工厂方法模式的应用之 MyBatis SqlSessionFactory

在 MyBatis 中，SqlSessionFactory 和 SqlSession 就是工厂方法模式的应用。SqlSessionFactory 是工厂接口，它定义了创建 SqlSession 的方法。

SqlSessionFactory：

package org.apache.ibatis.session;

import java.sql.Connection;

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();

}

SqlSession：

package org.apache.ibatis.session;

import java.io.Closeable;
import java.sql.Connection;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.BatchResult;

public interface SqlSession extends Closeable {

  <T> T selectOne(String statement);

  <T> T selectOne(String statement, Object parameter);

  <E> List<E> selectList(String statement);

  // ...
}

DefaultSqlSessionFactory 实现了 SqlSessionFactory 接口：

public class DefaultSqlSessionFactory implements SqlSessionFactory {
  @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

  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);
      final Executor executor = configuration.newExecutor(tx, execType);
      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();
    }
  }

  // ...
}

其实现方法的返回值是 DefaultSqlSession 对象，DefaultSqlSession 实现了 SqlSession 接口：

public class DefaultSqlSession implements SqlSession {
  @Override
  public <T> T selectOne(String statement) {
    return this.<T>selectOne(statement, null);
  }

  // ...
}