mybatis list in常见问题解析

1. What is MyBatis?

MyBatis is a Java persistence framework that simplifies the mapping of Java objects to SQL statements. It provides a way to interact with databases using SQL queries without writing complex JDBC code.

2. What is the “list” tag in MyBatis?

The “list” tag in MyBatis is used to iterate over a collection of items in a SQL query result. It allows you to map a single row in a database table to multiple objects in Java, or to map a single object to multiple rows in a table.

3. How to use the “list” tag in MyBatis?

To use the “list” tag in MyBatis, you need to define a collection property in your Java object, and then specify the “list” tag in your SQL query. The “list” tag takes two attributes: “property” which specifies the name of the collection property in the Java object, and “ofType” which specifies the type of objects in the collection.

For example:

<select id="getUserOrders" resultType="User">
    SELECT * FROM orders WHERE user_id = #{userId}
</select>

<resultMap id="UserResultMap" type="User">
    <id column="id" property="id"/>
    <result column="name" property="name"/>
    <collection property="orders" ofType="Order">
        <result column="order_id" property="id"/>
        <result column="order_name" property="name"/>
    </collection>
</resultMap>

In this example, the “getUserOrders” query retrieves user information and their orders from the database. The “User” Java object has a collection property called “orders” which contains a list of Order objects.

4. What are the common issues with the “list” tag in MyBatis?

Some common issues with the “list” tag in MyBatis include:

• Incorrect mapping of the collection property in the Java object.
• Inconsistent data types between the database and Java object.
• Missing or incorrect mapping of columns in the SQL query.
• Using the wrong type in the “ofType” attribute of the “list” tag.

To avoid these issues, make sure to carefully define the collection property in your Java object, map the columns correctly in your SQL query, and specify the correct type in the “ofType” attribute of the “list” tag.