例如，我们实现了一个web视频监控服务器，服务器端采集摄像头数据，客户端使用浏览器通过http请求接收数据。服务器使用推送的方式（multipart/x-mixed-replace）一直使用一个tcp连接向客户端传递数据。这种方式将持续占用一个线程，导致单线程服务器无法处理多客户端请求。
要求：改写程序，在每个线程中处理一个客户端请求，支持多客户端访问。
解决方案：
threading.local()函数可以创建线程本地数据空间，其下属性对每个线程独立存在。
- 对于线程本地数据：
线程本地数据是特定线程的数据。管理线程本地数据，只需要创建一个local（或者一个子类型）的实例并在实例中储存属性：
mydata = threading.local()mydata.x = 1
在不同的线程中，实例的值会不同。class threading.local，一个代表线程本地数据的类。
- 方案示例：
yum install -y numpy opencv* python-qt4 pip3 install -i https://pypi.tuna.tsinghua.edu.cn/simple opencv-contrib-python==3.4.2.17
import os, cv2, time, struct, threadingfrom http.server import HTTPServer, BaseHTTPRequestHandlerfrom socketserver import TCPServer, ThreadingTCPServerfrom threading import Thread, RLockfrom select import selectclass JpegStreamer(Thread):
def __init__(self, camrea):
super().__init__()
self.cap = cv2.VideoCapture(camrea)
self.lock = RLock()
self.pipes = {}
def register(self):
pr, pw = os.pipe()
self.lock.acquire()
self.pipes[pr] = pw
self.lock.release()
return pr def unregister(self, pr):
self.lock.acquire()
pw = self.pipes.pop(pr)
self.lock.release()
os.close(pr)
os.close(pw)
def capture(self):
cap = self.cap while cap.isOpened():
ret, frame = cap.read()
if ret:
ret, data = cv2.imencode('.jpg', frame, (cv2.IMWRITE_JPEG_QUALITY, 40))
yield data.tostring()
def send_frame(self, frame):
n = struct.pack('1', len(frame))
self.lock.acquire()
if len(self.pipes):
_, pipes, _ = select([], self.pipes.values(), [], 1)
for pipe in pipes:
os.write(pipe, n)
os.write(pipe, frame)
self.lock.release()
def run(self):
for frame in self.capture():
self.send_frame(frame)class JpegRetriever:
def __init__(self, streamer):
self.streamer = streamer def retrieve(self):
while True:
ns = os.read(self.pipe, 8)
n = struct.unpack('1', ns)[0]
data = os.read(self.pipe, n)
yield data def __enter__(self):
if hasattr(self, 'pipe'):
raise RuntimeError()
self.pipe = streamer.register()
return self.retrieve()
def __exit__(self, *args):
self.streamer.unregister(self.pipe)
del self.pipe return Trueclass WebHandler(BaseHTTPRequestHandler):
retriever = None
@staticmethod
def set_retriever(retriever):
WebHandler.retriever = retriever def do_GET(self):
if self.retriever is None:
raise RuntimeError('no retriever')
if self.path != '/':
return
self.send_response(200)
self.send_header('Content-type', 'multipart/x-mixed-replace;boundary=jpeg_frame')
self.end_headers()
with self.retriever as frames:
for frame in frames:
self.send_frame(frame)
def send_frame(self, frame):
sh = b'--jpeg_frame\r\n'
sh += b'Content-Type: image/jpeg\r\n'
sh += b'Content-Length: %d\r\n\r\n' % len(frame)
self.wfile.write(sh)
self.wfile.write(frame)if __name__ == '__main__':
# 创建Streamer，开启摄像头采集
streamer = JpegStreamer(0)
streamer.start()
# http服务器创建Retriever
retriever = JpegRetriever(streamer)
WebHandler.set_retriever(retriever)
# 开启http服务器
HOST = '192.168.30.128' #本机ip
POST = 9000
print('Start server...(http://%s:%d)' % (HOST, POST))
httpd = TCPServer((HOST, POST), WebHandler)
httpd.serve_forever()
我这里是Windows系统，linux虚拟机root运行提示：
VIDEOIO ERROR: V4L: can't open camera by index 0 Start server...(http://192.168.30.128:9000)
不过无所谓，当通过浏览器访问192.168.30.128:9000时，即使多个窗口同时访问，也只有一个访问记录产生，说明只有一个线程建立。
改进：通过threading.local()函数创建线程本地数据空间，数据对每个线程独立。
import os, cv2, time, struct, threadingfrom http.server import HTTPServer, BaseHTTPRequestHandlerfrom socketserver import TCPServer, ThreadingTCPServerfrom threading import Thread, RLockfrom select import selectclass JpegStreamer(Thread):
def __init__(self, camrea):
super().__init__()
self.cap = cv2.VideoCapture(camrea)
self.lock = RLock()
self.pipes = {}
def register(self):
pr, pw = os.pipe()
self.lock.acquire()
self.pipes[pr] = pw
self.lock.release()
return pr def unregister(self, pr):
self.lock.acquire()
pw = self.pipes.pop(pr)
self.lock.release()
os.close(pr)
os.close(pw)
def capture(self):
cap = self.cap while cap.isOpened():
ret, frame = cap.read()
if ret:
ret, data = cv2.imencode('.jpg', frame, (cv2.IMWRITE_JPEG_QUALITY, 40))
yield data.tostring()
def send_frame(self, frame):
n = struct.pack('1', len(frame))
self.lock.acquire()
if len(self.pipes):
_, pipes, _ = select([], self.pipes.values(), [], 1)
for pipe in pipes:
os.write(pipe, n)
os.write(pipe, frame)
self.lock.release()
def run(self):
for frame in self.capture():
self.send_frame(frame)class JpegRetriever:
def __init__(self, streamer):
self.streamer = streamer
self.local = threading.local() #创建local对象
def retrieve(self):
while True:
ns = os.read(self.local.pipe, 8)
n = struct.unpack('1', ns)[0]
data = os.read(self.local.pipe, n)
yield data def __enter__(self):
if hasattr(self.local, 'pipe'):
raise RuntimeError()
self.local.pipe = streamer.register()
return self.retrieve()
def __exit__(self, *args):
self.streamer.unregister(self.local.pipe)
del self.local.pipe return Trueclass WebHandler(BaseHTTPRequestHandler):
retriever = None
@staticmethod
def set_retriever(retriever):
WebHandler.retriever = retriever def do_GET(self):
if self.retriever is None:
raise RuntimeError('no retriever')
if self.path != '/':
return
self.send_response(200)
self.send_header('Content-type', 'multipart/x-mixed-replace;boundary=jpeg_frame')
self.end_headers()
with self.retriever as frames:
for frame in frames:
self.send_frame(frame)
def send_frame(self, frame):
sh = b'--jpeg_frame\r\n'
sh += b'Content-Type: image/jpeg\r\n'
sh += b'Content-Length: %d\r\n\r\n' % len(frame)
self.wfile.write(sh)
self.wfile.write(frame)if __name__ == '__main__':
# 创建Streamer，开启摄像头采集
streamer = JpegStreamer(0)
streamer.start()
# http服务器创建Retriever
retriever = JpegRetriever(streamer)
WebHandler.set_retriever(retriever)
# 开启http服务器
HOST = '192.168.30.128'
POST = 9000
print('Start server...(http://%s:%d)' % (HOST, POST))
httpd = ThreadingTCPServer((HOST, POST), WebHandler)
httpd.serve_forever()
接下来继续多个浏览器多个窗口访问，可以同时进行多个访问，且每个访问都会新产生一条访问记录。