MyBatis Patterns

Use MyBatis when SQL control and explicit mappings matter. Keep SQL readable, make mapper contracts small, and put business transactions in the service layer. Pair this skill with springboot-patterns for application structure, database-migrations for schema changes, postgres-patterns or mysql-patterns for engine-specific SQL, and security-review for untrusted input and access-control concerns.

When to Activate

• Adding or reviewing MyBatis mapper interfaces or XML files
• Choosing between XML mappers, annotation mappers, and generated SQL
• Designing resultMap mappings, joins, nested collections, or projections
• Reviewing dynamic SQL, parameter binding, pagination, or batch writes
• Debugging N+1 queries, slow statements, connection use, or transaction bugs
• Integrating MyBatis-Spring into Spring Boot or an XML-first legacy application

How It Works

Use a thin mapper for persistence operations and keep business rules in a service. A typical flow is:

Controller / batch job -> Service (@Transactional) -> Mapper -> Database

• Register mappers once, either with @Mapper on each interface or with @MapperScan for a package. Do not mix registration approaches casually.
• Keep the XML namespace equal to the mapper interface's fully qualified name.
• Give each statement a stable, descriptive id; treat mapper methods as a persistence API that can be tested independently.
• Return a domain DTO or projection for read paths instead of exposing a large mutable map when the shape is known.
• Keep schema changes in migrations. Do not make mapper startup silently create or alter production tables.

Examples

@MapperScan("com.example.user.persistence")
@Configuration
class MyBatisConfig {
}

public interface UserMapper {
  UserSummary findSummaryById(long userId);
}

<mapper namespace="com.example.user.persistence.UserMapper">
  <select id="findSummaryById"
          parameterType="long"
          resultType="com.example.user.persistence.UserSummary">
    SELECT id, display_name AS displayName, status
    FROM users
    WHERE id = #{userId}
  </select>
</mapper>

Parameter Binding and SQL Safety

Use #{...} for values. MyBatis binds it as a prepared-statement parameter. Treat ${...} as raw SQL text: it is only appropriate for a value selected from a closed, application-owned allowlist such as a sort-column map.

private static final Map<String, String> SORT_COLUMNS = createSortColumns();

private static Map<String, String> createSortColumns() {
    Map<String, String> columns = new HashMap<>();
    columns.put("name", "display_name");
    columns.put("created", "created_at");
    return Collections.unmodifiableMap(columns);
}

String sortColumn = SORT_COLUMNS.getOrDefault(request.sort(), "created_at");
String sortDirection = request.descending() ? "DESC" : "ASC";
// The mapper receives only an allowlisted identifier; user input is never
// passed directly to ${sortColumn} or ${sortDirection}.

<select id="findPage" resultType="com.example.user.persistence.UserSummary">
  SELECT id AS id,
         display_name AS displayName,
         status AS status,
         created_at AS createdAt
  FROM users
  WHERE tenant_id = #{tenantId}
  ORDER BY ${sortColumn} ${sortDirection}
  LIMIT #{limit}
</select>

Derive tenantId from the authenticated principal or a server-side tenant context, never from a request field. Authenticate the principal and verify that it is authorized for the tenant before invoking the mapper; keep the tenant_id predicate in SQL as defense in depth.

If possible, avoid ${} entirely by selecting among fixed statements or by using a database-specific query builder. Never interpolate user input into a table name, column name, ORDER BY, WHERE fragment, or SQL expression.

For multiple parameters, use a request object or explicit @Param names:

List<UserSummary> findActive(
    @Param("tenantId") long tenantId,
    @Param("statuses") Set<UserStatus> statuses
);

Do not use the deprecated external parameterMap element. Prefer inline parameter mappings and explicit jdbcType for nullable values when the JDBC driver needs it.

XML, Result Types, and Result Maps

• Use resultType for a simple one-to-one mapping whose column labels already match the target properties.
• Use resultMap when aliases, type handlers, nested objects, or collections need explicit mapping.

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