"Unveiling the java.lang.Out OfMemoryError"

What Is a java.lang.OutOfMemoryError? 

A java.lang.OutOfMemoryError is a subclass of java.lang.VirtualMachineError that is thrown when the Java Virtual Machine is broken or has run out of resources that are necessary to continue the operation of the Java Virtual Machine. Obviously, memory is the exhausted resource for a java.lang.OutOfMemoryError, which is thrown when the Java Virtual Machine cannot allocate an object due to memory constraints. Unfortunately, the Java specification of java.lang.OutOfMemoryError does not elaborate further on what kind of memory it's talking about.
There are six different types of runtime data areas, or memory areas, in the Java Virtual Machine


1. Program Counter Register
2. Java Virtual Machine Stack
3. Heap
4. Method Area
5. Runtime Constant Pool
6. Native Method Stack 

The Program Counter Register, also known as the pc register, stores the address of the Java byte code instruction that is currently being executed (just like the processor register in your central processing unit of the device from which you are reading or printing this article). You will not see a java.lang.OutOfMemoryError from the pc register since a program counter is not conventionally considered as a memory. 

Java Virtual Machine Stacks contain frames where data, return values, and partial execution results are stored. Java Virtual Machine Stacks can be expanded during runtime. If there's not enough memory for the expansion of an existing Java Virtual Machine stack, or for the creation of a new Java Virtual Machine stack for a new thread, the Java Virtual Machine will throw a java.lang.OutOfMemoryError.
The Heap is where instances of Java classes and arrays are allocated. A java.lang.OutOfMemoryError will be thrown when there is not enough memory available for instances of Java classes or arrays. 

The Method Area stores class-related information, the runtime constant pool, for instances, the code for methods and constructors, and field/method data. If there's not enough memory in the method area, you will encounter java.lang.OutOfMemoryError. 

The Runtime Constant Pool contains constants such as field references and literals (Java Literals are syntactic representations of boolean, character, numeric, or string data. ). A java.lang.OutOfMemoryError will be thrown when not enough memory is available for the construction of the runtime constant pool area. 

Native Memory Stacks store conventional stacks, also known as C stacks, to support native methods that are written in a non-Java language such as C/C++. Native memory stacks can be expanded during runtime. If there's not enough memory for the expansion of an existing native memory stack or for the creation of a new native memory stack for a new thread, you would see a java.lang.OutOfMemoryError. 

You may have seen a java.lang.StackOverflowError, which is completely different from a java.lang.OutOfMemoryError. A java.lang.StackOverflowError is thrown when native memory stacks or Java Virtual Machine stacks need more memory than is configured. In most IBM Java Virtual Machine implementations, the -Xmso command-line option controls the stack size for operation system threads or native thread, and the -Xss command-line option controls the stack size for Java threads. In some implementations, such as Sun Microsystems HotSpot Java Virtual Machine, the Java methods share stack frames with C/C++ native code. The maximum stack size for a thread can be configured with the -Xss Java command-line option. The default sizes of these options vary by platform and implementation, but are usually between 256 Kbytes-1024 Kbytes. Please refer to the documentation of your Java virtual machine for more specific information.
Now that we understand which memory areas could cause a java.lang.OutOfMemoryError, let's take a look at actual error messages. What does a java.lang.OutOfMemoryError look like and how can I address each symptom? Have you ever seen a java.lang.OutOfMemoryError similar to the following? 

java.lang.OutOfMemoryError: Requested array size exceeds VM limit:
This error message indicates that there is a memory request for an array but that's too large for a predefined limit of a virtual machine. What do we do if we encounter this kind of java.lang.OutOfMemoryError? We need to check the source code to make sure that there's no huge array created dynamically or statically. Fortunately, latest virtual machines usually do not have this limit.
java.lang.OutOfMemoryError: PermGen space: 

You will see an OutOfMemoryError when the Permanent Generation area of the Java heap is full, like the above message. 

On some Java Virtual Machines, such as Sun Microsystems' HotSpot Java Virtual Machine, a dedicated memory area called permanent generation (or permanent region) stores objects that describe classes and methods. We can visualize the usage of a permanent generation with the IBM Pattern Modeling and Analysis Tool for the Java Garbage Collector.