Uncovering Memory Leaks Using NetBeans Profiler

Contributed by Jiri Sedlacek
This articles discusses how to use the NetBeans Profiler to locate memory leaks in a Java application.

What's a Memory Leak?

"Memory leak is a particular kind of unintentional memory consumption by a computer program  where the program fails to release memory when no longer needed" as 
Wikipedia says.  This can happen for any program written in any programming language and Java is no exception.

Many users think that the Java VM releases unused objects from memory automatically, but that's not always the case.  There are several scenarios when objects cannot be released and memory leaks occur.  One common scenario is when a single object with large
memory consumption may be unintentionally held in memory, for example a

Project

instance in NetBeans IDE when the project has already been closed.  Another common scenario is when small objects are constantly being added to a

Collection

over time but they are never removed from that

Collection

, for example caching coordinates in a graphics editor or a game.

How Can I Detect a Memory Leak?

In the first scenario, when a single (or few) object remains in memory after some  action, memory leak detection is typically done by comparing memory snapshots taken before the action to snapshots taken after the action and then  searching for unnecessary
references to that object(s) in a heap walker tool.

This article will cover the second scenario, when there are many small objects that are continuously accumulating without being released. This type of leak can be more difficult to detect because typically it's not related to any concrete action and does
not use a noticeable amount of memory until the program has run for a long time.  This scenario is dangerous especially with Java EE applications that are expected to run  for long periods of time.  After some time the leak starts to consume memory and may
slow down performance of the  application or cause an

OutOfMemoryError

crash without any visible cause.

Even if your application seems to be working without any problems, before releasing your application it's always a good idea  to verify that your application does not contain any potential memory leaks.  For this purpose the NetBeans Profiler offers you
special metrics which can reliably uncover  large amounts of continuously leaking small objects - the
Surviving Generations metrics.  You can start the application in Monitoring mode without any profiling overhead and  watch the graph to see if the application leaks over time.

What Do The Surviving Generations Metrics Mean?

Surviving Generations metrics cannot be easily defined with just one line, so let's make a three-line definition:

a Generation is a set of instances created within the same GC interval (between two garbage collections)
a Surviving Generation is a Generation that survives at least one garbage collection. The number of   survived garbage collections - the generation's age - is its unique identifier
Surviving Generations (metrics) value is the number of different Surviving Generations   that are currently alive on the heap (number of Generations with different generation ages)
Typically there are several long-lived objects (like an application's main

JFrame

etc.) in an application.  The generation's age increases during the application's run time but still represents one or a few Surviving Generations.

Most applications also have short-lived objects which are created very frequently  (such as

Dimension

etc.) but these objects are released very soon, typically within only a few garbage collections.  This means that they represent only a few Surviving Generations (with generation's age of 1, 2, 3 etc.).

If we merge the two above cases, the total number of Surviving Generations in typical applications is  quite stable. You may have long-lived objects representing, for example, 3 Surviving Generations, and short-lived  objects representing, for example, 5
Surviving Generations, which means there will be about 8 Surviving Generations during the application's runtime.

Of course, in some application, for example a Swing application, dialogs and other components are created  during run time and as a result the number of Surviving Generations grows a bit.  But after some period of time the number of Surviving Generations
should become stable  because all long-lived objects have already been created and newly-created  short-lived objects are periodically being released from the heap.

However, if there is a memory leak in an application which prevents newly-created  objects from being released from the heap (for example objects stored in a

Collection

and never removed), the number of Surviving Generations grows. Why? Because between every two  garbage collections a new generation (of leaking objects) is created and it survives each  successive garbage collection. During run time
the number of Surviving Generations whose  generation's age differ by one unit increases, and that's exactly what the Surviving Generations  metrics is able to detect regardless of how much memory is wasted by such a leak.  That's why using the Surviving Generations
metrics can help you discover a memory leak much  sooner, before it consumes the entire heap and causes an

OutOfMemoryError

.

How To Do It With NetBeans Profiler?

The NetBeans Profiler is included as part of the regular installation of current versions of NetBeans IDE.    If you are using an older version of the IDE that does not include all the profiler features (NetBeans IDE 6.1 or earlier)    you might need to
download and install the    
NetBeans Profiler installer package.

Discovering the Leak

Download and install
NetBeans IDE.
Open the project that you want to check for possible memory leaks.
Choose Profile > Profile Project from the main menu to start profiling the project.      
Alternatively, choose Attach Profiler if you want to attach the NetBeans Profiler to an external application.

Choose the Monitor mode and click Run. (You don't need to monitor application's threads.)