In java technology,threads are usually PReemptive,but not necessarily Time-sliced(the process of giving each thread an equal amount of CPU time).It is common mistake to believe that "preemptive" is a fancy Word for "does time-slicing".
For the runtime on a Solaris Operating Environment platform,Java technology does not preempt threads of the same priority.However,the runtime on Microsoft Windows platforms uses time-slicing,so it preempts threads of the same priority and even threads of higher priority.Preemption is not guaranteed;however,most JVM implementations result in behavior that appears to be strictly preemptive.Across JVM implementations,there is no absolute guarantee of preemption or time-slicing.The only guarantees lie in the coder’s use of wait and sleep.
The model of a preemptive scheduler is that many threads might be runnable,but only one thread is actually running.This thread continues to run until it ceases to be runnable or another thread of higher priority becomes runnable.In the latter case,the lower priority thread is preempted by the thread of higher priority,which gets a chance to run instead.
A thread might cease to runnable (that is,because blocked) for a variety of reasons.The thread’s code can execute a Thread.sleep() call,deliberately aSKINg the thread to pause for a fixed period of time.The thread might have to wait to access a resource and cannot continue until that resource become available.
All thread that are runnable are kept in pools according to priority.When a blocked thread becomes runnable,it is placed back into the appropriate runnable pool.Threads from the highest priority nonempty pool are given CPU time.
The last sentence is worded loosed because: (1) In most JVM implementations,priorities seem to work in a preemptive manner,although there is no guarantee that priorities have any meaning at all; (2) Microsoft Window’s values affect thread behavior so that it is possible that a Java Priority 4 thread might be running,in spite of the fact that a runnable Java Priority 5 thread is waiting for the CPU. In reality,many JVMs implement pool as queues,but this is not guaranteed hehavior.
