Practical Java Thread Pool Management: Best Practices and Pitfalls

1. 國立交通大學資訊工程學系分散式系統實驗室 Distributed Computing Systems Lab Java Thread Pool實務與陷阱 葉秉哲 2008.08.23. http://william.cswiz.org

2. Agenda • Threads 數量上限 • 容量滿載的處理策略 • 異常處理 • 天有不測風雲 2.

3. 參考資料 • Scott Oaks & Henry Wong (2004), Java Threads, 3rd edition, O’Reilly. • Brian Goetz et. al (2006), Java Concurrency in Practice, Addison-Wesley. 3.

4. Threads數量上限

5. Demo 1 Native thread 上限 (C/C++) 5.

6. Demo 2 VM thread 上限 (Java) 6.

7. Thread Object run( ) start( ) sleep( ) … wait( ) notify( ) notifyAll( ) … « interface »Runnable run( ) Java Concurrency Facilities Keyword: synchronized java.lang

8. Thread MyThread run( ) start( ) sleep( ) … run( ) Java Threads, 1st Way java.lang n main

9. others java.lang MyThread « interface »Runnable run( ) run( ) 1 1 n Java Threads, 2nd Way Thread main

10. Thread 容量滿載 • Win32 API • CreateThread() return NULL • Java • Throw java.lang.OutOfMemoryError 10.

11. 容量滿載處理策略

12. java.util.concurrent « interface »Executor « interface »Runnable « utility »Executors execute( Runnable ) run( ) « factory » newFixedThreadPool( ) newCachedThreadPool( ) newScheduledThreadPool( ) … n 1 « instantiate » XXX « use » « use » MyWorker main 12.

13. Demo 3 固定容量的 Thread Pool 13.

14. newFixedThreadPool public class TestThreadPool { private static final int POOL_SIZE = 10; private static final int MAX_COUNT = 10000; public static void main(String[] args) { Executorpool = Executors.newFixedThreadPool(POOL_SIZE); for (int i = 0; i < MAX_COUNT; ++i) pool.execute( new ExecutorDemo(i) ); } } class ExecutorDemo implements Runnable { public ExecutorDemo(int sn) { /*…*/ } public void run() { /*…*/ } }

15. « utility »Executors « factory » newFixedThreadPool( ) newCachedThreadPool( ) newScheduledThreadPool( ) … Executors 懶人包 • FixedThreadPool • 固定數量 • 彈性不足 • CachedThreadPool • 自動伸縮 (0～Integer.MAX_VALUE) • 無法處理瞬間巨量 15.

16. Demo 4 兼顧並行性與系統負載的 Thread Pool 16.

17. ThreadPool – DIY! public class TestThreadPool { private static final int MAX_POOL_SIZE = 10; private static final int CORE_POOL_SIZE = 5 ; private static final long THREAD_KEEPALIVE_TIME = 60 ; private static final int QUEUE_SIZE = 3; //… public static void main(String[] args) { Executorpool = Executors.newThreadPoolExecutor( CORE_POOL_SIZE, MAX_POOL_SIZE, THREAD_KEEPALIVE_TIME, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(QUEUE_SIZE), new ThreadPoolExecutor.CallerRunsPolicy() ); /*…*/ } }

18. 異常處理

19. Demo 5 當 Thread Pool 遇見 exception… 19.

20. 他抓得住我？ public class TestThreadPool { //… public static void main(String[] args) { Executorpool = Executors.newFixedThreadPool(POOL_SIZE); for (int i = 0; i < MAX_COUNT; ++i) try { pool.execute( new ExecutorDemo(i) ); } catch (Throwable e) { /*…*/ } } } class ExecutorDemo implements Runnable { public ExecutorDemo(int sn) { /*…*/ } public void run() { throw new RuntimeException(); } }

21. Demo 6 除錯版：安裝自訂的 Thread.UncaughtExceptionHandler 21.

22. 安裝自訂的 ThreadFactory public class TestThreadPool { //… public static void main(String[] args) { Executorpool = Executors.newThreadPoolExecutor( CORE_POOL_SIZE, MAX_POOL_SIZE, THREAD_KEEPALIVE_TIME, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(QUEUE_SIZE), new SafeThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy() ); /*…*/ } } class SafeThreadFactory implements ThreadFactory { /*下一頁*/ }

23. 自訂 UncaughtExceptionHandler class SafeThreadFactory implements ThreadFactory { @Override public Thread newThread(Runnable r) { Thread thread = new Thread(r); thread.setUncaughtExceptionHandler( new Thread.UncaughtExceptionHandler() { @Override public void uncaughtException( Thread t, Throwable e) { //… } } ); return thread; } }

24. Demo 7 懶人版：不讓 thread 丟出 exception 24.

25. Thread 防呆策略 class ErrorProofThread implements Runnable { //… public void run() { try { /* main program logic here… */ } catch (Throwable ignore) { // you can do nothing here } } }

26. 天有不測風雲…

27. Heartbeat • Watchdog 27.

28. Any Questions?