In the following example there are two functionally equivalent methods:
public class Question {
public static String method1() {
String s = new String("s1");
// some operations on s1
s = new String("s2");
return s;
}
public static String method2() {
final String s1 = new String("s1");
// some operations on s1
final String s2 = new String("s2");
return s2;
}
}
however in first(method1) of them string "s1" is clearly available for garbage collection before return statement. In second(method2) string "s1" is still reachable (though from code review prospective it's not used anymore).
My question is - is there anything in jvm spec which says that once variable is unused down the stack it could be available for garbage collection?
EDIT:
Sometimes variables can refer to object like fully rendered image and that have impact on memory.
I'm asking because of practical considerations. I have large chunk of memory-greedy code in one method and thinking if I could help JVM (a bit) just by splitting this method into few small ones.
I really prefer code where no reassignment is done since it's easier to read and reason about.
UPDATE: per jls-12.6.1:
Java compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner
So it looks like it's possible for GC to claim object which still visible. I doubt, however that this optimisation is done during offline compilation (it would screw up debugging) and most likely will be done by JIT.
No, because your code could conceivably retrieve it and do something with it, and the abstract JVM does not consider what code is coming ahead. However, a very, very, very clever optimizing JVM might analyze the code ahead and find that there is no way s1 could ever be referenced, and garbage collect it. You definitely can't count on this, though.
If you're talking about the interpreter, then in the second case S1 remains "referenced" until the method exits and the stack frame is rolled up. (That is, in the standard interpreter -- it's entirely possible for GC to use liveness info from method verification. And, in addition (and more likely), javac may do its own liveness analysis and "share" interpreter slots based on that.)
In the case of the JITC, however, an even mildly optimizing one might recognize that S1 is unused and recycle that register for S2. Or it might not. The GC will examine register contents, and if S1 has been reused for something else then the old S1 object will be reclaimed (if not otherwise referenced). If the S1 location has not been reused then the S1 object might not be reclaimed.
"Might not" because, depending on the JVM, the JITC may or may not provide the GC with a map of where object references are "live" in the program flow. And this map, if provided, may or may not precisely identify the end of the "live range" (the last point of reference) of S1. Many different possibilities.
Note that this potential variability does not violate any Java principles -- GC is not required to reclaim an object at the earliest possible opportunity, and there's no practical way for a program to be sensitive to precisely when an object is reclaimed.
VM is free to optimized the code to nullify s1 before method exit (as long as it's correct), so s1 might be eligible for garbage earlier.
However that is hardly necessary. Many method invocations must have happened before the next GC; all the stack frames have been cleared anyway, no need to worry about a specific local variable in a specific method invocation.
As far as Java the language is concerned, garbages can live forever without impact program semantics. That's why JLS hardly talks about garbage at all.
in first of them string "s1" is clearly available for garbage collection before return statement
It isn't clear at all. I think you are confusing 'unused' with 'unreachable'. They aren't necessarily the same thing.
Formally speaking the variable is live until its enclosing scope terminates, so it isn't available for garbage collection until then.
However "a Java compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner" JLS #12.6.1.
Basically stack frames and static area are considered as roots by GC. So if object is referenced from any stack frame its considered alive. The problem with reclaiming some objects from active stack frame is that GC works in parallel with application(mutator). How do you think GC should find out that object is unused while method is in progress? That would require a synchronization which would be VERY heavy and complex, in fact this will break the idea of GC to work in parallel with mutator. Every thread might keep variables in processor registers. To implement your logic, they should also be added to GC roots. I cant even imagine how to implement it.
To answer you question. If you have any logic which produces a lot of objects which are unused in the future, separate it to a distinct method. This is actually a good practice.
You should also take int account optimizations by JVM(like EJP pointed out). There is also an escape analysis, which might prevent object from heap allocation at all. But rely your codes performance on them is a bad practice
In some library found on google code I came across this util method:
public static void gc(){
Object obj = new Object();
WeakReference ref = new WeakReference<Object>(obj);
obj = null;
while(ref.get()!=null)
System.gc();
}
Its doc says it provides a reliable way to call GC, because calling System#gc() is just a hint without any guarantees. I showed it my senior, and he said I should think about why this method is invalid.
I read some articles on weak references but I'm still confused.
Can somebody show me the point?
I have direct experience with the supposed "safe GC" idiom you have posted.
It doesn't work.
The reason is quite simple: just the fact that a weak ref is cleared is not a signal that the object has been collected; it only means that it has become unreachable through any strong or soft reference. In my experience this signal arrives before the object is reclaimed.
A better attempt would be to use a Phantom reference, which at least ensures that the object has entered the finalizable state, but once again, it can still be occupying the heap, and in practice it still is occupying it. Another explanation could be that this particular object, obviously residing in Eden space, did get reclaimed, but the GC run which did it was not exhaustive and there is more memory to reclaim in other generations.
On the other hand, I have very reliably used this trivial idiom:
for (int i = 0; i < 3; i++) { System.gc(); Thread.sleep(500); }
Try it and see if it works for you. The sleep part is optional: it is needed only if System.gc() uses concurrent sweeping.
If you object to the apparent fickleness of this approach, just remember that any approach to explicit GC-ing is fickle. This one is at least honest about it—and just happens to work on actual systems. It is, naturally, non-portable and can cease to work at any time for a large array of reasons. Even so, it is the best you'll ever get.
The point is, that System.gc() does not need to clean up all weak references. And consider some Java virtual machines too. If System.gc for once (the first time) does decide to not clean that reference, it is very likely to the next call. Hence you have a possibly infinite loop. Probably depending on other threads changing the state for the garbage collection to terminate the loop.
So: once is enough.
There is no way to guarantee a GC call because just as the documentation says System.gc is just a hint that can be ignored by the system.
So assume that the JVM ignores System.gc - in that case the whole thing just loops until some other part of the systems causes a GC. If you run single-threaded or nobody else allocates much memory you basically create an inifite loop here.
The point is that your thread will stop and wait until the weak reference is cleared, thus "simulating" garbage collection. There's no guarantee when (or indeed even IF) this will actually happen.
You could be stuck waiting on this while for a long, long time.
Programmatic we need to ensure that, when a obj is removed then its corresponding entry should be removed. Only then, that object becomes a candidate for garbage collection. Otherwise, even though it is not used at run-time, this stale object will not be garbage collected.
The object to which this reference refers, or null if this reference object has been cleared.
As your object reference of WeakReference class. so it will not give null. but after removing gc., it provide null.
Object obj = new Object();
WeakReference ref = new WeakReference<Object>(obj);
obj = null;
if(ref.get()!=null)
{
System.gc();
System.out.println("remove ref");
}
if(ref.get()!=null){
System.out.println("not execute");
}
Output:
remove ref
Don't assign null value to obj.
Object obj = new Object();
WeakReference ref = new WeakReference<Object>(obj);
if(ref.get()!=null)
{
System.gc();
System.out.println("remove ref");
}
if(ref.get()!=null){
System.out.println("execute");
}
Output:
remove ref
execute
Code that tries to force GC is usually a sign for an underlying bigger problem (i.e. design issue or missing knowledge on the developers part).
I have seen a few use cases where calling System.gc() in production code actually makes sense, for example, before printing the current memory usage - it doesn't matter if the values are off but we'd like to improve chances the values are as small as possible. Of course, we knew that GC was enabled - we used this to automatically detect memory leaks on a QA system.
In general, calling System.gc() yells "my code is buggy and I don't know how to fix it!".
I just had an interview where I was asked to create a memory leak with Java.
Needless to say, I felt pretty dumb, having no idea how to start creating one.
What would an example be?
Here's a good way to create a true memory leak (objects inaccessible by running code but still stored in memory) in pure Java:
The application creates a long-running thread (or use a thread pool to leak even faster).
The thread loads a class via an (optionally custom) ClassLoader.
The class allocates a large chunk of memory (e.g. new byte[1000000]), stores a strong reference to it in a static field, and then stores a reference to itself in a ThreadLocal. Allocating the extra memory is optional (leaking the class instance is enough), but it will make the leak work that much faster.
The application clears all references to the custom class or the ClassLoader it was loaded from.
Repeat.
Due to the way ThreadLocal is implemented in Oracle's JDK, this creates a memory leak:
Each Thread has a private field threadLocals, which actually stores the thread-local values.
Each key in this map is a weak reference to a ThreadLocal object, so after that ThreadLocal object is garbage-collected, its entry is removed from the map.
But each value is a strong reference, so when a value (directly or indirectly) points to the ThreadLocal object that is its key, that object will neither be garbage-collected nor removed from the map as long as the thread lives.
In this example, the chain of strong references looks like this:
Thread object → threadLocals map → instance of example class → example class → static ThreadLocal field → ThreadLocal object.
(The ClassLoader doesn't really play a role in creating the leak, it just makes the leak worse because of this additional reference chain: example class → ClassLoader → all the classes it has loaded. It was even worse in many JVM implementations, especially prior to Java 7, because classes and ClassLoaders were allocated straight into permgen and were never garbage-collected at all.)
A variation on this pattern is why application containers (like Tomcat) can leak memory like a sieve if you frequently redeploy applications which happen to use ThreadLocals that in some way point back to themselves. This can happen for a number of subtle reasons and is often hard to debug and/or fix.
Update: Since lots of people keep asking for it, here's some example code that shows this behavior in action.
Static field holding an object reference [especially a final field]
class MemorableClass {
static final ArrayList list = new ArrayList(100);
}
(Unclosed) open streams (file , network, etc.)
try {
BufferedReader br = new BufferedReader(new FileReader(inputFile));
...
...
} catch (Exception e) {
e.printStackTrace();
}
Unclosed connections
try {
Connection conn = ConnectionFactory.getConnection();
...
...
} catch (Exception e) {
e.printStackTrace();
}
Areas that are unreachable from JVM's garbage collector, such as memory allocated through native methods.
In web applications, some objects are stored in application scope until the application is explicitly stopped or removed.
getServletContext().setAttribute("SOME_MAP", map);
Incorrect or inappropriate JVM options, such as the noclassgc option on IBM JDK that prevents unused class garbage collection
See IBM JDK settings.
A simple thing to do is to use a HashSet with an incorrect (or non-existent) hashCode() or equals(), and then keep adding "duplicates". Instead of ignoring duplicates as it should, the set will only ever grow and you won't be able to remove them.
If you want these bad keys/elements to hang around you can use a static field like
class BadKey {
// no hashCode or equals();
public final String key;
public BadKey(String key) { this.key = key; }
}
Map map = System.getProperties();
map.put(new BadKey("key"), "value"); // Memory leak even if your threads die.
Below there will be a non-obvious case where Java leaks, besides the standard case of forgotten listeners, static references, bogus/modifiable keys in hashmaps, or just threads stuck without any chance to end their life-cycle.
File.deleteOnExit() - always leaks the string, if the string is a substring, the leak is even worse (the underlying char[] is also leaked) - in Java 7 substring also copies the char[], so the later doesn't apply; #Daniel, no needs for votes, though.
I'll concentrate on threads to show the danger of unmanaged threads mostly, don't wish to even touch swing.
Runtime.addShutdownHook and not remove... and then even with removeShutdownHook due to a bug in ThreadGroup class regarding unstarted threads it may not get collected, effectively leak the ThreadGroup. JGroup has the leak in GossipRouter.
Creating, but not starting, a Thread goes into the same category as above.
Creating a thread inherits the ContextClassLoader and AccessControlContext, plus the ThreadGroup and any InheritedThreadLocal, all those references are potential leaks, along with the entire classes loaded by the classloader and all static references, and ja-ja. The effect is especially visible with the entire j.u.c.Executor framework that features a super simple ThreadFactory interface, yet most developers have no clue of the lurking danger. Also a lot of libraries do start threads upon request (way too many industry popular libraries).
ThreadLocal caches; those are evil in many cases. I am sure everyone has seen quite a bit of simple caches based on ThreadLocal, well the bad news: if the thread keeps going more than expected the life the context ClassLoader, it is a pure nice little leak. Do not use ThreadLocal caches unless really needed.
Calling ThreadGroup.destroy() when the ThreadGroup has no threads itself, but it still keeps child ThreadGroups. A bad leak that will prevent the ThreadGroup to remove from its parent, but all the children become un-enumerateable.
Using WeakHashMap and the value (in)directly references the key. This is a hard one to find without a heap dump. That applies to all extended Weak/SoftReference that might keep a hard reference back to the guarded object.
Using java.net.URL with the HTTP(S) protocol and loading the resource from(!). This one is special, the KeepAliveCache creates a new thread in the system ThreadGroup which leaks the current thread's context classloader. The thread is created upon the first request when no alive thread exists, so either you may get lucky or just leak. The leak is already fixed in Java 7 and the code that creates thread properly removes the context classloader. There are few more cases (like ImageFetcher, also fixed) of creating similar threads.
Using InflaterInputStream passing new java.util.zip.Inflater() in the constructor (PNGImageDecoder for instance) and not calling end() of the inflater. Well, if you pass in the constructor with just new, no chance... And yes, calling close() on the stream does not close the inflater if it's manually passed as constructor parameter. This is not a true leak since it'd be released by the finalizer... when it deems it necessary. Till that moment it eats native memory so badly it can cause Linux oom_killer to kill the process with impunity. The main issue is that finalization in Java is very unreliable and G1 made it worse till 7.0.2. Moral of the story: release native resources as soon as you can; the finalizer is just too poor.
The same case with java.util.zip.Deflater. This one is far worse since Deflater is memory hungry in Java, i.e. always uses 15 bits (max) and 8 memory levels (9 is max) allocating several hundreds KB of native memory. Fortunately, Deflater is not widely used and to my knowledge JDK contains no misuses. Always call end() if you manually create a Deflater or Inflater. The best part of the last two: you can't find them via normal profiling tools available.
(I can add some more time wasters I have encountered upon request.)
Good luck and stay safe; leaks are evil!
Most examples here are "too complex". They are edge cases. With these examples, the programmer made a mistake (like don't redefining equals/hashcode), or has been bitten by a corner case of the JVM/JAVA (load of class with static...). I think that's not the type of example an interviewer want or even the most common case.
But there are really simpler cases for memory leaks. The garbage collector only frees what is no longer referenced. We as Java developers don't care about memory. We allocate it when needed and let it be freed automatically. Fine.
But any long-lived application tend to have shared state. It can be anything, statics, singletons... Often non-trivial applications tend to make complex objects graphs. Just forgetting to set a reference to null or more often forgetting to remove one object from a collection is enough to make a memory leak.
Of course all sort of listeners (like UI listeners), caches, or any long-lived shared state tend to produce memory leak if not properly handled. What shall be understood is that this is not a Java corner case, or a problem with the garbage collector. It is a design problem. We design that we add a listener to a long-lived object, but we don't remove the listener when no longer needed. We cache objects, but we have no strategy to remove them from the cache.
We maybe have a complex graph that store the previous state that is needed by a computation. But the previous state is itself linked to the state before and so on.
Like we have to close SQL connections or files. We need to set proper references to null and remove elements from the collection. We shall have proper caching strategies (maximum memory size, number of elements, or timers). All objects that allow a listener to be notified must provide both a addListener and removeListener method. And when these notifiers are no longer used, they must clear their listener list.
A memory leak is indeed truly possible and is perfectly predictable. No need for special language features or corner cases. Memory leaks are either an indicator that something is maybe missing or even of design problems.
The answer depends entirely on what the interviewer thought they were asking.
Is it possible in practice to make Java leak? Of course it is, and there are plenty of examples in the other answers.
But there are multiple meta-questions that may have been being asked?
Is a theoretically "perfect" Java implementation vulnerable to leaks?
Does the candidate understand the difference between theory and reality?
Does the candidate understand how garbage collection works?
Or how garbage collection is supposed to work in an ideal case?
Do they know they can call other languages through native interfaces?
Do they know to leak memory in those other languages?
Does the candidate even know what memory management is, and what is going on behind the scene in Java?
I'm reading your meta-question as "What's an answer I could have used in this interview situation". And hence, I'm going to focus on interview skills instead of Java. I believe you're more likely to repeat the situation of not knowing the answer to a question in an interview than you are to be in a place of needing to know how to make Java leak. So, hopefully, this will help.
One of the most important skills you can develop for interviewing is learning to actively listen to the questions and working with the interviewer to extract their intent. Not only does this let you answer their question the way they want, but also shows that you have some vital communication skills. And when it comes down to a choice between many equally talented developers, I'll hire the one who listens, thinks, and understands before they respond every time.
The following is a pretty pointless example if you do not understand JDBC. Or at least how JDBC expects a developer to close Connection, Statement, and ResultSet instances before discarding them or losing references to them, instead of relying on implementing the finalize method.
void doWork() {
try {
Connection conn = ConnectionFactory.getConnection();
PreparedStatement stmt = conn.preparedStatement("some query");
// executes a valid query
ResultSet rs = stmt.executeQuery();
while(rs.hasNext()) {
// ... process the result set
}
} catch(SQLException sqlEx) {
log(sqlEx);
}
}
The problem with the above is that the Connection object is not closed, and hence the physical Connection will remain open until the garbage collector comes around and sees that it is unreachable. GC will invoke the finalize method, but there are JDBC drivers that do not implement the finalize, at least not in the same way that Connection.close is implemented. The resulting behavior is that while the JVM will reclaim memory due to unreachable objects being collected, resources (including memory) associated with the Connection object might not be reclaimed.
As such, Connection's final method does not clean up everything. One might find that the physical Connection to the database server will last several garbage collection cycles until the database server eventually figures out that the Connection is not alive (if it does) and should be closed.
Even if the JDBC driver implemented finalize, the compiler can throw exceptions during finalization. The resulting behavior is that any memory associated with the now "dormant" object will not be reclaimed by the compiler, as finalize is guaranteed to be invoked only once.
The above scenario of encountering exceptions during object finalization is related to another scenario that could lead to a memory leak - object resurrection. Object resurrection is often done intentionally by creating a strong reference to the object from being finalized, from another object. When object resurrection is misused it will lead to a memory leak in combination with other sources of memory leaks.
There are plenty more examples that you can conjure up - like
Managing a List instance where you are only adding to the list and not deleting from it (although you should be getting rid of elements you no longer need), or
Opening Sockets or Files, but not closing them when they are no longer needed (similar to the above example involving the Connection class).
Not unloading Singletons when bringing down a Java EE application. The Classloader that loaded the singleton class will retain a reference to the class, and hence the singleton instance will never be collected by the JVM. When a new instance of the application is deployed, a new class loader is usually created, and the former class loader will continue to exist due to the singleton.
Probably one of the simplest examples of a potential memory leak, and how to avoid it, is the implementation of ArrayList.remove(int):
public E remove(int index) {
RangeCheck(index);
modCount++;
E oldValue = (E) elementData[index];
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index,
numMoved);
elementData[--size] = null; // (!) Let gc do its work
return oldValue;
}
If you were implementing it yourself, would you have thought to clear the array element that is no longer used (elementData[--size] = null)? That reference might keep a huge object alive ...
Any time you keep references around to objects that you no longer need you have a memory leak. See Handling memory leaks in Java programs for examples of how memory leaks manifest themselves in Java and what you can do about it.
You are able to make memory leak with sun.misc.Unsafe class. In fact this service class is used in different standard classes (for example in java.nio classes). You can't create instances of this class directly, but you may use reflection to get an instance.
Code doesn't compile in the Eclipse IDE - compile it using command javac (during compilation you'll get warnings)
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import sun.misc.Unsafe;
public class TestUnsafe {
public static void main(String[] args) throws Exception{
Class unsafeClass = Class.forName("sun.misc.Unsafe");
Field f = unsafeClass.getDeclaredField("theUnsafe");
f.setAccessible(true);
Unsafe unsafe = (Unsafe) f.get(null);
System.out.print("4..3..2..1...");
try
{
for(;;)
unsafe.allocateMemory(1024*1024);
} catch(Error e) {
System.out.println("Boom :)");
e.printStackTrace();
}
}
}
I can copy my answer from here:
Easiest way to cause memory leak in Java
"A memory leak, in computer science (or leakage, in this context), occurs when a computer program consumes memory but is unable to release it back to the operating system." (Wikipedia)
The easy answer is: You can't. Java does automatic memory management and will free resources that are not needed for you. You can't stop this from happening. It will always be able to release the resources. In programs with manual memory management, this is different. You can get some memory in C using malloc(). To free the memory, you need the pointer that malloc returned and call free() on it. But if you don't have the pointer any more (overwritten, or lifetime exceeded), then you are unfortunately incapable of freeing this memory and thus you have a memory leak.
All the other answers so far are in my definition not really memory leaks. They all aim at filling the memory with pointless stuff real fast. But at any time you could still dereference the objects you created and thus freeing the memory --> no leak. acconrad's answer comes pretty close though as I have to admit since his solution is effectively to just "crash" the garbage collector by forcing it in an endless loop).
The long answer is: You can get a memory leak by writing a library for Java using the JNI, which can have manual memory management and thus have memory leaks. If you call this library, your Java process will leak memory. Or, you can have bugs in the JVM, so that the JVM looses memory. There are probably bugs in the JVM, there may even be some known ones since garbage collection is not that trivial, but then it's still a bug. By design this is not possible. You may be asking for some Java code that is effected by such a bug. Sorry I don't know one and it might well not be a bug any more in the next Java version anyway.
Here's a simple/sinister one via http://wiki.eclipse.org/Performance_Bloopers#String.substring.28.29.
public class StringLeaker
{
private final String muchSmallerString;
public StringLeaker()
{
// Imagine the whole Declaration of Independence here
String veryLongString = "We hold these truths to be self-evident...";
// The substring here maintains a reference to the internal char[]
// representation of the original string.
this.muchSmallerString = veryLongString.substring(0, 1);
}
}
Because the substring refers to the internal representation of the original, much longer string, the original stays in memory. Thus, as long as you have a StringLeaker in play, you have the whole original string in memory, too, even though you might think you're just holding on to a single-character string.
The way to avoid storing an unwanted reference to the original string is to do something like this:
...
this.muchSmallerString = new String(veryLongString.substring(0, 1));
...
For added badness, you might also .intern() the substring:
...
this.muchSmallerString = veryLongString.substring(0, 1).intern();
...
Doing so will keep both the original long string and the derived substring in memory even after the StringLeaker instance has been discarded.
A common example of this in GUI code is when creating a widget/component and adding a listener to some static/application scoped object and then not removing the listener when the widget is destroyed. Not only do you get a memory leak, but also a performance hit as when whatever you are listening to fires events, all your old listeners are called too.
Take any web application running in any servlet container (Tomcat, Jetty, GlassFish, whatever...). Redeploy the application 10 or 20 times in a row (it may be enough to simply touch the WAR in the server's autodeploy directory.
Unless anybody has actually tested this, chances are high that you'll get an OutOfMemoryError after a couple of redeployments, because the application did not take care to clean up after itself. You may even find a bug in your server with this test.
The problem is, the lifetime of the container is longer than the lifetime of your application. You have to make sure that all references the container might have to objects or classes of your application can be garbage collected.
If there is just one reference surviving the undeployment of your web application, the corresponding classloader and by consequence all classes of your web application cannot be garbage collected.
Threads started by your application, ThreadLocal variables, logging appenders are some of the usual suspects to cause classloader leaks.
Maybe by using external native code through JNI?
With pure Java, it is almost impossible.
But that is about a "standard" type of memory leak, when you cannot access the memory anymore, but it is still owned by the application. You can instead keep references to unused objects, or open streams without closing them afterwards.
I have had a nice "memory leak" in relation to PermGen and XML parsing once.
The XML parser we used (I can't remember which one it was) did a String.intern() on tag names, to make comparison faster.
One of our customers had the great idea to store data values not in XML attributes or text, but as tagnames, so we had a document like:
<data>
<1>bla</1>
<2>foo</>
...
</data>
In fact, they did not use numbers but longer textual IDs (around 20 characters), which were unique and came in at a rate of 10-15 million a day. That makes 200 MB of rubbish a day, which is never needed again, and never GCed (since it is in PermGen). We had permgen set to 512 MB, so it took around two days for the out-of-memory exception (OOME) to arrive...
The interviewer was probably looking for a circular reference like the code below (which incidentally only leak memory in very old JVMs that used reference counting, which isn't the case anymore). But it's a pretty vague question, so it's a prime opportunity to show off your understanding of JVM memory management.
class A {
B bRef;
}
class B {
A aRef;
}
public class Main {
public static void main(String args[]) {
A myA = new A();
B myB = new B();
myA.bRef = myB;
myB.aRef = myA;
myA=null;
myB=null;
/* at this point, there is no access to the myA and myB objects, */
/* even though both objects still have active references. */
} /* main */
}
Then you can explain that with reference counting, the above code would leak memory. But most modern JVMs don't use reference counting any longer. Most use a sweep garbage collector, which will in fact collect this memory.
Next, you might explain creating an Object that has an underlying native resource, like this:
public class Main {
public static void main(String args[]) {
Socket s = new Socket(InetAddress.getByName("google.com"),80);
s=null;
/* at this point, because you didn't close the socket properly, */
/* you have a leak of a native descriptor, which uses memory. */
}
}
Then you can explain this is technically a memory leak, but really the leak is caused by native code in the JVM allocating underlying native resources, which weren't freed by your Java code.
At the end of the day, with a modern JVM, you need to write some Java code that allocates a native resource outside the normal scope of the JVM's awareness.
What's a memory leak:
It's caused by a bug or bad design.
It's a waste of memory.
It gets worse over time.
The garbage collector cannot clean it.
Typical example:
A cache of objects is a good starting point to mess things up.
private static final Map<String, Info> myCache = new HashMap<>();
public void getInfo(String key)
{
// uses cache
Info info = myCache.get(key);
if (info != null) return info;
// if it's not in cache, then fetch it from the database
info = Database.fetch(key);
if (info == null) return null;
// and store it in the cache
myCache.put(key, info);
return info;
}
Your cache grows and grows. And pretty soon the entire database gets sucked into memory. A better design uses an LRUMap (Only keeps recently used objects in cache).
Sure, you can make things a lot more complicated:
using ThreadLocal constructions.
adding more complex reference trees.
or leaks caused by 3rd party libraries.
What often happens:
If this Info object has references to other objects, which again have references to other objects. In a way you could also consider this to be some kind of memory leak, (caused by bad design).
I thought it was interesting that no one used the internal class examples. If you have an internal class; it inherently maintains a reference to the containing class. Of course it is not technically a memory leak because Java WILL eventually clean it up; but this can cause classes to hang around longer than anticipated.
public class Example1 {
public Example2 getNewExample2() {
return this.new Example2();
}
public class Example2 {
public Example2() {}
}
}
Now if you call Example1 and get an Example2 discarding Example1, you will inherently still have a link to an Example1 object.
public class Referencer {
public static Example2 GetAnExample2() {
Example1 ex = new Example1();
return ex.getNewExample2();
}
public static void main(String[] args) {
Example2 ex = Referencer.GetAnExample2();
// As long as ex is reachable; Example1 will always remain in memory.
}
}
I've also heard a rumor that if you have a variable that exists for longer than a specific amount of time; Java assumes that it will always exist and will actually never try to clean it up if cannot be reached in code anymore. But that is completely unverified.
I recently encountered a memory leak situation caused in a way by log4j.
Log4j has this mechanism called Nested Diagnostic Context(NDC) which is an instrument to distinguish interleaved log output from different sources. The granularity at which NDC works is threads, so it distinguishes log outputs from different threads separately.
In order to store thread specific tags, log4j's NDC class uses a Hashtable which is keyed by the Thread object itself (as opposed to say the thread id), and thus till the NDC tag stays in memory all the objects that hang off of the thread object also stay in memory. In our web application we use NDC to tag logoutputs with a request id to distinguish logs from a single request separately. The container that associates the NDC tag with a thread, also removes it while returning the response from a request. The problem occurred when during the course of processing a request, a child thread was spawned, something like the following code:
pubclic class RequestProcessor {
private static final Logger logger = Logger.getLogger(RequestProcessor.class);
public void doSomething() {
....
final List<String> hugeList = new ArrayList<String>(10000);
new Thread() {
public void run() {
logger.info("Child thread spawned")
for(String s:hugeList) {
....
}
}
}.start();
}
}
So an NDC context was associated with inline thread that was spawned. The thread object that was the key for this NDC context, is the inline thread which has the hugeList object hanging off of it. Hence even after the thread finished doing what it was doing, the reference to the hugeList was kept alive by the NDC context Hastable, thus causing a memory leak.
Create a static Map and keep adding hard references to it. Those will never be garbage collected.
public class Leaker {
private static final Map<String, Object> CACHE = new HashMap<String, Object>();
// Keep adding until failure.
public static void addToCache(String key, Object value) { Leaker.CACHE.put(key, value); }
}
Everyone always forgets the native code route. Here's a simple formula for a leak:
Declare a native method.
In the native method, call malloc. Don't call free.
Call the native method.
Remember, memory allocations in native code come from the JVM heap.
You can create a moving memory leak by creating a new instance of a class in that class's finalize method. Bonus points if the finalizer creates multiple instances. Here's a simple program that leaks the entire heap in sometime between a few seconds and a few minutes depending on your heap size:
class Leakee {
public void check() {
if (depth > 2) {
Leaker.done();
}
}
private int depth;
public Leakee(int d) {
depth = d;
}
protected void finalize() {
new Leakee(depth + 1).check();
new Leakee(depth + 1).check();
}
}
public class Leaker {
private static boolean makeMore = true;
public static void done() {
makeMore = false;
}
public static void main(String[] args) throws InterruptedException {
// make a bunch of them until the garbage collector gets active
while (makeMore) {
new Leakee(0).check();
}
// sit back and watch the finalizers chew through memory
while (true) {
Thread.sleep(1000);
System.out.println("memory=" +
Runtime.getRuntime().freeMemory() + " / " +
Runtime.getRuntime().totalMemory());
}
}
}
I don't think anyone has said this yet: you can resurrect an object by overriding the finalize() method such that finalize() stores a reference of this somewhere. The garbage collector will only be called once on the object so after that the object will never destroyed.
I came across a more subtle kind of resource leak recently.
We open resources via class loader's getResourceAsStream and it happened that the input stream handles were not closed.
Uhm, you might say, what an idiot.
Well, what makes this interesting is: this way, you can leak heap memory of the underlying process, rather than from JVM's heap.
All you need is a jar file with a file inside which will be referenced from Java code. The bigger the jar file, the quicker memory gets allocated.
You can easily create such a jar with the following class:
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.zip.ZipEntry;
import java.util.zip.ZipOutputStream;
public class BigJarCreator {
public static void main(String[] args) throws IOException {
ZipOutputStream zos = new ZipOutputStream(new FileOutputStream(new File("big.jar")));
zos.putNextEntry(new ZipEntry("resource.txt"));
zos.write("not too much in here".getBytes());
zos.closeEntry();
zos.putNextEntry(new ZipEntry("largeFile.out"));
for (int i=0 ; i<10000000 ; i++) {
zos.write((int) (Math.round(Math.random()*100)+20));
}
zos.closeEntry();
zos.close();
}
}
Just paste into a file named BigJarCreator.java, compile and run it from command line:
javac BigJarCreator.java
java -cp . BigJarCreator
Et voilà: you find a jar archive in your current working directory with two files inside.
Let's create a second class:
public class MemLeak {
public static void main(String[] args) throws InterruptedException {
int ITERATIONS=100000;
for (int i=0 ; i<ITERATIONS ; i++) {
MemLeak.class.getClassLoader().getResourceAsStream("resource.txt");
}
System.out.println("finished creation of streams, now waiting to be killed");
Thread.sleep(Long.MAX_VALUE);
}
}
This class basically does nothing, but create unreferenced InputStream objects. Those objects will be garbage collected immediately and thus, do not contribute to heap size.
It is important for our example to load an existing resource from a jar file, and size does matter here!
If you're doubtful, try to compile and start the class above, but make sure to chose a decent heap size (2 MB):
javac MemLeak.java
java -Xmx2m -classpath .:big.jar MemLeak
You will not encounter an OOM error here, as no references are kept, the application will keep running no matter how large you chose ITERATIONS in the above example.
The memory consumption of your process (visible in top (RES/RSS) or process explorer) grows unless the application gets to the wait command. In the setup above, it will allocate around 150 MB in memory.
If you want the application to play safe, close the input stream right where it's created:
MemLeak.class.getClassLoader().getResourceAsStream("resource.txt").close();
and your process will not exceed 35 MB, independent of the iteration count.
Quite simple and surprising.
As a lot of people have suggested, resource leaks are fairly easy to cause - like the JDBC examples. Actual memory leaks are a bit harder - especially if you aren't relying on broken bits of the JVM to do it for you...
The ideas of creating objects that have a very large footprint and then not being able to access them aren't real memory leaks either. If nothing can access it then it will be garbage collected, and if something can access it then it's not a leak...
One way that used to work though - and I don't know if it still does - is to have a three-deep circular chain. As in Object A has a reference to Object B, Object B has a reference to Object C and Object C has a reference to Object A. The GC was clever enough to know that a two deep chain - as in A <--> B - can safely be collected if A and B aren't accessible by anything else, but couldn't handle the three-way chain...
Another way to create potentially huge memory leaks is to hold references to Map.Entry<K,V> of a TreeMap.
It is hard to asses why this applies only to TreeMaps, but by looking at the implementation the reason might be that: a TreeMap.Entry stores references to its siblings, therefore if a TreeMap is ready to be collected, but some other class holds a reference to any of its Map.Entry, then the entire Map will be retained into memory.
Real-life scenario:
Imagine having a db query that returns a big TreeMap data structure. People usually use TreeMaps as the element insertion order is retained.
public static Map<String, Integer> pseudoQueryDatabase();
If the query was called lots of times and, for each query (so, for each Map returned) you save an Entry somewhere, the memory would constantly keep growing.
Consider the following wrapper class:
class EntryHolder {
Map.Entry<String, Integer> entry;
EntryHolder(Map.Entry<String, Integer> entry) {
this.entry = entry;
}
}
Application:
public class LeakTest {
private final List<EntryHolder> holdersCache = new ArrayList<>();
private static final int MAP_SIZE = 100_000;
public void run() {
// create 500 entries each holding a reference to an Entry of a TreeMap
IntStream.range(0, 500).forEach(value -> {
// create map
final Map<String, Integer> map = pseudoQueryDatabase();
final int index = new Random().nextInt(MAP_SIZE);
// get random entry from map
for (Map.Entry<String, Integer> entry : map.entrySet()) {
if (entry.getValue().equals(index)) {
holdersCache.add(new EntryHolder(entry));
break;
}
}
// to observe behavior in visualvm
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
public static Map<String, Integer> pseudoQueryDatabase() {
final Map<String, Integer> map = new TreeMap<>();
IntStream.range(0, MAP_SIZE).forEach(i -> map.put(String.valueOf(i), i));
return map;
}
public static void main(String[] args) throws Exception {
new LeakTest().run();
}
}
After each pseudoQueryDatabase() call, the map instances should be ready for collection, but it won't happen, as at least one Entry is stored somewhere else.
Depending on your jvm settings, the application may crash in the early stage due to a OutOfMemoryError.
You can see from this visualvm graph how the memory keeps growing.
The same does not happen with a hashed data-structure (HashMap).
This is the graph when using a HashMap.
The solution? Just directly save the key / value (as you probably already do) rather than saving the Map.Entry.
I have written a more extensive benchmark here.
There are many good examples of memory leaks in Java, and I will mention two of them in this answer.
Example 1:
Here is a good example of a memory leak from the book Effective Java, Third Edition (item 7: Eliminate obsolete object references):
// Can you spot the "memory leak"?
public class Stack {
private static final int DEFAULT_INITIAL_CAPACITY = 16;
private Object[] elements;
private int size = 0;
public Stack() {
elements = new Object[DEFAULT_INITIAL_CAPACITY];
}
public void push(Object e) {
ensureCapacity();
elements[size++] = e;
}
public Object pop() {
if (size == 0) throw new EmptyStackException();
return elements[--size];
}
/*** Ensure space for at least one more element, roughly* doubling the capacity each time the array needs to grow.*/
private void ensureCapacity() {
if (elements.length == size) elements = Arrays.copyOf(elements, 2 * size + 1);
}
}
This is the paragraph of the book that describes why this implementation will cause a memory leak:
If a stack grows and then shrinks, the objects that were popped off the
stack will not be garbage collected, even if the program using the
stack has no more references to them. This is because the
stack maintains obsolete references to these objects. An obsolete
reference is simply a reference that will never be dereferenced
again. In this case, any references outside of the “active portion” of
the element array are obsolete. The active portion consists of the
elements whose index is less than size
Here is the solution of the book to tackle this memory leak:
The fix for this sort of problem is simple: null out
references once they become obsolete. In the case of our Stack class,
the reference to an item becomes obsolete as soon as it’s popped
off the stack. The corrected version of the pop method looks like this:
public Object pop() {
if (size == 0) throw new EmptyStackException();
Object result = elements[--size];
elements[size] = null; // Eliminate obsolete reference
return result;
}
But how can we prevent a memory leak from happening? This is a good caveat from the book:
Generally speaking, whenever a class manages its own memory,
the programmer should be alert for memory leaks. Whenever an element
is freed, any object references contained in the element should be
nulled out.
Example 2:
The observer pattern also can cause a memory leak. You can read about this pattern in the following link: Observer pattern.
This is one implementation of the Observer pattern:
class EventSource {
public interface Observer {
void update(String event);
}
private final List<Observer> observers = new ArrayList<>();
private void notifyObservers(String event) {
observers.forEach(observer -> observer.update(event)); //alternative lambda expression: observers.forEach(Observer::update);
}
public void addObserver(Observer observer) {
observers.add(observer);
}
public void scanSystemIn() {
Scanner scanner = new Scanner(System.in);
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
notifyObservers(line);
}
}
}
In this implementation, EventSource, which is Observable in the Observer design pattern, can hold links to Observer objects, but this link is never removed from the observers field in EventSource. So they will never be collected by the garbage collector. One solution to tackle this problem is providing another method to the client for removing the aforementioned observers from the observers field when they don't need those observers anymore:
public void removeObserver(Observer observer) {
observers.remove(observer);
}
Threads are not collected until they terminate. They serve as roots of garbage collection. They are one of the few objects that won't be reclaimed simply by forgetting about them or clearing references to them.
Consider: the basic pattern to terminate a worker thread is to set some condition variable seen by the thread. The thread can check the variable periodically and use that as a signal to terminate. If the variable is not declared volatile, then the change to the variable might not be seen by the thread, so it won't know to terminate. Or imagine if some threads want to update a shared object, but deadlock while trying to lock on it.
If you only have a handful of threads these bugs will probably be obvious because your program will stop working properly. If you have a thread pool that creates more threads as needed, then the obsolete/stuck threads might not be noticed, and will accumulate indefinitely, causing a memory leak. Threads are likely to use other data in your application, so will also prevent anything they directly reference from ever being collected.
As a toy example:
static void leakMe(final Object object) {
new Thread() {
public void run() {
Object o = object;
for (;;) {
try {
sleep(Long.MAX_VALUE);
} catch (InterruptedException e) {}
}
}
}.start();
}
Call System.gc() all you like, but the object passed to leakMe will never die.
The interviewer might have been looking for a circular reference solution:
public static void main(String[] args) {
while (true) {
Element first = new Element();
first.next = new Element();
first.next.next = first;
}
}
This is a classic problem with reference counting garbage collectors. You would then politely explain that JVMs use a much more sophisticated algorithm that doesn't have this limitation.
Similar to Can JIT be prevented from optimising away method calls? I'm attempting to track memory usage of long-lived data store objects, however I'm finding that if I initialize a store, log the system memory, then initialize another store, sometimes the compiler (presumably the JIT) is smart enough to notice that these objects are no longer needed.
public class MemTest {
public static void main(String[] args) {
logMemory("Initial State");
MemoryHog mh = new MemoryHog();
logMemory("Built MemoryHog");
MemoryHog mh2 = new MemoryHog();
logMemory("Built Second MemoryHog"); // by here, mh may be GCed
}
}
Now the suggestion in the linked thread is to keep a pointer to these objects, but the GC appears to be smart enough to tell that the objects aren't used by main() anymore. I could add a call to these objects after the last logMemory() call, but that's a rather manual solution - every time I test an object, I have to do some sort of side-effect triggering call after the final logMemory() call, or I may get inconsistent results.
I'm looking for general case solutions; I understand that adding a call like System.out.println(mh.hashCode()+mh2.hashCode()) at the end of the main() method would be sufficient, but I dislike this for several reasons. First, it introduces an external dependency on the testing above - if the SOUT call is removed, the behavior of the JVM during the memory logging calls may change. Second, it's prone to user-error; if the objects being tested above change, or new ones are added, the user must remember to manually update this SOUT call as well, or they'll introduce difficult to detect inconsistencies in their test. Finally, I dislike that this solution prints at all - it seems like an unnecessary hack that I can avoid with a better understanding of the JIT's optimizations. To the last point, Patricia Shanahan's answer offers a reasonable solution (explicitly print that the output is for memory sanity purposes) but I'd still like to avoid it if possible.
So my initial solution is to store these objects in a static list, and then iterate over them in the main class's finalize method*, like so:
public class MemTest {
private static ArrayList<Object> objectHolder = new ArrayList<>();
public static void main(String[] args) {
logMemory("Initial State", null);
MemoryHog mh = new MemoryHog();
logMemory("Built MemoryHog", mh); // adds mh to objectHolder
MemoryHog mh2 = new MemoryHog();
logMemory("Built Second MemoryHog", mh2); // adds mh2 to objectHolder
}
protected void finalize() throws Throwable {
for(Object o : objectHolder) {
o.hashCode();
}
}
}
But now I've only offloaded the problem one step - what if the JIT optimizes away the loop in the finalize method, and decides these objects don't need to be saved? Admittedly, maybe simply holding the objects in the main class is enough for Java 7, but unless it's documented that the finalzie method can't be optimized away, there's still nothing theoretically preventing the JIT/GC from getting rid of these objects early, since there's no side effects in the contents of my finalize method.
One possibility would be to change the finalize method to:
protected void finalize() throws Throwable {
int codes = 0;
for(Object o : loggedObjects) {
codes += o.hashCode();
}
System.out.println(codes);
}
As I understand it (and I could be wrong here), calling System.out.println() will prevent the JIT from getting rid of this code, since it's a method with external side effects, so even though it doesn't impact the program, it can't be removed. This is promising, but I don't really want some sort of gibberish being output if I can help it. The fact that the JIT can't (or shouldn't!) optimize away System.out.println() calls suggests to me that the JIT has a notion of side effects, and if I can tell it this finalize block has such side effects, it should never optimize it away.
So my questions:
Is holdijng a list of objects in the main class enough to prevent them from ever being GCed?
Is looping over those objects and calling something trivial like .hashCode() in the finalize method enough?
Is computing and printing some result in this method enough?
Are there other methods (like System.out.println) the JIT is aware of that cannot be optimized away, or even better, is there some way to tell the JIT not to optimize away a method call / code block?
*Some quick testing confirms, as I suspected, that the JVM doesn't generally run the main class's finalize method, it abruptly exits. The JIT/GC may still not be smart enough to GC my objects simply because the finalize method exists, even if it doesn't get run, but I'm not confident that's always the case. If it's not documented behavior, I can't reasonably trust it will remain true, even if it's true now.
Here's a plan that may be overkill, but should be safe and reasonably simple:
Keep a List of references to the objects.
At the end, iterate over the list summing the hashCode() results.
Print the sum of the hash codes.
Printing the sum ensures that the final loop cannot be optimized out. The only thing you need to do for each object creation is put it in a List add call.
Yes, it would be legal for mh1 to be garbage collected at that point. At that point, there is no code that could possibly use the variable. If the JVM could detect this, then the corresponding MemoryHog object will be treated as unreachable ... if the GC were to run at that point.
A later call like System.out.println(mh1) would be sufficient to inhibit collection of the object. So would using it in a "computation"; e.g.
if (mh1 == mh2) { System.out.println("the sky is falling!"); }
Is holding a list of objects in the main class enough to prevent them from ever being GCed?
It depends on where the list is declared. If the list was a local variable, and it became unreachable before mh1, then putting the object into the list will make no difference.
Is looping over those objects and calling something trivial like .hashCode() in the finalize method enough?
By the time the finalize method is called, the GC has already decided that the object is unreachable. The only way that the finalize method could prevent the object being deleted would be to add it to some other (reachable) data structure or assign it to a (reachable) variable.
Are there other methods (like System.out.println) the JIT is aware of that cannot be optimized away,
Yea ... anything that makes the object reachable.
or even better, is there some way to tell the JIT not to optimize away a method call / code block?
No way to do that ... apart from making sure that the method call or code block does something that contributes to the computation being performed.
UPDATE
First, what is going on here is not really JIT optimization. Rather, the JIT is emitting some kind of "map" that the GC is using to determine when local variables (i.e. variables on the stack) are dead ... depending on the program counter (PC).
Your examples to inhibit collection all involve blocking the JIT via SOUT, I'd like to avoid that somewhat hacky solution.
Hey ... ANYTHING that depends on the exact timing of when things are garbage collected is a hack. You are not supposed to do that in a properly engineered application.
I updated my code to make it clear that the list that's holding my objects is a static variable of the main class, but it seems if the JIT's smart enough it could still theoretically GC these values once it knows the main method doesn't need them.
I disagree. In practice, the JIT cannot determine that a static will never be referenced. Consider these cases:
Before the JIT runs, it appears that nothing will use static s again. After the JIT has run, the application loads a new class that refers to s. If the JIT "optimized" the s variable, the GC would treat it as unreachable, and either null it or create a dangling references. When the dynamically loaded class then looked at s it would then see the wrong value ... or worse.
If the application ... or any libraries used by the application ... uses reflection, then it can refer to the value of any static variable without this being detectable by the JIT.
So while it is theoretically possible to do this optimization is a small number of cases:
in the vast majority of cases, you can't, and
in the few cases that you can, the pay-off (in terms of performance improvement) is most likely negligible.
I similarly updated my code to clarify that I'm talking about the finalize method of the main class.
The finalize method of the main class is irrelevant because:
you are not creating an instance of the main class, and
the finalize method CANNOT refer to the local variables of another method (e.g. the main method).
... it's existence prevents the JIT from nuking my static list.
Not true. The static list can't be nuked anyway; see above.
As I understand it, there's something special about SOUT that the JIT is aware of that prevents it from optimizing such calls away.
There is nothing special about sout. It is just something that we KNOW that influences the results of the computation and that we therefore KNOW that the JIT cannot legally optimize away.