Mybatis之Mapper呼叫原始碼分析

這一篇是承接前面兩篇的，分別為:Mybatis原始碼解析之配置載入(一), Mybatis原始碼解析之配置載入(二),前面兩篇講了在Mybatis啟動時如何載入配置，這一節就講在執行時，如何通過session獲取Mapper代理類，從而實現對資料庫的查詢操作。

程式

首先展示下之前寫的程式，main程式在第一篇中有展示，這裡就不再完全的貼出來了，獲取session並獲取mapper類程式碼如下：

SqlSession session = sessionFactory.openSession();
        UserMapper userMapper =
 
 session.getMapper(UserMapper.class);

        //執行查詢返回一個唯一user物件的sql
        User user = userMapper.getUser(1);
        System.out.println(user);

獲取session在這就不講了，就是將configuration中的配置資訊取出，然後例項化對應Executor類以及事務等相關資訊，這裡就直接從session中獲取mapper類開始說。

session物件呼叫getMapper方法獲取Mapper代理類，我們進入到getMapper方法。

@Override
 

  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

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 {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

在第三個getMapper方法中，從knownMappers中取出對應代理工廠類，knownMappers是一個HashMap物件，從第二篇文章中的addMapper方法得知，在載入mapper介面時，每一個Mapper介面類都被新增進knownMappers中，具體可以看以下一段程式碼，

public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

繼續回到之前的程式碼，在獲取到MapperProxyFactory物件後，進行了例項化操作，繼續分析，此處進入到newInstance方法中。

public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

此處根據mapperInterface獲取到Mapper的代理類物件，就是MapperProxy類。

當我們的演示程式main程式開始呼叫mapper中的方法時，即如下程式碼：

//執行查詢返回一個唯一user物件的sql
User user = userMapper.getUser(1);

對應我們轉到MapperProxy類的invoke方法。

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);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

我們直接看最下面的mapperMethod.execute(sqlSession, args),進入到execute方法中。

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 {
          Object param = method.convertArgsToSqlCommandParam(args);
          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;
  }

我們的演示程式中呼叫的方法為SELECT型別，同時我們要求返回的為resultMap，所以應該是最後一種方法呼叫，即else中的

Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);

其中的Object param = method.convertArgsToSqlCommandParam(args)

是將引數轉換成kv結構的map返回，這裡面就不再多看，直接到selectOne方法中。

@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.<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;
    }
  }

轉到了session中的selectOne方法，session中的selectOne還是呼叫了selectList方法，繼續轉到selectList方法。

@Override
  public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }

  @Override
  public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      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();
    }
  }

selectList最終呼叫了**MappedStatement ms =configuration.getMappedStatement(statement);從之前的文章分析得知此處getMappedStatement實際上就是從mappedStatements.get(id)**中取出對應SQL，這裡傳進去的statement就是方法名，也就是id，因此此處也與前面串聯起來。

接下來就是Executor執行sql的過程，我們進入到query方法中。

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

根據parameter獲取到BoundSql，繼續找query方法。

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;
  }

這個query方法中只看list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql)這一行，其他的在日後分析cache的時候再回來繼續看原始碼。

我們轉到queryFromDatabase方法。

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;
  }

轉向doQuery()方法，當然doQuery有多個實現類，這裡簡單點直接看SimpleExecutor，simpleExecutor中的doQuery實現如下:

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);
    }
  }

這裡的stmt = prepareStatement(handler, ms.getStatementLog());也不再詳細講述了，就是對引數的組裝，以後講其他的在此會涉及到，以後再說，直接看query方法。

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

上述程式碼中最終是到了Statemet進行execute查詢方法，而Statement物件即為java.sql包下的Statement物件，最終還是轉換成了Statement物件查詢，然後將返回結果進行了處理，從而返回一個list結果，然後有了上述程式碼中的selectOne方法的size判斷。

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;
    }

如果返回多條則報錯，如果返回一條則直接返回結果。

Mybatis呼叫方法原始碼分析就到這了，其中漏了很多重要的步驟的講述，比如plugin的使用，引數的組裝等等，這個到以後分析其他的時候再來講述。