memory-leaks interview questions
Top memory-leaks frequently asked interview questions
Recently I ran into this error in my web application:
java.lang.OutOfMemoryError: PermGen space
It's a typical Hibernate/JPA + IceFaces/JSF application running on Tomcat 6 and JDK 1.6.
Apparently this can occur after redeploying an application a few times.
What causes it and what can be done to avoid it?
How do I fix the problem?
Source: (StackOverflow)
I just had an interview, and I was asked to create a memory leak with Java.
Needless to say I felt pretty dumb having no clue on how to even start creating one.
What would an example be?
Source: (StackOverflow)
What is this error, and why does it happen?
05-17 18:24:57.069: ERROR/WindowManager(18850): Activity com.mypkg.myP has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@44c46ff0 that was originally added here
05-17 18:24:57.069: ERROR/WindowManager(18850): android.view.WindowLeaked: Activity ccom.mypkg.myP has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@44c46ff0 that was originally added here
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.view.ViewRoot.<init>(ViewRoot.java:231)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:148)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.view.Window$LocalWindowManager.addView(Window.java:424)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.Dialog.show(Dialog.java:239)
05-17 18:24:57.069: ERROR/WindowManager(18850): at com.mypkg.myP$PreparePairingLinkageData.onPreExecute(viewP.java:183)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.os.AsyncTask.execute(AsyncTask.java:391)
05-17 18:24:57.069: ERROR/WindowManager(18850): at com.mypkg.myP.onCreate(viewP.java:94)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2544)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2621)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.ActivityThread.access$2200(ActivityThread.java:126)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1932)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.os.Handler.dispatchMessage(Handler.java:99)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.os.Looper.loop(Looper.java:123)
05-17 18:24:57.069: ERROR/WindowManager(18850): at android.app.ActivityThread.main(ActivityThread.java:4595)
05-17 18:24:57.069: ERROR/WindowManager(18850): at java.lang.reflect.Method.invokeNative(Native Method)
05-17 18:24:57.069: ERROR/WindowManager(18850): at java.lang.reflect.Method.invoke(Method.java:521)
05-17 18:24:57.069: ERROR/WindowManager(18850): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
05-17 18:24:57.069: ERROR/WindowManager(18850): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
05-17 18:24:57.069: ERROR/WindowManager(18850): at dalvik.system.NativeStart.main(Native Method)
Source: (StackOverflow)
I've got an Android app developed, and I'm at the point of a phone app development where everything seems to be working well and you want to declare victory and ship, but you know there just have to be some memory and resource leaks in there; and there's only 16mb of heap on the Android and its apparently surprisingly easy to leak in an Android app.
I've been looking around and so far have only been able to dig up info on 'hprof' and 'traceview' and neither gets a lot of favorable reviews.
What tools or methods have you come across or developed and care to share maybe in an OS project?
Source: (StackOverflow)
I am using the perfmon windows utility to debug memory leaks in a process.
Perfmon explanation:
Working Set- Working Set is the current size, in bytes, of the Working Set of this process. The Working Set is the set of memory pages touched recently by the threads in the process. If free memory in the computer is above a threshold, pages are left in the Working Set of a process even if they are not in use. When free memory falls below a threshold, pages are trimmed from Working Sets. If they are needed they will then be soft-faulted back into the Working Set before leaving main memory.
Virtual Bytes- Virtual Bytes is the current size, in bytes, of the virtual address space the process is using. Use of virtual address space does not necessarily imply corresponding use of either disk or main memory pages. Virtual space is finite, and the process can limit its ability to load libraries.
Private Bytes- Private Bytes is the current size, in bytes, of memory that this process has allocated that cannot be shared with other processes.
Q1. Is it the Private Bytes which I should measure to be sure if the process is having any leaks as it does not involve any shared libraries and any leaks if happening will come from the process itself?
Q2. What is the total memory consumed by the process? Is it the Virtual Bytes or is it the sum of Virtual Bytes and Working Set
Q3. Is there any relation between Private Bytes, Working Set and Virtual Bytes.
Q4. Any tool which gives a better idea memory information?
Source: (StackOverflow)
I'm developing an Android 2.3.3 application with a service. I have this inside that service to communicate with Main activity:
public class UDPListenerService extends Service
{
private static final String TAG = "UDPListenerService";
//private ThreadGroup myThreads = new ThreadGroup("UDPListenerServiceWorker");
private UDPListenerThread myThread;
/**
* Handler to communicate from WorkerThread to service.
*/
private Handler mServiceHandler;
// Used to receive messages from the Activity
final Messenger inMessenger = new Messenger(new IncomingHandler());
// Use to send message to the Activity
private Messenger outMessenger;
class IncomingHandler extends Handler
{
@Override
public void handleMessage(Message msg)
{
}
}
/**
* Target we publish for clients to send messages to Incoming Handler.
*/
final Messenger mMessenger = new Messenger(new IncomingHandler());
[ ... ]
}
And here, final Messenger mMessenger = new Messenger(new IncomingHandler());, I get the following Lint warning:
This Handler class should be static or leaks might occur: IncomingHandler
What does it mean?
Source: (StackOverflow)
Is it ever acceptable to have a memory leak in your C or C++ application?
What if you allocate some memory and use it until the very last line of code in your application (for example, a global object's destructor)? As long as the memory consumption doesn't grow over time, is it OK to trust the OS to free your memory for you when your application terminates (on Windows, Mac, and Linux)? Would you even consider this a real memory leak if the memory was being used continuously until it was freed by the OS.
What if a third party library forced this situation on you? Would refuse to use that third party library no matter how great it otherwise might be?
I only see one practical disadvantage, and that is that these benign leaks will show up with memory leak detection tools as false positives.
Source: (StackOverflow)
I developed an application that uses lots of images on Android.
The app runs once, fills the information on the screen (Layouts, Listviews, Textviews, ImageViews, etc) and user reads the information.
There is no animation, no special effects or anything that can fill the memory.
Sometimes the drawables can change. Some are android resources and some are files saved in a folder in the SDCARD.
Then the user quits (the onDestroy method is executed and app stays in memory by the VM ) and then at some point the user enters again.
Each time the user enters to the app, I can see the memory growing more and more until user gets the java.lang.OutOfMemoryError.
So what is the best/correct way to handle many images?
Should I put them in static methods so they are not loaded all the time?
Do I have to clean the layout or the images used in the layout in a special way?
Source: (StackOverflow)
I was answering a question about the possibility of closures (legitimately) extending object-lifetimes when I ran into some extremely curious code-gen on the part of the C# compiler (4.0 if that matters).
The shortest repro I can find is the following:
- Create a lambda that captures a local while calling a static method of the containing type.
- Assign the generated delegate-reference to an instance field of the containing object.
Result: The compiler creates a closure-object that references the object that created the lambda, when it has no reason to - the 'inner' target of the delegate is a static method, and the lambda-creating-object's instance members needn't be (and aren't) touched when the delegate is executed. Effectively, the compiler is acting like the programmer has captured this without reason.
class Foo
{
private Action _field;
public void InstanceMethod()
{
var capturedVariable = Math.Pow(42, 1);
_field = () => StaticMethod(capturedVariable);
}
private static void StaticMethod(double arg) { }
}
The generated code from a release build (decompiled to 'simpler' C#) looks like this:
public void InstanceMethod()
{
<>c__DisplayClass1 CS$<>8__locals2 = new <>c__DisplayClass1();
CS$<>8__locals2.<>4__this = this; // What's this doing here?
CS$<>8__locals2.capturedVariable = Math.Pow(42.0, 1.0);
this._field = new Action(CS$<>8__locals2.<InstanceMethod>b__0);
}
[CompilerGenerated]
private sealed class <>c__DisplayClass1
{
// Fields
public Foo <>4__this; // Never read, only written to.
public double capturedVariable;
// Methods
public void <InstanceMethod>b__0()
{
Foo.StaticMethod(this.capturedVariable);
}
}
Observe that <>4__this field of the closure object is populated with an object reference but is never read from (there is no reason).
So what's going on here? Does the language-specification allow for it? Is this a compiler bug / oddity or is there a good reason (that I'm clearly missing) for the closure to reference the object? This makes me anxious because this looks like a recipe for closure-happy programmers (like me) to unwittingly introduce strange memory-leaks (imagine if the delegate were used as an event-handler) into programs.
Source: (StackOverflow)
In .NET perspective:
- What is a Memory Leak?
- How can you determine whether your application leaks? What are the effects?
- How can you prevent a memory leak?
- If your application has memory leak, does it go away when the process exits or is killed? Or do memory leaks in your application affect other processes on the system even after process completion?
- And what about unmanaged code accessed via COM Interop and/or P/Invoke?
I have some answers for these questions myself, but they are incomplete. What do you think?
Source: (StackOverflow)
I've run into memory leaks many times. Usually when I'm malloc-ing like there's no tomorrow, or dangling FILE *s like dirty laundry. I generally assume (read: hope desperately) that all memory is cleaned up at least when the program terminates. Are there any situations where leaked memory won't be collected when the program terminates, or crashes?
If the answer varies widely from language-to-language, then let's focus on C(++).
Please note hyperbolic usage of the phrase, 'like there's no tomorrow', and 'dangling ... like dirty laundry'. Unsafe* malloc*ing can hurt the ones you love. Also, please use caution with dirty laundry.
Source: (StackOverflow)
I learned C# first, and now I'm starting with C++. As I understand, operator new in C++ is not similar to the one in C#.
Can you explain the reason of the memory leak in this sample code?
class A { ... };
struct B { ... };
A *object1 = new A();
B object2 = *(new B());
Source: (StackOverflow)
I was told I can add the -XX:+HeapDumpOnOutOfMemoryError parameter to my JVM start up options to my JBoss start up script to get a heap dump when we get an out of memory error in our application. I was wondering where this data gets dumped? Is it just to the console, or to some log file? If it's just to the console, what if I'm not logged into the unix server through the console?
Thanks
Source: (StackOverflow)
I've created a very simple test case that creates a Backbone view, attaches a handler to an event, and instantiates a user-defined class. I believe that by clicking the "Remove" button in this sample, everything will be cleaned up and there should be no memory leaks.
A jsfiddle for the code is here: http://jsfiddle.net/4QhR2/
// scope everything to a function
function main() {
function MyWrapper() {
this.element = null;
}
MyWrapper.prototype.set = function(elem) {
this.element = elem;
}
MyWrapper.prototype.get = function() {
return this.element;
}
var MyView = Backbone.View.extend({
tagName : "div",
id : "view",
events : {
"click #button" : "onButton",
},
initialize : function(options) {
// done for demo purposes only, should be using templates
this.html_text = "<input type='text' id='textbox' /><button id='button'>Remove</button>";
this.listenTo(this,"all",function(){console.log("Event: "+arguments[0]);});
},
render : function() {
this.$el.html(this.html_text);
this.wrapper = new MyWrapper();
this.wrapper.set(this.$("#textbox"));
this.wrapper.get().val("placeholder");
return this;
},
onButton : function() {
// assume this gets .remove() called on subviews (if they existed)
this.trigger("cleanup");
this.remove();
}
});
var view = new MyView();
$("#content").append(view.render().el);
}
main();
However, I am unclear how to use Google Chrome's profiler to verify that this is, in fact, the case. There are a gazillion things that show up on the heap profiler snapshot, and I have no idea how to decode what's good/bad. The tutorials I've seen on it so far either just tell me to "use the snapshot profiler" or give me a hugely detailed manifesto on how the entire profiler works. Is it possible to just use the profiler as a tool, or do I really have to understand how the whole thing was engineered?
EDIT: Tutorials like these:
Gmail memory leak fixing
Using DevTools
Are representative of some of the stronger material out there, from what I've seen. However, beyond introducing the concept of the 3 Snapshot Technique, I find they offer very little in terms of practical knowledge (for a beginner like me). The 'Using DevTools' tutorial doesn't work through a real example, so its vague and general conceptual description of things aren't overly helpful. As for the 'Gmail' example:
So you found a leak. Now what?
Examine the retaining path of leaked objects in the lower half of the Profiles panel
If the allocation site cannot be easily inferred (i.e. event listeners):
Instrument the constructor of the retaining object via the JS console to save the stack trace for allocations
Using Closure? Enable the appropriate existing flag (i.e. goog.events.Listener.ENABLE_MONITORING) to set the creationStack property during construction
I find myself more confused after reading that, not less. And, again, it's just telling me to do things, not how to do them. From my perspective, all of the information out there is either too vague or would only make sense to someone who already understood the process.
Some of these more specific issues have been raised in @Jonathan Naguin's answer below.
Source: (StackOverflow)