Mybatis源码-SqlSession（二）

1、实例代码
在实例搭建文章中，通过 SqlSession 对象查询数据，可乐写了两种方法。
①、常规的需要我们拼接 statement 方式；
②、xxxMapper.interface 接口代理方式；

对应下面两种方法：

//根据id查询person表数据
@Test
public void testSelectPersonById() {
    /*这个字符串由 PersonMapper.xml 文件中 两个部分构成
        <mapper namespace="com.itcoke.mapper.PersonMapper"> 的 namespace 的值
        <select id="selectPersonById" > id 值
    */
    String namespace = "com.itcoke.mapper.PersonMapper";
    String method = "selectPersonById";
    //根据 sqlSessionFactory 产生 session
    SqlSession sqlSession = sessionFactory.openSession();
    Person person = sqlSession.selectOne(namespace + "." + method, 1L);
    System.out.println(person);
    sqlSession.close();
}

//根据id查询person表数据
//通过接口代理的方式
@Test
public void testInterfaceSelectPersonById() {
    //根据 sqlSessionFactory 产生 session
    SqlSession sqlSession = sessionFactory.openSession();
    PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);
    Person person = mapper.selectPersonById(1L);
    System.out.println(person);
    sqlSession.close();
}

2、构建过程图示

3、代码剖析
3.1 Executor
我们通过 DefaultSessionFactory.openSession() 方法获取 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();
  
  }

其实是可以通过构造方法指定 Executor 的类型，比如：

SqlSession sqlSession = sessionFactory.openSession(ExecutorType.SIMPLE);

再看生成 Executor 的源代码：在Configuration类中

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    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;
  }

如果不指定执行器类型，直接默认 openSession() 方法，生成的是 CachingExecutor 执行器，这里的 cacheEnabled 其实是默认开启二级缓存的配置,在 mybatis-config.xml 文件中.
并且需要注意的是这里 new CachingExecutor(executor)，传进去了一个 SimpleExecutor 对象，后面和数据库交互的实际上是该对象。

得到了 SqlSession，接下来看这段代码：

Person person = sqlSession.selectOne(namespace + "." + method, 1L);

  @Override
  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.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;
    }
  }

直接看上面源码的第 76 行代码：

List<T> list = this.selectList(statement, parameter);

  private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }

在上一篇文章介绍 SqlSessionFactory 的构建过程时，我们说了 configuration 对象的组成：

看上面的源码得到 MappedStatement 对象，包含了我们在 mapper.xml 文件中配置的 sql 语句。

执行 executor.query() 方法，注意，这里的 executor 是 CachingExecutor：

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

这段源码，我们可以得到两个信息：
①、获取我们指定配置的boundSql 对象，包含我们配置的 sql 语句和参数信息。
②、根据相关信息得到一个缓存 key，通过这个key，连续两次相同的查询，第二次可以不去查数据库，直接获取缓存的数据。
接着我们继续看 query() 方法：

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

看源码，也就是说先去查缓存，缓存命中了直接返回数据，没有命中就执行：delegate.query() 方法。
这里的 delegate 是上文我们说的构造 Executor 传进来得 SimpleExecutor 对象。

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

关于 mybatis 缓存后面会专门写一篇文章来仔细介绍，这里我们先梳理主线，看上面标红行代码，缓存查不到，从数据库里面查。
继续跟 queryFromDatabase() 方法：

  @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());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

3.2 StatementHandler
执行到上面第 61 行源码：

StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);

跟进去：

  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

这里构造的是一个 RoutingStatementHandler 对象，聪明的你一听听名字都知道会路由生成别的对象。
没错，是根据传入的 statementType 生成具体的对象：

  public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

在 MappedStatement 对象中，默认 statementType 是 PERPARED:

    public Builder(Configuration configuration, String id, SqlSource sqlSource, SqlCommandType sqlCommandType) {
      mappedStatement.configuration = configuration;
      mappedStatement.id = id;
      mappedStatement.sqlSource = sqlSource;
      mappedStatement.statementType = StatementType.PREPARED;
      mappedStatement.resultSetType = ResultSetType.DEFAULT;
      mappedStatement.parameterMap = new ParameterMap.Builder(configuration, "defaultParameterMap", null, new ArrayList<>()).build();
      mappedStatement.resultMaps = new ArrayList<>();
      mappedStatement.sqlCommandType = sqlCommandType;
      mappedStatement.keyGenerator = configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
      String logId = id;
      if (configuration.getLogPrefix() != null) {
        logId = configuration.getLogPrefix() + id;
      }
      mappedStatement.statementLog = LogFactory.getLog(logId);
      mappedStatement.lang = configuration.getDefaultScriptingLanguageInstance();
    }

也就是这里最终生成的 StatementHandler 实际上是 PreparedStatementHandler 对象。
通常情况下：
①、不带参数的 SQL语句执行，会生成 SimpleStatementHandler 对象。
②、带参数的 SQL 语句执行，会生成 PreparedStatementHandler 对象。
③、存储过程执行，会生成 CallableStatementHandler 对象。
3.3 ParameterHandler
再回到 SimpleExecutor 的 doQuery() 方法：

  @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());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

看第 62 行代码：

stmt = prepareStatement(handler, ms.getStatementLog());

跟进去：

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }

这里第 86 行获取数据库连接。
第 88 行，进行参数处理，说直接点，就是将 SQL 语句中的 “?” 替换成我们传入的具体值。
但是我们知道 Java对象参数和数据库参数是不一样的，那么肯定还会做参数类型转换，没错，就是通过下面将要介绍 TypeHandler 来完成。
3.4 TypeHandler
也就是将 Java 类型和 数据库类型进行互相转换。

Mybatis 提供给了很多内置的参数转换，基本上不需要我们自己去定义。

当然，聪明的你可能会问了，假如这些都不满足呢？假如我要自定义一些类型呢？
不用担心，Mybatis 给我们预留了自定义类型的接口，如果你想自定义类型，通常分为两步：
①、实现 org.apache.ibatis.type.TypeHandler 接口， 或继承类 org.apache.ibatis.type.BaseTypeHandler
②、在配置文件中配置自定义的 TypeHnadler。
https://mybatis.org/mybatis-3/zh/configuration.html#typeHandlers
3.4 ResultSetHandler
还是回到 SimpleExecutor 的 doQuery() 方法：

  @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());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

上一步我们得到了 PrepraedStatementHandler 对象，接着看 handler.query() 方法：

  @Override
  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.handleResultSets(ps);
  }

通过执行 ps.execute() 方法，得到结果集，然后通过 resultSetHandler 去处理结果集。
4、总结
本文讲解了如何通过 SqlSession 进行一次数据库查询操作，但是正如文章开头所言给大家介绍了两种查询方式，一种是需要自己拼接 namespace+method 语句，另一种是通过 接口的形式。其实这两种方式是一样的，namespace 就是接口的包名（对应xxxMapper.xml 的namespace名称），method 就是接口的方法名（对应到xxxMapper.xml 的每个SQL语句的 id 属性），通过两者拼接，我们能去 xxxMapper.xml 文件中定位到具体的 SQL 语句。很明显，通过接口的形式避免了我们写字符串错误的可能，实际开发中，我们基本上都是这种方式。