Java - How is memory leak in Java harmful? How can it possibly get used for a bad cause?

Question

Creating a memory leak with Java

I was going through above "interview" question. After reading it's answers I myself ended up having a few questions.

Let's guess there is already a memory leak in the code. How is that harmful? How can the data go in wrong hands?

I am pretty sure that System.read(); (or something like that) is not going to read the data from the memory leak. Is that even possible?

Please help with some reference/code/documents.

Answer 1

Memory leak is really a broad argument, to be honest I've voted to close your question (because too broad) but on the other hand I would try to give you a little spark of what behind this problem.

Consider that you're creating a session in memory for every user connected to your web service, but you don't throw away the session after some time, simply because you forget or because a bad design of your application, this would cause a memory leak.

And again, consider that you don't close your open files or sockets.

Or consider that somewhere you save a reference to all the intermediate data structures produced by your process. In this case there is no way for the garbage collector to free the allocated memory.

Memory leaks mostly happens in long running application, because in the short run a memory leak have little chances to generate a out of memory exception. But in the long run the thing changes, there are applications that runs for months or even years.

There are so many situation where a memory leaks could happen. Many framework or libraries and even the languages try to save the programmers by this "bad" situations, but I personally think that is the experience of the programmer that does the difference.

For example in Java the Try with resource Statement is an example of language features born to help programmers in such situation (this helps to not forget).

So when designing your own objects that should close some resource at end of their life, try to implement java.lang.AutoCloseable interface and add the appropriate methods. Have a look at how many classes are now implementing the Autocloseable interface, this also explain how is important the memory (leak) and resource handling.

I would also suggest to study the difference between Java stack and heap memory management.

Once I experienced a Tomcat instance that hanged a server every three months. After some time the server had to be restarted every three week, till the time the server had to be restarted every day.

Comes out that "someone", wrote a for loop instead to add a while clause in a sql query.

So, there are programmers that does this as full time job, that are expert in this kind of investigations and that are able to find and correct memory leaks.