JRE - Java Runtime Environment

Enhanced Definition

The Java Runtime Environment (JRE) on z/OS is a software package that provides the necessary components to run Java applications within the z/OS operating system. It includes the Java Virtual Machine (JVM), core class libraries, and supporting files, enabling the execution of compiled Java bytecode without requiring development tools.

Key Characteristics

- Platform-Specific Implementation: The z/OS JRE is specifically compiled and optimized for the IBM z/Architecture, leveraging hardware features for performance and efficiency, including support for zIIP (System z Integrated Information Processor) eligibility for certain workloads.
- Components: It primarily consists of the JVM (Java Virtual Machine), which interprets and executes Java bytecode, along with the standard Java class libraries (e.g., java.lang, java.util, java.io) and other runtime files.
- Installation Location: Typically installed within z/OS UNIX System Services (USS) file systems, often under directories like /usr/lpp/java/J8.0_64 or similar paths, making it accessible to various z/OS components and applications.
- Version Management: Multiple JRE versions (e.g., Java 8, Java 11, Java 17) can coexist on a single z/OS system, allowing different applications to run with their required Java levels without conflict.
- Execution-Only: Unlike the JDK (Java Development Kit), the JRE does not include compilers (javac), debuggers, or other development utilities; its sole purpose is to execute pre-compiled Java code.

Use Cases

- Running Java Batch Applications: Executing Java programs as traditional z/OS batch jobs, invoked via JCL using utilities like BPXBATCH or directly through OMVS commands, for tasks such as data processing, reporting, or utility functions.
- Hosting Application Servers: Providing the runtime foundation for Java application servers like IBM WebSphere Application Server for z/OS (including Liberty profile) or Apache Tomcat running within z/OS USS, hosting enterprise Java applications (e.g., servlets, EJBs).
- DB2 for z/OS Stored Procedures and UDFs: Enabling the execution of Java-based DB2 stored procedures and User-Defined Functions (UDFs) directly within the DB2 address space, allowing database logic to be implemented in Java.
- CICS Java Applications: Supporting Java applications deployed within CICS Transaction Server for z/OS, where CICS provides a JVM server environment to host and manage Java workloads for transactional processing.
- System Utilities and Automation: