MyBatis源码分析之——执行SQL语句的过程

User user = userMapper.getUserById(1)

调用invoke代理方法

由于所有的 Mapper 都是 MapperProxy 代理对象，所以任意的方法都是执行MapperProxy 的invoke()方法

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
  //判断是否需要去执行SQL还是直接执行方法
  if (Object.class.equals(method.getDeclaringClass())) {
    return method.invoke(this, args);
    //这里判断的是接口中的默认方法Default等
  } else if (isDefaultMethod(method)) {
    return invokeDefaultMethod(proxy, method, args);
  }
} catch (Throwable t) {
  throw ExceptionUtil.unwrapThrowable(t);
}
//获取缓存,保存了方法签名和接口方法的关系
final MapperMethod mapperMethod = cachedMapperMethod(method);
  return mapperMethod.execute(sqlSession, args);
}

调用execute方法

这里的例子用的是查询所以走的是else。

public Object execute(SqlSession sqlSession, Object[] args) {
  Object result;
  //根据命令类型走不行的操作command.getType()是select
  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 {
        //将参数转换为SQL的参数
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
        if (method.returnsOptional()
          && (result == null 
          || !method.getReturnType().equals(result.getClass()))) {
          result = Optional.ofNullable(result);
        }
      } 
      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;
}

调用selectOne其实是selectList

selectone查询一个和查询多个其实是一样的

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.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;
  }
}

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
    //从Configuration里的mappedStatements里根据key(id的全路径)获取MappedStatement对象
    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();
  }
}

mappedStatements对象如图

MappedStatement对象如图

执行query方法

1.创建CacheKey

从 BoundSql 中获取SQL信息,创建 CacheKey。这个CacheKey就是缓存的Key。

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds
rowBounds, ResultHandler resultHandler) throws SQLException {
  //创建缓存Key
  BoundSql boundSql = ms.getBoundSql(parameterObject);
  //key = -575461213:-771016147:mapper.UserMapper.getUserById:0:2147483647:select * from test_user where id = ?:1:development
  CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
  return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

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

2.清空本地缓存

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 aquery").object(ms.getId());
  if (closed) {
    throw new ExecutorException("Executor was closed.");
  }
  //queryStack 用于记录查询栈，防止递归查询重复处理缓存
  //flushCache=true 的时候，会先清理本地缓存（一级缓存）
  if (queryStack == 0 && ms.isFlushCacheRequired()) {
    //清空本地缓存
    clearLocalCache();
  } 
  List<E> list;
  try {
    queryStack++;
    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
    if (list != null) {
      handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
    } else {
      //如果没有缓存,会从数据库查询：queryFromDatabase()
      list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
    }
  } finally {
    queryStack--;
  } 
  if (queryStack == 0) {
    for (DeferredLoad deferredLoad : deferredLoads) {
      deferredLoad.load();
    } 
    // issue #601
    deferredLoads.clear();
    //如果 LocalCacheScope == STATEMENT,会清理本地缓存
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      // issue #482
      clearLocalCache();
    }
  } 
  return list;
}

3.从数据库查询

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter,
RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  List<E> list;
  //先在缓存用占位符占位
  localCache.putObject(key, EXECUTION_PLACEHOLDER);
  try {
    //执行Executor 的 doQuery(),默认是SimpleExecutor
    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
  } finally {
    //执行查询后,移除占位符
    localCache.removeObject(key);
  } 
  //从新放入数据
  localCache.putObject(key, list);
  if (ms.getStatementType() == StatementType.CALLABLE) {
    localOutputParameterCache.putObject(key, parameter);
  } 
  return list;
}

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.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

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