mybatis使用笔记

1. mapper.xml配置文件中，只要调用到了insert方法，不管name为不为空都会执行到preparedStatement.update()方法，如果name为null，此时就会报Query is Empty的异常

<insert id="insert">
              <if test="name != null">
              INSERT INTO user(name) VALUES(
               #{name}
              )
              </if>
</insert>

2. mapper.xml中过滤条件中可以调用方法，但不能使用< > <=等特殊符号(加CDATA也不行)， and或者or不能大写，这与XML要求有关

如下：

<select id="selectById" resultMap="BaseResultMap">
        SELECT *
        from account
        WHERE
        <if test="list != null and list.size() != 0">
            <foreach collection="list" item="model" open="id in (" close=")" separator=",">
                #{model}
            </foreach>
        </if>
        <if test="id != null">
            and id = #{id}
        </if>
        limit 0, 1
    </select> 

3.  对于SQL语句的执行mybatis使用的是StatementHandler，在newXxxStatementHandler()时会调用BaseStatementHandler的构造方法，详见随笔（http://www.cnblogs.com/stefanking/p/5102837.html）

　　PreparedStatementHandler执行过程：获取最终需要执行的SQL语句 --> prepare(sql) --> PreparedStatementHandler.update(Statement) --> PreparedStatement.execute() --> ... 

　　对第一步的解释：在BaseStatementHandler的构造函数中获取最终要执行的SQL语句

　　代码：

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
  }

/**
*  MappedStatement
*/
public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.size() <= 0) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }

/**
* DynamicSqlSource implements SqlSource
*/
public BoundSql getBoundSql(Object parameterObject) {
    DynamicContext context = new DynamicContext(configuration, parameterObject);
    rootSqlNode.apply(context); //此处处理if where trim等内部标签过滤条件，得到最终需要编译的SQL语句，参数未替换， 如： select * from user where userName = #{param.userName} and age = #{param.age}
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
    SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings()); //此处替换掉占位符，得到最终参数，和最终预编译的SQL语句： 如：select * from user where userName = ？ and age = ？
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {  //占位符对应位置的参数值
      boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());
    }
    return boundSql;
  }