Yes, we're now running our Black Friday Sale. All Access and Pro are 33% off until 2nd December, 2025:
Where Does Java’s String Constant Pool Live, the Heap or the Stack?
Last updated: February 24, 2021
Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.
Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.
With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.
Try a 14-Day Free Trial of Orkes Conductor today.
Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.
Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.
With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.
Try a 14-Day Free Trial of Orkes Conductor today.
Mocking is an essential part of unit testing, and the Mockito library makes it easy to write clean and intuitive unit tests for your Java code.
Get started with mocking and improve your application tests using our Mockito guide:
Spring 5 added support for reactive programming with the Spring WebFlux module, which has been improved upon ever since. Get started with the Reactor project basics and reactive programming in Spring Boot:
Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.
But these can also be overused and fall into some common pitfalls.
To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:
Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:
Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.
I built the security material as two full courses - Core and OAuth, to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project.
You can explore the course here:
Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.
Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.
With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.
Try a 14-Day Free Trial of Orkes Conductor today.
Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.
Get started with Spring Data JPA through the guided reference course:
Refactor Java code safely — and automatically — with OpenRewrite.
Refactoring big codebases by hand is slow, risky, and easy to put off. That’s where OpenRewrite comes in. The open-source framework for large-scale, automated code transformations helps teams modernize safely and consistently.
Each month, the creators and maintainers of OpenRewrite at Moderne run live, hands-on training sessions — one for newcomers and one for experienced users. You’ll see how recipes work, how to apply them across projects, and how to modernize code with confidence.
Join the next session, bring your questions, and learn how to automate the kind of work that usually eats your sprint time.
Yes, we're now running our Black Friday Sale. All Access and Pro are 33% off until 2nd December, 2025:
1. Introduction
Whenever we declare a variable or create an object, it is stored in the memory. At a high level, Java divides the memory into two blocks: stack and heap. Both memories store specific types of data and have different patterns for their storage and access.
In this tutorial, we’ll look at different parameters and learn which is the most appropriate area to store the String constant pool.
2. String Constant Pool
The String constant pool is a special memory area. When we declare a String literal, the JVM creates the object in the pool and stores its reference on the stack. Before creating each String object in memory, the JVM performs some steps to decrease the memory overhead.
The String constant pool uses a Hashmap in its implementation. Each bucket of the Hashmap contains a list of Strings with the same hash code. In earlier versions of Java, the storage area for the pool was a fixed size and could often lead to the “Could not reserve enough space for object heap” error.
When the system loads the classes, String literals of all classes go to the application-level pool. It is because of the fact that equal String literals of different classes have to be the same Object. In these situations, data in the pool should be available to each class without any dependency.
Usually, the stack stores the data that is short-lived. It includes local primitive variables, references of heap objects, and methods in execution. Heap allows dynamic memory allocation, stores the Java objects and JRE classes at the runtime.
The heap allows global access and data stores in the heap are available to all threads during the lifetime of the application, whereas the data stores on the stack have the private scope and only the owner thread can access them.
The stack stores the data in contiguous memory blocks and permits random access. If a class needs a random String from the pool, it might not be available due to the LIFO (last-in-first-out) rule of the stack. In contrast, the heap allocates the memory dynamically and allows us to access the data in any way.
Let’s assume we have a code snippet consisting of different types of variables. The stack will store the value of the int literal and references of String and Demo objects. The value of any object will be stored in the heap, and all the String literals go in the pool inside the heap:
The variables created on the stack are deallocated as soon as the thread completes execution. In contrast, a garbage collector reclaims the resources in the heap. Similarly, the garbage collector collects the un-referenced items from the pool.
The default size of the pool may differ on the different platforms. In any case, it is still much bigger than the available stack size. Before JDK 7, the pool was part of permgen space, and from JDK 7 to now, it is part of the main heap memory.
3. Conclusion
In this short article, we learned about the storage area for String constant pool. Stack and heap have different characteristics to store and access data. From memory allocation to its access and availability, a heap is the most suitable area to store the String constant pool.
In fact, the pool has never been a part of stack memory.
