This chapter describes CICS® performance considerations for Java™ programs
that require a Java Virtual Machine (JVM) in which to execute.
Java application programs can run under CICS control in Java Virtual Machines (JVMs) that are initialized
by CICS and run within the CICS region address space. When a Java program
executes in a JVM, the JVM interprets the Java bytecode. The type of JVM supported
by CICS is the JVM that is provided by the IBM Software Developer Kit for z/OS, Java 2 Technology Edition, Version 1.4.2 at
or later, which features the persistent reusable JVM technology. This JVM can be reused many
times by Java applications in CICS, avoiding the high startup overheads
that are typical for a single-use JVM.
"About JVMs" in Java Applications in CICS explains:
- The type of JVM that is supported by each release of CICS.
- The structure of JVMs, including the types of classes contained in a JVM,
and the different storage heaps in a JVM.
- How CICS creates JVMs and manages the JVM pool in which they reside, including:
how CICS obtains information from the PROGRAM resource definition to create
a JVM that is suitable for a Java program; how JVMs run on J8-mode open
TCBs (for JVMs in CICS key) or J9-mode open TCBs (for JVMs in user key),
in the open transaction environment (OTE); and how the MAXJVMTCBS limit controls
the number of JVMs in the JVM pool.
- How CICS assigns JVMs to applications that request them, including the mechanism
that CICS uses to control mismatches and steals in the JVM pool.
- The three levels of reusability for JVMs--resettable JVMs, continuous
JVMs, and single-use JVMs--and the performance implications of each level
of reusability.
- How the shared class cache works. The shared class cache enables the JVMs
in the CICS region to share commonly-used class files and compiled classes,
which would otherwise be stored in the system heap for each individual JVM.
Before attempting performance tuning for your JVMs, you should read this
information to help you understand how CICS deals with JVMs.
"Managing your JVMs" in Java Applications in CICS tells you
how to perform basic management tasks for the JVMs in your CICS region, including:
- Monitoring the JVMs and collecting statistics.
- Terminating or disabling the JVM pool.
- Redirecting the output from JVMs.
- Controlling JVM tracing.
You can also take the following action to tune the JVM pool and your
JVMs:
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