHandler {

  <E> List<E> handleResultSets(Statement stmt) throws SQLException;

  <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;

  void handleOutputParameters(CallableStatement cs) throws SQLException;

}

handleResultSets方法默认实现如下图所示：

 public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());

    final List<Object> multipleResults = new ArrayList<>();

    int resultSetCount = 0;
    // 从Statement对象中获取ResultSet对象，然后将ResultSet包装为ResultSetWrapper对象 
    ResultSetWrapper rsw = getFirstResultSet(stmt);
    // 获取解析Mapper接口及Mapper SQL配置生成的ResultMap信息，一条语句一般对应一个ResultMap
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      // 调用handleResultSet()方法对ResultSetWrapper对象进行处理，将生成的实体对象存放在multipleResults列表中  
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }

    String[] resultSets = mappedStatement.getResultSets();
    if (resultSets != null) {
      while (rsw != null && resultSetCount < resultSets.length) {
        ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
        if (parentMapping != null) {
          String nestedResultMapId = parentMapping.getNestedResultMapId();
          ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
          handleResultSet(rsw, resultMap, null, parentMapping);
        }
        rsw = getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount++;
      }
    }

    return collapseSingleResultList(multipleResults);
  }

总结

至此，对于Mybatis的执行流程已经有一个大致的了解。SqlSesison提供了对用户暴露的API，使用了外观模式，Executor才是真正的执行器，Executor调用了StatementHandler去执行相关操作，使用了ParameterHandler、TypeHandler、ResultSetHandler进行参数占位符的替换、结果的转换操作。