java - MyBatis 可以不用执行就生成 Dynamic SQL 吗？

Question

我有一些复杂的查询要用一些可选的过滤器来构建，MyBatis 似乎是生成动态 SQL 的理想选择。

但是，我仍然希望我的查询与应用程序的其余部分(不使用 MyBatis)在相同的框架中执行。

所以我希望做的是严格使用 MyBatis 来生成 SQL，但从那里使用我的应用程序的其余部分来实际执行它。这可能吗？如果是，怎么办？

Answer 1

虽然 MyBatis 被设计为在构建查询后执行查询，但您可以利用它的配置和一点“内部知识”来获得您需要的内容。

MyBatis 是一个非常好的框架，不幸的是它缺少文档，所以源代码是你的 friend 。如果你四处挖掘，你应该会遇到这些类:org.apache.ibatis.mapping.MappedStatement 和 org.apache.ibatis.mapping.BoundSql，它们是构建的关键参与者动态 SQL。这是一个基本的用法示例:

MySQL 表 user 中包含以下数据:

name    login
-----   -----
Andy    a
Barry   b
Cris    c

用户类:

package pack.test;
public class User {
    private String name;
    private String login;
    // getters and setters ommited
}

UserService 接口(interface):

package pack.test;
public interface UserService {
    // using a different sort of parameter to show some dynamic SQL
    public User getUser(int loginNumber);
}

UserService.xml 映射器文件:

<mapper namespace="pack.test.UserService">
    <select id="getUser" resultType="pack.test.User" parameterType="int">
       <!-- dynamic change of parameter from int index to login string -->
       select * from user where login = <choose>
                                           <when test="_parameter == 1">'a'</when>
                                           <when test="_parameter == 2">'b'</when>
                                           <otherwise>'c'</otherwise>
                                        </choose>   
    </select>
</mapper>

sqlmap-config.file:

<configuration>
    <settings>
        <setting name="lazyLoadingEnabled" value="false" />
    </settings>
    <environments default="development"> 
        <environment id="development"> 
            <transactionManager type="JDBC"/> 
            <dataSource type="POOLED"> 
                <property name="driver" value="com.mysql.jdbc.Driver"/> 
                <property name="url" value="jdbc:mysql://localhost/test"/> 
                <property name="username" value="..."/> 
                <property name="password" value="..."/> 
            </dataSource> 
        </environment> 
      </environments>
    <mappers>
        <mapper resource="pack/test/UserService.xml"/>
    </mappers>
</configuration>

AppTester 显示结果:

package pack.test;

import java.io.Reader;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class AppTester {
    private static String CONFIGURATION_FILE = "sqlmap-config.xml";

    public static void main(String[] args) throws Exception {
        Reader reader = null;
        SqlSession session = null;
        try {

            reader = Resources.getResourceAsReader(CONFIGURATION_FILE);
            session = new SqlSessionFactoryBuilder().build(reader).openSession();
            UserService userService = session.getMapper(UserService.class);

            // three users retreived from index
            for (int i = 1; i <= 3; i++) {
                User user = userService.getUser(i);
                System.out.println("Retreived user: " + user.getName() + " " + user.getLogin());

                // must mimic the internal statement key for the mapper and method you are calling
                MappedStatement ms = session.getConfiguration().getMappedStatement(UserService.class.getName() + ".getUser");
                BoundSql boundSql = ms.getBoundSql(i); // parameter for the SQL statement
                System.out.println("SQL used: " + boundSql.getSql());
                System.out.println();
            }

        } finally {
            if (reader != null) {
                reader.close();
            }
            if (session != null) {
                session.close();
            }
        }
    }
}

结果:

Retreived user: Andy a
SQL used: select * from user where login =  'a'

Retreived user: Barry b
SQL used: select * from user where login =  'b'

Retreived user: Cris c
SQL used: select * from user where login =  'c'