JAVA-BASED REAL-TIME PROGRAMMING

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4. Exercises and Labs 4.4 Exercise 3 - Monitor and Wait/Notify In this exercise you will re-implement the Buffer class from exercise 1 as a monitor and you will write a monitor for a queue application. Buffer Implement the getLine method below. Note that the counting semaphores have been replaced with attributes used together with the Java signalling mechanism wait/notify. class Buffer { int available; // Number of lines that are available. final int size=8; // The max number of buffered lines. String[] buffData; // The actual buffer. int nextToPut; // Writers index. int nextToGet; // Readers index. } Buffer() { buffData = new String[size]; } synchronized void putLine(String inp) { try { while (available==size) wait(); } catch (InterruptedException exc) { throw new RTError("Buffer.putLine interrupted: "+exc); }; buffData[nextToPut] = new String(inp); if (++nextToPut >= size) nextToPut = 0; available++; notifyAll(); // Only notify() could wake up another producer. } synchronized String getLine() { // Write the code that implements this method ... } A note on InterruptedException When we in a synchronized method wait for some condition to become true, that is, when we call wait, we are forced to (by the compiler) to take care of an InterruptedException by • having the method declared to throw that exception, or • catching the exception and completing the method under the actual situation, or • catching the exception and terminating the program, or • catching the exception and generate an unchecked exception like ‘divide by zero’, or 112 2012-08-29 16:05
4.4. Exercise 3 - Monitor and Wait/Notify • catching the exception and throwing an instance of (a subclass of) Error. Having examined these alternatives we recommend (in most cases) to throw an Error. The purpose of the standard Error class in Java is to throw unchecked exceptions that the ordinary programmer is not expected to handle, and when a monitor operation fails there is normally nothing else to do than to enter some type of fail-safe or recovery mode (or simply a thread exit in simple applications). To simply throw an se.lth.cs.realtime.RTError facilitates this. Queue system A queue system consists of the following hardware: • A ticket-printing unit with a button that customers press in order to get a numbered queue ticket. • Buttons at several counters. A clerk pushes the button to announce that he/she is free to handle a customer. • A display showing counter number and queue number for the next customer to be served. Develop the control program for the queue-handling system. The thread classes for input and output have already been written. Your task is to develop the monitor class that the threads use. The hardware is encapsulated in an available static class HW. To solve the task you need to answer some questions before implementation: 1. What data is needed in the monitor? 2. Which conditions must be fulfilled to permit each monitor operation to complete? 3. Which events may imply that the conditions have been fulfilled? 4. An impatient clerk could possibly press its button several times. How do you handle that? 5. Write the monitor! Input to the control system consists of signals from the customer and clerk buttons. Output consists of the numbers to the display. Reading input is blocking which prevents use of polling. Instead, you need one thread for each of the inputs. This agrees with the rule to have one thread for each independent external sequence of events. It is also specified that the display may not be updated more often than each 10 seconds. Therefore, it is most convenient to have a dedicated thread for updating the display. Thus, we have the following threads: 113
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JAVA-BASED REAL-TIME PROGRAMMING Kl
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• Each piece of software must be
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Contents I Concurrent programming i
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Part I Concurrent programming in Ja
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1. Introduction pany depends on the
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1. Introduction control may be dela
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Chapter 2 Fundamentals Goal: Review
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2.2. Our physical world is parallel
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2.4. Concurrency a trend that the p
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2.4. Concurrency Behind the term co
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2.4. Concurrency in functions like
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2.5. Interrupts, pre-emption, and r
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2.5. Interrupts, pre-emption, and r
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2.6. Models of concurrent execution
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Object properties Implicit mutual e
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2.6. Models of concurrent execution
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2.7 Multi-process programming 2.7.
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2.9. Software issues In this chapte
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Chapter 3 Multi-Threaded Programmin
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3.1.1 Thread creation At start of a
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to foresee the value actually retur
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3.1. Threads first obtains the curr
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the run-time system (that is, the s
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3.1.6 Thread interruption and termi
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3.2. Resources and mutual exclusion
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3.2. Resources and mutual exclusion
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give give give 3.2. Resources and m
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