1.使用DB门面操作数据库，因为项目中每个应用都会单独建立一个数据库，所以使用单例模式封装了一个类， 实例化这个类就可以操作对应数据库中的数据表。
但是出现的问题就是，第一次查数据的时候能查出来，第二次就无法查出数据了，需要重新实例化这个类
例如 ：

                    //将这个单例类中的Instance置为null
                    $this->objModel->nullInstance();
                    //重新实例化类
                    $dbName = “xxxx11”;
                    $this->objFileModel = FileModel::init($dbName);

2.`

class FileModel
{

private $TableObj = null;
private $DbObj = null;
public static $instance = null;
/**
 * 初始化数据表链接对象
 * ImgModel constructor.
 * @param null $dbName
 */
private function __construct($dbName = null)
{

    try {
        if (null == $this->TableObj) {
            $dbInfo = array(
                'driver' => 'mysql',

                'read' => [
                    'host' => env('DB_READ_HOST', 'localhost')
                ],
                'write' => [
                    'host' => env('DB_WRITE_HOST', 'localhost')
                ],

                'port' => env('DB_PORT'),
                'database' => $dbName,
                'username' => env('DB_USERNAME'),
                'password' => env('DB_PASSWORD'),
                'charset' => 'utf8',
                'collation' => 'utf8_unicode_ci',
                'prefix' => '',
                'strict' => true,
                'engine' => null,
            );
            Config::set("database.connections.cs_mysql", $dbInfo);
            $this->DbObj = DB::connection('cs_mysql');
            $this->TableObj = $this->DbObj->table("file");

        }
    }catch (\Exception $e) {
    }
}

/**
 * @param null $dbName
 * @return FileModel|null
 */
public static function init($dbName = null) {
    if (null == self::$instance || !isset(self::$instance)) {
        self::$instance = new self($dbName);
    }
    return self::$instance;
}

public function getA($name) {
  return $this->TableObj->where("name", $name)->get();
}

public function getB($name) {
  return $this->TableObj->where("name", $name)->get();
}


public function getA() {
  return $this->TableObj->where("uid", $uid)->where("is_commit", $is_commit)->get();
}


public function nullInstance() {
    self::$instance = null;
}

}
`

$dbName = “xxx”;
$this->objModel = FileModel::init($dbName);

$dataA = $this->objModel->getA("ssss"); //这次可以获取数据

$dataB = $this->objModel->getB("ssss"); //这次获取的数据就是空

但是如果我在调用getB之前重新实例化FileModel这个类就ok了

$dbName = “xxx”;
$this->objModel = FileModel::init($dbName);

$dataA = $this->objModel->getA("ssss"); //这次可以获取数据

 $this->objModel->nullInstance();
 $dbName = “xxx”;
 $this->objModel = FileModel::init($dbName);
 
 $dataB = $this->objModel->getB("ssss");  //这次就可以获取到数据了
 
 
 
 
 
 **请问大神为什么会出现这种问题呢，怎么才能更好的解决这个问题？**