Extension refers to adding functions not provided by general Mapper.
Give an example:
An example of using bulk insert as an extension function in the official document of general Mapper is:   tk.mybatis.Mapper.additional.insert.insertlistmapper < T >   tk.mybatis.mapper.additional.insert.InsertListProvider
Let's copy a batch update. Suppose we want to generate the following SQL statements:

UPDATE tabple_emp SET emp_name=?,emp_age=?,emp_salary=? where emp_id=?;
UPDATE tabple_emp SET emp_name=?,emp_age=?,emp_salary=? where emp_id=?; 
UPDATE tabple_emp SET emp_name=?,emp_age=?,emp_salary=? where emp_id=?; 

In order to generate the above SQL statements, we need to use the foreach tag of MyBatis.

<foreach collection="list" item="record" separator=";" >
	UPDATE tabple_emp
	<set>
		emp_name=#{record.empName},
		emp_age=#{record.empAge},
		emp_salary=#{record.empSalary},
	</set>
	where emp_id=#{record.empId}
</foreach>

1. Create the extension interface MyBatchUpdateMapper

public interface MyBatchUpdateMapper<T> {
	
	@UpdateProvider(type=MyBatchUpdateProvider.class, method="dynamicSQL")
	void batchUpdate(List<T> list);
}

2. Create MyBatchUpdateProvider assembly SQL

public class MyBatchUpdateProvider extends MapperTemplate {

	public MyBatchUpdateProvider(Class<?> mapperClass, MapperHelper mapperHelper) {
		super(mapperClass, mapperHelper);
	}
	
	/**
		<foreach collection="list" item="record" separator=";" >
			UPDATE tabple_emp
			<set>
				emp_name=#{record.empName},
				emp_age=#{record.empAge},
				emp_salary=#{record.empSalary},
			</set>
			where emp_id=#{record.empId}
		</foreach>
	*/
	public String batchUpdate(MappedStatement statement) {
		//1. Create StringBuilder to splice various components of SQL statement
		StringBuilder builder = new StringBuilder();
		//2. Splicing foreach label
		builder.append("<foreach collection=\"list\" item=\"record\" separator=\";\" >");
		//3. Get the Class object corresponding to the entity Class
		Class<?> entityClass = super.getEntityClass(statement);
		//4. Get the corresponding table name of entity class in the database
		String tableName = super.tableName(entityClass);
		//5. Generate update clause
		String updateClause = SqlHelper.updateTable(entityClass, tableName);
		builder.append(updateClause);
		builder.append("<set>");
		//6. Get all field information
		Set<EntityColumn> columns = EntityHelper.getColumns(entityClass);
		String idColumn = null;
		String idHolder = null;
		for (EntityColumn entityColumn : columns) {
			boolean isPrimaryKey = entityColumn.isId();
			//7. Judge whether the current field is a primary key
			if(isPrimaryKey) {
				//8. Cache the field name and value of the primary key
				idColumn = entityColumn.getColumn();
				//The return format is as follows: {record. Age, JDBC type = numeric, typehandler = mytypehandler}
				idHolder = entityColumn.getColumnHolder("record");
			}else {
				//9. Splicing SET clauses with non primary key fields
				String column = entityColumn.getColumn();
				String columnHolder = entityColumn.getColumnHolder("record");
				builder.append(column).append("=").append(columnHolder).append(",");
			}
		}
		builder.append("</set>");
		//10. Use the previously cached primary key name and value to splice where clause
		builder.append("where ").append(idColumn).append("=").append(idHolder);
		builder.append("</foreach>");
		//11. Return the spliced string
		return builder.toString();
	}
}

3. MyMapper inherits the extension interface MyBatchUpdateMapper

public interface MyMapper<T> 
		extends SelectAllMapper<T>,SelectByExampleMapper<T>,MyBatchUpdateMapper<T> {
}

Test:
Note: JDBC executes semicolon separated SQL statements. Parameter allowMultiQueries=true must be added when connecting to the database

public class MyBatchUpdateTest {
	
	public static void main(String[] args) {
		ClassPathXmlApplicationContext iocContainer = new ClassPathXmlApplicationContext("spring-context.xml");
		EmployeeService employeeService = iocContainer.getBean(EmployeeService.class);
		
		List<Employee> empList = new ArrayList<Employee>();
		
		empList.add(new Employee(25, "newName01", 111.11, 10));
		empList.add(new Employee(26, "newName02", 222.22, 20));
		empList.add(new Employee(27, "newName03", 333.33, 30));
		
		employeeService.batchUpdateEmp(empList);
		iocContainer.close();
	}
}

Console output:

Preparing: UPDATE tabple_emp SET emp_name=?,emp_salary=?,emp_age=? where emp_id=? ; UPDATE tabple_emp SET emp_name=?,emp_salary=?,emp_age=? where emp_id=? ; UPDATE tabple_emp SET emp_name=?,emp_salary=?,emp_age=? where emp_id=?   
Parameters: newName01(String), 111.11(Double), 10(Integer), 1(Integer), newName02(String), 222.22(Double), 20(Integer), 2(Integer), newName03(String), 333.33(Double), 30(Integer), 3(Integer)  
Updates: 1