Mybatis之Mapper调用源码分析

Posted 2022-08-22 叶长风

Mybatis之Mapper调用源码分析

这一篇是承接前面两篇的，分别为:Mybatis源码解析之配置加载(一), Mybatis源码解析之配置加载(二),前面两篇讲了在Mybatis启动时如何加载配置，这一节就讲在运行时，如何通过session获取Mapper代理类，从而实现对数据库的查询操作。

程序

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首先展示下之前写的程序，main程序在第一篇中有展示，这里就不再完全的贴出来了，获取session并获取mapper类代码如下：

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

        //执行查询返回一个唯一user对象的sql
        User user = userMapper.getUser(1);
        System.out.println(user);

获取session在这就不讲了，就是将configuration中的配置信息取出，然后实例化对应Executor类以及事务等相关信息，这里就直接从session中获取mapper类开始说。

session对象调用getMapper方法获取Mapper代理类，我们进入到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);
    
  

在第三个getMapper方法中，从knownMappers中取出对应代理工厂类，knownMappers是一个HashMap对象，从第二篇文章中的addMapper方法得知，在加载mapper接口时，每一个Mapper接口类都被添加进knownMappers中，具体可以看以下一段代码，

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);
        
      
    
  

继续回到之前的代码，在获取到MapperProxyFactory对象后，进行了实例化操作，继续分析，此处进入到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);
  

此处根据mapperInterface获取到Mapper的代理类对象，就是MapperProxy类。

当我们的演示程序main程序开始调用mapper中的方法时，即如下代码：

//执行查询返回一个唯一user对象的sql
User user = userMapper.getUser(1);

对应我们转到MapperProxy类的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);
  

我们直接看最下面的mapperMethod.execute(sqlSession, args),进入到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;
  

我们的演示程序中调用的方法为SELECT类型，同时我们要求返回的为resultMap，所以应该是最后一种方法调用，即else中的

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

其中的Object param = method.convertArgsToSqlCommandParam(args)

是将参数转换成kv结构的map返回，这里面就不再多看，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;
    
  

转到了session中的selectOne方法，session中的selectOne还是调用了selectList方法，继续转到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最终调用了**MappedStatement ms =configuration.getMappedStatement(statement);从之前的文章分析得知此处getMappedStatement实际上就是从mappedStatements.get(id)**中取出对应SQL，这里传进去的statement就是方法名，也就是id，因此此处也与前面串联起来。

接下来就是Executor执行sql的过程，我们进入到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);
 

根据parameter获取到BoundSql，继续找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;
  

这个query方法中只看list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql)这一行，其他的在日后分析cache的时候再回来继续看源码。

我们转到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;
  

转向doQuery()方法，当然doQuery有多个实现类，这里简单点直接看SimpleExecutor，simpleExecutor中的doQuery实现如下:

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);
    
  

这里的stmt = prepareStatement(handler, ms.getStatementLog());也不再详细讲述了，就是对参数的组装，以后讲其他的在此会涉及到，以后再说，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);
  

上述代码中最终是到了Statemet进行execute查询方法，而Statement对象即为java.sql包下的Statement对象，最终还是转换成了Statement对象查询，然后将返回结果进行了处理，从而返回一个list结果，然后有了上述代码中的selectOne方法的size判断。

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;
    

如果返回多条则报错，如果返回一条则直接返回结果。

Mybatis调用方法源码分析就到这了，其中漏了很多重要的步骤的讲述，比如plugin的使用，参数的组装等等，这个到以后分析其他的时候再来讲述。