This post is completely about out of memory(OOM) issues that causes app crashes , request failures for any  java based applications including spring-boot, vert.x, spark .

Let us first understand how JVM process that is running our application acquires memory from the Hardware using the Operating System.

Every Java application is nothing but a single process for Operating System with Java Virtual Machine as additional executable code .

So to put it in simple terms,

A process for operating system to execute our code with java virtual machine support
.

While any process can have it’s own logic to acquire required memory for execution or rely on operating system , the java based processes use the former one using some arguments which can be overridden by the user .

Here are the list of such arguments from Java .
-Xms set initial Java heap size
-Xmx set maximum Java heap size


As java mainly uses heap for object management, we will focus more on this in this post.

Now let’s focus on the main issue , So What Causes JAVA OOM ?

Before we dive into it, let us first recall the facts from few memory management capabilities of JVM .

• Any java process has it’s own logic to cleanup the unused heap memory frequently using Garbage collector.
• Heap memory internally has areas like Younger Generation, Old Generation, Eden  etc…

During the runtime of a java application , there are various ways java heap memory can go beyond the limits set by above parameters .

1. Total heap usage is higher than the -Xmx value passed during the submission.
2. Gradual increase of heap memory usage due to pile up of referenced objects that garbage collector cannot cleanup.
3. Sudden increase in the number of object creations at a faster pace than garbage collector frequency .
4.  Single object loading something bigger than the actual heap memory available.
5. Some java in-built object like ArrayList unable to acquire memory that needs continuous slot within the heap space.