MyBatis提供了简单的Java注解,使得我们可以不配置XML格式的Mapper文件,方便的编写简单的数据库操作代码:
public interface UserMapper {
@Select("SELECT * FROM users WHERE id = #{userId}")
User getUser(@Param("userId") String userId);
}
<configuration>
<settings>
<setting name="mapUnderscoreToCamelCase" value="true"/>
</settings>
</configuration>
但是没有Dynamic SQL的注解是不完整的,故这里向大家介绍下如何通过实现LanguageDriver,优雅的在MyBatis注解中使用Dynamic SQL。
自定义Select In注解
@Lang(SimpleSelectInExtendedLanguageDriver.class)
@Select("SELECT * FROM users WHERE id IN (#{userIds})")
List<User> selectUsers(@Param("userIds") List<String> userIds);
// 一次编写,受益终生
public class SimpleSelectInExtendedLanguageDriver
extends XMLLanguageDriver implements LanguageDriver {
private final Pattern inPattern = Pattern.compile("\\(#\\{(\\w+)\\}\\)");
@Override
public SqlSource createSqlSource(Configuration configuration,
String script, Class<?> parameterType) {
Matcher matcher = inPattern.matcher(script);
if (matcher.find()) {
script = matcher.replaceAll("(<foreach collection=\"$1\" item=\"__item\" separator=\",\" >#{__item}</foreach>)");
}
script = "<script>" + script + "</script>";
return super.createSqlSource(configuration, script, parameterType);
}
}
我们通过实现自己的LanguageDriver,在MyBatis编译语句前,将我们自定义的标签替换为了动态SQL语句,其等同于:
@Select({"<script>",
"SELECT *",
"FROM user",
"WHERE id IN",
"<foreach item='item' index='index' collection='list'",
"open='(' separator=',' close=')'>",
"#{item}",
"</foreach>",
"</script>"})
List<User> selectUsers(@Param("userIds") List<Intger> userIds);
通过实现LanguageDriver,剥离出了冗长的动态SQL语句,简化Select In的注解代码。
自定义Update Bean注解
类似的,通过重写LanguageDriver,我们还能扩展出远比其它方案(e.g. XML SQL Mapper配置、在注解语句中写动态SQL)简洁的自定义操作。
一个常用的操作是更新数据库中的一条记录。通常而言,每张表(采用下划线命名法)会有一个对应的Domain对象(采用驼峰式命名法),当我们更新一条记录时,需要为对象中的每个字段配置映射关系,会写出如下的代码:
int updateUser(User user);
<update id="updateUser">
update users
<set>
<if test="username != null">username=#{username},</if>
<if test="password != null">password=#{password},</if>
<if test="email != null">email=#{email},</if>
<if test="homeAddress != null">home_address=#{homeAddress}</if>
</set>
where id=#{userId}
</update>
冗长的代码只是把驼峰式命名的变量名映射为下划线式命名的列名,显然我们可以将这种映射规律自动化:
@Update("UPDATE user (#{user}) WHERE id =#{userId}")
@Lang(SimpleUpdateExtendedLanguageDriver.class)
int updateUser(Store store);
/**
* Created by benxue on 3/1/16.
*/
public class SimpleUpdateExtendedLanguageDriver extends XMLLanguageDriver
implements LanguageDriver {
private final Pattern inPattern = Pattern.compile("\\(#\\{(\\w+)\\}\\)");
@Override
public SqlSource createSqlSource(Configuration configuration, String script, Class<?> parameterType) {
Matcher matcher = inPattern.matcher(script);
if (matcher.find()) {
StringBuffer ss = new StringBuffer();
ss.append("<set>");
for (Field field : parameterType.getDeclaredFields()) {
String temp = "<if test=\"__field != null\">__column=#{__field},</if>";
ss.append(temp.replaceAll("__field", field.getName())
.replaceAll("__column", CaseFormat.LOWER_CAMEL.to(CaseFormat.LOWER_UNDERSCORE, field.getName())));
}
ss.deleteCharAt(ss.lastIndexOf(","));
ss.append("</set>");
script = matcher.replaceAll(ss.toString());
script = "<script>" + script + "</script>";
}
return super.createSqlSource(configuration, script, parameterType);
}
}
一个常见的情况是,Domain中的部分属性在数据库表中并不存在对应的列,我们增加一个自定义的注释并对LanguageDriver的实现稍作修改:
public class User{
...
@Invisible
private UserSearchDO userSearchDO;
...
}
/**
* Created by benxue on 3/10/16.
* The field marked as Invisible will not be scanned by customized simple extended language drivers
* of myBatis
*/
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Invisible {
}
...
for (Field field : parameterType.getDeclaredFields()) {
if (!field.isAnnotationPresent(Invisible.class)) {
String temp = "<if test=\"__field != null\">__column=#{__field},</if>";
ss.append(temp.replaceAll("__field", field.getName())
.replaceAll("__column", CaseFormat.LOWER_CAMEL.to(CaseFormat.LOWER_UNDERSCORE, field.getName())));
}
}
...
由此,以后就可以用一句话完成动态Update语句了。
总结
通过实现LanguageDriver,我们可以实现方便的自定义注解。在遵循一些约定的情况下(e.g. Domain使用驼峰命名法,数据库表使用下划线命名法),就可以和麻烦的XML配置和动态SQL编写say 88了:)
// 清爽的数据库操作
@Select("SELECT * FROM user WHERE user_id IN (#{userIds})")
@Lang(SimpleSelectInExtendedLanguageDriver.class)
List<User> getUsers(@Param("userIds") List<Long> userIds);
@Update("UPDATE user (#{user}) WHERE user_id =#{userId}")
@Lang(SimpleUpdateExtendedLanguageDriver.class)
int updateUser(User user);
@Insert("INSERT INTO user (#{user})")
@Lang(SimpleInsertExtendedLanguageDriver.class)
void insertUser(User user);
引用
How to use Annotations with iBatis (myBatis) for an IN query