Php 是否每次都使用PDO数据库类而不创建新连接？

我得到了这个PDO数据库类

class clsDatabase{
  // db settings
  private $host   = 'localhost';
  private $user   = 'test';
  private $dbname = 'test';
  private $pass   = 'test1';

  private $dbh;
  private $error;

  public function __construct(){
        // Set DSN
        $dsn = 'mysql: host=' . $this->host . ';dbname=' . $this->dbname;
        // Set options
        $options = array(
            PDO::ATTR_PERSISTENT            => true,
            PDO::ATTR_ERRMODE               => PDO::ERRMODE_EXCEPTION,
            PDO::MYSQL_ATTR_INIT_COMMAND    => 'SET NAMES UTF8'
        );
        // Create a new PDO instanace
        try{
            $this->dbh = new PDO($dsn, $this->user, $this->pass, $options); 
        }
        // Catch any errors
        catch(PDOException $e){
            $this->error = $e->getMessage();
            echo $this->error;
            exit;
        }       
    }

    public function query($query){
        $this->stmt = $this->dbh->prepare($query);
    }
}   

我尝试在不同的类中分离代码，例如，我得到了一个连接到clsUserController的clsDBUser。我这样做是为了知道什么类使用什么数据库代码。我的clsDBUser课是这样的

class clsDBUser extends clsDatabase {
    // construct
    public function __construct() {
        parent::__construct();
    }

    // get users
    public function getUsers($users_id){
        $query = "
            SELECT 
                email
            FROM 
                users
            WHERE 
               users_id = :users_id
        ";          
        $this->query($query);
        $this->bind(':users_id', $users_id);

        if($row = $this->single()){
            $this->close();
            return $row;
        }
        $this->close();
        return false;       
    }
}

class DB {
   // connectionStuff goes Here
}

class Model {
   private $db

   public function __construct($db) {
       $this->db = $db;
   }
}

我想知道这是一种方法，还是我现在正在每个类中创建一个新的数据库连接？因为通常在PHP4中（是的，我知道旧的），我无法识别每次都必须建立一个新的数据库连接

---

我需要改进吗？我需要如何改进呢？

不要从连接类（

clsDatabase

）扩展实体（

clsDBUser

）

对

clsDatabase

使用单例（或更高级的模式）

例如：

class clsDatabase {

    static private $instance = null;

    // some other private fields

    private function __construct(/* parameters*/) {
        // do it
    }

    public static function instance() {
        if (is_null(self::$instance)) {
            self::$instance = new self(/* pass any parameters */);
        }
        return self::$instance;
    }

    public function queryRow($query) {
        $oStatement = $this->dbh->prepare($query);

        // ...

        return $row;
    }
}

class clsDBUser {

    public function getUser($id) {
        $query = "...";
        return $clsDatabase::instance()->queryRow($query);
    }

}

---

嘿，我会这样做的

class clsDBUser extends clsDatabase {
    // construct
    public function __construct() {
        parent::__construct();
    }

    // get users
    public function getUsers($users_id){
        $query = "
            SELECT 
                email
            FROM 
                users
            WHERE 
               users_id = :users_id
        ";          
        $this->query($query);
        $this->bind(':users_id', $users_id);

        if($row = $this->single()){
            $this->close();
            return $row;
        }
        $this->close();
        return false;       
    }
}

class DB {
   // connectionStuff goes Here
}

class Model {
   private $db

   public function __construct($db) {
       $this->db = $db;
   }
}

使用：

重建：

仅包含连接内容的clsDB类

class clsDB{
    // db settings
    private $host   = 'localhost';
    private $user   = 'test';
    private $dbname = 'test';
    private $pass   = 'test';

    private $dbh;
    private $error;

    public function __construct(){
        // Set DSN
        $dsn = 'mysql: host=' . $this->host . ';dbname=' . $this->dbname;
        // Set options
        $options = array(
            PDO::ATTR_PERSISTENT            => true,
            PDO::ATTR_ERRMODE               => PDO::ERRMODE_EXCEPTION,
            PDO::MYSQL_ATTR_INIT_COMMAND    => 'SET NAMES UTF8'
        );
        // Create a new PDO instanace
        try{
            $this->dbh = new PDO($dsn, $this->user, $this->pass, $options); 
        }
        // Catch any errors
        catch(PDOException $e){
            $this->error = $e->getMessage();
            echo $this->error;
            exit;
        }       
    }           
}

clsDBLogin：

class clsDBLogin{   
   private $db;

   public function __construct($db) {
       $this->db = $db;
   }
}

在index.php中，我执行以下操作：

$clsDB      = new clsDB();
$clsDBLogin = new clsDBLogin($clsDB);

在clsDBLogin中，我会：

public function validateLogin($email){  
    $email = str_replace(' ', '', $email);
    $email = strtolower($email);

    // Check user in db to start verification
    $query = '
        SELECT
            u.*, ui.*
        FROM
            users as u,
            users_info as ui
        WHERE
            u.users_id = ui.users_id
        AND
            u.email = :email                
    ';
    $this->db->prepare($query);
    $this->db->bindValue(':email', $email, PDO::PARAM_STR);

    if($this->db->execute()){
        if($row = $this->db->fetch(PDO::FETCH_ASSOC)){
            return $row;            
        }       
    }   
}

---

你应该走void先生回答中所示的路。简言之：

摆脱clsDatabase

创建PDO的实例

将其传递到clsDBLogin的财产中，如void先生的回答所示

然后以$this->db->prepare（）等形式使用这个pdo实例

所以应该是这样

class clsDBLogin
{
    public function __construct($db)
    {
        $this->db = $db;
    }

    public function validateLogin($email)
    {  
        $email = trim($email);

        // Check user in db to start verification
        $query = 'SELECT * FROM users u, users_info ui 
                  WHERE u.users_id = ui.users_id AND u.email = ?';
        $stmt = $this->db->prepare($query);
        $stmt->execute([$email]);
        return $stmt->fetch();
    }
}

$dsn = 'mysql: host=localhost;dbname=test;charset=utf8';
$options = array(
        PDO::ATTR_PERSISTENT            => true,
        PDO::ATTR_ERRMODE               => PDO::ERRMODE_EXCEPTION,
);
// Create a new PDO instanace
$pdo = new PDO($dsn, $this->user, $this->pass, $options); 

$DBLogin = new clsDBLogin($pdo);
$user = $DBLogin->validateLogin($email);

---

这里有三层：

• 数据库连接器：您可以将纯PDO用于此或数据库抽象层库（）
• 实体存储库：换句话说，某种ORM。提供高级ORM功能。当然，您可以编写自己的轻量级解决方案
• 实体：可以是逻辑记录的简单、简单或任何其他对象表示形式

当我们手动执行此操作时。。。首先，不要相互延伸。一般情况下：切勿延伸与其他层不同的层。使用（DI）代替

当您将所有特定信息（依赖项）作为构造函数参数传递时，DI的情况非常简单。我的活动对象（如示例

Entity

）只知道实体的一般行为（在存储库中的一个键处）。为了简单起见，我使用原始SQL

存储库类：

class Repository {

    private $oPDO;
    private $tableName;
    private $keyFieldName;

    public function __construct($oPDO, $tableName, $keyFieldName) {
        $this->oPDO = $oPDO;
        $this->tableName = $tableName;
        $this->keyFieldName = $keyFieldName;
    }

    public function getPDO() {
        return $this->oPDO;
    }

    public function getTableName() {
        return $this->tableName;
    }

    public function getKeyFieldName() {
        return $this->keyFieldName;
    }

    public function getEntity($id) {
        return new Entity($this, $id);
    }

    public function createEntity() {
        return new Entity($this, null);
    }

}

class Entity implements ArrayAccess {

    private $oRepository;
    private $id;

    private $record = null;

    public function __construct($oRepository, $id) {
        $this->oRepository = $oRepository;
        $this->id = $id;
    }

    public function load($reload = false) {
        if (!$this->record && !$this->id) {
            return false;
        }

        if (!$reload && !is_null($this->record)) {
            return true;
        }

        $quotedTableName = $this->quoteIdentifier($this->oRepository->getTableName());
        $quotedKeyFieldName = $this->quoteIdentifier($this->oRepository->getKeyFieldName());
        $selectSql = "SELECT * FROM {$quotedTableName} WHERE {$quotedKeyFieldName} = ?";
        $oStatement = $this->oRepository->getPDO()->prepare($selectSql);
        $this->bindParam($oStatement, 1, $this->id);
        $oStatement->execute();

        $result = $oStatement->fetch(PDO::FETCH_ASSOC);

        if ($result === false || is_null($result)) {
            return false;
        }

        $this->record = $result;
        return true;
    }

    public function save() {
        $oPDO = $this->oRepository->getPDO();

        $tableName = $this->oRepository->getTableName();
        $keyFieldName = $this->oRepository->getKeyFieldName();
        $quotedTableName = $this->quoteIdentifier($tableName);
        $quotedKeyFieldName = $this->quoteIdentifier($keyFieldName);

        if (is_null($this->id)) {
            $insertSql = "INSERT INTO {$quotedTableName} (";
            $insertSql .= implode(", ", array_map([$this, "quoteIdentifier"], array_keys($this->record)));
            $insertSql .= ") VALUES (";
            $insertSql .= implode(", ", array_fill(0, count($this->record), "?"));
            $insertSql .= ")";
            $oStatement = $oPDO->prepare($insertSql);

            $p = 1;
            foreach ($this->record as $fieldName => $value) {
                $this->bindParam($oStatement, $p, $value);
                $p++;
            }

            if ($oStatement->execute()) {
                $this->id = $oPDO->lastInsertId();
                return true;
            } else {
                return false;
            }
        } else {
            $updateSql = "UPDATE {$quotedTableName} SET ";
            $updateSql .= implode(" = ?, ", array_map([$this, "quoteIdentifier"], array_keys($this->record)));
            $updateSql .= " = ? WHERE {$quotedKeyFieldName} = ?";
            $oStatement = $oPDO->prepare($updateSql);

            $p = 1;
            foreach ($this->record as $fieldName => $value) {
                $this->bindParam($oStatement, $p, $value);
                $p++;
            }
            $this->bindParam($oStatement, $p, $this->id);

            if ($oStatement->execute()) {
                if (isset($this->record[$keyFieldName])) {
                    $this->id = $this->record[$keyFieldName];
                }
                return true;
            } else {
                return false;
            }
        }
    }

    public function isExisting($reload = false) {
        if (!$this->record && !$this->id) {
            return false;
        }

        if (!$reload && !is_null($this->record)) {
            return true;
        }

        $quotedTableName = $this->quoteIdentifier($this->oRepository->getTableName());
        $quotedKeyFieldName = $this->quoteIdentifier($this->oRepository->getKeyFieldName());
        $selectSql = "SELECT 1 FROM {$quotedTableName} WHERE {$quotedKeyFieldName} = ?";
        $oStatement = $this->oRepository->getPDO()->prepare($selectSql);
        $oStatement->bindParam(1, $this->id);
        $oStatement->execute();

        $result = $oStatement->fetch(PDO::FETCH_ASSOC);

        if ($result === false || is_null($result)) {
            return false;
        }

        return true;
    }

    public function getId() {
        return $this->id;
    }

    public function getRecord() {
        $this->load();
        return $this->record;
    }

    public function offsetExists($offset) {
        $this->load();
        return isset($this->record[$offset]);
    }

    public function offsetGet($offset) {
        $this->load();
        return $this->record[$offset];
    }

    public function offsetSet($offset, $value) {
        $this->load();
        $this->record[$offset] = $value;
    }

    public function offsetUnset($offset) {
        $this->load();
        $this->record[$offset] = null;
    }

    private function quoteIdentifier($name) {
        return "`" . str_replace("`", "``", $name) . "`";
    }

    private function bindParam($oStatement, $key, $value) {
        $oStatement->bindParam($key, $value);
    }

}

$oRepo = new Repository($oPDO, "user", "user_id");

var_dump($oRepo->getEntity(2345235)->isExisting());

$oSameUser = $oRepo->getEntity(1);
var_dump($oSameUser->isExisting());
var_dump($oSameUser->getRecord());

$oNewUser = $oRepo->createEntity();
$oNewUser["username"] = "smith.john";
$oNewUser["password"] = password_hash("ihatesingletons", PASSWORD_DEFAULT);
$oNewUser["name"] = "John Smith";
$oNewUser->save();

$oNewUser["name"] = "John Jack Smith";
$oNewUser->save();

实体类：

class Repository {

    private $oPDO;
    private $tableName;
    private $keyFieldName;

    public function __construct($oPDO, $tableName, $keyFieldName) {
        $this->oPDO = $oPDO;
        $this->tableName = $tableName;
        $this->keyFieldName = $keyFieldName;
    }

    public function getPDO() {
        return $this->oPDO;
    }

    public function getTableName() {
        return $this->tableName;
    }

    public function getKeyFieldName() {
        return $this->keyFieldName;
    }

    public function getEntity($id) {
        return new Entity($this, $id);
    }

    public function createEntity() {
        return new Entity($this, null);
    }

}

class Entity implements ArrayAccess {

    private $oRepository;
    private $id;

    private $record = null;

    public function __construct($oRepository, $id) {
        $this->oRepository = $oRepository;
        $this->id = $id;
    }

    public function load($reload = false) {
        if (!$this->record && !$this->id) {
            return false;
        }

        if (!$reload && !is_null($this->record)) {
            return true;
        }

        $quotedTableName = $this->quoteIdentifier($this->oRepository->getTableName());
        $quotedKeyFieldName = $this->quoteIdentifier($this->oRepository->getKeyFieldName());
        $selectSql = "SELECT * FROM {$quotedTableName} WHERE {$quotedKeyFieldName} = ?";
        $oStatement = $this->oRepository->getPDO()->prepare($selectSql);
        $this->bindParam($oStatement, 1, $this->id);
        $oStatement->execute();

        $result = $oStatement->fetch(PDO::FETCH_ASSOC);

        if ($result === false || is_null($result)) {
            return false;
        }

        $this->record = $result;
        return true;
    }

    public function save() {
        $oPDO = $this->oRepository->getPDO();

        $tableName = $this->oRepository->getTableName();
        $keyFieldName = $this->oRepository->getKeyFieldName();
        $quotedTableName = $this->quoteIdentifier($tableName);
        $quotedKeyFieldName = $this->quoteIdentifier($keyFieldName);

        if (is_null($this->id)) {
            $insertSql = "INSERT INTO {$quotedTableName} (";
            $insertSql .= implode(", ", array_map([$this, "quoteIdentifier"], array_keys($this->record)));
            $insertSql .= ") VALUES (";
            $insertSql .= implode(", ", array_fill(0, count($this->record), "?"));
            $insertSql .= ")";
            $oStatement = $oPDO->prepare($insertSql);

            $p = 1;
            foreach ($this->record as $fieldName => $value) {
                $this->bindParam($oStatement, $p, $value);
                $p++;
            }

            if ($oStatement->execute()) {
                $this->id = $oPDO->lastInsertId();
                return true;
            } else {
                return false;
            }
        } else {
            $updateSql = "UPDATE {$quotedTableName} SET ";
            $updateSql .= implode(" = ?, ", array_map([$this, "quoteIdentifier"], array_keys($this->record)));
            $updateSql .= " = ? WHERE {$quotedKeyFieldName} = ?";
            $oStatement = $oPDO->prepare($updateSql);

            $p = 1;
            foreach ($this->record as $fieldName => $value) {
                $this->bindParam($oStatement, $p, $value);
                $p++;
            }
            $this->bindParam($oStatement, $p, $this->id);

            if ($oStatement->execute()) {
                if (isset($this->record[$keyFieldName])) {
                    $this->id = $this->record[$keyFieldName];
                }
                return true;
            } else {
                return false;
            }
        }
    }

    public function isExisting($reload = false) {
        if (!$this->record && !$this->id) {
            return false;
        }

        if (!$reload && !is_null($this->record)) {
            return true;
        }

        $quotedTableName = $this->quoteIdentifier($this->oRepository->getTableName());
        $quotedKeyFieldName = $this->quoteIdentifier($this->oRepository->getKeyFieldName());
        $selectSql = "SELECT 1 FROM {$quotedTableName} WHERE {$quotedKeyFieldName} = ?";
        $oStatement = $this->oRepository->getPDO()->prepare($selectSql);
        $oStatement->bindParam(1, $this->id);
        $oStatement->execute();

        $result = $oStatement->fetch(PDO::FETCH_ASSOC);

        if ($result === false || is_null($result)) {
            return false;
        }

        return true;
    }

    public function getId() {
        return $this->id;
    }

    public function getRecord() {
        $this->load();
        return $this->record;
    }

    public function offsetExists($offset) {
        $this->load();
        return isset($this->record[$offset]);
    }

    public function offsetGet($offset) {
        $this->load();
        return $this->record[$offset];
    }

    public function offsetSet($offset, $value) {
        $this->load();
        $this->record[$offset] = $value;
    }

    public function offsetUnset($offset) {
        $this->load();
        $this->record[$offset] = null;
    }

    private function quoteIdentifier($name) {
        return "`" . str_replace("`", "``", $name) . "`";
    }

    private function bindParam($oStatement, $key, $value) {
        $oStatement->bindParam($key, $value);
    }

}

$oRepo = new Repository($oPDO, "user", "user_id");

var_dump($oRepo->getEntity(2345235)->isExisting());

$oSameUser = $oRepo->getEntity(1);
var_dump($oSameUser->isExisting());
var_dump($oSameUser->getRecord());

$oNewUser = $oRepo->createEntity();
$oNewUser["username"] = "smith.john";
$oNewUser["password"] = password_hash("ihatesingletons", PASSWORD_DEFAULT);
$oNewUser["name"] = "John Smith";
$oNewUser->save();

$oNewUser["name"] = "John Jack Smith";
$oNewUser->save();

用法：

class Repository {

    private $oPDO;
    private $tableName;
    private $keyFieldName;

    public function __construct($oPDO, $tableName, $keyFieldName) {
        $this->oPDO = $oPDO;
        $this->tableName = $tableName;
        $this->keyFieldName = $keyFieldName;
    }

    public function getPDO() {
        return $this->oPDO;
    }

    public function getTableName() {
        return $this->tableName;
    }

    public function getKeyFieldName() {
        return $this->keyFieldName;
    }

    public function getEntity($id) {
        return new Entity($this, $id);
    }

    public function createEntity() {
        return new Entity($this, null);
    }

}

class Entity implements ArrayAccess {

    private $oRepository;
    private $id;

    private $record = null;

    public function __construct($oRepository, $id) {
        $this->oRepository = $oRepository;
        $this->id = $id;
    }

    public function load($reload = false) {
        if (!$this->record && !$this->id) {
            return false;
        }

        if (!$reload && !is_null($this->record)) {
            return true;
        }

        $quotedTableName = $this->quoteIdentifier($this->oRepository->getTableName());
        $quotedKeyFieldName = $this->quoteIdentifier($this->oRepository->getKeyFieldName());
        $selectSql = "SELECT * FROM {$quotedTableName} WHERE {$quotedKeyFieldName} = ?";
        $oStatement = $this->oRepository->getPDO()->prepare($selectSql);
        $this->bindParam($oStatement, 1, $this->id);
        $oStatement->execute();

        $result = $oStatement->fetch(PDO::FETCH_ASSOC);

        if ($result === false || is_null($result)) {
            return false;
        }

        $this->record = $result;
        return true;
    }

    public function save() {
        $oPDO = $this->oRepository->getPDO();

        $tableName = $this->oRepository->getTableName();
        $keyFieldName = $this->oRepository->getKeyFieldName();
        $quotedTableName = $this->quoteIdentifier($tableName);
        $quotedKeyFieldName = $this->quoteIdentifier($keyFieldName);

        if (is_null($this->id)) {
            $insertSql = "INSERT INTO {$quotedTableName} (";
            $insertSql .= implode(", ", array_map([$this, "quoteIdentifier"], array_keys($this->record)));
            $insertSql .= ") VALUES (";
            $insertSql .= implode(", ", array_fill(0, count($this->record), "?"));
            $insertSql .= ")";
            $oStatement = $oPDO->prepare($insertSql);

            $p = 1;
            foreach ($this->record as $fieldName => $value) {
                $this->bindParam($oStatement, $p, $value);
                $p++;
            }

            if ($oStatement->execute()) {
                $this->id = $oPDO->lastInsertId();
                return true;
            } else {
                return false;
            }
        } else {
            $updateSql = "UPDATE {$quotedTableName} SET ";
            $updateSql .= implode(" = ?, ", array_map([$this, "quoteIdentifier"], array_keys($this->record)));
            $updateSql .= " = ? WHERE {$quotedKeyFieldName} = ?";
            $oStatement = $oPDO->prepare($updateSql);

            $p = 1;
            foreach ($this->record as $fieldName => $value) {
                $this->bindParam($oStatement, $p, $value);
                $p++;
            }
            $this->bindParam($oStatement, $p, $this->id);

            if ($oStatement->execute()) {
                if (isset($this->record[$keyFieldName])) {
                    $this->id = $this->record[$keyFieldName];
                }
                return true;
            } else {
                return false;
            }
        }
    }

    public function isExisting($reload = false) {
        if (!$this->record && !$this->id) {
            return false;
        }

        if (!$reload && !is_null($this->record)) {
            return true;
        }

        $quotedTableName = $this->quoteIdentifier($this->oRepository->getTableName());
        $quotedKeyFieldName = $this->quoteIdentifier($this->oRepository->getKeyFieldName());
        $selectSql = "SELECT 1 FROM {$quotedTableName} WHERE {$quotedKeyFieldName} = ?";
        $oStatement = $this->oRepository->getPDO()->prepare($selectSql);
        $oStatement->bindParam(1, $this->id);
        $oStatement->execute();

        $result = $oStatement->fetch(PDO::FETCH_ASSOC);

        if ($result === false || is_null($result)) {
            return false;
        }

        return true;
    }

    public function getId() {
        return $this->id;
    }

    public function getRecord() {
        $this->load();
        return $this->record;
    }

    public function offsetExists($offset) {
        $this->load();
        return isset($this->record[$offset]);
    }

    public function offsetGet($offset) {
        $this->load();
        return $this->record[$offset];
    }

    public function offsetSet($offset, $value) {
        $this->load();
        $this->record[$offset] = $value;
    }

    public function offsetUnset($offset) {
        $this->load();
        $this->record[$offset] = null;
    }

    private function quoteIdentifier($name) {
        return "`" . str_replace("`", "``", $name) . "`";
    }

    private function bindParam($oStatement, $key, $value) {
        $oStatement->bindParam($key, $value);
    }

}

$oRepo = new Repository($oPDO, "user", "user_id");

var_dump($oRepo->getEntity(2345235)->isExisting());

$oSameUser = $oRepo->getEntity(1);
var_dump($oSameUser->isExisting());
var_dump($oSameUser->getRecord());

$oNewUser = $oRepo->createEntity();
$oNewUser["username"] = "smith.john";
$oNewUser["password"] = password_hash("ihatesingletons", PASSWORD_DEFAULT);
$oNewUser["name"] = "John Smith";
$oNewUser->save();

$oNewUser["name"] = "John Jack Smith";
$oNewUser->save();

---

当然，您可以从

存储库

扩展

morecuterepository

，从

实体

扩展

morecuteentity

，并使用特定的行为。

观察单例实现和其他类似工厂的设计模式，或者查看其他解决方案：PDO已经有了一个非常好的类，为什么要彻底改造一个轮子？为什么要调用一个方法

query（）

，当它所做的一切都是

prepare

时，你要添加到PDO中的就是混淆Singleton是一个反模式，你应该避免它。你想要的是DI（）。您可以使用php di。它是一个DIC（依赖注入容器），可以自动连接您的依赖项是的，DI是正确的解决方案，但是“不要扩展”是正确的答案。我写过：“或者更高级的模式。”好吧，我可以用你的例子吗？因为我明天会试试这个，可能会给你更多的问题，如果可以的话@Dávid Horváth你能给我一个完整的例子吗？@poNgz0r请看我的另一个答案，一个高级的例子。不完全是。您的代码非常凌乱（到处都是clsDatabase:：getInstance（）），令人困惑（使用query作为prepare的别名，我会称之为破坏）且不灵活（此代码将只限于一个实例，您将无法使用另一个实例）。此外，在关闭连接后，您将无法在脚本中运行多个查询，这相当可笑，那么该怎么办呢？1。摆脱clsDatabase。2.创建PDO的实例。3.将其传递到clsDBLogin的财产中，如void先生的回答所示。4.然后以$this->db->prepare（）等形式使用这个pdo实例。这个实例本质上更好。虽然仍然有很多无用的代码，但总体来说，这种方法是可行的，这不正是问题所在吗？因为在这段代码中，您每次都会在所有类中的每个操作之前调用

$db=new db..

——这不是效率低下吗？@教父不，在初始问题中，会为继承父类的对象的每个实例建立连接。在我的示例中，只有一个用于创建连接的实例，它被传递给依赖于sql连接的所有实例