Java Stand-alone Applications on z/OS, Volume I

Chapter 2. Job management 13

The first step was described briefly in the previous section and it should not

present much difficulty for Java developers because the process of writing and

compiling Java programs is the same on any platforms.

2.2.1 Overview of Job Control Language

Job Control Language (JCL) is a scripting language used to control the

execution of programs in mainframe operating environments. A JCL script

describes the unit of work to be performed (job steps), resource allocations (for

example, data set and memory), and much more.

A JCL script typically consists of a job card, data definition (DD) statements, and

EXEC statements. A

job card marks the beginning of a job and tells the system

how to process the job. It includes information such as a job name, accounting

information, and message level and class.

An

EXEC statement identifies a step within a job and describes a program to be

executed in the step. A job typically consists of several job steps.

The

REGION parameter specifies the virtual storage allocation required for this

step. For example, REGION=32M means the step requires 32 megabytes of

virtual storage.

The

COND parameter is used to specify when this step should be executed. For

example, COND=(0,NE) means this step gets executed only when all of the

previous steps finished with a return code of 0.

DD statements describe the I/O resources required for the step.

The syntax of JCL scripts is very strict. It is especially sensitive regarding the

starting positions of various keywords and the continuation of a statement on the

next line. Basic JCL syntax is summarized below:

򐂰 All JCL statements must start in column 1 and be identified by // (two slashes)

at the beginning of the line.

򐂰 All statements must start in column 1 and end in column 71.

򐂰 A comma indicates the statement has a continuation.

򐂰 A continuation of a statement must start between column 4 and 16.

򐂰 //* at the start of the line marks a comment line.

14 Java Stand-alone Applications on z/OS Volume 1

Once a job is submitted to the system, it is the responsibility of the Job Entry

Subsystem (JES)

to schedule and execute the job. There are two versions of JES

systems supported, JES2 and JES3. An overview of the job submission and

execution process is shown in Figure 2-1. A user submits a job to the system via

JES. JES interprets the JCL and passes the job to the system for processing with

required resources. Once the job is completed by the system, JES collects the

output and information about the job and returns it to the user.

Figure 2-1 Overview of job submission and execution

Example 2-2 on page 15 shows a typical JCL script that invokes a REXX exec

called MYREXX. REXX execs are invoked using a program called IRXJCL.

IRXJCL must know where to find the REXX exec to execute, which is specified

using a DD statement called SYSEXEC. The MYREXX exec uses two

Tip: Since the syntax of JCL scripts is very strict, the syntax highlighting

feature of the ISPF editor may help identify problems. The syntax highlighting

feature of the ISPF editor can be turned on by typing the following command

at the command prompt:

HILIGHT ON

User

User

JES

Job

Job

Results

Results

Resource

Resource

Resource

zSeries