mybatis 执行SQL的过程

当咱们在调用mybatis mapper的接口时发生了点什么事呢  简单的跟着源码看了看

@Service("userService")  
public class UserServiceImpl implements IUserService {  
    //UserMapper是个人接口，mybatis给它生成一个动态代理
    @Resource
    private UserMapper userMapper;  
    
    public User getUserById(int userId) {  
        // TODO Auto-generated method stub  
       //执行咱们接口所对应的sql
        return this.userMapper.selectByPrimaryKey(userId);  
    }  
}              

咱们查看一下调用堆栈

咱们直接到invoke中看一下发生了什么事

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
//查看是否有缓存
    final MapperMethod mapperMethod = cachedMapperMethod(method);
//继续执行
    return mapperMethod.execute(sqlSession, args);
  }

咱们接着进execute看一下代码

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        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 if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          //把输入参数转换一下
          Object param = method.convertArgsToSqlCommandParam(args);
          //咱们查询最后会走到这里
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        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;
  }

根据咱们需求不一样  会case到不一样的地方，咱们跟的是select 因此最后会走到sqlSession.selectOne部分，接着进入代码

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

咱们能够看出sqlSessionProxy也是一个代理对象，它代理的是谁呢，是咱们的DefaultSqlSession 咱们进入DefaultSqlSession查看

//咱们经过动态代理  会走到这里  
@Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
   //而后会调用selectList
    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;
    }
  }

进入selectList

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
//这个是否是特别熟悉，咱们在加载xml的时候，会存储到这个里面
      MappedStatement ms = configuration.getMappedStatement(statement);
//执行sql
      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();
    }
  }

咱们会经过configuration.getMappedStatement(statement);把xml跟sql相关的 数据取出来，而后咱们进入executor.query

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

boundSql 中存着的就是咱们真实的sql（下图），createCacheKey我猜想是建立查询缓存

而后咱们进入query函数

 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, parameterObject, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

咱们能够看到，最初是查询缓存，若是没有的话 ，咱们又会进入一个query

  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 {
      //从这进入queryFromDatabase数据库查询
        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;
  }

queryFromDatabase

  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

咱们会先把缓存的key存起来，而后执行doQuery

  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);
    }
  }

获取configuration 以后 执行handler.query

  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    return delegate.<E>query(statement, resultHandler);
  }

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

执行到这里  是否是看到jdbc的执行语句了，没错 就是PreparedStatement ！

这篇简单的说了一下mybatis的执行过程，若是有不对之处敬请指出，关于xml的加载还有动态代理的问题  请看我其余文章