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Java 8 parallel stream
- 1. JAVA 8 ParallelStream 第五組：蔡詠捷、王登文、游憶文
- 2. outline • Java 8Lambda Expression • Stream & Parallel Stream • Code Demo • Java Fork and Join using ForkJoinPool(Java7) • Conclusion
- 3. Java 8 LambdaExpression
- 5. Lambda Expression • Java8 support Lambda Expression then officially support functional programming paradigm • Lambda comes from the Lambda Calculus and refers to anonymous functions in programming • Lambda Expression • Method argument 必須為一個 Functional Interface • Functional Interface : Single Abstract Method interface（SAM interface） • Ex : java.lang.Runnable, java.util.Comparator
- 7. Lambda Expression • FunctionalInterface 定義：只有一個 public abstract method 的 Java Interface • 取代Functional Interface所產出的匿名類別，簡化程式碼
- 8. Lambda Expression • @FunctionalInterfaceAnnotation
- 9. Stream • Stream 是一個CollectionWrapper，可以讓把集合內的元素 ( Elements ) 以 串流 ( Stream ) 形式交到 Stream Methods 以 Lambda Expression 執行 • Stream 執行時期如同 Iterator，單向而不可往返，資料只讀取一次，讀取過一 次後即用盡了
- 10. Stream • Lazy evalutation •JVM 在沒有遇到 terminal operations 前，並不會執行任何 Stream method • Short-circuit execution • JVM 會自行判斷 short-circuit，提早完成集合元素遍歷 ( Element traversal ) • Automatic parallelism • JVM 可以把 Java Lambda Streams 同時交給多核心處理器 ( Multi-Core )或多線程 (Multi-Thread) 來執行，無須去編寫任何多線程代碼
- 11. Stream • Lazy evaluation •Code example
- 12. Stream • Short circuitexample • ( fn1() || fn2() ) • if fn1() is true , fn2() will not execute • ( fn1() && fn2() ) • if fn1() is false , fn2() will not execute
- 13. Stream • Collection support •Array support
- 14. Parallel Stream • 直接將Stream() 改為 ParallelStream() • 要視硬體及JVM配置與JVM負載而定 • 也有可能造成負面作用(Side-effect)
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- 17. Let’s look someexamples~ In Java7, We do that...... https://www.dropbox.com/s/dyx9rpxj53ofyxb/%E8%9E%A2%E5 %B9%95%E6%88%AA%E5%9C%96%202015-11- 01%2000.19.44.png?dl=0
- 18. In Java8, Wejust need!
- 19. We can easilychange to Parallel
- 20. Java Fork andJoin using ForkJoinPool
- 21. Fork • each subtaskcan be executed in parallel by different CPUs, or different threads on the same CPU. • The limit for when it makes sense to fork a task into subtasks is also called a threshold. It is up to each task to decide on a sensible threshold. It depends very much on the kind of work being done.
- 22. Join Once the subtaskshave finished executing, the task may join (merge) all the results into one result.
- 23. ForkJoinPool A special threadpool which is designed to work well with fork-and-join task splitting. Example:As a parameter to the ForkJoinPool constructor you pass the indicated level of parallelism you desire. ForkJoinPool forkJoinPool = new ForkJoinPool(8); As a parameter to the ForkJoinPool constructor you pass the indicated level of parallelism you desire.
- 24. ForkJoinPool You can submittwo types of tasks. RecursiveAction and RecursiveTask. Action won’t return any result, just compute but task will return something.
- 25. ForkJoinPool You can submittwo types of tasks. RecursiveAction and RecursiveTask. Action won’t return any result, just compute but task will return something.
- 26. Use case: MyRecursiveAction myRecursiveAction= new MyRecursiveAction(24); forkJoinPool.invoke(myRecursiveAction); Advanced Topic: http://coopsoft.com/ar/CalamityArticle.html
- 27. Parallel Stream isgood or not good？ Discuss Parallel Stream Good and Bad
- 28. Parallel Stream Problem •Java 8 parallel streams 也許會讓 程式跑得更快，或者不會，甚至是 跑的更慢 • streams 方法以低成本達到平行處 理(parallel processing)的方便性， but they do almost in a ‘Black Box’ • parallel processing 並非等同於 concurrent processing. 但許多在 Java8中「automatic parallelization」範例 裡事實上都 是concurrent processing.
- 29. Concurrent Processing VSParallel Processing
- 30. Streams Good for •They allow functional programming style using bindings. • They allow for better performance by removing iteration. Iteration occurs with evaluation. With streams, we can bind dozens of functions without iterating. • Streams may be infinite. Functions may be bound to infinite streams without problem. =>a short circuiting operation
- 31. And Not GoodFor • All streams use the same ForkJoinPool=>very long running task=> many long running sub-tasks => use each thread in pool • one parallel subtask performing I/O has side effects on others • Not many threads running at the same time, and in particular no other parallel stream
- 32. 使用Stream 建議 • 適合使用 •需在多核心的CPU下執行才能表現效能 • 無I/O存取下 • 切割Pool & Num Setting • Collection,Array… • 不適合使用 • 需在單核心的CPU下執行效能差 • I/O存取佔執行之大部份作業下 • 建議： • 不使用Default ForkJoin Pool(除非在執行作業的Task及伺服器設定可容許…)
- 33. 結論 • Parallel Stream看似方便、好處多，但背後有多少問題是我們還沒發現的？ • Parallel streams 是不可預知的，難以最正確的方式在運作. • 大部份使用Parallel Stream時無法預知可能會影響到正在 運作不相關的程式的 執行效能。 • 在請「parallel Stream」來時，請三思而後行(除非功力深厚OR…)
- 34. Reference • https://dzone.com/articles/think-twice-using-java-8 • http://zeroturnaround.com/rebellabs/java-parallel-streams-are-bad-for-your-health/ •http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/ • https://wefollownews.appspot.com/cittopnews201408_48/14848.html • http://openhome.cc/Gossip/Java/ParallelStream.html • http://www.slideshare.net/dgomezg/parallel-streams-en-java- 8http://www.slideshare.net/JustinSDK/java-se-8-lambda • http://magiclen.org/java-8-lambda/http://developer.51cto.com/art/201311/417451.htm • Custom Thread Pool 參考 : http://stackoverflow.com/questions/21163108/custom-thread- pool-in-java-8-parallel-stream • ForkJoinPool submit, execute, invoke : http://stackoverflow.com/questions/17881183/difference-between-execute-submit-and- invoke-in-a-forkjoinpool