Mybatis SqlSession案例详解

public class MybatisApplication {
    public static final String URL = "jdbc:mysql://localhost:3306/mblog";
    public static final String USER = "root";
    public static final String PASSWORD = "123456";

    public static void main(String[] args) {
        String resource = "mybatis-config.xml";
        InputStream inputStream = null;
        SqlSession sqlSession = null;
        try {
            inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            sqlSession = sqlSessionFactory.openSession();
            UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
            System.out.println(userMapper.selectById(1));

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            sqlSession.close();
        }
    }
}

SqlSession sqlSession = sqlSessionFactory.openSession();

public class DefaultSqlSessionFactory implements SqlSessionFactory {

  private final Configuration configuration;

  public DefaultSqlSessionFactory(Configuration configuration) {
    this.configuration = configuration;
  }
  //创建session，这个方法直接调用本类中的另外一个方法
  @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }
  //其实是调用这个方法
  private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      //对应xml标签<environments> ,这个在配置文件解析的时候就已经存放到configuration中了。
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      //创建一个executor来执行SQL  
      final Executor executor = configuration.newExecutor(tx, execType);
      //这里也说明了，为什么我们代码里的SqlSession是DefaultSqlSession
      return 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();
    }
  }

    private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
        if (environment == null || environment.getTransactionFactory() == null) {
            return new ManagedTransactionFactory();
        }
        return environment.getTransactionFactory();
    }
}

final Executor executor = configuration.newExecutor(tx, execType);
//Configuration中
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    //第一步
    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);
    }
    //第二步
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    //第三步
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
}

public enum ExecutorType {
  SIMPLE, REUSE, BATCH
}

protected abstract int doUpdate(MappedStatement ms, Object parameter) throws SQLException;

protected abstract List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException;

protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException;

protected abstract <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql)  throws SQLException;

if (cacheEnabled) {
      executor = new CachingExecutor(executor);
}

executor = (Executor) interceptorChain.pluginAll(executor);

public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
  this.configuration = configuration;
  this.executor = executor;
  this.dirty = false;
  this.autoCommit = autoCommit;
}