Overview

In MyBatis mapper XML files, fully qualified class names for parameterType and resultType attributes can become lengthy. Type aliases provide a cleaner solution by allowing you to reference classes by simple names.

Defining Type Aliases

Aliases are configured in the mybatis-config.xml file within the <typeAliases> section. This section must be placed after <properties> and before <environments>.

Single Class Alias

<typeAliases>
    <typeAlias type="com.example.entity.User" alias="User"/>
</typeAliases>

After defining this alias, you can use it in mapper files:

<insert id="addUser" parameterType="User">
    INSERT INTO users(name, email) VALUES(#{name}, #{email})
</insert>

Package-Level Alias

When working with multiple classes in the same package, defining aliases individualyl becomes tedious. The <package> element automatically registers all classes in a package using their simple class names:

<typeAliases>
    <package name="com.example.entity"/>
</typeAliases>

With this configuration, User, Product, and Order classes are automatically available as aliases.

Practical Example

Entity Class

package com.example.entity;

public class Product {
    private Long id;
    private String productName;
    private BigDecimal price;
    
    public Product() {}
    
    public Product(String productName, BigDecimal price) {
        this.productName = productName;
        this.price = price;
    }
    
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getProductName() { return productName; }
    public void setProductName(String productName) { this.productName = productName; }
    public BigDecimal getPrice() { return price; }
    public void setPrice(BigDecimal price) { this.price = price; }
}

DAO Interface

package com.example.dao;

import com.example.entity.Product;

public interface ProductDao {
    void save(Product product);
}

DAO Implementation

package com.example.dao;

import com.example.entity.Product;
import org.apache.ibatis.session.SqlSession;

public class ProductDaoImpl implements ProductDao {
    @Override
    public void save(Product product) {
        try (SqlSession session = MyBatisSessionFactory.openSession()) {
            session.insert("ProductMapper.insert", product);
            session.commit();
        }
    }
}

Mapper XML

<?xml version="1.0" encoding="UTF-8"?>
<mapper namespace="ProductMapper">
    <insert id="insert" parameterType="Product">
        INSERT INTO products(product_name, price) 
        VALUES(#{productName}, #{price})
    </insert>
</mapper>

MyBatis Configuration

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <properties resource="db.properties"/>
    
    <typeAliases>
        <package name="com.example.entity"/>
    </typeAliases>
    
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    
    <mappers>
        <mapper resource="mappers/ProductMapper.xml"/>
    </mappers>
</configuration>

Dtaabase Properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/myapp
jdbc.username=root
jdbc.password=secret

Key Points

Aspect	Description
Location	Must be in mybatis-config.xml between <properties> and <environments>
Single alias	Use <typeAlias> with type and alias attributes
Package alias	Use <package> to auto-register all classes in a package
Case sensitivity	Aliases are case-insensitive by default

The package-level approach is recommended for most projects as it reduces configuration overhead and keeps the codebase maintainable.