A custom Velocity directive for MyBatis to generate column aliases. Related to https://github.com/mybatis/mybatis-3/pull/136

ColumnAliasDirective.java

/*
 * Copyright 2014 MyBatis.org.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.mybatis.scripting.velocity.use;

import java.io.IOException;
import java.io.StringWriter;
import java.io.Writer;

import org.apache.velocity.context.InternalContextAdapter;
import org.apache.velocity.exception.MethodInvocationException;
import org.apache.velocity.exception.ParseErrorException;
import org.apache.velocity.exception.ResourceNotFoundException;
import org.apache.velocity.runtime.directive.Directive;
import org.apache.velocity.runtime.parser.node.ASTBlock;
import org.apache.velocity.runtime.parser.node.Node;

public class ColumnAliasDirective extends Directive {

  @Override
  public String getName() {
    return "columnalias";
  }

  @Override
  public int getType() {
    return BLOCK;
  }

  @Override
  public boolean render(InternalContextAdapter context, Writer writer, Node node) throws IOException, ResourceNotFoundException, ParseErrorException, MethodInvocationException {
    String body = null;
    String table = null;
    String prefix = null;
    for (int i = 0; i < node.jjtGetNumChildren(); i++) {
      if (node.jjtGetChild(i) != null) {
        if (node.jjtGetChild(i) instanceof ASTBlock) {
          StringWriter stringWriter = new StringWriter();
          node.jjtGetChild(i).render(context, stringWriter);
          body = stringWriter.toString();
          break;
        } else {
          if (i == 0) {
            table = String.valueOf(node.jjtGetChild(i).value(context));
          } else if (i == 1) {
            prefix = String.valueOf(node.jjtGetChild(i).value(context));
          } else {
            break;
          }
        }
      }
    }

    writer.write(translate(body, table, prefix));

    return true;
  }

  private String translate(String body, String table, String prefix) {
    if (body == null || body.length() == 0 || (table == null && prefix == null))
      return body;
    StringBuilder buffer = new StringBuilder();
    String tablePrefix = table != null && table.length() > 0 ? table + "." : "";
    String columnPrefix = prefix != null && prefix.length() > 0 ? prefix : "";
    String[] columns = body.split(",");
    for (String column : columns) {
      String trimmed = column.trim();
      if (trimmed.length() > 0) {
        if (buffer.length() > 0)
          buffer.append(", ");
        buffer.append(tablePrefix).append(trimmed);
        if (columnPrefix.length() > 0) {
          buffer.append(" AS ").append(columnPrefix).append(trimmed);
        }
      }
    }
    return buffer.toString();
  }
}

example_result.sql

select
comment.a AS comment_a, comment.b AS comment_b, comment.c AS comment_c
from tab comment

example_usage.xml

<sql id="cols">a, b, c</sql>

<select lang="velocity">
select
#columnalias("comment", "comment_")
<include refid="cols" />
#end
from tab comment
</select>