Changeset 31717 for main/trunk/gli/src
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- 2017-05-30T21:25:17+12:00 (7 years ago)
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main/trunk/gli/src/org/greenstone/gatherer/util/Readme_Using_SafeProcess.txt
r31713 r31717 1 SAFEPROCESS README 1 First drafts of this document 2 ak19, 30 May 2017 3 4 ========================================= 5 SAFEPROCESS.JAVA README 6 ========================================= 2 7 3 8 GS Java developers should use SafeProcess.java in place of directly using Java's Process class, unless any issues are discovered with SafeProcess.java hereafter. … … 8 13 - greenstone3/src/java/org/greenstone/util/SafeProcess.java 9 14 15 16 This document contains the following sections 17 A. WHY WE SHOULD GO THROUGH SAFEPROCESS INSTEAD OF USING JAVA'S PROCESS DIRECTLY 18 B. MODEL OF SAFEPROCESS THREADS AND INTERRUPT BEHAVIOUR 19 C. USAGE: DEBUGGING 20 D. USAGE: INSTANTIATION & DEFAULT RUNPROCESS VARIANTS 21 E. INTERNAL IMPLEMENTATION DETAILS - USEFUL IF CUSTOMISING SAFEPROCESS' BEHAVIOUR OR CALLING CANCEL ON SAFEPROCESS 22 F. USAGE: CUSTOMISING 23 G. USAGE: CANCELLING 24 H. USAGE: IMPLEMENTING HOOKS AROUND CANCELLING OR PROCESS' END 25 I. USEFUL STATIC METHODS AND STATIC INNER CLASSES 26 J. OTHER USEFUL INSTANCE METHODS 27 K. LINKS AND NOTES ON PROCESSES, PROCESS STREAMS, INTERRUPTING AND DESTROYING PROCESSES 28 L. USEFUL SYNCHRONISATION NOTES AND SUGGESTED READING ON THREADS AND THREAD SAFETY 29 M. TICKETS, COMMITS, TAGS 30 N. GLI vs GS3 CODE's SAFEPROCESS.JAVA - THE DIFFERENCES 31 O. FUTURE WORK, IMPROVEMENTS 32 10 33 _______________________________________________________________________________ 11 WHY WE SHOULD GO THROUGH SAFEPROCESS INSTEAD OF USING JAVA'S PROCESS DIRECTLY34 A. WHY WE SHOULD GO THROUGH SAFEPROCESS INSTEAD OF USING JAVA'S PROCESS DIRECTLY 12 35 _______________________________________________________________________________ 13 36 … … 20 43 21 44 22 _____________________________________________________ 23 MODEL OF SAFEPROCESS THREADS AND INTERRUPT BEHAVIOUR24 _____________________________________________________ 45 _________________________________________________________ 46 B. MODEL OF SAFEPROCESS THREADS AND INTERRUPT BEHAVIOUR 47 _________________________________________________________ 25 48 26 49 This section is especially IMPORTANT TO READ IF you're thinking of customising SafeProcess' handling of the internal Process' iostreams, since each of them are always handled in their own distinct worker threads, and you need to remember to deal with ThreadSafety issues. … … 52 75 53 76 __________________________________________________ 54 USAGE: DEBUGGING55 __________________________________________________ 56 57 There's a lot of info that a SafeProcess instance will log during its execution and termination phases, including Exceptions but also very basic things, such as the command that the SafeProcess instance is running .58 59 By default, such debugging-related logging is turned off. You can turn it on by adjusting the static SafeProcess.DEBUG = 1 and recompiling.77 C. USAGE: DEBUGGING 78 __________________________________________________ 79 80 There's a lot of info that a SafeProcess instance will log during its execution and termination phases, including Exceptions but also very basic things, such as the command that the SafeProcess instance is running and other messages. 81 82 Exceptions are always logged, but debugging related messages can be turned off or on. By default, it's turned off. You can turn it on by adjusting the static SafeProcess.DEBUG = 1 and recompiling. 60 83 61 84 For GS3, the log output uses log4j. … … 64 87 If you're using SafeProcess yourself and wish to log debug statements that are related to how well you're using SafeProcess, you can call one of the static SafeProcess.log() functions yourself. Remember to set the static SafeProcess.DEBUG = 1. 65 88 66 __________________________________________________ 67 USAGE: INSTANTIATION & DEFAULT RUNPROCESS VARIANTS 68 __________________________________________________ 89 90 There are 4 variants of the log() function: 91 92 - public static void log(String msg) 93 logs the message if debugging is on, DEBUG = 1 94 95 - public static void log(Exception e) 96 logs the stacktrace for the exception, regardless of if debugging is on 97 98 - public static void log(String msg, Exception e) 99 logs the message and logs the stacktrace for the exception, regardless of if debugging is on 100 101 - public static void log(String msg, Exception e, boolean printStackTrace) 102 if debugging is on or if the printStackTrace parameter is true, both the message and the stacktrace for the exception are logged. 103 if debugging is off and the printStackTrace parameter is false, nothing is logged. Call this variant if an exception is expected and you only want to log it in debugging mode. An example of an expected exception are some InterruptedExceptions that may occur over the course of SafeProcess 104 105 _______________________________________________________ 106 D. USAGE: INSTANTIATION & DEFAULT RUNPROCESS VARIANTS 107 _______________________________________________________ 69 108 70 109 Usage of the SafeProcess class generally follows the following sequence: … … 156 195 The exit value will also be -1 if you call getExitValue() before setting off the SafeProcess via any of the runProcess() variants. 157 196 158 _________________________________________________________________________________________________________________ 159 INTERNAL IMPLEMENTATION DETAILS - USEFUL IF CUSTOMISING SAFEPROCESS' BEHAVIOUR OR CALLING CANCEL ON SAFEPROCESS160 _________________________________________________________________________________________________________________ 197 _____________________________________________________________________________________________________________________ 198 E. INTERNAL IMPLEMENTATION DETAILS - USEFUL IF CUSTOMISING SAFEPROCESS' BEHAVIOUR OR CALLING CANCEL ON SAFEPROCESS 199 _____________________________________________________________________________________________________________________ 161 200 162 201 This section is for understanding how SafeProcess runs a Java Process internally. These methods are not public and their details are hidden away in the code. … … 239 278 240 279 __________________________________________________ 241 USAGE: CUSTOMISING280 F. USAGE: CUSTOMISING 242 281 __________________________________________________ 243 282 … … 318 357 319 358 If, during the execution of the primary thread, you want to do some complex things during a cancel operation, such as or before and after a process is destroyed, then 320 - implement SafeProcess.MainProcessHandler (see below)359 - implement SafeProcess.MainProcessHandler. See the Interface definition below. For further details, refer to the sections "CANCELLING" and "IMPLEMENTING HOOKS AROUND CANCELLING OR PROCESS' END". 321 360 - configure your SafeProcess instance by calling setMainHandler(MainProcessHandler mph) on it *before* calling a runProcess() method on that instance. 322 361 - For EXAMPLES OF USE, see GLI's DownloadJob.java. … … 370 409 } 371 410 372 373 __________________________________________________ 374 USAGE: CANCELLING 375 __________________________________________________ 411 __________________________________________________ 412 G. USAGE: CANCELLING 413 __________________________________________________ 414 415 SafeProcess provides a way to cancel an internal Process that is run by SafeProcess, and which will ensure that the internal process and worker threads all safely terminate and tidily clean up after themselves. 416 417 As explained, SafeProcess internally accomplishes this by sending an InterruptedException on the primary thread that's running the internal Process. An InterruptedException is sent only when SafeProcess is in an "interruptible" phase, as no interruption is necessary at any other time such as cleanup. 418 419 Only a thread *external* to the SafeProcess' primary thread, such as a GUI thread, can call cancelRunningProcess() on a reference to the SafeProcess instance. Two variants are available: 420 421 - boolean cancelRunningProcess() 422 - boolean cancelRunningProcess(boolean forceWaitUntilInterruptible) 423 424 The boolean value returned is true if an interrupt had to be sent, or false if one didn't need to be sent. An interrupt does not need to be sent if 425 - the process was never yet run by the time cancel was called or 426 - the internal Process has already naturally terminated and the SafeProcess has moved on to some stage of the cleanup phase by the time cancel was called. 427 428 If you want to inspect the return value, a good variable name is sentInterrupt. For example, 429 boolean sentInterrupt = mySafeProc.cancelRunningProcess(); 430 431 432 Programming pattern: 433 A graphical interface may have a Cancel button associated with an actionPerformed() method, which will be run from a GUI/EventDispatchThread when the user presses cancel. Either directly or indirectly, actionPerfomed() during a cancel action needs to lead to one of the cancelRunningProcess() method variants being called on the SafeProcess instance. 434 435 The variant you choose depends on whether the thread that invokes cancel on the SafeProcess is an EventDispatchThread (a GUI thread), or else, whether you are willing to wait until SafeProcess is interruptible. If you're calling the cancel method from a GUI thread or otherwise don't care to wait, call the zero-argument variant. Otherwise, you can call the single argument version and pass in true for forceWaitUntilInterruptible. 436 437 438 Usage: 439 - Usually, there is no need to bother waiting for the uninterruptible phase to finish, so you can call the zero-argument version of cancelRunningProcess(), and continue as before. 440 441 - If you're calling from a GUI thread and want to disable your cancel button during the uninterruptible phase, however short this may be, in order to give proper feedback to the user, then you can implement the hooks of the SafeProcess.MainHandler interface as explained in the next section. 442 443 - Call the cancelRunningProcess(forceWaitUntilInterruptible=true) variant if you're not calling from a GUI thread and you want to wait until SafeProcess becomes interruptible again (if you happened to have invoked the cancel method during an uninterruptible phase). 444 445 446 Further details: 447 Calling cancelRunningProcess(forceWaitUntilInterruptible=true) forces the method to only return when the SafeProcess has internally become "interruptible". There is a very brief phase at the start of a SafeProcess' cleanup period that blocks until the worker threads have cleanly terminated. This phase should not be interrupted (as would have happened upon an InterruptedException being sent just then), as the uninterruptible phase happens after the internal process has finished and the process is properly terminating. 448 449 If the cancel method happened to be called during this "blocking" phase and if this phase were to theoretically take long and if cancelRunningProcess() were not to return immediately if the calling thread is a GUI thread, then your graphical interface, or at least the cancel button, would have been unresponsive until the cancelRunningProcess() method finally returned. But this uninterruptible phase is short, and it always terminates, so calling cancelRunningProcess() is sufficient. And it always returns immediately if the caller is a GUI thread. The zero-argument variant of cancelRunningProecss() internally calls cancelRunningProcess(forceWaitUntilInterruptible=false) and causes the method to return immediately: either cancelRunningProcess was called when SafeProcess was interruptible and it sends an InterruptedException to the primary thread and returns, or cancelRunningProcess got called when SafeProcess was in an uninterruptible phase. And if the caller was a GUI thread (or if forceWaitUntilInterruptible was set to false), the cancelRunningProcess() method would also return immediately. 450 376 451 377 452 For EXAMPLES OF USE, see GLI's GShell.java and DownloadJob.java. 378 453 379 __________________________________________________ 380 USAGE: IMPLEMENTING HOOKS AROUND CANCELLING 381 __________________________________________________ 382 383 For EXAMPLES OF USE, see GLI's DownloadJob.java. 384 385 __________________________________________________ 386 USEFUL STATIC METHODS AND STATIC INNER CLASSES 387 __________________________________________________ 454 _________________________________________________________________ 455 H. USAGE: IMPLEMENTING HOOKS AROUND CANCELLING OR PROCESS' END 456 _________________________________________________________________ 457 458 - Implement the SafeProcess.MainProcessHandler Interface, see the decription below and the section CUSTOMISING. 459 - Write the hooks you want. 460 - If you don't really intend to override beforeWaitingForStreamsToEnd(boolean forciblyTerminating) or afterStreamsEnded(boolean forciblyTerminating), then return the boolean parameter forciblyTerminating. 461 - For EXAMPLES OF USE, see GLI's DownloadJob.java. 462 463 464 The SafeProcess.MainProcessHandler provides hooks into various key stages of the SafeProcess instance's life cycle. Specifically, the clean up and termination stages of its life cycle. Currently, all stages that you can write hooks for come after the internal process has come to an end either naturally or prematurely. 465 466 467 If your code needs to do some special processes during any of these stages, override them. 468 469 The stages are: 470 471 1. boolean beforeWaitingForStreamsToEnd(boolean forciblyTerminating) 472 473 Called after Process.waitFor() (when the internal Process has naturally or prematurely ended) and before the worker threads are join()ed (when the cleanup phase has begun, which always occurs, regardless of whether the Process had ended naturally or was cancelled). The parameter forciblyTerminating is true if the Process had been cancelled during an interruptible phase, or if an unexpected exception had occurred. In both cases, the internal Process object may need to be destroy()ed. But if the Process had only been cancelled during an uninterruptible phase, it means it had already naturally terminated and has started cleaning up. In such cases, the internal Process object does not need to be destroy()ed and forciblyTerminating will be false. 474 475 If not overriding, return the parameter. If overriding, even if the parameter is true, you may determine the process has come to a natural end after all. In that case, the process need not be destroyed, so you'd return false. For an example, see GLI's DownloadJob.java. 476 477 If you want to disable your cancel button temporarily while the join() is taking place, this is the method to do it. 478 479 480 2. boolean afterStreamsEnded(boolean forciblyTerminating) 481 482 Called after the worker threads are join()ed, which is the start of the clean up phase. This method is also always called and the parameter indicates whether it's so far been determined that the process needs to be destroyed or not. If not overriding, return the parameter. 483 484 If you want to re-enable your cancel button since the join() have by now taken place, this is the method to do so. 485 486 487 3. void beforeProcessDestroy() 488 4. void afterProcessDestroy() 489 490 These two methods are only called if destroy() needed to be called on the internal Process object (if the Process did not come to a natural end for any reason, including being cancelled during an interruptible phase or if an unexpected exception occurred). 491 492 493 5. void doneCleanup(boolean wasForciblyTerminated) 494 495 This method is always called at the very end of the SafeProcess' lifecycle, regardless of whether the internal Process naturally terminated or needed to be destroyed as happens upon premature termination. If the process needed to be destroyed, then the parameter wasForciblyTerminated would be true. 496 497 498 ___________________________________________________ 499 I. USEFUL STATIC METHODS AND STATIC INNER CLASSES 500 ___________________________________________________ 388 501 389 502 public static boolean isAvailable(String programName) 390 503 - Runs the `which` cmd over the program and returns true or false 391 `which` is included in winbin for Windows, and is part of unix systems504 - `which` is included in winbin for Windows, and is part of unix systems 392 505 393 506 static public boolean processRunning(Process process) 394 - returns true if the Process is currently running and hasn't come to an end. 395 - It does not 507 - returns true if the internal Process is currently running and hasn't come to an end. That is, this method returns true if Process.waitFor() is still blocking 508 - If the SafeProcess is in its cleanup/termination phase, then this method will return false, even though SafeProcess is still doing stuff. 509 - If you want to know when SafeProcess has finished in its entirety, call the instance method processRunning() on the SafeProcess object. 396 510 397 511 public static long getProcessID(Process p) … … 411 525 412 526 public static boolean closeSocket(Socket resourceHandle) 413 - will attempt to cleanly close your Socket, logging on Exception. In Java 6, Sockets didn't yet implement Closeable,414 so this method is to ensure backwards compatibility527 - will attempt to cleanly close your Socket, logging on Exception. 528 - From Java 7 onwards, calling closeResource() would suffice on Sockets too. But in Java 6, Sockets didn't yet implement Closeable, so this method is to ensure backwards compatibility. 415 529 416 530 public static class OutputStreamGobbler extends Thread … … 423 537 424 538 425 Package access: 426 static methods to prematurely terminate any process denoted by processID and any subprocesses it may have launched: 427 static boolean killUnixProcessWithID(long processID) 428 static void killWinProcessWithID(long processID) 429 430 431 __________________________________________________ 432 OTHER USEFUL INSTANCE METHODS 539 Package access: static methods to prematurely terminate any process denoted by processID and any subprocesses it may have launched 540 541 static void killWinProcessWithID(long processID) 542 543 static boolean killUnixProcessWithID(long processID) 544 - for Mac/Linux 545 546 547 __________________________________________________ 548 J. OTHER USEFUL INSTANCE METHODS 433 549 __________________________________________________ 434 550 … … 437 553 438 554 public boolean cancelRunningProcess() 555 public boolean cancelRunningProcess(boolean forceWaitUntilInterruptible) 439 556 - cancel the SafeProcess instance 440 557 - untimately synchronized, so threadsafe 441 __________________________________________________ 442 USEFUL SYNCHRONISATION NOTES 443 __________________________________________________ 444 [Also add in links] 445 446 447 __________________________________________________ 448 TICKETS, COMMITS, TAGS 449 __________________________________________________ 450 451 558 - returns true if the process were still running so that an interrupt needed to be sent to terminate it, returns false if sending an interrupt was unnecessary because the process had already entered the start of its termination phase when cancel was called. Will also return false if the process was never run by the time cancel was called. 559 - usually you want to call the zero-argument version 560 - See the section CANCELLING for further details 561 562 _________________________________________________________________________________________________ 563 K. LINKS AND NOTES ON PROCESSES, PROCESS STREAMS, INTERRUPTING AND DESTROYING PROCESSES 564 _________________________________________________________________________________________________ 565 566 Processes and streams 567 568 - http://www.javaworld.com/article/2071275/core-java/when-runtime-exec---won-t.html?page=2 569 explains why SafeProcess.java is implemented with worker threads to handle a Process' iostreams. 570 - http://steveliles.github.io/invoking_processes_from_java.html 571 - http://mark.koli.ch/leaky-pipes-remember-to-close-your-streams-when-using-javas-runtimegetruntimeexec 572 573 574 Interrupting a Process: 575 576 - http://stackoverflow.com/questions/2126997/who-is-calling-the-java-thread-interrupt-method-if-im-not 577 - http://stackoverflow.com/questions/13623445/future-cancel-method-is-not-working?noredirect=1&lq=1 578 - http://stackoverflow.com/questions/3976344/handling-interruptedexception-in-java 579 - http://stackoverflow.com/questions/4906799/why-invoke-thread-currentthread-interrupt-when-catch-any-interruptexception 580 - http://stackoverflow.com/questions/4906799/why-invoke-thread-currentthread-interrupt-when-catch-any-interruptexception 581 - https://praveer09.github.io/technology/2015/12/06/understanding-thread-interruption-in-java/ 582 583 584 Destroying a process and subprocesses on various OS: 585 586 - Need process id, pid, for which Java Native Access libraries (jar files) are required. 587 http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program 588 - To find the processID of the process launched by SafeProcess, need to use Java Native Access (JNA) jars, available jna.jar and jna-platform.jar. 589 http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program 590 http://stackoverflow.com/questions/35842/how-can-a-java-program-get-its-own-process-id 591 http://www.golesny.de/p/code/javagetpid 592 https://github.com/java-native-access/jna/blob/master/www/GettingStarted.md 593 We're using JNA v 4.1.0, downloaded from https://mvnrepository.com/artifact/net.java.dev.jna/jna 594 595 WINDOWS NOTES: 596 - Can't artificially send Ctrl-C: stackoverflow.com/questions/1835885/send-ctrl-c-to-process-open-by-java 597 598 On Windows, p.destroy() terminates process p that Java launched, 599 but does not terminate any processes that p may have launched. Presumably since they didn't form a proper process tree. 600 https://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/e3cb7532-87f6-4ae3-9d80-a3afc8b9d437/how-to-kill-a-process-tree-in-cc-on-windows-platform?forum=vclanguage 601 https://msdn.microsoft.com/en-us/library/windows/desktop/ms684161(v=vs.85).aspx 602 603 Searching for: "forcibly terminate external process launched by Java on Windows" 604 Not possible: stackoverflow.com/questions/1835885/send-ctrl-c-to-process-open-by-java 605 But can use taskkill or tskill or wmic commands to terminate a process by processID 606 stackoverflow.com/questions/912889/how-to-send-interrupt-key-sequence-to-a-java-process 607 Taskkill command can kill by Image Name, such as all running perl, e.g. taskkill /f /im perl.exe 608 But what if we kill perl instances not launched by GS? 609 /f Specifies to forcefully terminate the process(es). We need this flag switched on to kill childprocesses. 610 /t Terminates the specified process and any child processes which were started by it. 611 /t didn't work to terminate subprocesses. Maybe since the process wasn't launched as 612 a properly constructed processtree. 613 /im is the image name (the name of the program), see Image Name column in Win Task Manager. 614 615 We don't want to kill all perl running processes. 616 Another option is to use wmic, available since Windows XP, to kill a process based on its command 617 which we sort of know (SafeProcess.command) and which can be seen in TaskManager under the 618 "Command Line" column of the Processes tab. 619 https://superuser.com/questions/52159/kill-a-process-with-a-specific-command-line-from-command-line 620 The following works kill any Command Line that matches -site localsite lucene-jdbm-demo 621 C:>wmic PATH win32_process Where "CommandLine like '%-site%localsite%%lucene-jdbm-demo%'" Call Terminate 622 "WMIC Wildcard Search using 'like' and %" 623 https://codeslammer.wordpress.com/2009/02/21/wmic-wildcard-search-using-like-and/ 624 However, we're not even guaranteed that every perl command GS launches will contain the collection name 625 Nor do we want to kill all perl processes that GS launches with bin\windows\perl\bin\perl, though this works: 626 wmic PATH win32_process Where "CommandLine like '%bin%windows%perl%bin%perl%'" Call Terminate 627 The above could kill GS perl processes we don't intend to terminate, as they're not spawned by the particular 628 Process we're trying to terminate from the root down. 629 630 Solution: We can use taskkill or the longstanding tskill or wmic to kill a process by ID. Since we can 631 kill an external process that SafeProcess launched OK, and only have trouble killing any child processes 632 it launched, we need to know the pids of the child processes. 633 634 We can use Windows' wmic to discover the childpids of a process whose id we know. 635 And we can use JNA to get the process ID of the external process that SafeProcess launched. 636 637 See links further above on how to find the processID of the process launched by SafeProcess on various OS, 638 and where to find the JNA jars needed for this. 639 640 WMIC can show us a list of parent process id and process id of running processes, and then we can 641 kill those child processes with a specific process id. 642 https://superuser.com/questions/851692/track-which-program-launches-a-certain-process 643 http://stackoverflow.com/questions/7486717/finding-parent-process-id-on-windows 644 WMIC can get us the pids of all childprocesses launched by parent process denoted by parent pid. 645 And vice versa: 646 if you know the parent pid and want to know all the pids of the child processes spawned: 647 wmic process where (parentprocessid=596) get processid 648 if you know a child process id and want to know the parent's id: 649 wmic process where (processid=180) get parentprocessid 650 651 The above is the current solution. 652 653 Linux ps equivalent on Windows is "tasklist", see 654 http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program 655 656 657 MAC/UNIX NOTES: 658 - Kill signals, their names and numerical equivalents: http://www.faqs.org/qa/qa-831.html 659 - Killing a processtree (a process group) on Unix: 660 https://stackoverflow.com/questions/8533377/why-child-process-still-alive-after-parent-process-was-killed-in-linux 661 Works on Linux but not Mac: kill -TERM -pid 662 Works on Mac but not Linux: pkill -TERM -P pid did work 663 - https://stackoverflow.com/questions/28332888/return-value-of-kill 664 "kill returns an exit code of 0 (true) if the process still existed it and was killed. 665 kill returns an exit code of 1 (false) if the kill failed, probably because the process was no longer running." 666 - https://superuser.com/questions/343031/sigterm-with-a-keyboard-shortcut 667 Ctrl-C sends a SIGNINT, not SIGTERM or SIGKILL. And on Ctrl-C, "the signal is sent to the foreground *process group*." 668 - https://linux.die.net/man/1/kill (manual) 669 - https://unix.stackexchange.com/questions/117227/why-pidof-and-pgrep-are-behaving-differently 670 - https://unix.stackexchange.com/questions/67635/elegantly-get-list-of-children-processes 671 - https://stackoverflow.com/questions/994033/mac-os-x-quickest-way-to-kill-quit-an-entire-process-tree-from-within-a-cocoa-a 672 - https://unix.stackexchange.com/questions/132224/is-it-possible-to-get-process-group-id-from-proc 673 - https://unix.stackexchange.com/questions/99112/default-exit-code-when-process-is-terminated 674 675 _______________________________________________________________________________________ 676 L. USEFUL SYNCHRONISATION NOTES AND SUGGESTED READING ON THREADS AND THREAD SAFETY 677 _______________________________________________________________________________________ 678 679 Things worth reading on Concurrency and Thread safety: 680 681 - Deitel and Deitel's newer editions of their Java books have updated their chapter on Concurrency. 682 - http://docs.oracle.com/javase/tutorial/uiswing/concurrency/ 683 series of Java articles on concurrency again. 684 - https://docs.oracle.com/javase/tutorial/essential/concurrency/atomicvars.html 685 On "primitive" like variables that provide instrinsic locks. 686 687 688 Notes on synchronization: 689 690 - You can declare methods are synchronized. For example, 691 public synchronized void doStuff() 692 This implicitly synchronizes on the *instance* of the class on which this method is called. 693 694 - Within any methods, you can synchronize on specific objects to just lock on them. You have a synchronized code block within which you can manipulate the object. 695 696 Keep synchronized blocks and methods as short as you can. 697 There are some subtle issues related to using synchronized methods, as explained below, that can result in deadlocks. 698 699 700 More reading: 701 702 - http://stackoverflow.com/questions/574240/is-there-an-advantage-to-use-a-synchronized-method-instead-of-a-synchronized-blo 703 704 "Not only do synchronized methods not lock the whole class, but they don't lock the whole instance either. Unsynchronized methods in the class may still proceed on the instance." 705 "Only the syncronized methods are locked. If there are fields you use within synced methods that are accessed by unsynced methods, you can run into race conditions." 706 707 "synchronizing on "this" is considered in some circles to be an anti-pattern. The unintended consequence is that outside of the class someone can lock on an object reference that is equal to "this" and prevent other threads from passing the barriers within the class potentially creating a deadlock situation. Creating a "private final Object = new Object();" variable purely for locking purposes is the often used solution. Here's another question relating directly to this issue. 708 709 - http://stackoverflow.com/questions/442564/avoid-synchronizedthis-in-java?lq=1" 710 711 "A private lock is a defensive mechanism, which is never a bad idea. 712 713 Also, as you alluded to, private locks can control granularity. One set of operations on an object might be totally unrelated to another but synchronized(this) will mutually exclude access to all of them." 714 715 716 - http://stackoverflow.com/questions/8393883/is-synchronized-keyword-exception-safe 717 "In any scoped thread-safe block, the moment you get out of it, the thread-safety is gone." 718 "In case of an exception the lock will be released." 719 720 - http://stackoverflow.com/questions/8259479/should-i-synchronize-listener-notifications-or-not 721 "Use a CopyOnWriteArrayList for your listener arrays." 722 "If you use the CopyOnWriteArrayList, then you don't have to synchronize when iterating." 723 "CopyOnWriteArrayList is thread-safe, so there is no need to synchronize." 724 725 "Use a ConcurrentLinkedQueue<Listener> ... for this kind of problems: adding, removing and iterating simultaneously on a collection. 726 A precision : this solution prevents a listener from being called from the very moment it is deregistered." 727 "It means that you start iterating, an element is added, it will be called, another is removed, it won't, all this in the same iteration cycle. 728 It's the best of both world: ensuring synchronization, while being fine grained on who gets called and who's not." 729 730 Examples of using CopyOnWriteArrayList: in GLI's shell/GShell.java and GS3's gsdl3/build/CollectionConstructor.java 731 732 - http://stackoverflow.com/questions/8260205/when-a-listener-is-removed-is-it-okay-that-the-event-be-called-on-that-listener 733 734 - http://stackoverflow.com/questions/2282166/java-synchronizing-on-primitives 735 736 1. You can't lock on a primitive and 737 2. Don't lock on a Long unless you're careful how you construct them. Long values created by autoboxing or Long.valueOf() in a certain range are guaranteed to be the same across the JVM which means other threads could be locking on the same exact Long object and giving you cross-talk. This can be a subtle concurrency bug (similar to locking on intern'ed strings). 738 739 Cross-talk: 740 "In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another." 741 742 743 __________________________________________________ 744 M. TICKETS, COMMITS, TAGS 745 __________________________________________________ 746 747 The following was already documented in ticket http://trac.greenstone.org/ticket/895 748 749 Other Java classes of GLI and GS3 src code were shifted from using Java's Process class directly to using our new SafeProcess class. 750 751 GLI src code classes that were changed: 752 /Scratch/ak19/gs3-svn-15Nov2016/gli>fgrep -rl "Process" src 753 754 + src/org/greenstone/gatherer/util/SafeProcess.java 755 + src/org/greenstone/gatherer/gui/FormatConversionDialog.java 756 + src/org/greenstone/gatherer/gui/DownloadPane.java 757 + src/org/greenstone/gatherer/util/GS3ServerThread.java 758 + 2 src/org/greenstone/gatherer/Gatherer.java [2 MORE but don't want to mess with those] 759 + 1 src/org/greenstone/gatherer/download/ServerInfoDialog.java 760 + 2 src/org/greenstone/gatherer/greenstone/Classifiers.java 761 + 2 src/org/greenstone/gatherer/greenstone/Plugins.java 762 + 1 src/org/greenstone/gatherer/collection/ScriptOptions.java 763 + !! src/org/greenstone/gatherer/util/ExternalProgram.java && file/FileAssociationManager.java 764 765 After updating GS3 src code to using SafeProcess, returned to GLI: 766 - VERY HARD: 767 + 1 src/org/greenstone/gatherer/shell/GShell.java 768 - VERY, VERY HARD: 769 + 2 src/org/greenstone/gatherer/download/DownloadJob.java 770 771 772 GS3 src code classes that were changed: 773 > fgrep -r "Process" . | grep -v ".svn" | grep -v "~" | less 774 775 + 2 ./java/org/greenstone/admin/guiext/Command.java 776 X ./java/org/greenstone/gsdl3/util/Processing.java 777 + MANY OCCURRENCES ./java/org/greenstone/gsdl3/service/MapRetrieve.java 778 + Uses Processing 2 times: ./java/org/greenstone/gsdl3/util/GDBMWrapper.java 779 + ./java/org/greenstone/util/BrowserLauncher.java 780 + ./java/org/greenstone/util/RunTarget.java 781 782 In the GLI and GS3 src files modified to use SafeProcess above, only the modifications to GS3's Command.java, MapRetrieve.java and GDMBWrapper.java are untested. All others have been tested, at least on Linux. Many have also been tested on Windows. 783 784 Tested GLI's collection building (uses GShell.java's modifications) and DownloadJob.java on all 3 OS. Both have a Cancel button that should cancel any processes and subprocesses launched. Before the modifications, subprocesses would still be running on Windows and during (full-)import on the Mac when cancel was pressed. But now have added code to SafeProcess to issue OS specific system calls to terminate the process and its subprocesses successfully on Windows and Mac too. 785 786 Also tested GS3 user comments and the online GS3's doc editor's meta editing followed by rebuilding. All worked successfully on Linux, Windows and Mac with the modifications to use SafeProcess. 787 788 The modifications to GLI started with GS3ServerThread.java and other *easily* changed classes in 789 - http://trac.greenstone.org/changeset/31582 (beginning of GLI modification) 790 - until http://trac.greenstone.org/changeset/31665 (GS3 src code modification). 791 792 The version of GS3 and GLI Java code before the GS3 src was modified is at 793 - http://trac.greenstone.org/browser/main/tags/GS3-src-SafeProcess 794 795 Next, GLI's GShell.java and any associated files were modified along with SafeProcess, to support cancelling. 796 797 Then, around 22 May, GLI's DownloadJob.java and any associated class files were modified along with SafeProcess to ensure subprocesses were safely terminated on Windows when the download action was cancelled OR when building (GShell.java) was cancelled. The Mac modifications to SafeProcess to ensure subprocesses were terminated was finished on 26 May 2017. 798 799 The tagged version of the GS3 and GLI Java code for when all the changes related to using SafeProcess were made to GS3 and GLI code is at 800 - http://trac.greenstone.org/browser/main/tags/UsingSafeProcess 801 802 _________________________________________________________ 803 N. GLI vs GS3 CODE's SAFEPROCESS.JAVA - THE DIFFERENCES 804 _________________________________________________________ 805 806 - package name 807 GLI: package org.greenstone.gatherer.util; 808 GS3: package org.greenstone.util; 809 - imports: 810 GLI: import org.greenstone.gatherer.DebugStream; 811 - GS3 uses Misc.java and GLI uses Utility.java for determining the OS and the appropriate newline character representation for the OS. 812 - GS3 uses its Logger object for logging messages, GLI uses System.err (Debugstream code is presently commented out). 813 814 815 $ diff gli/src/org/greenstone/gatherer/util/SafeProcess.java src/java/org/greenstone/util/SafeProcess.java 816 ----------------------------------------------------------------------------------------------------------- 817 1c1 818 < package org.greenstone.gatherer.util; 819 --- 820 > package org.greenstone.util; 821 27c27 822 < import org.greenstone.gatherer.DebugStream; 823 --- 824 > //import org.greenstone.gatherer.DebugStream; 825 56c56 826 < ///static Logger logger = Logger.getLogger(org.greenstone.util.SafeProcess.class.getName()); 827 --- 828 > static Logger logger = Logger.getLogger(org.greenstone.util.SafeProcess.class.getName()); 829 153a154 830 > 831 816c817 832 < if(Utility.isMac()) { 833 --- 834 > if(Misc.isMac()) { 835 880c881 836 < if(Utility.isWindows()) { 837 --- 838 > if(Misc.isWindows()) { 839 909c910 840 < if(!Utility.isMac() && canSkipExtraWorkIfLinux) { 841 --- 842 > if(!Misc.isMac() && canSkipExtraWorkIfLinux) { 843 1284c1285 844 < outputstr.append(Utility.NEWLINE); // "\n" is system dependent (Win must be "\r\n") 845 --- 846 > outputstr.append(Misc.NEWLINE); // "\n" is system dependent (Win must be "\r\n") 847 1381c1382 848 < /*if(Utility.isWindows()) { 849 --- 850 > /*if(Misc.isWindows()) { 851 1416c1417 852 < //logger.info(msg); 853 --- 854 > logger.info(msg); 855 1418c1419 856 < System.err.println(msg); 857 --- 858 > //System.err.println(msg); 859 1424c1425 860 < //logger.error(msg, e); 861 --- 862 > logger.error(msg, e); 863 1426,1427c1427,1428 864 < System.err.println(msg); 865 < e.printStackTrace(); 866 --- 867 > //System.err.println(msg); 868 > //e.printStackTrace(); 869 1434c1435 870 < //logger.error(e); 871 --- 872 > logger.error(e); 873 1436c1437 874 < e.printStackTrace(); 875 --- 876 > //e.printStackTrace(); 877 878 ----------------------------------------------------------------------------------------------------------- 879 880 881 __________________________________________________ 882 O. FUTURE WORK, IMPROVEMENTS 883 __________________________________________________ 884 885 - On Windows, Perl could launch processes as proper ProcessTrees: http://search.cpan.org/~gsar/libwin32-0.191/ 886 Then killing the root process will kill child processes naturally, and we won't need to call OS commands to terminate a process. 887 - Need to find a Mac (or Unix) equivalent way creating Process Tree in Perl too. 888 889 Eventually, instead of running a windows/mac or even linux command to kill the process ourselves, investigate if it is useful to change over to use 890 https://github.com/flapdoodle-oss/de.flapdoodle.embed.process/blob/master/src/main/java/de/flapdoodle/embed/process/runtime/Processes.java 891 - mentioned by http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program 892 - works with Apache license, http://www.apache.org/licenses/LICENSE-2.0 893 - This is a Java class that uses JNA to terminate processes. It also has the getProcessID() method. 894 - However, does it kill subprocesses? (The OS specific commands presently issued in SafeProcess.destroyProcess() ensure subprocesses are killed.) 895 896 897 Currently, SafeProcess does Java things the way that older Java versions recognise and accept, not only for backwards compatibility with earlier versions of Java, but in order to minimise the number of drastic changes from porting the existing Greenstone GLI and GS3 code to using SafeProcess. 898 899 - In future, think of changing the method doRuntimeExec() over to using ProcessBuilder, instead of Runtime.exec(). ProcessBuilder seems to have been introduced from Java 5. 900 https://docs.oracle.com/javase/7/docs/api/java/lang/ProcessBuilder.html 901 See also https://zeroturnaround.com/rebellabs/how-to-deal-with-subprocesses-in-java/ 902 which suggests using Apache Common Exec to launch processes and says what will be forthcoming in Java 9 903 904 - Instead of creating and running Threads the usual way, should we use features of newer Java versions like the Concurrency API and Executor which uses Threadpools, and let Executor handle these details? Improvements would be with multicore systems 905 https://stackoverflow.com/questions/3330430/does-java-have-support-for-multicore-processors-parallel-processing 906 907 908 909
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