You can but it will be a performance hog.
If you implement SingleThreadModel, the container runs only one thread in the service method, but to service multiple requests parallelly, the container is free to instantiate multiple instances of your servlet class.
If, instead of implementing SingleThreadModel interface, you synchronize your doXXX methods, you will get the worst of both worlds. Neither the requests will be served parallelly (because of synchronized) and nor the container will be free to instatiate multiple objects (because of not implementing STM). Of course, in some cases, this is what you might want!
HTH,
Paul.
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