Web server iis - Chapter 15 Multithreading 839 [Note: On many computer
Chapter 15 Multithreading 839 [Note: On many computer platforms, C and C++ programs can perform multithreading by using system specific code libraries.] Software Engineering Observation 15.1 Unlike many languages that do not have built-in multithreading (such as C and C++) and must therefore make calls to operating system multithreading primitives, Java includes multithreading primitives as part of the language itself (actually in classes Thread, ThreadGroup, ThreadLocal and ThreadDeath of the java.lang package). This encourages the use of multithreading among a larger part of the applications-programming community. We will discuss many applications of concurrent programming. When programs download large files such as audio clips or video clips from the World Wide Web, we do not want to wait until an entire clip is downloaded before starting the playback. So we can put multiple threads to work: One that downloads a clip and another that plays the clip so that these activities, or tasks, may proceed concurrently. To avoid choppy playback, we will coordinate the threads so that the player thread does not begin until there is a sufficient amount of the clip in memory to keep the player thread busy. Another example of multithreading is Java s automatic garbage collection. In C and C++, the programmer is responsible for reclaiming dynamically allocated memory. Java provides a garbage collector thread that reclaims dynamically allocated memory that the program no longer needs. Testing and Debugging Tip 15.1 In C and C++, programmers must provide explicit statements that reclaim dynamically allocated memory. When memory is not reclaimed (because a programmer forgets to do so, because of a logic error or because an exception diverts program control), this results in an all-too-common error called a memory leak that can eventually exhaust the supply of free memory and may cause premature program termination. Java s automatic garbage collection eliminates the vast majority of memory leaks, that is, those that are due to orphaned (unreferenced) objects. Java s garbage collector runs as a low-priority thread. When Java determines that there are no longer any references to an object, it marks the object for eventual garbage collection. The garbage-collector thread runs when processor time is available and when there are no higher priority runnable threads. However, the garbage collector will run immediately when the system is out of memory. Performance Tip 15.1 Setting an object reference to null marks that object for eventual garbage collection (if there are no other references to the object). This can help conserve memory in a system in which a local variable that refers to an object does not go out of scope because the method in which it appears executes for a lengthy period. Writing multithreaded programs can be tricky. Although the human mind can perform many functions concurrently, humans find it difficult to jump between parallel trains of thought. To see why multithreading can be difficult to program and understand, try the following experiment: open three books to page 1. Now try reading the books concurrently. Read a few words from the first book, then read a few words from the second book, then read a few words from the third book, then loop back and read the next few words from the first book, and so on. After a brief time, you will appreciate the challenges of multithreading: switching between books, reading briefly, remembering your place in each
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