Instructore: Tasneem Darwish1 University of Palestine Faculty of Applied Engineering and Urban Planning Software Engineering Department Concurrent and Real Time programming Completing the Java concurrency model

Instructore: Tasneem Darwish2 Outlines  Thread Priorities and Thread Scheduling  Delaying Threads  Thread Groups  Java event handling  The Process and Runtime Classes  Thread-related Exceptions  Strengths and Limitations of the Java Concurrency Model

Instructore: Tasneem Darwish3 Thread Priorities and Thread Scheduling  Although priorities can be given to Java threads, they are only used as a guide to the underlying scheduler when allocating resources

Instructore: Tasneem Darwish4 Thread Priorities and Thread Scheduling  Java allows a thread's priority to be queried and set via the getPriority and setPriority methods.  By default, a thread has the same priority as its parent.  If the programmer has not allocated a priority to any thread, then all threads are given the NORM_PRIORITY  the Java thread that executes the main method runs with a NORM_PRIORITY

Instructore: Tasneem Darwish5 Thread Priorities and Thread Scheduling  From a real-time perspective, Java's scheduling and priority models are weak; in particular: 1.no guarantee is given that the highest priority runnable thread is always executing. 2.equal priority threads may or may not be time sliced; and 3.different Java priorities may be mapped to the same operating system priority.  Strengthening the scheduling and priority models is a key aim of the RTSJ (Chapter 10).

Instructore: Tasneem Darwish6 Delaying Threads  A relative delay allows a thread to wait for a future time  It can be implemented by using one of the two sleep static methods (defined in the Thread class).  Sleep only guarantees that the thread is released (made executable) after the period has expired  Once the thread has been made runnable, it will then have to wait to be scheduled for execution.  The length of the waiting time will depend on the other runnable threads in the system and their relative priorities

Instructore: Tasneem Darwish7 Delaying Threads  A relative delay allows a thread to wait for a future time  Java's sleep mechanisms support relative delays, but there is no support for an absolute delay.  Absolute delay is to ask the JVM to delay a thread until 2 p.m. on January 31, 2010 (for example).

Instructore: Tasneem Darwish8 Delaying Threads A delay example using sleep  consider a city's traffic light control system.  In the morning "rush hour", say between 7 and 10 a.m., the controllers need to give more time to traffic entering the city centre than to traffic leaving the city centre.  Between the 4 and 7 p.m. "rush hour", the situation is reversed.

Instructore: Tasneem Darwish9 Delaying Threads  Suppose that the following class contains the appropriate duration values:

Instructore: Tasneem Darwish10 Delaying Threads

Instructore: Tasneem Darwish11 Delaying Threads

Instructore: Tasneem Darwish12 Delaying Threads  There are two problems with this approach: 1. no attempt has been made to take into account the time taken to execute the setInboundTimes and setOutboundTimes method calls; 2.The thread may be preempted between finishing the calls to set the durations and the call to the next sleep method.  Support for absolute delays is another key extension provided by the RTSJ (Chapter 7).

Instructore: Tasneem Darwish13 Delaying Threads  a thread usually wait for an arbitrarily long period of time within a synchronized method for an associated notify  there are occasions when the absence of the notify, within a specified period of time, requires that the thread take some alternative action  This is achieved by using the wait method with a specified timeout

Instructore: Tasneem Darwish14 Delaying Threads  the timeout is a relative time  it is not possible to know for certain if the thread has been woken by the timeout expiring or by a notify

Instructore: Tasneem Darwish15 Thread groups  Thread groups allow collections of threads to be grouped together and manipulated as a group rather than as individuals.  Thread groups also provide a means of restricting who does what to which thread.  Every thread in Java is a member of a thread group.  There is a default group associated with the main program, all created threads are placed in this group (by default).  ThreadGroup is the class which supports the thread groups in Java

Instructore: Tasneem Darwish16 Thread groups  When a thread creates a new thread group, it does so from within a thread group.  the new thread group is a child of the current thread group unless a different thread group is passed as a parameter to the constructor.  Using the constructor methods in the ThreadGroup class allows hierarchies of thread groups to be created.  Requests to destroy a thread group will be applied to all threads in the group.

Instructore: Tasneem Darwish17 Thread groups  Setting the maximum priority of a thread group will cause the setPriority method in the Thread class to silently truncate the priorities if they are above the maximum value for the group.

Instructore: Tasneem Darwish18 Java event handling  In Java, threads are the entities that are scheduled by the virtual machine  there are entities scheduled for execution and have an imposed ordering.  User-interface events are good examples for entities that have imposed ordering.  the events are executed by a single thread that imposes the required ordering.  The classes Timer and TimerTask are used for this style of programming.

Instructore: Tasneem Darwish19 Java event handling  Timer object is an execution engine  The Timer accepts requests to schedule tasks for execution  After scheduling the Timer executes these tasks sequentially in an order determined by parameters passed with the schedule request.  A task is an instance of any subclass of the TimerTask

Instructore: Tasneem Darwish20 The process and runtime classes  Java virtual machine executes under control of an operating system.  all modern operating systems support the concept of a process  Java allows the programmer to create and interact with other processes under that host operating system.  this interaction is heavily dependent on the actual host (the O.S) and consequently use of the facilities might reduce the portability of the Java program.

Instructore: Tasneem Darwish21 The process and runtime classes  Java defines two classes to aid interaction with other processes: 1.the java.lang.Process class 2.the java.lang.Runtime class.