1 / 38

Java Memory Management

Java Memory Management. Charles-François THUEUX Sitraka Inc. cft@sitraka.com. Overview. Review of Java’s Memory Model Loitering Objects Effective Memory Management Design Implementation Documentation Conclusion and Further Reading. Memory Safety in Java.

taffy
Download Presentation

Java Memory Management

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Java Memory Management Charles-François THUEUX Sitraka Inc. cft@sitraka.com

  2. Overview • Review of Java’s Memory Model • Loitering Objects • Effective Memory Management • Design • Implementation • Documentation • Conclusion and Further Reading Stuttgart JUG 2001

  3. Memory Safety in Java • A key aspect in the design of... • the Java Language • no pointer arithmetic • the Java Virtual Machine (JVM) • bytecode instruction set • runtime checks (array bounds, ref casts) • garbage collection Stuttgart JUG 2001

  4. Memory Safety in Java • Eliminated many memory-related problems • Buffer overruns • De-referencing stale pointers • Memory leaks • However Java programs can exhibit the macro-level symptoms of traditional memory leaks • Process size seemingly grows without bounds Stuttgart JUG 2001

  5. Java’s Memory Model • The intent of Garbage Collection is to remove objects that are no longer needed • Undecidable in general • Java uses an approximation • remove objects that are no longer reachable • The reachability test starts at the Heap’s root set Stuttgart JUG 2001

  6. The Root Set • Set of foundational object references • static reference fields within class definitions • local reference variables within the method frames of each Java thread stack • The contents of the Root Set change dynamically as your program runs • As threads enter and exit methods, local reference variables enter and leave the Root Set Stuttgart JUG 2001

  7. The Dynamic Root Set - 1 1 public 2 class MyApp 3 { 4 private static MyAppmyApp = null; 5 6 public static 7 void 8 main( String[] args ) 9 { 10 myApp = new MyApp( ); 11 myApp.method1( ); 12 myApp.method2( ); 13 } Consider a single thread of execution... Root Set: MyApp myApp String[] args Stuttgart JUG 2001

  8. The Dynamic Root Set - 2 14 private void 15 method1( ) 16 { 17 BigObjectbigObj = new BigObject( ); 18 ... 19 } 20 21 private void 22 method2( ) 23 { 24 BiggerObjectbiggerObj = new BiggerObject( ); 25 ... 26 } 27 } Root Set: MyApp myApp String[] args BigObject bigObj Stuttgart JUG 2001

  9. The Dynamic Root Set - 3 1 public 2 class MyApp 3 { 4 private static MyAppmyApp = null; 5 6 public static 7 void 8 main( String[] args ) 9 { 10 myApp = new MyApp( ); 11 myApp.method1( ); 12 myApp.method2( ); 13 } Root Set: MyApp myApp String[] args Stuttgart JUG 2001

  10. The Dynamic Root Set - 4 14 private void 15 method1( ) 16 { 17 BigObjectbigObj = new BigObject( ); 18 ... 19 } 20 21 private void 22 method2( ) 23 { 24 BiggerObjectbiggerObj = new BiggerObject( ); 25 ... 26 } 27 } Root Set: MyApp myApp String[] args BiggerObject biggerObj Stuttgart JUG 2001

  11. The Dynamic Root Set - 5 1 public 2 class MyApp 3 { 4 private static MyAppmyApp = null; 5 6 public static 7 void 8 main( String[] args ) 9 { 10 myApp = new MyApp( ); 11 myApp.method1( ); 12 myApp.method2( ); 13 } Root Set: MyApp myApp Stuttgart JUG 2001

  12. Reachable Objects • Elements within the Root Set refer to objects within the JVM’s Heap • Reference variables within those objects refer to further objects within the Heap Root Set Object Reference Stuttgart JUG 2001

  13. Object States • Define three progressive object “states” • Allocated • Exists within the JVM’s heap • Reachable • A path exists (directly or indirectly) from a member of the root set, through a sequence of references, to that object. • Live • From the intent of the application’s design, the program will use the object (meaning at least one of its fields will be accessed and/or one of its methods will be invoked) along some future path of execution. Stuttgart JUG 2001

  14. Memory leak in C/C++ (handled by JVM’s GC) “Memory leak” in Java The JVM Heap allocated reachable live Stuttgart JUG 2001

  15. C/C++ vs. Java • Memory leak in C/C++ • Object was allocated, but it’s not reachable • malloc()/new, but forgot to free()/delete before destroying the memory pointer • “Memory leak” in Java • The object is reachable, but it’s not live • a reference was set, but it hasn’t been eliminated • Object is reachable to the GC, but the source code to fix the leak may not be available to you Stuttgart JUG 2001

  16. “Memory Leaks” in Java • Impact is often more severe than C/C++ • Rarely a single object, but a wholesub-graph • A single lingering reference can have massive memory impact (and a significant performance impact) Unintentional reference Stuttgart JUG 2001

  17. Loitering Objects • The term “Memory Leak” has too much historical baggage from C/C++ • and it doesn’t accurately describe the formulation of the problem as it pertains to Java • A new term: Loitering Object • An object that remains within the Heap past its useful life Stuttgart JUG 2001

  18. Reference Management • The key to effective memory management in Java is effective reference management • What undermines effective reference management ? • Awareness of the issue • Bad habits from C/C++ development • Class Libraries and Application Frameworks • Ill-defined reference management policies • Encapsulate flawed reference assignments Stuttgart JUG 2001

  19. Reference Management • As a Java programmer, how can I effectively manage references within my software, and promote better reference management ? 1. Design 2. Implementation 3. Documentation Stuttgart JUG 2001

  20. 1. Design for Ref. Mgmt. • For each Use Case within your application, explicitly characterize: a. The lifecycle of each object b. The inter-relationships between various objects Stuttgart JUG 2001

  21. Lifecycle Relationships • Object B adopts A’s lifecycle and has no control over it A B Stuttgart JUG 2001

  22. 1a. Object Lifecycles • For each object within the Use Case you are implementing, you need to explicitly define: • its point of creation • the duration of its usefulness • the point at which it should be eliminated from the runtime environment Stuttgart JUG 2001

  23. 1a. Object Lifecycles • In Java, creating an object within the runtime environment is an explicit act, while its elimination is an implicit one • Defining - within your design - the point when your object should be eliminated will help you validate the correctness of your Java implementation Stuttgart JUG 2001

  24. 1b. Inter-Object Relationships • Objects establish relationships as they collaborate to accomplish their goals • Examples: • Composition (a has-a relationship) • Association (a uses-a relationship) • Relationship lifecycles Stuttgart JUG 2001

  25. 1b. Inter-Object Relationships • Think “Symmetry” • If you define a method that establishes a relationship, ensure you define a method that revokes it. • The Observer Pattern subject.addObserver( Observer ) subject.removeObserver( Observer ) Stuttgart JUG 2001

  26. 2. Implementation • Loitering objects often arise from simple coding oversights or omissions • forgot to null-ify a variable • didn’t remove an object from a list • Use a Heap Analysis Tool to validate that your implementation adheres to your design Stuttgart JUG 2001

  27. Reference Variable Scope • Three forms of reference variables: • Class-based: • Reference variables within a class definition that have a static attribute associated with them • Object-based: • Non static reference variables within a class definition • Method-based: • Reference variables defined within the scope of a method Stuttgart JUG 2001

  28. Reference Variable Scope • Don’t be concerned about assignments to method-based reference variables within methods of short execution time • Be attentive of assignments to class-based and object-based reference variables, and method-based reference variables within methods of long execution times Stuttgart JUG 2001

  29. Lingerer • Reference used transiently by long-term object • Reference always reset on next use • e.g. associations with menu items • e.g. global action or service Stuttgart JUG 2001

  30. Lingerer Example • Print action as singleton • class Printer… • has data member Printable target; • calls target.doPrint(); • target inside printer not set to null on completion • target is a lingering reference • target cannot be GC’ed until next print Stuttgart JUG 2001

  31. Lingerer Strategies • Don’t use a set of fields to maintain state • enclose in object • easier to maintain • one reference to clean up • Draw state diagram • quiescent state == no outgoing references • Early exit methods or multi-stage process • setup; process; cleanup Stuttgart JUG 2001

  32. 3. Documentation • For methods that establish a relationship to another object, identify (in your javadoc description) the symmetric method that revokes the relationship void addObserver( Observer ) Adds the Observer argument to the subject's internal set of observers. To remove the observer from this internal set, invoke the removeObserver( Observer ) method. Stuttgart JUG 2001

  33. Dealing with Flaws of Others • After determining you have a loitering object, your investigation reveals that the object holding the reference to your loiterer is one which you do not have the source code to. • How do you handle this ? Stuttgart JUG 2001

  34. Using Their Code Properly ? • Are you using their framework properly ? • Understand their notion of object life cycles • Think symmetry (again) • An add() operation implies a remove() • A register() implies an unregister() • Does the symmetric operation (to the one that established the reference) remove it ? Stuttgart JUG 2001

  35. Notify the Author • You’ve established that they are erroneously holding an object reference • Create a simple test case • Make it easy for the vendor to clearly identify the issue. • Ask for • Work arounds (if possible) • Resolution to the fundamental problem Stuttgart JUG 2001

  36. Eliminate Subgraph Elements • If you can not fix the root cause, free up as much of the loitering subgraph as you can. Unintentional reference Unintentional reference Stuttgart JUG 2001

  37. Conclusion • The key to effective memory management in Java is effective reference management • Incorporate reference management in your design • Validate your Java implementation • Document your reference management policies so others will know what to do Stuttgart JUG 2001

  38. Further Reading • Loitering Objects and Java Framework Design by Leonard Slipp, JavaReport, Volume 6, Number 1 (January 2001) http://www.javareport.com/html/from_pages/article.asp?id=249&mon=1&yr=2001 • How Do You Plug Java Memory Leaks ? by Ethan Henry and Ed Lycklama, Dr. Dobb’s Journal, Java Q&A Column,Volume 25, Number 2 (February 2000) www.ddj.com/articles/2000/0002/0002l/0002l.htm • Memory Leaks in Java Programs by Joel Nylund, JavaReport, Volume 4, Number 11 (November 1999). www.javareport.com/archive/9911/html/from_pages/ftp_feature.shtml Stuttgart JUG 2001

More Related