Java Memory Management

1. Java Memory Management Charles-Fran�ois THUEUX Sitraka Inc. cft@sitraka.com

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

3. 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

4. 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

5. Stuttgart JUG 2001 5 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 Java�s Memory Model

6. 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

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

8. Stuttgart JUG 2001 8 The Dynamic Root Set - 2 14 private void 15 method1( ) 16 { 17 BigObject bigObj = new BigObject( ); 18 ... 19 } 20 21 private void 22 method2( ) 23 { 24 BiggerObject biggerObj = new BiggerObject( ); 25 ... 26 } 27 }

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

10. Stuttgart JUG 2001 10 The Dynamic Root Set - 4 14 private void 15 method1( ) 16 { 17 BigObject bigObj = new BigObject( ); 18 ... 19 } 20 21 private void 22 method2( ) 23 { 24 BiggerObject biggerObj = new BiggerObject( ); 25 ... 26 } 27 }

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

12. 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

13. 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.

14. Stuttgart JUG 2001 14 The JVM Heap

15. 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

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

17. 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

18. 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

19. 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

20. 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

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

22. 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

23. 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

24. 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

25. 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 )

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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 ?

34. 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 ?

35. 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

36. 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.

37. 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

38. 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