Master MyBatis: Core Principles, Architecture, and Interview Tips

What is MyBatis?

MyBatis is a powerful persistence framework for Java that supports custom SQL, stored procedures, and advanced mapping. It is a semi‑ORM tool that wraps JDBC, allowing developers to focus on SQL statements while handling connection, statement creation, and result mapping automatically.

Advantages

SQL‑centric programming offers high flexibility and performance control.

Reduces JDBC boilerplate by over 50%.

Compatible with any JDBC‑supported database.

Integrates smoothly with Spring.

Provides mapping tags for object‑relational relationships.

Disadvantages

Requires manual SQL writing, demanding strong SQL skills.

SQL is database‑specific, limiting portability.

Suitable Scenarios

MyBatis is ideal for projects that need fine‑grained SQL control, high performance, or frequent requirement changes, such as many internet applications.

Architecture Overview

The initialization starts from mybatis‑config.xml, which is parsed into a Configuration object.

Key steps include:

Loading configuration from XML files and annotations into MappedStatement objects.

SQL parsing: locating the MappedStatement by ID and generating the final SQL with parameters.

SQL execution: sending the SQL to the database.

Result mapping: converting the result set into Java objects such as HashMap, POJOs, or primitive types.

Three‑Layer Architecture

API Interface Layer – Exposes methods for external callers and forwards requests to the data‑processing layer.

Data Processing Layer – Handles SQL lookup, parsing, execution, and result mapping.

Foundation Layer – Provides core services like connection management, transaction handling, configuration loading, and caching.

Core Components

SqlSessionFactoryBuilder

Creates a Configuration from XML or programmatic settings and builds a SqlSessionFactory.

SqlSessionFactory

Singleton‑style factory that produces SqlSession instances; each factory corresponds to one environment configuration.

SqlSession

Top‑level API for database interaction; obtains MappedStatement by ID and executes CRUD operations. It is not thread‑safe and should be closed after use.

Executor

Core of MyBatis execution. Implementations include SimpleExecutor, ReuseExecutor, and BatchExecutor, handling statement creation, caching, and batch processing.

StatementHandler

Wraps JDBC Statement operations, sets parameters, and converts results to Java collections.

ResultSetHandler

ParameterHandler – converts method arguments to JDBC parameters.

ResultSetHandler – transforms ResultSet into Java collections.

TypeHandler – maps between Java types and JDBC types.

MappedStatement

Holds mapping information for a single SQL node, including the SQL string, input and output parameter mappings.

SqlSource

Generates dynamic SQL based on the provided parameter object and produces a BoundSql instance.

Common Interview Questions

JDBC steps: load driver, obtain connection, create statement, execute CRUD, retrieve results, close resources.

Difference between #{} and ${}: #{} uses prepared‑statement placeholders (prevents SQL injection), while ${} performs direct string replacement.

How MyBatis implements pagination: RowBounds for in‑memory pagination or physical pagination via SQL or plugins.

How to write a MyBatis plugin: implement Interceptor, override intercept(), annotate with @Intercepts, and configure in mybatis‑config.xml.

Dynamic SQL tags: trim, where, set, foreach, if, choose, when, otherwise, bind.

Mapping XML tags: <resultMap>, <parameterMap>, <sql>, <include>, <selectKey>, plus the standard CRUD tags.

First‑ and second‑level cache behavior and configuration.

Sample Batch Insert with Generated Keys

<insert id="insertAuthor" useGeneratedKeys="true" keyProperty="id">
  insert into Author(username, password, email, bio) values
  <foreach item="item" collection="list" separator=",">
    (#{item.username}, #{item.password}, #{item.email}, #{item.bio})
  </foreach>
</insert>