Java Memory Layout

by Dongsoo Kim

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Posted on November 30, 2022

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//image made by DongsooKim

How JVM works on the Memory?

1. Overview of Java Memory

Once .java file is compiled, it becomes .class file which includes its name, the number of class’s fields, methods, and byte code. There is a class loader which load all the .class files and put it into Method area in Runtime Data Area.

Runtime Data Area consists of 5 areas. Method area which stores the class information from .class files and of course, static fileds. Heap stores instances created by “new” keywords, and becomes target of Garbage Collection. JVM Stacks stores each method when it is called as a “Frame” structure. And as its name, it has a stack data structure which is characterized with LIFO(Last In First Out). For example, once the methodA call methodB then, methodB stacks above methodA. After methodA is done its process, then it is removed from stack and thread goes back to the methodA. PC Registers stores current command of thread and Native Method Area manages the method written by C or C++.

Method and Heap area are shared by threads so developer needs to consider concurrency in the multi-thread environment. Otherwise, JVM Stacks, PC Registers, and Native Method Stacks belongs to each thread, so when the new thread is created, each of 3 areas are created as well.

2. How it works?

After .class file is loaded by the Class Loader and put into Method, JVM brings byte codes from .class file to Execute Engine and translate it into the binary machine codes. With this proces, JVM enables the same Java application run in the different OS and that is why JAVA is well known for “Platform-independent”.

While processing, if there is new instances initialized, those are stored in Heap area and every time the thread meets the method, it encapsulate with the Frame and put it into stack.

3. What is Frame in JVM Stacks?

Frame consists of Current Class Constant pool references, local variables array, operand stacks. The reason it uses the datastructure array is to find the elements as fast as possible with index. At the very first index, it always contains this indicating current class. And input parameters and variables comes in a row. When it is required to sum the given input parameter(value 27) and localV(value 23), the sum operand would be stored in a stack. After the sum calculation is done, then it assign integer variable ‘sum’ with the result 50 as the byte code says. After it returns the certain result, and once method is done, it is removed from the JVM Stacks.

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