IDEA创建MyBatis项目--实现简单的查操作

旧城等待， 2022-05-08 06:22 182阅读 0赞

本篇文章只是为了快速创建一个Mybatis实现查的项目，并没有对知识点进行讲。我个人认为学习框架先是动手实现一下，先不用管为什么要这么做，等运行成功后再去琢磨比较容易懂和记得牢。

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> ### 通过maven加载Mybatis依赖包 ###

![70 2][]

<dependencies>
            <dependency>
                <groupId>org.mybatis</groupId>
                <artifactId>mybatis</artifactId>
                <version>3.3.0</version>
            </dependency>
            <dependency>
                <groupId>mysql</groupId>
                <artifactId>mysql-connector-java</artifactId>
                <version>5.1.29</version>
            </dependency>
            <dependency>
                <groupId>junit</groupId>
                <artifactId>junit</artifactId>
                <version>4.11</version>
                <scope>test</scope>
            </dependency>
            <dependency>
                <groupId>log4j</groupId>
                <artifactId>log4j</artifactId>
                <version>1.2.17</version>
            </dependency>
            <dependency>
                <groupId>org.slf4j</groupId>
                <artifactId>slf4j-api</artifactId>
                <version>1.7.12</version>
            </dependency>
        </dependencies>

> ### 新建config.properties配置文件 ###

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> ### 新建Mybatis配置文件 ###

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE configuration
            PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-config.dtd">
    <configuration>
        <properties resource="config.properties"/>
        <typeAliases>
    
            <package name=""/>
        </typeAliases>
        <environments default="development">
            <environment id="development">
                <transactionManager type="JDBC"/>
                <dataSource type="POOLED">
                    <property name="driver" value="${database.driver}"/>
                    <property name="url" value="${database.url}"/>
                    <property name="username" value="${database.username}"/>
                    <property name="password" value="${database.password}"/>
                </dataSource>
            </environment>
        </environments>
        <mappers>
            <mapper resource="mapper/StudentMapper.xml"/>
        </mappers>
    
    </configuration>

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> ### 编写SqlSessionFactoryUtils工具类 ###

package com.Utils;
    
    import org.apache.ibatis.session.SqlSession;
    import org.apache.ibatis.session.SqlSessionFactory;
    import org.apache.ibatis.session.SqlSessionFactoryBuilder;
    import org.apache.ibatis.io.Resources;
    
    import java.io.IOException;
    import java.io.InputStream;
    
    public class SqlSessionFactoryUtils {
    
        private final static Class<SqlSessionFactoryUtils> lock=SqlSessionFactoryUtils.class;
    
        private static SqlSessionFactory sqlSessionFactory=null;
    
        private SqlSessionFactoryUtils(){};
    
        public static SqlSessionFactory getSqlSessionFactory(){
            synchronized (lock){
                if (sqlSessionFactory!=null){
                    return sqlSessionFactory;
                }
    
                String resource="mybatis-config.xml";
                InputStream inputStream;
                try {
                    inputStream=Resources.getResourceAsStream(resource);
                    sqlSessionFactory =new SqlSessionFactoryBuilder().build(inputStream);
                }catch (IOException e){
                    e.printStackTrace();
                    return null;
                }
    
                return sqlSessionFactory;
            }
        }
    
        public static SqlSession openSqlSession(){
            if (sqlSessionFactory==null){
                getSqlSessionFactory();
            }
            return  sqlSessionFactory.openSession();
        }
    
    
    }

### ![70 6][] ###

> ### 编写Student实体类 ###

package com.entity;
    
    public class Student {
    
        private int id;
        private String name;
        private int age;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        @Override
        public String toString() {
            return "Student{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }

### ![70 7][] ###

> ### 编写StudentMapper接口 ###

package com.mapper;
    
    import com.entity.Student;
    
    public interface StudentMapper {
        public Student getStudent(long id);
    }

### ![70 8][] ###

> ### 编写StudentMapper.xml映射器 ###

<?xml version="1.0" encoding="utf-8" ?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
            "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.mapper.StudentMapper">
    
        <select id="getStudent" parameterType="long" resultType="com.entity.Student">
            select id,name,age from student where id=#{id}
        </select>
    
    </mapper>

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> ### 数据库表结构 ###

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> ### 编写Main类 ###

package com;
    
    import com.entity.Student;
    import com.mapper.StudentMapper;
    import com.utils.SqlSessionFactoryUtils;
    import org.apache.ibatis.session.SqlSession;
    
    public class Main {
    
        public static void main(String[] args){
            SqlSession sqlSession=SqlSessionFactoryUtils.openSqlSession();
            StudentMapper studentMapper=sqlSession.getMapper(StudentMapper.class);
            System.out.println(studentMapper.getStudent(1).toString());
        }
    }

### ![70 11][] ###

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> ### 注意的点 ###

1.  Mapper.xml的namespace与Mapper.java类的包名要一致
2.  Mapper.xml中select元素的id属性值必须跟Mapper.java中方法名要一致

> ### 报错 ###

出现这个错误，可以看一下报错的内容：Could not find resource com/xml/StudentMapper.xml。一个很不好的习惯，把xml放在了java/com/xml包下，java包目录下就应该放java代码，放xml实在不应该。把xml文件放在resources目录下，便解决这个问题。<mapper resource="mapper/StudentMapper.xml"/>实际就是查找resources/mapper目录的文件。

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"C:\Program Files\Java\jdk1.8.0_131\bin\java.exe" "-javaagent:E:\IntelliJ IDEA 2018.1.6\lib\idea_rt.jar=59220:E:\IntelliJ IDEA 2018.1.6\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_131\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_131\jre\lib\rt.jar;C:\Users\林颖\Desktop\conmybatis\target\classes;C:\Users\林颖\.m2\repository\org\mybatis\mybatis\3.3.0\mybatis-3.3.0.jar;C:\Users\林颖\.m2\repository\mysql\mysql-connector-java\5.1.29\mysql-connector-java-5.1.29.jar;C:\Users\林颖\.m2\repository\log4j\log4j\1.2.17\log4j-1.2.17.jar;C:\Users\林颖\.m2\repository\org\slf4j\slf4j-api\1.7.12\slf4j-api-1.7.12.jar;C:\Users\林颖\.m2\repository\org\slf4j\slf4j-log4j12\1.7.12\slf4j-log4j12-1.7.12.jar" com.mian
    log4j:WARN No appenders could be found for logger (org.apache.ibatis.logging.LogFactory).
    log4j:WARN Please initialize the log4j system properly.
    log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
    Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: 
    ### Error building SqlSession.
    ### The error may exist in com/xml/StudentMapper.xml
    ### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.io.IOException: Could not find resource com/xml/StudentMapper.xml
    	at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)
    	at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:82)
    	at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:66)
    	at com.Utils.SqlSessionFactoryUtils.getSqlSessionFactory(SqlSessionFactoryUtils.java:29)
    	at com.Utils.SqlSessionFactoryUtils.openSqlSession(SqlSessionFactoryUtils.java:41)
    	at com.mian.main(mian.java:11)
    Caused by: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. Cause: java.io.IOException: Could not find resource com/xml/StudentMapper.xml
    	at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:115)
    	at org.apache.ibatis.builder.xml.XMLConfigBuilder.parse(XMLConfigBuilder.java:95)
    	at org.apache.ibatis.session.SqlSessionFactoryBuilder.build(SqlSessionFactoryBuilder.java:80)
    	... 4 more
    Caused by: java.io.IOException: Could not find resource com/xml/StudentMapper.xml
    	at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:114)
    	at org.apache.ibatis.io.Resources.getResourceAsStream(Resources.java:100)
    	at org.apache.ibatis.builder.xml.XMLConfigBuilder.mapperElement(XMLConfigBuilder.java:338)
    	at org.apache.ibatis.builder.xml.XMLConfigBuilder.parseConfiguration(XMLConfigBuilder.java:113)
    	... 6 more
    
    进程完成，退出码 1

**修改之前：**

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--------------------

Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: 
    ### Error querying database.  Cause: java.sql.SQLException: Error setting driver on UnpooledDataSource. Cause: java.lang.ClassNotFoundException: Cannot find class: ${database.driver}

遇到这种问题肯定是config.properties跟 value="$\{database.xx\}"两个文件其中一个写错了。

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