]> Introduction TickerTimer ComputerTimer TickerTimer Driver ComputerTimer Driver Runtime Behaviour One of the early promises of Jini was that it would find its way into all sorts of devices which could advertise their presence. However, Jini does not run on the really small Java virtual machines such as the KVM. But if it could, how would it be used? Most people have a number of electronic clocks in their house: alarm clocks, a clock on the oven, another on the microwave, and so on. When the electricity resumes after a power failure, they all start flashing, and you have to go round one after another setting them manually. Wouldn't it be nice if you only had to reset one (or if it got a value from a time server somewhere) and all the others reset themselves from it. In this chapter we look at this "flashing clocks" problem from a Jini viewpoint, to see what a Jini solution would look like. This example uses JoinManager and ServiceDiscoveryManager to advertise and discover services. On my site http://jan.newmarch.name/internetdevices/upnp/upnp-more-programming.html is an alternative solution using UPnP - a middleware system that is making more grounds in the area of small devices than Jini is, probably due to lighter resource requirements and an active coordinating body. Each clock is available as a service which we call a Ticker. A ticker has methods to get and set the time, and in addition it knows if it has a valid time or if it has an invalid time (and so should be shown flashing). A ticker can have its time set: when it does, it becomes valid, so that a display can stop flashing. The interface for a ticker is We shall give two implementations of this service: the first is the "dumb" one: when it starts it guesses at a start time and enters an invalid state. It uses a separate thread to keep imcreasing its time every second (approximately). When its time is set, it becomes valid, but will probably drift from the correct time due to its use of sleep to keep changing the time. The dumb timer is This timer uses the computer's internal clock to always return the correct time on request. It is always valid. To make this more visual, we can put the timers into a Swing frame and watch them ticking away. The following code is based on that of Satoshi Konno for UPnP A clock pane is A clock frame is A driver for the ticker timer in the frame above is This can be run by java clock.clock.TickerClock "Ticking Clock" A driver for the computer timer in the frame above is This can be run by java clock.clock.ComputerClock "Computer Clock" Two (or more) clocks can be started. If ticking clocks ae started then they will all be flashing. Once a computer clock is started though, the clocks will discover each other. Either the computer clock will discover ticking clocks and reset them, or the tickong clocks will discover the computer clock and reset themselves. I don't know which occurs and it doesn't matter. When running, the two clocks look like The final part is to advertise each timer as a Jini service, to try to locate other timer services and to listen to events from each one. This is handled by the clock device (really it is what we have been calling a Jini server, we have just adopted the UPnP. The device has a timer installed by setTimer(), and advertises this using a JoinManager. In the meantime it uses a ServiceDiscoveryManager to find other timers. terminology here). If several clocks are started, they will advertise themselves and also attempt to find other clocks. When one finds another it tries to determine its state: if one is valid and the other invalid then either the valid one sets the time on the invalid one, or the invalid one gets the correct time from the valid one. Which takes place depends on whether the valid one discovers the invalid one or vice versa - it doesn't matter, since the result is the same!. Two valid clocks do nothing to each other, as do two invalid ones. The ant file clock.clock.xml runs a ticker clock, pauses sixty seconds and then runs a computer clock. When one discovers the other, the ticker clock has its time reset. ©right;