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Memory Allocation in Java: Everything You Need To Know in 2023

Memory allocation in java refers to the process where the computer programs and services are allocated dedicated to virtual memory spaces. The Java Virtual Machine divides the memory into Stack and Heap Memory. For Java Virtual Machine, executing an application in its maximum potential can happen from stack and heap memory. Every time a new variable or object is declared, the memory allocates memory dedicated to such operations. If you often ask people what is JVM and explain me the Java memory allocation, this blog will guide you!

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What is the Structure of Memory in Java and Memory Management?

Before you understand memory management in Java, let us recollect the basics of the Java memory structure. In programming languages, memory refers to the spaces that store values, instructions or data. There are two types of memory in Java – stack memory and heap memory. Stack memory is the physical space or the RAM assigned to various Java objects during the run time. It is created for static memory allocation before executing a thread. The stack memory contains short-lived, method-specific values. It is based on the Last-in-first-out order, commonly called the LIFO order. The stack memory creates a new block when we invoke a method. It holds local primitive values. When we end the method, the block created in the stack memory becomes unoccupied.

Heap memory is the area in which all Java objects reside. The Java virtual machine (JVM) creates the heap space. The program uses heap space throughout the period during which the application runs. The heap space is further divided into several generations. The Young generation heap space is the place that stores new objects. Next is the old generation. The objects in the young generation have a timeline. When the timeline gets over, the objects are moved to the old generation. Last is the permanent generation, where the metadata for runtime classes and application methods is stored.

The heap space gets filled as new Java objects are created, and more memory is required for new objects. Therefore, we have to clean the heap space and provide memory for new objects. This process is called memory management. In simple words, memory management in Java is the process of allocating memory space to new Java objects. In memory management, the unused objects are removed and space is created for new Java objects.

How is Memory Management in Java Done?

One of the greatest advantages of using the Java programming language is that the programmers do not need to stress about memory management.  This job is done by garbage collector, a mechanism that enables automatic memory management in Java. There are two main components of memory management in Java:

1. Java Virtual Machine (JVM): You can understand JVM as an engine or a program that provides a runtime environment for Java bytecodes. It translates the bytecodes into computer language. It creates a memory in the heap space. This memory gets destroyed when the JVM exits.

2. Garbage collector: The garbage collection process starts when the heap memory becomes full. The garbage collector automatically destroys the objects that are no longer needed. Thus, it creates space for memory allocation of new objects. The garbage collector usually clears memory from the old generation.

What is JVM and explain me the Java Memory Allocation?

As discussed above, JVM or the Java virtual machine is the specification which provides a runtime environment to execute a bytecode. One of its primary functions is memory management. JVM creates runtime areas in the heap structure and provides instances for an object in the heap whenever we use a new keyword. It is essential for memory allocation in a heap.

As the name suggests, Java memory allocation is the process of allocating or assigning memory to various Java objects. This process involves setting aside virtual memory sections in a program. This is done by the JVM. There are two ways in which memory can be allocated in Java:

  1. Static memory allocation in Java: In static memory allocation, we have to declare the variables before executing the program. Static memory is allocated during the compile time.
  2. Dynamic memory allocation in Java: Dynamic memory allocation in Java means that memory is allocated to Java objects during the run time or the execution time. It is contrary to static memory allocation. The dynamic memory allocation takes place in the heap space. The heap space is where new objects are always created and their references are stored in the stack memory.

Now let us discuss Java memory allocation for both stack and heap memory.

Stack Memory

The Stock Memory allocation in java is used for static memory and thread execution. The values contained in this memory are temporary and limited to specific methods as they keep getting referenced in Last-In-First-Out fashion.

As soon as the memory is called and a new block gets created in the stack memory, the stack memory then holds primitive values and references until the method lasts. After its ending, the block is flushed and is available for a new process to take place. In general, the overall size of the stack memory is insignificant to that of the heap memory. 

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Characteristics of Stack Memory

Based on the different sections of the memory allocation in Java Virtual Machine (JVM), here are some of the discrete features of the stack memory:

  • The stack memory can grow or contract as any new methods get called and returned accordingly.
  • Any variable in the stack can run as long as the scope of the method exists.
  • It gets auto-allocation and deallocation as and when a method undergoes execution. 
  • In the case of full memory, the java.lang.StackOverFlowError sets off.
  • It is faster in access when compared to the heap memory.

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Methods used in the stack memory allocation in java

  • Object push(Object element): Here, an item gets pushed to the top of the stack.
  • Object pop(): Any element located at the top of the stack gets flushed and returned. In case of a stack being vacant as the pop() gets invoked, the exception – EmptyStackException occurs. 
  • Object peek(): Here, the top element gets returned but doesn’t undergo flushing.
  • Boolean empty(): If the loop doesn’t have any top value in its stack, the function returns 1 (true), otherwise 0 (false).
  • In search(Object element): This is used to understand if an object is present in the stack. In case the value is found, the function returns the location of the element from the top of the stack, otherwise returns -1.

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Java Heap Space

Mainly used by java runtime, Java Heap Space comes into play every time an object is created and allocated in it. The discrete function, like Garbage Collection, keeps flushing the memory used by the previous objects that hold no reference. For an object created in the Heap Space can have free access across the application. 

The Memory allocation in java is divided into parts, namely Heap, Stack, Code, and Static.

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Characteristics of the Java Heap Memory

  • Accessible from the complicated memory management technique, including the Young Generation, Old or Tenured Generation, and Permanent Generation.
  • In heap memory, when it gets full, it returns java.lang.OutOfMemoryError.
  • The access in this memory is comparatively slower than that of the stack memory.
  • It doesn’t undergo automatic deallocation and requires a similar function like Garbage Collector to remove foreign objects for the memory to work in its optimal stage.

An example of the stack and heap memory allocation in java is: 

Source

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Heap Space and Stack Memory: Fundamental Differences

Heap Space Memory Stack Memory
All parts of the application invoke the heap memory. The stack memory execution is limited to a single thread.
Anytime an object gets created, it is stored in the heap space.    The stack memory only comprises its reference and local primitive variables. 
Objects here are accessible globally across the application.  Other threads cannot access stack memory objects.
Here, memory is defined according to young and old generations. Memory management occurs on a Last-In-First-Out basis.
The memory remains as per the scope of the application. Memory is temporary.
The methods like – XMX and XMS JVM are used to define the optimal size of the heap memory.  For stack memory, it gets determined by the -XSS method.
Here, the exception of java.lang.OutOfMemoryError occurs in the case of full memory. Here, the error java.lang.StackOverFlowError happens in case the memory is full.
The size is more but takes time to process compared to the stack memory.                   The size is lesser but faster in execution for its smooth LIFO operation.

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Conclusion

Memory allocation in java occurs in two ways, mainly, stack and heap space. We hope it helped you in understanding the process of it all.

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How does memory management happen in Java?

Java Virtual Machine (JVM) is a computer program used to run programs written in Java programming language. It makes the process of using memory management efficient. Also, it is designed to be robust so that even if there is a crash in the operating system, the program that is running on the JVM is not affected. Memory management means managing every piece of memory (RAM) in a computer system. JVM monitors allocating and releasing memory when needed. As soon as a program is loaded into the memory, the JVM allocates memory space for it. This process is called heap. The heap is a common memory space for many parts of the application.

What is Garbage Collection in Java?

The Java Virtual Machine (JVM) uses reference counting to track the number of Java objects. Before any object can be collected, the number of references to this object must reach zero. The user program can explicitly delete objects by calling finalize() method. Finalize is a static method in the Object class which is called by Garbage Collector (GC). Finalize method will release all the resources of the object before Garbage Collector will take care of this object. Garbage Collector is the process of cleaning unwanted objects. The process of Garbage Collector will be triggered when the Java run-time system detects that the Java heap is almost full. Every object has a bitmap in JVM. The bitmap is set for each object to track whether it's been used or not. When the bitmap is turned to 0, GC will take care of this object.

What are the features of Java programming language?

Java Programming Language is a general-purpose, high-level programming language that is used to build applications and applets. Java software can run on any platform that supports Java without being reprogrammed. Features of Java programming language are: Object-oriented features, Robust, High Security, and Platform-independent, Free of Cost, Easy to Learn and use. Java is an object-oriented programming language which is platform independent and can also be used on any OS or platform. Java is compiled language and bytecode is produced which is then executed in the Java Virtual Machine.

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