Eclipse Memory Analyzer (MAT) is a powerful tool for heap dump analysis. It has several great features to debug memory problems effectively. ‘Incoming references’ and ‘outgoing references’ is one such feature. In this article, let’s discuss ‘incoming references’, ‘outgoing references’ and difference between them. In Eclipse MAT, when you right-click on any object, you will see drop down menu. If you select ‘List Objects’ menu item, you will notice two options:
- ‘with outgoing references’
- ‘with incoming references’
As it’s easier to understand new concepts through examples, let’s learn ‘Incoming References,’ ‘Outgoing References’ and also through an example. Suppose your application’s source code looks like this:
publicclass A { private C c1 = C.getInstance();}publicclass B { private C c2 = C.getInstance(); }publicclass C { privatestatic C myC = newC(); publicstatic C getInstance() { returnmyC; } private D d1 = newD(); private E e1 = newE(); }publicclass D {}publicclass E {}publicclassSimpleExample { publicstaticvoid main (String argsp[]) throws Exception { A a = newA(); B b = newB(); }} Now if we are going draw the objects diagrammatically, for the above example application, it’s going to look like this:
Figure 1: Sample application’s objects references
- Object A and Object B are holding reference to Object C
- Object C is holding reference to Object D and Object E
Now in this sample project let’s study the ‘Incoming references’ and ‘Outgoing references’ of object C.
Incoming References of Object C
All the objects that holds references object C is called ‘incoming references’. In this example, Object C’s incoming references are object A, object B and class C.
To confirm this theory, we captured heap dump from the above sample application and uploaded to Eclipse MAT. Below are the ‘Incoming References’ reported by Eclipse MAT for Object C.
Figure 2: Incoming references of Object C
When you right-click on Object C in the ‘Dominator Tree’ and select ‘List Objects with incoming references’ option you will notice that Eclipse MAT generates the above report. You can notice Object A, Object B and class C reported as incoming references. Eclipse MAT also displays the variables used to reference Object C. You can see object A referencing object C using the variable ‘c1.’ Similarly, other variables used to reference object C are also reported.
Outgoing References of Object C
All the objects that Object C references is called ‘outgoing references’. In this example, Object C’s outgoing referenceareobject D, object E and class C.
Below is the ‘Outgoing References’ reported by Eclipse MAT for Object C.
Figure 3: Outgoing references of Object C
When you right click on Object C in the ‘Dominator Tree’ and select ‘List Objects’ with outgoing references’ option you will notice Eclipse MAT to generate above report. You can notice Object D, Object E and class C reported as incoming references. Eclipse MAT also displays the variables by Object C to reference other objects. You can see object C referencing object D using the variable ‘d1.’ Similarly, other variables used in object C are also reported.
We hope this article has clarified the differences between ‘Incoming references’ and ‘Outgoing references.’
Author bio: Ram Lakshmanan
Every single day, millions & millions of people in North America — bank, travel, and commerce — use the applications that Ram Lakshmanan has architected. Ram is an acclaimed speaker in major conferences on scalability, availability, and performance topics. Recently, he has founded a startup, which specializes in troubleshooting performance problems.
