Preface
MyBatis is based on the idea of "the database structure is uncontrollable", that is, we hope that the database follows the third normal form or BCNF, but in fact it goes against our wishes, then the result set mapping It is MyBatis that provides us with this means of converting between ideal and reality, and resultMap is the configuration label of the result set mapping.
Before going deep into the ResultMap tag, we need to understand the process from the SQL query result set to the JavaBean or POJO entity.
From SQL query results to domain model entities
Get the ResultSet object through JDBC query
Traverse the ResultSet object and temporarily store each row of data into a HashMap instance, using the field name or field alias of the result set as the key, and the field value as the value
According to the type attribute of the ResultMap tag Instantiate the domain model through reflection
-
Fill the key-value pairs in the HashMap into the domain model instance according to the type attribute of the ResultMap tag and tag information such as id and result and return
1. resultMap
1. Attribute description
id attribute, the identification of the resultMap tag.
type attribute , the fully qualified class name of the return value, or a type alias.
autoMapping attribute, value range true (default value) | false, set whether to start the automatic mapping function. The automatic mapping function is to automatically find the attribute name with the same name as the field name in lowercase, and call the setter method. After setting it to false, you need to clearly indicate the mapping relationship in
resultMap
before calling the corresponding setter method.
2. Basic function: Establish a mapping relationship between SQL query result fields and entity attributes
Example 1: Construct a domain model through setters
public class EStudent{ private long id; private String name; private int age; // getter,setter方法 /** * 必须提供一个无参数的构造函数 */ public EStudent(){} }
<select> SELECT ID, Name, Age FROM TStudent </select> <resultmap> <id></id> <result></result> <result></result> </resultmap>
Sub-element description:
id element, used to set the mapping relationship between primary key fields and domain model attributes
result element, used Set the mapping relationship between ordinary fields and domain model attributes
id, result statement attribute configuration details:
Property | Description |
---|---|
Needs to be mapped to JavaBean Property name. | |
The column name or label alias of the data table. | |
A complete class name, or a type alias. If you match a JavaBean, MyBatis will usually detect it on its own. Then, if you want to map to a HashMap, then you need to specify the purpose of javaType. | |
List of types supported by the data table. This attribute is only useful for columns that allow nulls during insert, update, or delete. JDBC requires this, but MyBatis does not. If you are coding directly against JDBC and have columns that allow nulls, you will want to specify this. | |
Use this attribute to override the type handler. This value can be a complete class name or a type alias. |
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