JVM vs JDK vs JRE: What's the difference? #java #springboot #apache #tomcat #intellij #kotlin #scala

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The Java ecosystem relies on three key components that are often discussed together: JVM (Java Virtual Machine), JRE (Java Runtime Environment), and JDK (Java Development Kit). While these terms are interconnected, they serve distinct roles in Java application development and execution. Understanding the differences and purposes of JVM, JRE, and JDK is crucial for developers working with Java.

JVM (Java Virtual Machine)
The JVM is the core component responsible for executing Java bytecode. When you write a Java program, it is first compiled into bytecode by the Java compiler. The JVM interprets this bytecode and translates it into machine code that your hardware can understand, allowing your Java application to run on any device with a compatible JVM, regardless of the underlying platform (Windows, Linux, macOS, etc.). This is known as the "write once, run anywhere" principle.

Role of the JVM: The JVM manages the execution of Java applications, handles memory allocation and garbage collection, and ensures security through bytecode verification.
Language Agnostic: While designed for Java, the JVM can also run programs written in other languages that compile to bytecode, such as Kotlin, Scala, and Groovy.
JRE (Java Runtime Environment)

Role of the JRE: It contains everything needed to run Java programs but does not provide tools for compiling or debugging code.
End of the Standalone JRE: As of Java 9, Oracle no longer releases a standalone JRE. The introduction of the Java Platform Module System (JPMS) made the traditional JRE unnecessary. The modular design of Java 9 and beyond allows applications to bundle only the necessary modules needed for their execution, eliminating the need for a monolithic runtime like the old JRE.
JDK (Java Development Kit)
The JDK is the full development environment required to build and run Java applications. It includes the JRE, JVM, and additional tools for developers, such as the javac compiler, debuggers, profilers, and various utilities for packaging and deploying applications. Essentially, the JDK is the superset of the JRE and provides everything needed for both developing and running Java applications.

Role of the JDK: It provides the tools necessary for Java development. This includes the ability to compile Java source code into bytecode and run it in a JVM.
JDK vs. JRE: The JDK contains the JRE within it, meaning that you can use the JDK to run Java programs just like you would with the JRE. The key difference is that the JDK also provides development tools like the Java compiler (javac).
Java 9 and Beyond: The Modular JRE
With the release of Java 9, Oracle introduced the Java Platform Module System, also known as Project Jigsaw. This modular system divides the JDK into smaller, manageable components called modules. The traditional monolithic JRE became obsolete because applications can now use only the modules they require. This leads to smaller application sizes and more efficient deployment. As a result, Oracle stopped providing a separate JRE download, as the modularization allows both the JDK and runtime to be more lightweight and adaptable.

The modular JDK provides tools like jlink, which allows developers to create custom runtime images that include only the necessary modules. This modular approach is more efficient and better suited for modern applications, especially in environments like containers and microservices.

Conclusion
JVM: The engine that runs Java bytecode, making it platform-independent.
JRE: A collection of libraries and the JVM necessary to run Java applications, now replaced by modular runtimes.
JDK: The comprehensive toolkit containing everything needed for both developing and running Java applications, including the JRE, JVM, and development tools.
Since the introduction of Java 9's module system, Oracle has moved away from distributing a standalone JRE. Instead, developers and users can now rely on custom runtimes tailored to their application's needs, streamlining both the size and performance of Java programs. Understanding the interplay between the JVM, JDK, and the now-redundant JRE is essential for developing and deploying Java applications in the modern Java ecosystem.