1. Introduction The Java Runtime Environment (JRE) is a software layer that provides the minimum requirements for executing Java applications. It is often confused with the Java Development Kit (JDK), but while the JDK is a toolkit for developing Java programs, the JRE is the engine for running them. Without the JRE, a computer cannot execute Java bytecode.
| Command | Purpose | |---------|---------| | java | Launches a Java application. | | javaw | Launches a Java app without a console window (Windows). | | keytool | Manages keystores and certificates. | | policytool | Edits security policy files (deprecated in recent versions). | | jjs | Nashorn JavaScript engine (deprecated). | | jrunscript | Runs scripts (deprecated). | For development tools (e.g., javac , jdb , javadoc ), you need the JDK. # Check installed JRE version java -version Sample output: openjdk version "17.0.5" 2022-10-18 LTS Runtime Environment (build 17.0.5+8) runtime environment in java
javac Hello.java Run (requires JRE):
public class Hello public static void main(String[] args) System.out.println("JRE is working!"); Without the JRE, a computer cannot execute Java bytecode
| Memory Area | Purpose | |-------------|---------| | | Stores all objects and arrays. Shared across threads. Managed by garbage collection. | | Stack | Each thread has its own stack storing method calls, local variables, and partial results. | | Method Area | Stores class metadata, static variables, constants, and method bytecode. | | PC Registers | Holds the address of the current executing instruction per thread. | | Native Method Stack | Supports execution of native (C/C++) code via JNI. | | | keytool | Manages keystores and certificates
Running a simple program: