MyBatis 3源码解析

Posted 2021-02-17 xpz-python

Mybatis是支持定制化SQL、存储过程和高级映射的持久层框架。主要完成两件事：

• 封装JDBC的操作
• 利用反射完成Java类和SQL之间的转换

mybatis的主要目的就是管理执行SQL是参数的输入和输出，编写SQL和结果集的映射是mybatis的主要优点

mybatis中主要类和接口

• Configuration：将mybatis配置文件中的信息保存到该类中
• SqlSessionFactory：解析Configuration类中的配置信息，获取SqlSession
• SqlSession：负责和数据库交互，完成增删改查
• Executor：mybatis的调度核心，负责SQL的生成
• StatementHandler：封装了JDBC的statement操作
• ParameterHandler：负责完成JavaType到jdbcType的转换
• ResultSetHandler：负责完成结果集到Java Bean的转换
• MappedStatement：代表一个select|update|insert|delete元素
• SqlSource：根据传入的ParamterObject生成SQL
• BoundSql：包含SQL和参数信息

SqlSessionFactory和SqlSession源码

SqlSessionFactory的创建是mybatis的第一步，SqlSession完成数据库增删改查。我们先来看看二者的创建

首先创建SqlSessionFactoryBudiler对象，在调用builder方法读取mybatis配置文件，并创建SqlSessionFactory：

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);
  }
 
}

不管是调用SqlSessionFactoryBuilder哪个build重载方法，最后调用的都是上面的两种，这两种的区别只是采用不同的流读取配置文件，最后都会调用build(Configuration config)创建SqlSessionFactory接口的实现类对象，返回的是DefaultSqlSessionFactory 对象。

当创建SqlSessionFactory完成后下一步就是创建SqlSession：

public class DefaultSqlSessionFactory implements SqlSessionFactory {
 
  private final Configuration configuration;
 
  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);
      //这一步可以发现每个SqlSession会配一个Executor
      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();
    }
  }
}

接下来我们看看SqlSession的源码：

public class DefaultSqlSession implements SqlSession {
 
  private final Configuration configuration;
  private final Executor executor;
 
  private final boolean autoCommit;
 
  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
  
  @Override
  public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
  
  @Override
  public int update(String statement, Object parameter) {
    try {
      dirty = true;
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.update(ms, wrapCollection(parameter));
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error updating database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
}

SqlSession的所有增删改查操作，最后都会落到上面三个方法，而最后的SQL执行都是使用Executor执行。

接下来我们在看看Executor中的query()和update方法，Executor接口有一个抽象实现类BaseExecutor，我们调用的query()和update()方法实际属于该类，而该类的query()和update()最后都落到三个子类SimpleExecutor、ReuseExecutor、BatchExecutor中。这里我们选SimpleExecutor看看

public class SimpleExecutor extends BaseExecutor {
 
  @Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      //把锅甩给StatementHandler
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }
 
  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      //把锅甩给StatementHandler
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
 
}

上面prepareStatement()方法内部调用StatementHandler的prepare()方法，这一般是我们定制插件的拦截方法。

可以发现最后都交给StatementHandler处理。StatementHandler有三个实现类：PreparedStatementHandler、CallableStatementHandler、RoutingStatementHandler，我们选择PreparedStatementHandler看一下：

public class PreparedStatementHandler extends BaseStatementHandler {
 
  @Override
  public int update(Statement statement) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    int rows = ps.getUpdateCount();
    Object parameterObject = boundSql.getParameterObject();
    KeyGenerator keyGenerator = mappedStatement.getKeyGenerator();
    keyGenerator.processAfter(executor, mappedStatement, ps, parameterObject);
    return rows;
  }
 
  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }
 
}

好了，这里就是我们熟悉的JDBC操作了。