How can I create a memory leak in Java?

Asked 2023-09-20 20:15:00 View 5,086

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?

  • Ironically, the harder question for every non-trivial Java program is how not to create a memory leak! - anyone
  • Keep adding new objects to a container, but forget to add the code that removes them or implement partially working code that doesn't clean up all of them as the program progresses. - anyone
  • The most common memory leaks in Java server systems are in shared state -- caches and services which are shared between requests. Many of the answers here seem to be over-complex, ignoring this obvious and common area. One rather common leak pattern is probably an application-scoped Map with request-scoped keys (eg, some kind of hand-rolled cache). - anyone

Answers

Here's a good way to create a true memory leak (objects inaccessible by running code but still stored in memory) in pure Java:

  1. The application creates a long-running thread (or use a thread pool to leak even faster).
  2. The thread loads a class via an (optionally custom) ClassLoader.
  3. 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.
  4. The application clears all references to the custom class or the ClassLoader it was loaded from.
  5. 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.

Answered   2023-09-20 20:15:00

  • +1 ClassLoader leaks are some of the most commonly painful memory leaks in the JEE world, often caused by 3rd party libs that transform data (BeanUtils, XML/JSON codecs). This can happen when the lib is loaded outside your application's root classloader but holds references to your classes (eg. by caching). When you undeploy/redeploy your app the JVM is unable to garbage collect the app's classloader (and therefore all classes loaded by it), so with repeat deploys the app server eventually borks. If lucky you get a clue with ClassCastException z.x.y.Abc cannot be cast to z.x.y.Abc - anyone
  • +1: Classloader leaks are a nightmare. I spent weeks trying to figure them out. The sad thing is, as what @earcam has said, they are mostly caused by 3rd party libs and also most profilers can't detect these leaks. There's a good and clear explanation on this blog about Classloader leaks. blogs.oracle.com/fkieviet/entry/… - anyone

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.

Answered   2023-09-20 20:15:00

  • I'd disagree that context and session attributes are "leaks." They're just long-lived variables. And the static final field is more or less just a constant. Maybe large constants should be avoided, but I don't think it's fair to call it a memory leak. - anyone
  • (Unclosed) open streams ( file , network etc... ), doesn't leak for real, during finalization (which will be after the next GC cycle) close() is going to be scheduled (close() is usually not invoked in the finalizer thread since might be a blocking operation). It's a bad practice not to close, but it doesn't cause a leak. Unclosed java.sql.Connection is the same. - anyone
  • In most sane JVMs, it appears as though the String class only has a weak reference on its intern hashtable contents. As such, it is garbage collected properly and not a leak. (but IANAJP) mindprod.com/jgloss/interned.html#GC - anyone

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.

Answered   2023-09-20 20:15:00

  • Actually, you can remove the elements from a HashSet even if the element class gets hashCode and equals wrong; just get an iterator for the set and use its remove method, as the iterator actually operates on the underlying entries themselves and not the elements. (Note that an unimplemented hashCode/equals is not enough to trigger a leak; the defaults implement simple object identity and so you can get the elements and remove them normally.) - anyone
  • @Donal your last statement is incorrect. You can't get the element because you have no reference to it. The only reference to the key put in the map is the one that the map itself holds. Only if you went BadKey myKey = new BadKey("key"); map.put(key,"value"); would you ever be able to get it out. You are correct you could use the iterator to remove it, but you cannot always do that with all data structures. For example, if you don't null out the reference in @meriton's answer, that will be lost forever. You have no access to the backing array, and iterator will stop short of it. - anyone
  • The only way to remove elements when equals/hashCode is incorrect is to use Iterator.remove() when you find a match using a different method of comparison. - anyone
  • @Donal what I'm trying to say, I guess, is I disagree with your definition of a memory leak. I would consider (to continue the analogy) your iterator-removal technique to be a drip-pan under a leak; the leak still exists regardless of the drip pan. - anyone
  • I agree, this is not a memory "leak", because you can just remove references to the hashset and wait for the GC to kick in, and presto! the memory goes back. - anyone

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!

Answered   2023-09-20 20:15:00

  • Creating but not starting a Thread... Yikes, I was badly bitten by this one some centuries ago! (Java 1.3) - anyone
  • @leonbloy, before it was even worse as the thread was added straight to the threadgroup, not starting meant very hard leak. Not it just increases the unstarted count but that prevents the thread group from destroying (lesser evil but still a leak) - anyone
  • Thank you! "Calling ThreadGroup.destroy() when the ThreadGroup has no threads itself..." is an incredibly subtle bug; I've been chasing this for hours, led astray because enumerating the thread in my control GUI showed nothing, but the thread group and, presumably, at least one child group would not go away. - anyone
  • @bestsss : I'm curious, why would you want to remove a shutdown hook, given that it runs at, well, JVM shutdown? - anyone
  • @user253751 : This is in the context of memory leaks. In that context failing to remove a shutdown hook after it's used is not a leak because the JVM is shutting down. Granted, though, if you only temporarily need to do something on shutdown, then not removing it and continually adding new handlers would leak. - anyone

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.

Answered   2023-09-20 20:15:00

  • I find it funny that on other answers people are looking for those edge cases and tricks and seem to be completely missing the point. They could just show code that keep useless references to objects that will never use again, and never remove those references; one may say those cases are not "true" memory leaks because there are still references to those objects around, but if the program never use those references again and also never drop them, it is completely equivalent to (and as bad as) a "true memory leak". - anyone

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.

Answered   2023-09-20 20:15:00

  • Whenever I have asked that question, I am looking for a pretty simple answer - keep growing a queue, no finally close db etc, not odd classloader/thread details, implies they understand what the gc can and cannot do for you. Depends on the job you are interviewing for I guess. - anyone

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.

Answered   2023-09-20 20:15:00

  • You will reach maximum open connection limit before you hit memory limits usually. Don't ask me why I know... - anyone

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 ...

Answered   2023-09-20 20:15:00

  • And where is the memory leak here? - anyone
  • @maniek: I did not mean to imply that this code exhibits a memory leak. I quoted to it to show that sometimes non-obvious code is required to avoid accidental object retention. - anyone

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.

Answered   2023-09-20 20:15:00

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();
        }
    }
}

Answered   2023-09-20 20:15:00

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.

Answered   2023-09-20 20:15:00

  • That's an extremely limited (and not very useful) definition of memory leaks. The only definition that makes sense for practical purposes is "a memory leak is any condition in which the program continues to hold memory allocated after the data it holds it is no longer needed." - anyone
  • The definition of the term on Wikipedia has changed. It is now this: "In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that memory which is no longer needed is not released." This is the definition used by almost all posters, but not this one. - anyone

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.

Answered   2023-09-20 20:15:00

  • The method substring() creates a new String in java7 (it is a new behavior) - anyone

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.

Answered   2023-09-20 20:15:00

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.

Answered   2023-09-20 20:15:00

  • This is not because of a memory leak, but because the class loader does not unload the previous set of classes. Therefor it is not recommended to redeploy an application server without restarting the server (not the physical machine, but the app server). I have seen the same issue with WebSphere. - anyone
  • @Sven: This is what a memory leak is. The class loader not unloading the previous set of classes leads to a memory leak in this case. - anyone

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.

Answered   2023-09-20 20:15:00

  • That depends on the definition of "memory leak". If "memory that's held on to, but no longer needed", then it's easy to do in Java. If it's "memory that's allocated but not accessible by the code at all", then it gets slightly harder. - anyone

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...

Answered   2023-09-20 20:15:00

  • Just to nitpick your example code: I think numbers (or strings starting with numbers) are not allowed as element names in XML. - anyone
  • Note that this is no longer true for JDK 7+, where String interning happens on the heap. See this article for a detailed writeup: java-performance.info/string-intern-in-java-6-7-8 - anyone
  • So, i think using StringBuffer in place of String would resolve this problem? wont it? - anyone

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.

Answered   2023-09-20 20:15:00

  • No JVM ever used reference counting. You may be confusing things with an early implementation of Javascript by Microsoft. - anyone

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).

Answered   2023-09-20 20:15:00

  • On a slightly unrelated note: There's a popular saying: There are only 2 things difficult in programming: naming things and cache invalidation. - anyone

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.

Answered   2023-09-20 20:15:00

  • inner classes are rarely an issue. They are a straightforward case and very easy to detect. The rumor is just a rumor too. - anyone
  • The "rumor" sounds like someone half-read about how generational GC works. Long-lived-but-now-unreachable objects can indeed stick around and take up space for a while, because the JVM promoted them out of the younger generations so it could stop checking them every pass. They will evade the piddly "clean up my 5000 temp strings" passes, by design. But they're not immortal. They're still eligible for collection, and if the VM is strapped for RAM, it will eventually run a full GC sweep and repossess that memory. - anyone

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.

Answered   2023-09-20 20:15:00

  • That sucks. You should check this logging library that allocates ZERO memory while logging to a file: mentalog.soliveirajr.com - anyone
  • +1 Do you know offhand whether there is a similar issue with the MDC in slf4j/logback (successor products by the same author)? I'm about to do a deep dive on the source but wanted to check first. Either way, thanks for posting this. - anyone

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); }
}

Answered   2023-09-20 20:15:00

  • How is that a leak? It's doing exactly what you're asking it to do. If that's a leak, creating and storing objects anywhere is a leak. - anyone
  • I agree with @Falmarri. I don't see a leak there, you are just creating objects. You could certainly 'reclaim' the memory that you just allocated with another method called 'removeFromCache'. A leak is when you can't reclaim the memory. - anyone
  • My point is that somebody who keeps creating objects, perhaps putting them into a cache, could end up with an OOM error if they aren't careful. - anyone
  • @duffymo: But that's not really what the question was asking. It has nothing to do with simply using up all your memory. - anyone
  • The comments dissing this answer are pretty funny, given that the accepted answer with over 2000 votes is saying the exact same thing, albeit in a highly obfuscated way. ThreadLocals aren't magic -- they are just entries in a Map, pointed to by a member variable in Thread. - anyone

Everyone always forgets the native code route. Here's a simple formula for a leak:

  1. Declare a native method.
  2. In the native method, call malloc. Don't call free.
  3. Call the native method.

Remember, memory allocations in native code come from the JVM heap.

Answered   2023-09-20 20:15:00

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());
        }
    }
}

Answered   2023-09-20 20:15:00

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.

Answered   2023-09-20 20:15:00

  • This is untrue. finalize() will not be called but the object will be collected once there won't be more references. Garbage collector is not 'called' either. - anyone
  • This answer is misleading, the finalize() method can only be called once by the JVM, but this does not mean that it cannot be re-garbage collected if the object is resurrected and then dereferenced again. If there is resource closing code in the finalize() method then this code will not get run again, this may cause a memory leak. - anyone

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.

Answered   2023-09-20 20:15:00

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...

Answered   2023-09-20 20:15:00

  • Hasn't been the case for some time now. Modern GCs know how to handle circular references. - anyone

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.

Memory dump - TreeMap

The same does not happen with a hashed data-structure (HashMap).

This is the graph when using a HashMap.

Memory dump - 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.

Answered   2023-09-20 20:15:00

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);
}

Answered   2023-09-20 20:15:00

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.

Answered   2023-09-20 20:15:00

  • @Spidey Nothing is "stuck". The calling method returns promptly, and the passed object will never be reclaimed. That's precisely a leak. - anyone
  • You'll have a thread "running" (or sleeping, whatever) for the lifetime of your program. That doesn't count as a leak to me. As well as a pool doesn't count as a leak, even if you don't use it entirely. - anyone
  • @Spidey "You'll have a [thing] for the lifetime of your program. That doesn't count as a leak to me." Do you hear yourself? - anyone
  • @Spidey If you would count memory that the process knows about as not being leaked, then all the answers here are wrong, since the process always tracks which pages in its virtual address space are mapped. When the process terminates, the OS cleans up all the leaks by putting the pages back on the free page stack. To take that to the next extreme, one could beat to death any argued leak by pointing out that none of the physical bits in the RAM chips or in the swap space on disk have been physically misplaced or destroyed, so you can switch the computer off and on again to clean up any leak. - anyone
  • The practical definition of a leak is that it's memory which has been lost track of such that we don't know and thus can't perform the procedure necessary to reclaim only it; we would have to tear down and rebuild the entire memory space. A rogue thread like this could arise naturally through a deadlock or dodgy threadpool implementation. Objects referenced by such threads, even indirectly, are now prevented from ever being collected, so we have memory which will not be naturally reclaimed or reusable during the lifetime of the program. I'd call that a problem; specifically it's a memory leak. - anyone

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.

Answered   2023-09-20 20:15:00

  • This is a classic problem with reference counting garbage collectors. Even 15 years ago Java didn't use ref counting. Ref. counting is also slower than GC. - anyone
  • Not a memory leak. Just an infinite loop. - anyone
  • @Esben At each iteration, the previous first is not useful and should be garbage collected. In reference counting garbage collectors, the object wouldn't be freeed because there is an active reference on it (by itself). The infinite loop is here to desmonstrate the leak: when you run the program, the memory will raise indefinitely. - anyone
  • @rds @ Wesley Tarle suppose the loop was not infinite. Would there still be a memory leak? - anyone