Mybatis源码解读：sqlSession执行流程

前言

前面的章节主要是分析了Mybatis初始化启动以及如何解析配置文件，本篇章开始讲解应用程序是如何通过sqlSession去执行SQL语句的。

sqlSessionFactory与SqlSession

sqlSession对应着一次数据库会话，由于数据库会话不是永久性的，因此SqlSession的生命周期也不是永久的，当一旦关闭了Sqlsession就需要重新创建它。

前面讲到SqlSessionFactoryBuilder去读取mybatis的配置文件，然后build一个DefaultSqlSessionFactory，获取到SqlSessionFactory之后，就可以通过调用openSessionFactoryFromDataSource获取SqlSession对象

相关源码如下：

/**
  * @param execType 
  * @param level
  * @param autoCommit
  * @return
  */
 private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
   Transaction tx = null;
    try {
        //通过Confuguration对象去获取Mybatis相关配置信息, Environment对象包含了数据源和事务的配置
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        //通过Confuguration对象去获取Mybatis相关配置信息, Environment对象包含了数据源和事务的配置
     final Executor executor = configuration.newExecutor(tx, execType);
     //创建了一个DefaultSqlSession对象 
     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();
    }
 }

创建好sqlSession经过以下几个步骤：

• 从配置中获取Environment；
• 从Environment取得DataSource；
• 从Environment取得TransactionFactory；
• 从DataSource里获得数据库连接对象Connection；
• 在取得的数据库连接上创建事务对象Transaction；
• 创建Executor对象(该对象非常重要，事实上sqlsession的所有操作都是通过它完成的)；
• 创建sqlsession对象

创建好sqlSession对象后，就可以调用curd了

MapperProxy

在mybatis中，通过MapperProxy动态代理应用程序中的dao，我们代码执行自己写的dao里面方法的时候，其实是对应的mapperProxy在代理。

看下源码：

/**
 * @author Clinton Begin
 * @author Eduardo Macarron
 * @author Lasse Voss
 */
public class MapperRegistry {

  private final Configuration config;
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
        // 创建MapperProxyFacory，传递给它
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

  /**
   * @since 3.2.2
   */
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

}

再看下MapperProxyFactory的源码：

public class MapperProxyFactory<T> {

  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
      // 动态代理到应用程序写的dao接口上
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

Executor

Sqlsession的curd查询全部是交给Executor去实现的，Executor也是在执行过程中动态创建的

源代码

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    //1.批量执行sql操作
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    //2.重复使用statement执行sql操作
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    // 开启缓存
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

从源码可以看出，可以看出，如果不开启cache的话，创建的Executor只是3中基础类型之一，BatchExecutor专门用于执行批量sql操作，ReuseExecutor会重用statement执行sql操作，SimpleExecutor只是简单执行sql没有什么特别的。开启cache的话（默认是开启的并且没有任何理由去关闭它），就会创建CachingExecutor，它以前面创建的Executor作为唯一参数。CachingExecutor在查询数据库前先查找缓存，若没找到的话调用delegate（就是构造时传入的Executor对象）从数据库查询，并将查询结果存入缓存中。

Executor对象是可以被插件拦截的，如果定义了针对Executor类型的插件，最终生成的Executor对象是被各个插件插入后的代理对象。

MapperProxy

前面被代理对象的方法访问会落到代理者的invoke上

@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if (Object.class.equals(method.getDeclaringClass())) {
      try {
        return method.invoke(this, args);
      } catch (Throwable t) {
        throw ExceptionUtil.unwrapThrowable(t);
      }
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    // MapperMethod自己处理
    return mapperMethod.execute(sqlSession, args);
  }

MapperMethod

可以理解为是一个工厂模式，根据不同参数和返回类型选择不同的sqlsession方法来执行。这样mapper对象跟sqlsession真正关联起来了

//根据不同的类型选择执行sqlsession中的哪个方法
public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    if (SqlCommandType.INSERT == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.insert(command.getName(), param));
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));
    } else if (SqlCommandType.SELECT == command.getType()) {
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else if (SqlCommandType.FLUSH == command.getType()) {
        result = sqlSession.flushStatements();
    } else {
      throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

这样执行链路又回到了sqlsession，前面说过，sqlsession的真正执行作用的是executor，对它方法的调用最终都会落到executor的相应方法上(CacheExecutor 和普通Executor)。

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      MappedStatement ms = configuration.getMappedStatement(statement);
      //CRUD实际上是交给Excetor去处理
      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();
    }
  }

CacheExecutor

cacheExecutor又一个属性delegate,它保存的是某类普通的Executor，执行update/delete/insert时，直接调用delegate的update方法，执行query方法是先从缓存中取值，不存在再调用delegate的查询方法，并将查询结果存入cache中

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,ResultHandler resultHandler) throws SQLException {  
    if (ms != null) {  
        Cache cache = ms.getCache();  
        if (cache != null) {  
            flushCacheIfRequired(ms);  
            cache.getReadWriteLock().readLock().lock();  
            try {  
                if (ms.isUseCache() && resultHandler ==null) {  
                    CacheKey key = createCacheKey(ms, parameterObject, rowBounds);  
                    final List cachedList = (List)cache.getObject(key);  
                    if (cachedList != null) {  
                        return cachedList;  
                    } else {  
                        List list = delegate.query(ms,parameterObject, rowBounds, resultHandler);  
                        tcm.putObject(cache,key, list);  
                        return list;  
                    }  
                } else {  
                    return delegate.query(ms,parameterObject, rowBounds, resultHandler);  
                }  
            } finally {  
                cache.getReadWriteLock().readLock().unlock();  
            }
        }  
    }  
    return delegate.query(ms,parameterObject, rowBounds, resultHandler);  
}

普通Execute

有三类，都是继承BaseExecute；ReuseExecutor(重用statement执行sql操作)、SimpleExecutor、BatchExecutor

@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.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

默认为该SimpleExecutor

StatementHandler

从上面的源码可以看出，

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

Exector具体的事情执行还是交给StatementHandler来完成的

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;
  }

可以看到每次创建的StatementHandler都是RoutingStatementHandler,它只是一个分发者，属性delegate用于指定哪种具体的StatementHandler。可选的StatementHandler有

• SimpleStatementHandler
• PreparedStatementHandler
• CallableStatementHandler

三种。选用哪种在mapper配置文件的每个statement里指定，默认的是PreparedStatementHandler。同时StatementHandler是可以被拦截器拦截的，和Executor一样，被拦截器拦截后的对像是一个代理对象。由于mybatis没有实现数据库的物理分页，众多物理分页的实现都是在这个地方使用拦截器实现

StatementHandler创建后需要执行一些初始操作，比如statement的开启和参数设置、对于PreparedStatement还需要执行参数的设置操作

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

其中， handler.parameterize(stmt)通过调用ParameterHandler的setParameters完成参数的设置，ParameterHandler随着StatementHandler的创建而创建，默认的实现是DefaultParameterHandler：

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

参数设置完成后，执行数据库操作(update/query)。如果是query最后还有个查询结果的处理，我们看下常用的PreparedStatementHandler

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

结果处理使用的ResultSethandler来完成，默认的是FastResultSetHandler

public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
      ResultHandler resultHandler, BoundSql boundSql) {
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }