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This article will explain in detail how to use Mybatis for you. The editor thinks it is very practical, so I share it with you as a reference. I hope you can get something after reading this article.

1. Overview of Mybatis

MyBatis is an excellent persistence layer framework that supports ordinary SQL queries (compared to Hibernate encapsulation, Mybatis is a semi-automatic JDBC encapsulation, one of the features is that the SQL query statements executed by Mybatis need to be written in the configuration file), stored procedures and high-level mapping. MyBatis eliminates almost all manual setting of JDBC code and parameters and retrieval of result sets. MyBatis can use simple XML or annotations for configuration and raw mapping, mapping interfaces and Java's POJO (Plain Old Java Objects, plain Java objects) to records in the database.

2. Analysis of the principle of Mybatis

The following is a simple execution process with Mybatis

1. Load mybatis global configuration files (data sources, mapper mapping files, etc.), parse configuration files, MyBatis generates Configuration based on XML configuration files, and MappedStatement one by one (including parameter mapping configuration, dynamic SQL statement, result mapping configuration), which correspond to label items.

2. SqlSessionFactoryBuilder generates SqlSessionFactory from Configuration object, which is used to open SqlSession.

3. The SqlSession object completes the interaction with the database:

A. The user program calls the mybatis interface layer api (i.e. the method in the Mapper interface)

B. SqlSession finds the corresponding MappedStatement object by calling the Statement ID of api

C. Parsing MappedStatement objects through Executor (responsible for the generation of dynamic SQL and maintenance of query cache), transforming sql parameters, and dynamic sql splicing to generate jdbc Statement objects

D, JDBC executes sql.

E. With the help of the result mapping relationship in MappedStatement, the returned results are transformed into storage structures such as HashMap, JavaBean and returned.

The following is the frame schematic of Mybatis

3. Simple example of Mybatis

(1) Import relevant jar packages, core jar packages of Mybatis runtime environment and packages that connect to databases

(2) create a simple data table

(3) create Java object (PO type)

Package cn.mybatis.po;public class User {private int id; private String username; private String password; private String address; private String sex; public int getId () {return id;} public String getUsername () {return username;} public String getPassword () {return password;} public String getAddress () {return address;} public String getSex () {return sex;} public void setId (int id) {this.id = id;} public void setUsername (String username) {this.username = username } public void setPassword (String password) {this.password = password;} public void setAddress (String address) {this.address = address;} public void setSex (String sex) {this.sex = sex } @ Override public String toString () {return "User {" + "id=" + id + ", username='" + username +'\'+ ", password='" + password +'\'+ ", address='" + address +'\'+ ", sex='" + sex +'\'+'};}} User entity classes

(4) create Mybatis core configuration file (SqlMapConfig.xml)

Configure the information about the connection database in the core configuration file (if it is integrated with Spring, you can configure the database in the Spring configuration file)

Mybatis Core profile

(5) create a Mapper.xml file to correspond to the required Sql query operations

SELECT * FROM t_user WHERE id = # {id} UserMapper configuration file

(7) create a test program to test the select query you just wrote

Package cn.mybatis.first;import cn.mybatis.po.User;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import java.io.IOException;import java.io.InputStream;public class Test {public User findUserById () throws IOException {/ / get the mybatis configuration file String resource = "SqlMapConfig.xml"; / / get the file stream information of the configuration file InputStream inputStream = Resources.getResourceAsStream (resource) / / create the configuration file information passed into mybatis by the session factory SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder (). Build (inputStream); / / get SqlSession SqlSession sqlSession = sqlSessionFactory.openSession () through the session factory; / / manipulate the database through sqlSession / / the first parameter is the id:namespace + statment id / / of statment in the mapping file. The second parameter is to set the parameter User user = sqlSession.selectOne ("test.findUserById", 1) of type parameterType in the mapping file. / / System.out.println (user); / / release session resource try {sqlSession.close ();} catch (Exception e) {e.printStackTrace ();} return user;} public static void main (String [] args) {/ / TODO Auto-generated method stub Test test = new Test (); try {System.out.println (test.findUserById ());} catch (IOException e) {e.printStackTrace ();} Test test program

(8) add Log4j log file

# direct log messages to stdout # # log4j.rootLogger=DEBUG, stdoutlog4j.appender.stdout=org.apache.log4j.ConsoleAppenderlog4j.appender.stdout.layout=org.apache.log4j.PatternLayoutlog4j.appender.stdout.layout.ConversionPattern=%5p [% t] -% m%nLog4j.properties

(9) Test results

4. Other CRUD operations

(1) insert operation

Add the SQL configuration of the response in the Mapper file, and use the LAST_INSERT_ID () function in MySQL to get the primary key value of the added data

SELECT LAST_INSERT_ID () INSERT INTO t_user (id,username,password,address,sex) VALUES (# {id}, # {username}, # {password}, # {address}, # {sex})

Insert the log information of the data without using sqlSession.commit (); previous log situation

As you can see from the figure above, when no commit is added, the transaction is rolled back, so if you want to add data, you need to commit it manually (before integrating Spring).

Attach the function of insertUser

Public void inserUser () throws IOException {/ / get file stream information InputStream inputStream = Resources.getResourceAsStream ("SqlMapConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder (). Build (inputStream); SqlSession sqlSession = sqlSessionFactory.openSession (); User user = new User ("World", "1234", "Wuhan", "male"); sqlSession.insert ("test.addUser", user); System.out.println (user.getId ()); sqlSession.commit () / / release session resource try {sqlSession.close ();} catch (Exception e) {e.printStackTrace ();}} inserUser function

(2) Fuzzy query

First configure the Mapper file. The simple difference between ${} and # {} is as follows:

SELECT * FROM t_user WHERE username LIKE'% ${value}%'

A small error was encountered when using the query. Due to the insert method tested earlier, in which a parameter constructor was added to the User entity class, the following error occurred. The reason is: when using Mybatis query, you need to add a parameter-free constructor to the entity class (of course, if the entity class itself does not have a constructor, it will be the default one)

Attach the function implementation of findByUsername

Public void findUserByUsername () throws IOException {/ / get the file stream information of the configuration file InputStream inputStream = Resources.getResourceAsStream ("SqlMapConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder (). Build (inputStream); SqlSession sqlSession = sqlSessionFactory.openSession (); List userList = sqlSession.selectList ("test.findUserByUsername", "u"); System.out.println (userList); / / release session resource try {sqlSession.close ();} catch (Exception e) {e.printStackTrace ();}} findByUsername function implementation

(3) Delete operation

First configure the deletion operation in Mapper

DELETE FROM t_user WHERE id = # {value}

To run the test program, as in insert, you need to commit the transaction manually, as shown below

End result:

The data record numbered 10 has been deleted from the data table

5. Sort out the details

(1) understanding of some related classes in the sample program

A) SqlSessionFactoryBuilder

Used to create a SqlSessionFactory. Because SqlSessionFactory uses singleton mode, you don't need to use singleton mode to manage SqlSessionFactoryBuilder, you only need to use it once when creating SqlSessionFactory

B) SqlSessionFactory

Session factory, used to create SqlSession. The singleton pattern can be used to manage the SqlSessionFactory session factory, and an instance has been used since the factory was created.

C) SqlSession

An interface for programmers that provides a way to operate a database. SqlSession is not thread safe (the reason: in the SqlSession implementation class, in addition to the methods in the interface to manipulate the database, there are also properties of the data domain, such as some submitted data, etc., so it will cause thread unsafety in the case of multithreaded concurrent requests), so we can use SqlSession in the method body, so that each thread has its own method and will not conflict

(2) mapper mapping file in Mybatis

As when explaining the principle of Mybatis execution, the Sql statements configured in the Mapper mapping file are actually encapsulated as MapperStatment objects at the time of execution, that is, the Mapper mapping file manages different Sql according to statment. When writing the program, when we use the method of manipulating the database in SqlSession (selectOne,selectList, etc.), the parameters passed in, in addition to the arguments (id, fuzzy query string, etc.), also need to pass in the corresponding Sql location, and Sql is managed by Statment, so it is passed in namespace+statmentId.

(3) placeholder

# {id}: where id represents the accepted input parameter, and the parameter name is id. It is indicated here that if the input parameter is a simple type, the parameter name in # {} can be set arbitrarily (value or other names).

${value}: it means that the input parameters will be concatenated as strings in SQL without any modification, but this will easily lead to the hidden danger of SQL injection. The value in ${value} represents the accepted input parameters. Note that if the input parameters are of simple type, the formal parameters can only be value.

(4) alias definition

Definition of a single alias for ①

Use after defining aliases

SELECT * FROM t_user WHERE id = # {id}

Definition of ② batch aliases

(5) when you load the Mapper mapping file in SqlMapConfig.xml, you can also use the mapper interface to load the mapping file in addition to resource.

① first pays attention to one thing:

When configuring mybatis-config.xml, the nodes are in order, and the configuration order is as follows:

Properties/settings/typeAliases/typeHandlers/objectFactory/objectWrapperFactory/plugins/environments/databaseIdProvider/mappers

② uses mapper loading method to put the mapper interface and mapper configuration file in the same directory, and the file name is the same, and the development should follow the way of mapper agent.

Brief introduction to the method of developing dao with 6.Mybatis

(1) the way to use dao interface + to implement classes

A) write the interface first, just as you would normally write the schema

Package cn.mybatis.dao;import cn.mybatis.po.User;/** * original Dao development: dao interface + dao implementation class * / public interface UserDao {/ / query information according to id public User findUserById (int id) throws Exception; / / add information public void insertUser (User user) throws Exception; / / delete information public void deleteUser (int id) throws Exception;} dao interface

B) then write the interface implementation

Package cn.mybatis.dao.daoImpl;import cn.mybatis.dao.UserDao;import cn.mybatis.po.User;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.junit.Test;public class UserDaoImpl implements UserDao {/ / use the constructor to inject SqlSessionFactory private SqlSessionFactory sqlSessionFactory; public UserDaoImpl (SqlSessionFactory sqlSessionFactory) {this.sqlSessionFactory = sqlSessionFactory;} @ Override @ Test public User findUserById (int id) throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession () User user = sqlSession.selectOne ("test.findUserById", id); sqlSession.close (); return user;} @ Override public void insertUser (User user) throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession (); / / User user1 = new User ("test1", "123"," Hongshan "," male "); sqlSession.insert (" test.findUserById ", user); sqlSession.commit (); sqlSession.close () } @ Override public void deleteUser (int id) throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession (); sqlSession.delete ("test.findUserById", id); sqlSession.commit (); sqlSession.close ();}} dao interface implementation class

C) Mapper configuration file and SqlConfig configuration file remain unchanged

D) use Junit for testing

Package cn.mybatis.testdao;import cn.mybatis.dao.UserDao;import cn.mybatis.dao.daoImpl.UserDaoImpl;import cn.mybatis.po.User;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.After;import org.junit.Before;import org.junit.Test;import java.io.InputStream;public class UserDaoImplTest {private SqlSessionFactory sqlSessionFactory; @ Before public void setUp () throws Exception {InputStream inputStream = Resources.getResourceAsStream ("SqlMapConfig.xml") SqlSessionFactory = new SqlSessionFactoryBuilder () .build (inputStream);} @ Test public void testFindUserById () throws Exception {/ / create the object of UserDao UserDao userDao = new UserDaoImpl (sqlSessionFactory); / / call the method User user = userDao.findUserById (1) of UserDao; System.out.println (user);}} Junit test

E) Test results

F) problems with the original dao method

There are a large number of template methods (that is, a lot of repetitive code) in the implementation of the ① dao interface

② hard-coded statmentid when calling the SqlSession method

The parameters passed in when the ③ bar uses the SqlSession method will not report an error during the compilation phase (even if the parameters are incorrect) because of the use of generics.

(2) the method of using Mapper proxy (that is, only the Mapper interface is required)

(a) Specification for the use of mapper

① in the way the mapper proxy is used, the value of namespace should be the path of the mapper interface

The interface method name of ② in mapper.java interface file is the same as the id of statment in mapper.xml.

The input parameters of the interface method of ③ in mapper.java interface file are the same as the parameterType of statment in mapper.xml.

The return type of the interface method of ④ in the mapper.java interface file is the same as the resultType of statment in mapper.xml.

(B) query and delete operation examples

① writes mapper.xml configuration files that contain sql configurations for select and delete

SELECT * FROM t_user WHERE id = # {id} DELETE FROM t_user WHERE id = # {value} mapper.xml configuration file

② writes the mapper interface, and develops the specification to write the mapper interface in the way of mapper proxy.

Package cn.mybatis.testmapper;import cn.mybatis.mapper.UserMapper;import cn.mybatis.po.User;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.After;import org.junit.Before;import org.junit.Test;import java.io.InputStream;public class UserMapperTest {private SqlSessionFactory sqlSessionFactory; @ Before public void setUp () throws Exception {InputStream inputStream = Resources.getResourceAsStream ("SqlMapConfig.xml") SqlSessionFactory = new SqlSessionFactoryBuilder (). Build (inputStream);} @ Test public void testFindUserById () throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession (); / / get the proxy object of UserMapper UserMapper userMapper = sqlSession.getMapper (UserMapper.class); User user = userMapper.findUserById (9); System.out.println (user);} @ Test public void testDeleteUser () throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession (); UserMapper userMapper = sqlSession.getMapper (UserMapper.class); userMapper.deleteUser (9); sqlSession.commit () } @ After public void tearDown () throws Exception {}} mapper interface

③ Junit Test

Package cn.mybatis.testmapper;import cn.mybatis.mapper.UserMapper;import cn.mybatis.po.User;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.After;import org.junit.Before;import org.junit.Test;import java.io.InputStream;public class UserMapperTest {private SqlSessionFactory sqlSessionFactory; @ Before public void setUp () throws Exception {InputStream inputStream = Resources.getResourceAsStream ("SqlMapConfig.xml") SqlSessionFactory = new SqlSessionFactoryBuilder (). Build (inputStream);} @ Test public void testFindUserById () throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession (); / / get the proxy object of UserMapper UserMapper userMapper = sqlSession.getMapper (UserMapper.class); User user = userMapper.findUserById (8); System.out.println (user);} @ Test public void testDeleteUser () throws Exception {SqlSession sqlSession = sqlSessionFactory.openSession (); UserMapper userMapper = sqlSession.getMapper (UserMapper.class); userMapper.deleteUser (8) } @ After public void tearDown () throws Exception {}} Junit Test

④ query result display

⑤ deletion result display

This is the end of the article on "how to use Mybatis". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, please share it for more people to see.

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