Since most users will never develop applications, it doesn’t make sense to include these development tools into JRE. However, to develop these applications, you’ll need special development tools to transform Java source code into executable bytecode classes, libraries and full-blown applications. Now you understand that you need JRE to run Java applications on host devices. In other words, JVM is the component which allows Java bytecode to be platform-agnostic. In some sense, you can think of JVM as an adapter between platform-independent Java bytecode and platform-specific hardware and software. “hides”) the specifics of the hardware and the operating system installed on the host device. This component executes Java bytecode and abstracts out (i.e. I won’t elaborate more on class libraries and class loader (search the web if you’re curious what they do), but JVM is very prominent concept in Java world, so we need to discuss it as well. Generally speaking, the main tools contained inside JRE are class libraries, class loader and JVM. In short: JRE is a set of tools needed to run Java programs. Instead, you’ll need to install JRE on the target machine and then use this software to execute your Java application. That’s really nice feature, but such flexibility comes with a cost: you can’t just run Java applications “natively”. Since Java was designed with “write once, run anywhere” idea in mind, you should be able to run Java programs in many different environments. To understand what it does, imagine that you have a full-blown Java application and you want to run it. The most important parts of Java platform are so-called “JVM programming languages” (Java, Kotlin, Scala, etc.), JRE, JDK and JVM.
Therefore, the language itself is just one piece of a larger toolchain. First, you need to transform it into so-called bytecode and then use special tools to execute the bytecode on target devices. However, as you might know, Java source code isn’t executable in its original form. Developers who write code in Java programming language usually call it just “Java”, so it’s natural to think of Java as just that: a programming language.
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