How to find a Java Memory Leak

Question

How do you find a memory leak in Java  using  for example  JHat   I have tried to load the heap dump up in JHat to take a basic look  However  I do not understand how I am supposed to be able to find the root reference  ref  or whatever it is called  Basically  I can tell that there are several hundred megabytes of hash table entries   java util HashMap Entry or something like that   but maps are used all over the place    Is there some way to search for large maps  or perhaps find general roots of large object trees    Edit  Ok  I ve read the answers so far but let s just say I am a cheap bastard  meaning I am more interested in learning how to use JHat than to pay for JProfiler   Also  JHat is always available since it is part of the JDK  Unless of course there is no way with JHat but brute force  but I can t believe that can be the case   Also  I do not think I will be able to actually modify  adding logging of all map sizes  and run it for long enough for me to notice the leak

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Well  there s always the low tech solution of adding logging of the size of your maps when you modify them  then search the logs for which maps are growing beyond a reasonable size

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A tool is a big help   However  there are times when you can t use a tool  the heap dump is so huge it crashes the tool  you are trying to troubleshoot a machine in some production environment to which you only have shell access  etc   In that case  it helps to know your way around the hprof dump file   Look for SITES BEGIN  This shows you what objects are using the most memory  But the objects aren t lumped together solely by type  each entry also includes a  trace  ID  You can then search for that  TRACE nnnn  to see the top few frames of the stack where the object was allocated  Often  once I see where the object is allocated  I find a bug and I m done  Also  note that you can control how many frames are recorded in the stack with the options to -Xrunhprof   If you check out the allocation site  and don t see anything wrong  you have to start backward chaining from some of those live objects to root objects  to find the unexpected reference chain  This is where a tool really helps  but you can do the same thing by hand  well  with grep   There is not just one root object  i e   object not subject to garbage collection   Threads  classes  and stack frames act as root objects  and anything they reference strongly is not collectible   To do the chaining  look in the HEAP DUMP section for entries with the bad trace id  This will take you to an OBJ or ARR entry  which shows a unique object identifier in hexadecimal  Search for all occurrences of that id to find who s got a strong reference to the object  Follow each of those paths backward as they branch until you figure out where the leak is  See why a tool is so handy   Static members are a repeat offender for memory leaks  In fact  even without a tool  it d be worth spending a few minutes looking through your code for static Map members  Can a map grow large  Does anything ever clean up its entries

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I use following approach to finding memory leaks in Java  I ve used jProfiler with great success  but I believe that any specialized tool with graphing capabilities  diffs are easier to analyze in graphical form  will work    Start the application and wait until it get to  stable  state  when all the initialization is complete and the application is idle  Run the operation suspected of producing a memory leak several times to allow any cache  DB-related initialization to take place  Run GC and take memory snapshot  Run the operation again  Depending on the complexity of operation and sizes of data that is processed operation may need to be run several to many times  Run GC and take memory snapshot  Run a diff for 2 snapshots and analyze it    Basically analysis should start from greatest positive diff by  say  object types and find what causes those extra objects to stick in memory   For web applications that process requests in several threads analysis gets more complicated  but nevertheless general approach still applies   I did quite a number of projects specifically aimed at reducing memory footprint of the applications and this general approach with some application specific tweaks and trick always worked well

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There are tools that should help you find your leak  like JProbe  YourKit  AD4J or JRockit Mission Control  The last is the one that I personally know best  Any good tool should let you drill down to a level where you can easily identify what leaks  and where the leaking objects are allocated    Using HashTables  Hashmaps or similar is one of the few ways that you can acually leak memory in Java at all  If I had to find the leak by hand I would peridically print the size of my HashMaps  and from there find the one where I add items and forget to delete them

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As most of us use Eclipse already for writing code  Why not use the Memory Analyser Tool MAT  in Eclipse  It works great   The Eclipse MAT is a set of plug-ins for the Eclipse IDE which provides tools to analyze heap dumps from Java application and to identify memory problems in the application    This helps the developer to find memory leaks with the following features   Acquiring a memory snapshot  Heap Dump  Histogram Retained Heap Dominator Tree Exploring Paths to the GC Roots Inspector Common Memory Anti-Patterns Object Query Language

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You really need to use a memory profiler that tracks allocations  Take a look at JProfiler - their  heap walker  feature is great  and they have integration with all of the major Java IDEs  It s not free  but it isn t that expensive either   499 for a single license  - you will burn  500 worth of time pretty quickly struggling to find a leak with less sophisticated tools

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NetBeans has a built-in profiler

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Questioner here  I have got to say getting a tool that does not take 5 minutes to answer any click makes it a lot easier to find potential memory leaks   Since people are suggesting several tools   I only tried visual wm since I got that in the JDK and JProbe trial   I though I should suggest a free   open source tool built on the Eclipse platform  the Memory Analyzer  sometimes referenced as the SAP memory analyzer  available on http   www eclipse org mat      What is really cool about this tool is that it indexed the heap dump when I first opened it which allowed it to show data like retained heap without waiting 5 minutes for each object  pretty much all operations were tons faster than the other tools I tried    When you open the dump  the first screen shows you a pie chart with the biggest objects  counting retained heap  and one can quickly navigate down to the objects that are to big for comfort  It also has a Find likely leak suspects which I reccon can come in handy  but since the navigation was enough for me I did not really get into it

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You can find out by measuring memory usage size after calling garbage collector multiple times   Runtime runtime   Runtime getRuntime     while true                if System currentTimeMillis     4000    0           System gc            float usage    float   runtime totalMemory   - runtime freeMemory      1024   1024          System out println  Used memory      usage    Mb               If the output numbers were equal  there is no memory leak in your application  but if you saw difference between the numbers of memory usage  increasing numbers   there is memory leak in your project  For example   Used memory  14 603279Mb Used memory  14 737213Mb Used memory  14 772224Mb Used memory  14 802681Mb Used memory  14 840599Mb Used memory  14 900841Mb Used memory  14 942261Mb Used memory  14 976143Mb   Note that sometimes it takes some time to release memory by some actions like streams and sockets  You should not judge by first outputs  You should test it in a specific amount of time

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Most of the time  in enterprise applications the Java heap given is larger than the ideal size of max 12 to 16 GB  I have found it hard to make the NetBeans profiler work directly on these big java apps   But usually this is not needed  You can use the jmap utility that comes with the jdk to take a  live  heap dump   that is jmap will dump the heap after running GC   Do some operation on the application  wait till the operation is completed  then take another  live  heap dump  Use tools like Eclipse MAT to load the heapdumps  sort on the histogram  see which objects have increased  or which are the highest  This would give a clue   su  proceeuser  bin jmap -dump live format b file  tmp 2930javaheap hrpof 2930 pid of process    There is only one problem with this approach  Huge heap dumps  even with the live option  may be too big to transfer out to development lap  and may need a machine with enough memory RAM to open   That is where the class histogram comes into picture  You can dump a live class histogram with the jmap tool  This will give only the class histogram of memory usage Basically it won t have the information to chain the reference  For example it may put char array at the top  And String class somewhere below  You have to draw the connection yourself    jdk jdk1 6 0 38 bin jmap -histo live 60030  gt   tmp 60030istolive1330 txt   Instead of taking two heap dumps  take two class histograms  like as described above  Then compare the class histograms and see the classes that are increasing  See if you can relate the Java classes to your application classes  This will give a pretty good hint  Here is a pythons script that can help you compare two  jmap histogram dumps  histogramparser py  Finally tools like JConolse and VisualVm are essential to see the memory growth over time  and see if there is a memory leak  Finally sometimes your problem may not be a memory leak   but high memory usage For this enable GC logging use a more advanced and new compacting GC like G1GC  and you can use jdk tools like jstat to see the GC behaviour live  jstat -gccause pid  lt optional time interval gt    Other referecences to google for -jhat  jmap  Full GC  Humongous allocation  G1GC

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Checkout this screen cast about finding memory leaks with JProfiler  It s visual explanation of  Dima Malenko Answer   Note  Though JProfiler is not freeware  But Trial version can deal with current situation

[java] How to find a Java Memory Leak

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