50. How would you detect and minimise memory leaks in Java

In Java memory leaks are caused by poor program design where object references are long lived and the garbage collector is unable to reclaim those objects.

Detecting memory leaks:

  • Use tools like JProbe, OptimizeIt etc to detect memory leaks.

  • Use operating system process monitors like task manager on NT systems, ps, vmstat, iostat, netstat etc on UNIX systems.

  • Write your own utility class with the help of totalMemory() and freeMemory() methods in the Java Runtime class. Place these calls in your code strategically for pre and post memory recording where you suspect to be causing memory leaks. An even better approach than a utility class is using dynamic proxies or Aspect Oriented Programming (AOP) for pre and post memory recording where you have the control of activating memory measurement only when needed.

Minimising memory leaks:

In Java, typically memory leak occurs when an object of a longer lifecycle has a reference to objects of a short life cycle. This prevents the objects with short life cycle being garbage collected. The developer must remember to remove the references to the short-lived objects from the long-lived objects. Objects with the same life cycle do not cause any issues because the garbage collector is smart enough to deal with the circular references (Refer Q33 in Java section).

  • Design applications with an object’s life cycle in mind, instead of relying on the clever features of the JVM. Letting go of the object’s reference in one’s own class as soon as possible can mitigate memory problems. Example: myRef = null;

  • Unreachable collection objects can magnify a memory leak problem. In Java it is easy to let go of an entire collection by setting the root of the collection to null. The garbage collector will reclaim all the objects (unless some objects are needed elsewhere).

  • Use weak references (Refer Q32 in Java section) if you are the only one using it. The WeakHashMap is a combination of HashMap and WeakReference. This class can be used for programming problems where you need to have a HashMap of information, but you would like that information to be garbage collected if you are the only one referencing it.

  • Free native system resources like AWT frame, files, JNI etc when finished with them. Example: Frame, Dialog, and Graphics classes require that the method dispose() be called on them when they are no longer used, to free up the system resources they reserve.

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