MyBatis是一个持久层框架，使用简单，学习成本较低。可以执行自己手写的SQL语句，比较灵活。但是MyBatis的自动化程度不高，移植性也不高，有时从一个数据库迁移到另外一个数据库的时候需要自己修改配置。
一个Mybatis最简单的使用列子如下：
public class UserDaoTest {private SqlSessionFactory sqlSessionFactory;@Beforepublic void setUp() throws Exception{ClassPathResource resource = new ClassPathResource("mybatis-config.xml");InputStream inputStream = resource.getInputStream();sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);}@Testpublic void selectUserTest(){String id = "{0003CCCA-AEA9-4A1E-A3CC-06D884BA3906}";SqlSession sqlSession = sqlSessionFactory.openSession();CbondissuerMapper cbondissuerMapper = sqlSession.getMapper(CbondissuerMapper.class);Cbondissuer cbondissuer = cbondissuerMapper.selectByPrimaryKey(id);System.out.println(cbondissuer);sqlSession.close();}}
本博客只涉及创建SessionFactory，以及从SessionFactory获取SqlSession的流程。具体执行Sql的流程会在其他博客中分析。
ClassPathResource resource = new ClassPathResource("mybatis-config.xml");
InputStream inputStream = resource.getInputStream();
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
通过上面代码发现，创建SqlSessionFactory的代码在SqlSessionFactoryBuilder中，进去一探究竟：
//整个过程就是将配置文件解析成Configration对象，然后创建SqlSessionFactory的过程
//Configuration是SqlSessionFactory的一个内部属性
public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {try {XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);return build(parser.parse());} catch (Exception e) {throw ExceptionFactory.wrapException("Error building SqlSession.", e);} finally {ErrorContext.instance().reset();try {inputStream.close();} catch (IOException e) {// Intentionally ignore. Prefer previous error.}}}public SqlSessionFactory build(Configuration config) {return new DefaultSqlSessionFactory(config);}
下面我们看下解析配置文件过程中的一些细节。
先给出一个配置文件的列子：
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><!--SqlSessionFactoryBuilder中配置的配置文件的优先级最高；config.properties配置文件的优先级次之；properties标签中的配置优先级最低 --><properties resource="org/mybatis/example/config.properties"><property name="username" value="dev_user"/><property name="password" value="F2Fa3!33TYyg"/></properties><!--一些重要的全局配置--><settings><setting name="cacheEnabled" value="true"/><!--<setting name="lazyLoadingEnabled" value="true"/>--><!--<setting name="multipleResultSetsEnabled" value="true"/>--><!--<setting name="useColumnLabel" value="true"/>--><!--<setting name="useGeneratedKeys" value="false"/>--><!--<setting name="autoMappingBehavior" value="PARTIAL"/>--><!--<setting name="autoMappingUnknownColumnBehavior" value="WARNING"/>--><!--<setting name="defaultExecutorType" value="SIMPLE"/>--><!--<setting name="defaultStatementTimeout" value="25"/>--><!--<setting name="defaultFetchSize" value="100"/>--><!--<setting name="safeRowBoundsEnabled" value="false"/>--><!--<setting name="mapUnderscoreToCamelCase" value="false"/>--><!--<setting name="localCacheScope" value="STATEMENT"/>--><!--<setting name="jdbcTypeForNull" value="OTHER"/>--><!--<setting name="lazyLoadTriggerMethods" value="equals,clone,hashCode,toString"/>--><!--<setting name="logImpl" value="STDOUT_LOGGING" />--></settings><typeAliases></typeAliases><plugins><plugin interceptor="com.github.pagehelper.PageInterceptor"><!--默认值为 false，当该参数设置为 true 时，如果 pageSize=0 或者 RowBounds.limit = 0 就会查询出全部的结果--><!--如果某些查询数据量非常大，不应该允许查出所有数据--><property name="pageSizeZero" value="true"/></plugin></plugins><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://10.59.97.10:3308/windty"/><property name="username" value="windty_opr"/><property name="password" value="windty!234"/></dataSource></environment></environments><databaseIdProvider type="DB_VENDOR"><property name="MySQL" value="mysql" /><property name="Oracle" value="oracle" /></databaseIdProvider><mappers><!--这边可以使用package和resource两种方式加载mapper--><!--<package name="包名"/>--><!--<mapper resource="./mappers/SysUserMapper.xml"/>--><mapper resource="./mappers/CbondissuerMapper.xml"/></mappers></configuration>
下面是解析配置文件的核心方法：
private void parseConfiguration(XNode root) {try {//issue #117 read properties first//解析properties标签，并set到Configration对象中//在properties配置属性后，在Mybatis的配置文件中就可以使用${key}的形式使用了。propertiesElement(root.evalNode("properties"));//解析setting标签的配置Properties settings = settingsAsProperties(root.evalNode("settings"));//添加vfs的自定义实现，这个功能不怎么用loadCustomVfs(settings);//配置类的别名，配置后就可以用别名来替代全限定名//mybatis默认设置了很多别名，参考附录部分typeAliasesElement(root.evalNode("typeAliases"));//解析拦截器和拦截器的属性，set到Configration的interceptorChain中//MyBatis 允许你在已映射语句执行过程中的某一点进行拦截调用。默认情况下，MyBatis 允许使用插件来拦截的方法调用包括：//Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed)//ParameterHandler (getParameterObject, setParameters)//ResultSetHandler (handleResultSets, handleOutputParameters)//StatementHandler (prepare, parameterize, batch, update, query)pluginElement(root.evalNode("plugins"));//Mybatis创建对象是会使用objectFactory来创建对象，一般情况下不会自己配置这个objectFactory，使用系统默认的objectFactory就好了objectFactoryElement(root.evalNode("objectFactory"));objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));reflectorFactoryElement(root.evalNode("reflectorFactory"));//设置在setting标签中配置的配置settingsElement(settings);//解析环境信息，包括事物管理器和数据源，SqlSessionFactoryBuilder在解析时需要指定环境id，如果不指定的话，会选择默认的环境；//最后将这些信息set到Configration的Environment属性里面environmentsElement(root.evalNode("environments"));//databaseIdProviderElement(root.evalNode("databaseIdProvider"));//无论是 MyBatis 在预处理语句（PreparedStatement）中设置一个参数时，还是从结果集中取出一个值时， 都会用类型处理器将获取的值以合适的方式转换成 Java 类型。解析typeHandler。typeHandlerElement(root.evalNode("typeHandlers"));//解析MappermapperElement(root.evalNode("mappers"));} catch (Exception e) {throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);}
}
上面解析流程结束后会生成一个Configration对象，包含所有配置信息，然后会创建一个SqlSessionFactory对象，这个对象包含了Configration对象。
下面是openSession的过程:
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {Transaction tx = null;try {final Environment environment = configuration.getEnvironment();final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);//获取执行器，这边获得的执行器已经代理拦截器的功能（见下面代码）final Executor executor = configuration.newExecutor(tx, execType);//根据获取的执行器创建SqlSessionreturn new DefaultSqlSession(configuration, executor, autoCommit);} catch (Exception e) {closeTransaction(tx); // may have fetched a connection so lets call close()throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);} finally {ErrorContext.instance().reset();}}
//interceptorChain生成代理类，具体参见Plugin这个类的方法
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {executorType = executorType == null ? defaultExecutorType : executorType;executorType = executorType == null ? ExecutorType.SIMPLE : executorType;Executor executor;if (ExecutorType.BATCH == executorType) {executor = new BatchExecutor(this, transaction);} else if (ExecutorType.REUSE == executorType) {executor = new ReuseExecutor(this, transaction);} else {executor = new SimpleExecutor(this, transaction);}if (cacheEnabled) {executor = new CachingExecutor(executor);}executor = (Executor) interceptorChain.pluginAll(executor);return executor;}
到此为止，我们已经获得了SqlSession，拿到SqlSession就可以执行各种CRUD方法了。
对于MyBatis启动的流程（获取SqlSession的过程）这边简单总结下：
SQL的具体执行流程见后续博客。
一些重要类总结：
//TypeAliasRegistry
registerAlias("string", String.class);registerAlias("byte", Byte.class);
registerAlias("long", Long.class);
registerAlias("short", Short.class);
registerAlias("int", Integer.class);
registerAlias("integer", Integer.class);
registerAlias("double", Double.class);
registerAlias("float", Float.class);
registerAlias("boolean", Boolean.class);registerAlias("byte[]", Byte[].class);
registerAlias("long[]", Long[].class);
registerAlias("short[]", Short[].class);
registerAlias("int[]", Integer[].class);
registerAlias("integer[]", Integer[].class);
registerAlias("double[]", Double[].class);
registerAlias("float[]", Float[].class);
registerAlias("boolean[]", Boolean[].class);registerAlias("_byte", byte.class);
registerAlias("_long", long.class);
registerAlias("_short", short.class);
registerAlias("_int", int.class);
registerAlias("_integer", int.class);
registerAlias("_double", double.class);
registerAlias("_float", float.class);
registerAlias("_boolean", boolean.class);registerAlias("_byte[]", byte[].class);
registerAlias("_long[]", long[].class);
registerAlias("_short[]", short[].class);
registerAlias("_int[]", int[].class);
registerAlias("_integer[]", int[].class);
registerAlias("_double[]", double[].class);
registerAlias("_float[]", float[].class);
registerAlias("_boolean[]", boolean[].class);registerAlias("date", Date.class);
registerAlias("decimal", BigDecimal.class);
registerAlias("bigdecimal", BigDecimal.class);
registerAlias("biginteger", BigInteger.class);
registerAlias("object", Object.class);registerAlias("date[]", Date[].class);
registerAlias("decimal[]", BigDecimal[].class);
registerAlias("bigdecimal[]", BigDecimal[].class);
registerAlias("biginteger[]", BigInteger[].class);
registerAlias("object[]", Object[].class);registerAlias("map", Map.class);
registerAlias("hashmap", HashMap.class);
registerAlias("list", List.class);
registerAlias("arraylist", ArrayList.class);
registerAlias("collection", Collection.class);
registerAlias("iterator", Iterator.class);registerAlias("ResultSet", ResultSet.class);
https://blog.csdn.net/luanlouis/article/details/40422941
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