Towards A Light-Weight Tool for Locating Unintentional Object Retention in Java Programs
MotivationMotivation
• Even with automatic garbage collector, memory leaks do occur in Java programs in the form of loitering objects.
• Such loitering objects can be hard to locate and fix in a complex Java program.
• Current tools for diagnosing Java memory leaks address this problem mostly by digging information buried in JVM heap. This approach is weak at
▫ assisting programmer to establish strong hypothesis about loitering objects.
▫ providing effective clues to where in the source code these loitering objects are retained.
ObjectivesObjectives
• Build a light-weight tool for▫ Identifying the suspected classes of
loitering objects, and further.▫ locating unintended object references in
the source code.
Key IdeasKey Ideas
• The object Destruction/Creation ratio of a class is a good indicator for identifying loitering objects.
• A log of reference assignments that keeps track of all the references linking to the suspected objects will enable us to locate where those unintentional references are set.
• AspectJ’s code weaving facility provides a good basis for building a light-weight tool for the tasks above.
Major StepsMajor Steps
To locate unintended object references:
1. Weave in the object life monitoring aspect into the target application. Run the application for a period of time in order to calculate the D/C ratio value for selected application classes.
2. Identify some the suspected classes based on the D/C statistics, and then weave in the reference logging aspect on objects of those chosen classes.
3. Likewise, run the application for a period of time to log all references linking to the objects of suspected classes.
4. Analyze the reference log and extract reference patterns that look suspicious.
5. Use the reference patterns and precise source code location information to locate the code causing unintended references.
Work AheadWork Ahead
• Investigate enhancements to the scope of AspectJ’s join points.▫ AspectJ does not pick out join points of
array-element access.
• Develop a visual interface for the reference analysis tool.
Chin-Hung ChienFoxconn Electronics [email protected]
Kung Chen and Ju-Bing ChenDept. of Computer ScienceNational Chengchi [email protected]@cs.nccu.edu.tw
Code Weaving in AspectJ Code Weaving in AspectJ
where (pointcut)
do what (advice)aspect
crosscutting concernclass class
weaving
testCases.Queue : 0/1 = 0.0% testCases.QueueNode : 0/100 = 0.0%
Destruction/Creation Ratio
suspected classsuspected class
class : testCases.QueueNode reference count : 110 class statistics : testCases.Queue : 2 testCases.QueueNode : 108 field statistics : QueueNode testCases.QueueNode.next : 9 QueueNode testCases.QueueNode.prev : 99 QueueNode testCases.Queue.headNode : 1 QueueNode testCases.Queue.tailNode : 1 location statistics : Queue.java:22 : 1 Queue.java:46 : 1 QueueNode.java:42 : 9 QueueNode.java:16 : 99
Reference Statistics from the Log
A Flawed Implementation of Queue with Loitering Objects
Execution Flow
myObj = myQueue.dequeue();
myObj = myQueue.dequeue();
myObj = myQueue.dequeue();
myObj = myQueue.dequeue();
Live Object
Loitering Object
Object field
Intended Ref.
Unintended Ref.
myQueue = new Queue();…
myQueue.enqueue(..);myQueue.enqueue(..);
…
myQueue = new Queue();…
myQueue.enqueue(..);myQueue.enqueue(..);
…
QN1----------------
QN prevQN next
Object elem
QN1----------------
QN prevQN next
Object elem
QN2----------------
QN prevQN next
Object elem
QN2----------------
QN prevQN next
Object elem
QN3----------------
QN prevQN next
Object elem
QN3----------------
QN prevQN next
Object elem
headhead
......
object Aobject A object Bobject B object Cobject C
QN1----------------
QN prevQN next
Object elem
QN1----------------
QN prevQN next
Object elem
QN2----------------
QN prevQN next
Object elem
QN2----------------
QN prevQN next
Object elem
QN3----------------
QN prevQN next
Object elem
QN3----------------
QN prevQN next
Object elem
headhead
......
object Aobject A object Bobject B object Cobject C
myObjmyObj
QN1----------------
QN prevQN next
Object elem
QN1----------------
QN prevQN next
Object elem
QN2----------------
QN prevQN next
Object elem
QN2----------------
QN prevQN next
Object elem
QN3----------------
QN prevQN next
Object elem
QN3----------------
QN prevQN next
Object elem
headhead
......
object Aobject A object Bobject B object Cobject C
myObjmyObj
public class TestQueue { public static void main(String[] args) { Queue myQueue = new Queue();
for (int i=1; i<=100; i++) myQueue.enqueue(new Integer(i));
for (int i=1; i<=90; i++) System.out.println(myQueue.dequeue()); }}
public class Queue { ... public Object dequeue() { QueueNode tmp = head; head = tmp.getNextNode(); tmp.setNextNode(null); return tmp.getContainedObject(); }}
Test Program
missing code: head.setPreviousNode(null)
;
missing code: head.setPreviousNode(null)
;
Tool ComponentsTool Components
• Object life monitoring aspect▫ calculates D/C ratio
• Reference logging aspect▫ tracks unintended references
• Reference pattern analysis tool▫ extracts suspicious reference patterns
Reference Count:: 1Field: headNodeLocation: Queue.java, line 22
Reference Count:: 1Field: headNodeLocation: Queue.java, line 22
Reference Pattern Extraction
Reference Count:: 1Field: tailNodeLocation: Queue.java, line 46
Reference Count:: 1Field: tailNodeLocation: Queue.java, line 46
Reference Count:: 99Field: prevLocation: QueueNode.java, line 16
Reference Count:: 99Field: prevLocation: QueueNode.java, line 16
testCases.QueueNodetestCases.QueueNode
testCases.QueuetestCases.Queue
Reference Count:: 9Field: nextLocation: QueueNode.java, line 42
Reference Count:: 9Field: nextLocation: QueueNode.java, line 42
What is a Memory Leak in Java
Unreachable
Reachable but not Live
Reachable and Live
Root Set
loitering objects
garbage
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