mybatis工作流程&源码详解

该篇主要讲解的是mybatis从seesion创建到执行sql语句的流程

流程主线：

1.创建SqlSessionFactoryBuilder

2.创建会话工厂SqlSessionFactory

3.创建执行实例Executor（实现类有BatchExecutor，ReuseExecutor，SimpleExecutor以及CachingExecutor）

4.创建会话SqlSession

5. 创建执行器StatementHandler（实现类有SimpleStatementHandler，CallableStatementHandler，PreparedStatementHandler以及RoutingStatementHandler）

6.执行JDBC操作

SqlSessionFactoryBuilder

public class SqlSessionFactoryBuilder {
  //创建SqlSessionFactory
  public SqlSessionFactory build(Reader reader) {
    return build(reader, null, null);
  }
    //创建SqlSessionFactory
  public SqlSessionFactory build(Reader reader, String environment) {
    return build(reader, environment, null);
  }
  //创建SqlSessionFactory
  public SqlSessionFactory build(Reader reader, Properties properties) {
    return build(reader, null, properties);
  }
  //创建SqlSessionFactory
  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
    try {      //解析配置文件
      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.
      }
    }
  }
  //创建SqlSessionFactory
  public SqlSessionFactory build(InputStream inputStream) {
    return build(inputStream, null, null);
  }
 //创建SqlSessionFactory
  public SqlSessionFactory build(InputStream inputStream, String environment) {
    return build(inputStream, environment, null);
  }
  //创建SqlSessionFactory
  public SqlSessionFactory build(InputStream inputStream, Properties properties) {
    return build(inputStream, null, properties);
  }
  //创建SqlSessionFactory
  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {     //解析配置文件
      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.
      }
    }
  }
  //创建SqlSessionFactory
  public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

}

DefaultSqlSessionFactory

 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);　　　 //创建事务
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);      //创建执行实例
      final Executor executor = configuration.newExecutor(tx, execType);      //创建会话session
      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 SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
    try {
      boolean autoCommit;
      try {
        autoCommit = connection.getAutoCommit();
      } catch (SQLException e) {
        // Failover to true, as most poor drivers
        // or databases won't support transactions
        autoCommit = true;
      }
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);      //创建事务
      final Transaction tx = transactionFactory.newTransaction(connection);      //创建执行实例
      final Executor executor = configuration.newExecutor(tx, execType);      //创建会话session
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

创建执行实例
final Executor executor = configuration.newExecutor(tx, execType);

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;    //生成不同的实现类
    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;
  }

创建会话session
new DefaultSqlSession(configuration, executor, autoCommit);

// 这里是一部分源码public class DefaultSqlSession implements SqlSession {
  //持有配置对象（内部就包含了sql语句的元数据）
  private Configuration configuration;  //持有执行实例
  private Executor executor;

  private boolean autoCommit;
  private boolean dirty;

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

  public DefaultSqlSession(Configuration configuration, Executor executor) {
    this(configuration, executor, false);
  }

  public <T> T selectOne(String statement) {
    return this.<T>selectOne(statement, null);
  }

  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>selectList(statement, parameter);
    if (list.size() == 1) {
      return list.get(0);
    } else if (list.size() > 1) {
      throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
      return null;
    }
  }  //以下源码部分省略......
}

执行实例Executor持有执行器StatementHandler

以SimpleExecutor为例

public class SimpleExecutor extends BaseExecutor {

  public SimpleExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();      //根据对应场景生成不同的执行器
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

  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();　　　 //根据不同的场景生成不同的执行器
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
    return Collections.emptyList();
  }
  //执行JDBC
  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection);
    handler.parameterize(stmt);
    return stmt;
  }

}

备注：

　　如果自定义sql拦截器

　　当我们需要改变sql的时候，显然我们要在预编译SQL(prepare方法前加入修改的逻辑)。

　　当我们需要修改参数的时候我们可以在调用parameterize方法前修改逻辑。或者使用ParameterHandler来改造设置参数。

　　我们需要控制组装结果集的时候，也可以在query方法前后加入逻辑，或者使用ResultHandler来改造组装结果。