SerialPortUtil | Android串口通信工具

How can I tell the checker thread that the communication is already opened and make him wait until the main thread finish.

I don't fully understand your code but the critical thing with threads and locking is to make sure that all threads are calling code that is synchronized on the same object instance.

If I synchronize an object, there still is a window when the object is not synchronized.

Not if you use the same instance of the object. Making each of the public methods in SerialChannel synchronized will make sure that only 1 thread can be using the object at once.

I suspect that your real problem is not about protecting the SerialChannel object but more about race-conditions between the threads. They need to make multiple calls to the methods and they can block each other or interleave in an improper manner.

You can get around this with a couple of changes. You can make the send(...) and receive(...) methods auto-opening. Threads would just call send() or receive() which in turn would internally call open() if the fileInputStream or fileOutputStream was null. The thread would be inside of a synchronized so this would not be interrupted by another thread.

Another completely different model to consider would be to have one thread reading from the serial port and another writing to it that are dedicated to that task -- they would be built into the SerialChannel object. They would share data with the external threads using a read BlockingQueue and a write BlockingQueue. Then the serial port is opened early in your application which starts the IO threads and the external threads never worry about the IO. They just put() and take() from the queues. I typically do this (for example) when reading and writing to the console.

Hope something here helps.