JAVA-BASED REAL-TIME PROGRAMMING

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3. Multi-Threaded Programming To improve on these aspects, communication via messages (instead of via methods and shared data) will be introduced in this section. Originally and traditionally, this was referred to as mailboxes. From one point of view, a so called mailbox is nothing but a special type of monitor, so we do not need any additional support from the language (no new keywords like synchronized which supports monitors). From another point of view, when the above aspects are important, message based object interaction can be considered to be fundamental, so let us start with some considerations about the abstraction we deal with. 3.4.1 More on object interaction Method calls in a language/system providing only passive objects are by definition a synchronous type of communication. That is, passive objects work synchronously since they share the same thread of execution. In other words, this type of communication between objects is synchronous since the called object remains in its well defined state during the data transfer. During a method call, the caller enters the scope of the method within the scope of the called object, performing exactly what the method states. In a single-threaded system this is fine; data access via methods provide encapsulation and data abstraction. When all objects are executed by the same thread, we actually do not think of any thread at all. Data is transferred via arguments and return values, in both directions between the objects, which is straightforward and efficient in single-threaded applications. To support multi-threaded applications, the programming language could provide active (concurrently executing) objects, and models other than method calls for data transfer between objects. In fact, within object-oriented programming in general, communication between objects are (as in the language Smalltalk) often referred to as message passing. In Java, object interaction stems from the traditional (computationally efficient) C/C++ approach with method calls which are basically the same as function calls in C. Hence, in Java as well as in most other languages, asynchronous communication should be accomplished by a class implementing some type of mailbox or buffer supporting the principle of message passing. Thinking about real-life actors, sending a message is a natural way of transferring information, in particular when the sender and the receiver are acting concurrently but not synchronized (that is, performing actions independently of each other). The message need to be temporarily stored (buffered) somehow after being sent until being received. Today, information is most often stored and transmitted electronically. On the Internet, messages between humans are mainly accomplished by e-mail, but in technical systems we also use many other message principles depending on application and development needs. 86 2012-08-29 16:05
3.4.2 Events and buffers 3.4. Message-based communication – Mailboxes Assuming the sending and receiving thread belong to the same program, which is the case in this entire chapter, the sender and receiver can refer to shared objects. Thus, the buffer we need can be a monitor. We may compare with messages within user interaction packages such as the AWT. Such messages are called events since they represent an application event, but they are not buffered except before being fed into the application. Additionally, the AWT InputEvents are time-stamped so the graphics classes and the application can react accordingly to when the event (such as a mouse click) occurred physically, which can be quite different from when the event is processed (in particular on a heavily loaded system). For instance, assume two mouse clicks should be considered a a doubleclick if there is less than 1 s between them, and that the user clicks twice with the second click 0.9 s after the first. Then if there is a 0.2 s delay right after processing/registering the first click, the second click will be processed 1.1 s after the first one. Using the time-stamps it can still be deduced that the user actually double-clicked. Similar situations appear in feedback control where the delay (difference between current time and event time) results is so called phase lag, which in turn results in bad damping or even instability. Since our aim is both concurrent and real-time programming, time-stamping our messages (at the time of creation) will always be done. Events as messages The emission of an event object denotes some explicit change in state or execution of an application. Since the JDK1.0 there is the class java.awt.Event which was used as the base class for all the events in the old AWT1.0 containmentbased event model. That model was to a large extent implemented by inheritance. It was convenient when programming small applets, but did not scale up at all to larger application which implied extensive traversals of the containment hierarchy for each event (such as a mouse click). The java.awt.Event is kept only for backward compatibility and should not be used. Since JDK1.1, the so called delegation-based event model is used, both in the AWT (java.awt) as well as in Swing (JFC; javax.swing) and Java Beans (java.beans). The base class for events in all these packages is the (package and application area neutral) java.util.EventObject. The delegation model means that event handlers register themselves as listeners (synchronous subscribers) to certain events, and the emitter of the events calls some eventprocessing method according to the interface for that particular type of event. For instance, clicking with the mouse on a button in the GUI (Graphical User Interface) results in an InputEvent into the Java GUI package (e.g. AWT) where the associated java.awt.Button object calls the method action- 87
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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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4. Exercises and Labs Miscellaneous
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4. Exercises and Labs Scheduling wi
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4. Exercises and Labs Getting appro
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5. Solutions 2. With some additiona
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5. Solutions class YourMonitor { pr
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5. Solutions 5.4 Solution to exerci
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5. Solutions b) After the rewriting
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