Using DFHJVMRO to modify the Language Environment enclave for a JVM

DFHJVMRO specifies the run-time options that are used to create the Language Environment® enclave in which a JVM runs. It defines storage allocation parameters for heap and stack and a number of other options. For CICS®, the storage settings that are supplied in DFHJVMRO are more efficient than the default Language Environment storage settings. Java™ Applications in CICS explains how the storage for a JVM is allocated within its Language Environment enclave.

You might want to change the supplied version of the program if you need to:

Note that the initial heap size and the heap increment sizes defined by DFHJVMRO are for minimal values, and less than the minimum heap size for a typical JVM. This is because the actual amount of storage allocated for the Java heap is controlled by the Xmx initialization option specified in a JVM profile.

Language Environment run-time options are provided for the JVM by the following mechanisms, of which you have control over the first two only:

  1. The installation-defined defaults generated by the CEEDOPT macro.
  2. The DFHJVMRO user-replaceable module.
  3. The CICS JVM domain, which enforces certain options.

The DFHJVMRO user-replaceable-module:

The z/OS Version 1.4 Language Environment Programming Guide, SA22-7561-03, has complete information about Language Environment run-time options. The options are specified as character strings, comprising a 2-byte string length followed by the run-time option. The maximum length allowed for all Language Environment run-time options is 255 bytes, so you are recommended to use the abbreviated version of each option, and restrict your changes to a total of under 200 bytes.

Start of changeCICS checks the length of the runtime options before passing them to Language Environment. If the length is greater than 255 bytes, CICS does not attempt to start the JVM. Error messages are written to CSMT in this case. Note that the values you specify are not checked by CICS before being passed to Language Environment.End of change

If you decide to modify DFHJVMRO, note the following:

See the fully-commented source code for DFHJVMRO for an example of how to set these options.

Related tasks
Tuning Language Environment enclave storage for a JVM
Writing Java classes to redirect JVM stdout and stderr output
Using DFHJVMAT to modify options in a JVM profile
Rewriting user-replaceable programs
Assembling and link-editing user-replaceable programs
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