Memory leaks


Just need some discussion on general topic of memory leaks in java. I am not 100% sure how garbage collections works in java especially when we reassign strings to new values in a continuous loop because I believe strings are immutable! For example lets say in the following loop:

String buffer;
buffer= “”;
for (i=o; i<1000;i++) buffer+="some string ";

My understanding is that the variable buffer will be freed and recreated with new value in every iteration in the loop due to the immutable nature of strings in JAVA (same with JavaScript but that’s irrelevant for this discussion)! If the whole code is in a unending periodic timer loop (like for example say an Ignition client refreshing the values or screen every fixed interval of say once a second or a few second, will it cause memory leaks? Can I safely assume that the JVM’s garbage collector take care of garbage collection without causing any memory leaks?! In C or C++ I would have defined a large array for buffer (sufficient to to hold the largest buffer size i expect in any of the timer loops) and reused the array in every timer cycle. That way I use the max buffer size in every cycle but in each cycle I use different buffer size . There is no question of creating a new buffer every cycle and the resulting memory leaks.

Another example is let us say a method returns a list of items for a given list of items passed as input argument to it. I assign the returned list variable to a local list variable of same type. This method called in a never-ending timer loop every few seconds. I guess the local list variable will get freed and new list will be created automatically by the JVM? Or do I have to delete the list explicitly after its use is over in the code. Will the garbage collector take care of the memory management of freed list of variables. Do I need to take care of anything special to avoid memory leaks? This is especially important for Ignition type of systems which are continuously running applications triggered by timer loops unlike other OLTP applications which triggered by manual actions.

Any replies to these doubts will be appreciated. We need to be clear about these issue to design robust real time systems especially those developed in JAVA which is not designed with hard real time system in mind!

Best regards

Hi Kevin,
Once again I want to raise this topic. When I read a list of tags periodically every say 1 sec from Sever using, it returns list of corrosponding list of Qvalues. Now I am passing these values on TCP socket as a big string by concatinating values returned by getValues() method of each object of the Qvalue list. The buffer string is formed by the pseudo code given in my previous code. There could be 1000’s of tagpaths in the list. So will such creation of buffer string in each second cycle cause any problem of memmory leaks or performance degradation? What is the best way to do such a transfer?

Will appreciate your reply.

Hi Kevin,
I am somehow unsure about how efficient the garbage collection of string manipulation methods are in JAVA especially for a real time perpetually running application like SCADA are. Certainly these methods are very convenient and JAVA is excellent for String maipulation in applications, but they are OK for applications which are driven by user triggered events but for applications that are triggered every 250 or 500 milli seconds they will keep generating garbage millions of time on 24/7 basis! Is it safe to rely on java’s automati garbage collection?
In case of UDP the datagram (buffer) size is fixed for every packet so we can reuse a buffer. So I am OK with UDP for communication in real time applications where as in Socket approach we have to generate a new string for packet to be sent every time we send the data to other nodes. I am not sure if its the best approach.
How does Ignition’s communication between server & clients work? Is it TCP/IP based or UDP based or HTTP based or some other propriatory binary protocol based?
Will appreciate your early reply.

You don’t have any other choice than to rely on Java’s garbage detection. There’s no manual memory management. If you’re really concerned you can profile your application using something like JProfiler or YourKit.

Communication between Ignition clients and the gateway is done via HTTP.

Hi Kevin,
Thanks a lot for the reply. :thumb_right: Also will RMI be an option for communication between Ignition server and my servers?