2017.12.27 sqlSessionFactory和sqlSession（to be continued）

参考来自：《深入浅出MyBatis技术原理与实践-第6章 》

1.SqlSessionFactory

SqlSessionFactory是一个接口，最重要的功能是提供SqlSession。

 public interface SqlSessionFactory {

   SqlSession openSession();

   SqlSession openSession(boolean autoCommit);
   SqlSession openSession(Connection connection);
   SqlSession openSession(TransactionIsolationLevel level);

   SqlSession openSession(ExecutorType execType);
   SqlSession openSession(ExecutorType execType, boolean autoCommit);
   SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
   SqlSession openSession(ExecutorType execType, Connection connection);

   Configuration getConfiguration();

 }

SqlSessionFactory有一个默认的实现类DefaultSqlSessionFactory。

 package org.apache.ibatis.session.defaults;

 import java.sql.Connection;
 import java.sql.SQLException;

 import org.apache.ibatis.exceptions.ExceptionFactory;
 import org.apache.ibatis.executor.ErrorContext;
 import org.apache.ibatis.executor.Executor;
 import org.apache.ibatis.mapping.Environment;
 import org.apache.ibatis.session.Configuration;
 import org.apache.ibatis.session.ExecutorType;
 import org.apache.ibatis.session.SqlSession;
 import org.apache.ibatis.session.SqlSessionFactory;
 import org.apache.ibatis.session.TransactionIsolationLevel;
 import org.apache.ibatis.transaction.Transaction;
 import org.apache.ibatis.transaction.TransactionFactory;
 import org.apache.ibatis.transaction.managed.ManagedTransactionFactory;

 /**
  * @author Clinton Begin
  */
 public class DefaultSqlSessionFactory implements SqlSessionFactory {

   private final Configuration configuration;

   public DefaultSqlSessionFactory(Configuration configuration) {
     this.configuration = configuration;
   }
   @Override
   public Configuration getConfiguration() {
     return configuration;
   }
 89   //其余略
 
 }

在对SqlSessionFactory的构建中，直接使用构造方法去构建是有困难的，这样会将大量的逻辑放在构造方法中。因此采用的是builder模式，使用SqlSessionFactoryBuilder来构建。

SqlSessionFactory中提供了很多build方法，最终调用的都是下面两个中的一个，然后调用第三个build：生成一个defaultSqlSessionFactory(configuration config)。

Configuration类中则保存了配置相关的参数，全局唯一。

 package org.apache.ibatis.session;

 import java.io.IOException;
 import java.io.InputStream;
 import java.io.Reader;
 import java.util.Properties;

 import org.apache.ibatis.builder.xml.XMLConfigBuilder;
 import org.apache.ibatis.exceptions.ExceptionFactory;
 import org.apache.ibatis.executor.ErrorContext;
 import org.apache.ibatis.session.defaults.DefaultSqlSessionFactory;

 public class SqlSessionFactoryBuilder {

   public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
     try {
       XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
       return build(parser.parse());
     } catch (Exception e) {
       throw ExceptionFactory.wrapException("Error building SqlSession.", e);
     } finally {
       ErrorContext.instance().reset();
       try {
         reader.close();
       } catch (IOException e) {
         // Intentionally ignore. Prefer previous error.
       }
     }
   }

   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);
   }
   //其余略
 }

综上所述，SqlSessionFactory的构建过程：

• mybatis通过SqlSessionFactoryBuilder去构建SqlSessionFactory。
• SqlSessionFactoryBuilder中提供了SqlXMLConfigBuilder读取XML配置文件中的信息，保存至Configuration类的单例中。
• 然后SqlSessionFactoryBuilder实例化DefaultSqlSessionFactory。