Memory = Java heap + Native Memory

Java Memory Leak Reasons:

1. Not cleaning up after 'Exception';

2. Handling large result set in memory: 'pagination' or 'lazy loading' to avoid it.

3. OutOfMemory Error  not  equals 'Memory Leak', sometimes you can have memory leak without any errors, for example, performance will tank due to excessive GC though.

Reasons for OOM occuring:

1).  it can occur due to 'fragmentation', cause whenever JVM allocates memory, it must be contiguous blocks of memory; 

2). if 'PermGen' , which is the area of the heap where class objects and interned strings are stored, is full, OOM will happen.

Memory leak: which means the application is allocating memory and holding on them unnecessarily. After certain period, the heap will be filled with objects which are being used by the application and GC will NOT be able to reclaim them. Then OOM appears.

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