设计Python数据库连接池1-对象的循环引用问题

在Python中如果对象定义了__del__方法的话,在对象的引用记数为0时会自动调用__del__方法(很象c++中的析构函数),但如果A对象引用B对象,B对象又引用A对象,就形成循环引用,此时A,B对象引用次数都为1。python就无法正常调用__del__方法,原计划在__del__方法里释放的资源自然也就无法释放。

一个连接池拥有多个连接,而每个连接又拥有这个连接池的实例(一个叫pool的属性)。这样就产生了刚刚说的哪个问题。我想到的办法就是在每次从池中获取连接的时候将连接的pool设置为当前实例,然后在归还这个连接的时候再将其设置为None,并且要在这个连接对象的__del__方法中将pool属性设置为None。具体看代码吧。(目前只实现了SQLite3的)

'''
Created on 2009-4-17

@author: phyeas
'''
import time
from queue import Queue

class PoolException(Exception):
    pass

class Pool(object):
    '''一个数据库连接池'''
    def __init__(self, maxActive=5, maxWait=None, init_size=0, db_type="SQLite3", **config):
        self.__freeConns = Queue(maxActive)
        self.maxWait = maxWait
        self.db_type = db_type
        self.config = config
        if init_size > maxActive:
            init_size = maxActive
        for i in range(init_size):
            self.free(self._create_conn())
    
    def __del__(self):
        print("__del__ Pool..")
        self.release()
    
    def release(self):
        '''释放资源,关闭池中的所有连接'''
        print("release Pool..")
        while self.__freeConns and not self.__freeConns.empty():
            con = self.get()
            con.release()
        self.__freeConns = None

    def _create_conn(self):
        '''创建连接 '''
        if self.db_type in dbcs:
            return dbcs[self.db_type](**self.config);
        
    def get(self, timeout=None):
        '''获取一个连接
        @param timeout:超时时间
        '''
        if timeout is None:
            timeout = self.maxWait
        conn = None
        if self.__freeConns.empty():#如果容器是空的,直接创建一个连接
            conn = self._create_conn()
        else:
            conn = self.__freeConns.get(timeout=timeout)
        conn.pool = self
        return conn
    
    def free(self, conn):
        '''将一个连接放回池中
        @param conn: 连接对象
        '''
        conn.pool = None
        if(self.__freeConns.full()):#如果当前连接池已满,直接关闭连接
            conn.release()
            return
        self.__freeConns.put_nowait(conn)
        
from abc import ABCMeta, abstractmethod

class PoolingConnection(object, metaclass=ABCMeta):
    def __init__(self, **config):
        self.conn = None
        self.config = config
        self.pool = None
        
    def __del__(self):
        self.release()
        
    def __enter__(self):
        pass
    
    def __exit__(self, exc_type, exc_value, traceback):
        self.close()
        
    def release(self):
        print("release PoolingConnection..")
        if(self.conn is not None):
            self.conn.close()
            self.conn = None
        self.pool = None
            
    def close(self):
        if self.pool is None:
            raise PoolException("连接已关闭")
        self.pool.free(self)
        
    def __getattr__(self, val):
        if self.conn is None and self.pool is not None:
            self.conn = self._create_conn(**self.config)
        if self.conn is None:
            raise PoolException("无法创建数据库连接 或连接已关闭")
        return getattr(self.conn, val)

    @abstractmethod
    def _create_conn(self, **config):
        pass

class SQLit3PoolConnection(PoolingConnection):
    def _create_conn(self, **config):
        import sqlite3
        return sqlite3.connect(**config)

dbcs = {"SQLite3":SQLit3PoolConnection}

pool = Pool(database="F:\\test\\a")

def test():
    conn = pool.get()
    with conn:
        for a in conn.execute("SELECT * FROM A"):
            print(a)

if __name__ == "__main__":
    test()

  

以上代码在python3.0中测试通过……

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