根据id查询单条记录的过程

概览

• 测试代码
• 执行过程
• 总结

测试代码

UserMapper mapper = session.getMapper(UserMapper.class);
//数据库中只有一条ID为1的数据
User user = mapper.selectUser(1);

执行过程

由上一篇Mybatis源码系列（二）Mybatis获取Mapper的过程 可知，最终获取的UserMapper只是个经过org.apache.ibatis.binding.MapperProxy代理的对象，所以根据代理的原则，在执行查询的过程中，它会执行到以下方法org.apache.ibatis.binding.MapperProxy#invoke，具体代码如下:

//org.apache.ibatis.binding.MapperProxy#invoke
@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);
      }
    }
    //创建MapperMethod 并缓存  
    //这一点儿下次细说   里面的内容真的不少
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //执行
    return mapperMethod.execute(sqlSession, args);
  }

根据上面mapperMethod.execute(sqlSession, args)代码，我们来到了如下的方法中org.apache.ibatis.binding.MapperMethod#execute,具体代码如下：

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 if (method.returnsCursor()) {
        result = executeForCursor(sqlSession, args);
      } else {
         //因为是查询  并且响应值为单条记录，所以走到这一个查询
        //至于为什么会走到这里，后面的文章会说，这里先过个大概
        //convertArgsToSqlCommandParam(args)是转换参数的，代码不难，可以看看为什么多参数需要用@Param进行指定
        Object param = method.convertArgsToSqlCommandParam(args);
        //执行
        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;
  }

从代码sqlSession.selectOne(command.getName(), param);,我们进入org.apache.ibatis.session.defaults.DefaultSqlSession，看看这个里面真正执行的方法org.apache.ibatis.session.defaults.DefaultSqlSession#selectOne(java.lang.String, java.lang.Object) ，具体代码如下：

 @Override
  public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
   //注意list方法
    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;
    }
  }

进入上方list真正执行的方法中，内容如下：

@Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      //这一块儿ms，就是在org.apache.ibatis.binding.MapperProxy#invoke中缓存的ms
      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();
    }
  }

上方的query方法，会进入到org.apache.ibatis.executor.CachingExecutor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)这个方法中：

@Override
 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
   BoundSql boundSql = ms.getBoundSql(parameterObject);
    //这个方法可以进去仔细看看，mybatis涉及到缓存的，看看究竟是一级还是二级缓存，这个以后会说
   CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
   return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
 }

继续上面的query，进入到下面的方法中org.apache.ibatis.executor.CachingExecutor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler, org.apache.ibatis.cache.CacheKey, org.apache.ibatis.mapping.BoundSql),具体内容如下：

 @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
      //可以不关闭sqlsession 多调用几次看看有什么和不同
    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);
  }

继续上访的query方法，进入如下的方法中org.apache.ibatis.executor.BaseExecutor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler, org.apache.ibatis.cache.CacheKey, org.apache.ibatis.mapping.BoundSql)，内容如下：

@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) {
       //注意看看这个
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //查询数据库
        //下面的代码就不在介绍了，基本上到了底层的jdbc操作数据库的过程了
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      //清空本地缓存的条件
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

总结

好了，这一次的源码debug讲解基本上就到这里了，大体上只是讲了一个主干，其中一些细节，比如MapperMethod的创建和参数的解析、操作类型的判断、查询方法与sql绑定、以及之后的缓存相关的，这里都基本上一笔带过，都会在以后的篇章中，进行讲解的。