Bridge control applet

Applet blues

Applet explanation

• The main applet thread that first performs initialization for the entire applet, creates and starts the three real-time (embedded) threads, and then becomes the roadway simulation that supplies the "real world" in which the real-time threads operate.
• The CaptureData interrupt thread that waits for changes in the roadway sensors. The thread runs at a very high priority. When the thread resumes, it creates a new VehicleData buffer, stores the sensor data within that buffer, and adds the buffer to a chain of similar buffers that is maintained via a monitor. CaptureData then waits for the next interrupt. The interrupting agent is the main applet thread. This method of starting CaptureData simulates the generation of a hardware interrupt that is handled by a thread (CaptureData in this case).
• The DigestData thread that remains in a loop waiting for a VehicleData buffer. When a buffer becomes available, DigestData uses any changes in the bridge sensors (one at each end of the bridge) to maintain a tally of the number of vehicles that are currently on the bridge. It also maintains the current status of a pair of roadway sensors that note when there are vehicles approaching the left-bound and right-bound entries to the bridge. The thread runs at a middle level of priority. The on-bridge tally (and the direction of bridge traffic) are protected by a monitor that guarantees that only one thread at a time can examine or use that information. The direction of bridge traffic is set by the ControlLights thread but is needed by DigestData to determine if a bridge sensor pulse means a vehicle entering or leaving the bridge.
• The ControlLights thread that performs the actual control of the lights that block left-bound and right-bound traffic from attempting to cross the single-lane bridge at the same time. The thread runs at a low priority. It uses the on-bridge tally and car-waiting sensor information maintained by DigestData.