The Java Runtime Environment, or JRE, is a software layer that runs on top of a computer’s operating system software and provides the class libraries and other resources that a specific Java program needs to run.
The JRE is one of three interrelated components for developing and running Java programs. The other two components are as follows:
The JRE combines Java code created using the JDK with the necessary libraries required to run it on a JVM and then creates an instance of the JVM that executes the resulting program. JVMs are available for multiple operating systems, and programs created with the JRE will run on all of them. In this way, the Java Runtime Environment is what enables a Java program to run in any operating system without modification.
You can download the JDK, including a compatible JRE, from Oracle (link resides outside of ibm.com).
The JDK and JRE interact with one another to create a sustainable runtime environment that enables the seamless execution of Java-based applications in virtually any operating system. The following make up the JRE runtime architecture:
The Java ClassLoader dynamically loads all classes necessary to run a Java program. Since Java classes are only loaded into memory when they're required, the JRE uses ClassLoaders to automate this process on demand.
The bytecode verifier ensures the format and accuracy of Java code before it passes to the interpreter. In the event that code violates system integrity or access rights, the class will be considered corrupted and won't be loaded.
After the bytecode successfully loads, the Java interpreter creates an instance of the JVM that allows the Java program to be executed natively on the underlying machine.
Besides the Java Virtual Machine, JRE is composed of a variety of other supporting software tools and features to get the most out of your Java applications.
Included as part of the JRE installation are deployment technologies like Java Web Start and Java Plugin that simplify the activation of applications and provide advanced support for future Java updates.
The JRE also contains toolkits designed to help developers improve their user interface. Some of these toolkits include:
Java Runtime Environment provides a number of integration libraries to assist developers in creating seamless data connections between their applications and services. Some of these libraries include:
Included with the JRE are java.lang. and java.util. packages that are fundamental for the design of Java applications, package versioning, management, and monitoring. Some of these packages include:
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