This section describes stereotypes relating to the concept of a batch job. A Job is an entity that encapsulates an entire batch process. As is common with other Spring projects, a Job will be wired together via an XML configuration file or Java based configuration. This configuration may be referred to as the "job configuration". However, Job is just the top of an overall hierarchy:

In Spring Batch, a Job is simply a container for Steps. It combines multiple steps that belong logically together in a flow and allows for configuration of properties global to all steps, such as restartability. The job configuration contains:

A default simple implementation of the Job interface is provided by Spring Batch in the form of the SimpleJob class which creates some standard functionality on top of Job, however the batch namespace abstracts away the need to instantiate it directly. Instead, the <job> tag can be used:

<job id="footballJob">
    <step id="playerload" next="gameLoad"/>
    <step id="gameLoad" next="playerSummarization"/>
    <step id="playerSummarization"/>
</job>

3.1.2 JobParameters

Having discussed JobInstance and how it differs from Job, the natural question to ask is: "how is one JobInstance distinguished from another?" The answer is: JobParameters. JobParameters is a set of parameters used to start a batch job. They can be used for identification or even as reference data during the run:

In the example above, where there are two instances, one for January 1st, and another for January 2nd, there is really only one Job, one that was started with a job parameter of 01-01-2008 and another that was started with a parameter of 01-02-2008. Thus, the contract can be defined as: JobInstance = Job + identifying JobParameters. This allows a developer to effectively control how a JobInstance is defined, since they control what parameters are passed in.

	Note
	Not all job parameters are required to contribute to the identification of a JobInstance. By default they do, however the framework allows the submission of a Job with parameters that do not contribute to the identity of a JobInstance as well.

3.1.3 JobExecution

A JobExecution refers to the technical concept of a single attempt to run a Job. An execution may end in failure or success, but the JobInstance corresponding to a given execution will not be considered complete unless the execution completes successfully. Using the EndOfDay Job described above as an example, consider a JobInstance for 01-01-2008 that failed the first time it was run. If it is run again with the same identifying job parameters as the first run (01-01-2008), a new JobExecution will be created. However, there will still be only one JobInstance.

A Job defines what a job is and how it is to be executed, and JobInstance is a purely organizational object to group executions together, primarily to enable correct restart semantics. A JobExecution, however, is the primary storage mechanism for what actually happened during a run, and as such contains many more properties that must be controlled and persisted:

Table 3.1. JobExecution Properties

status	A BatchStatus object that indicates the status of the execution. While running, it's BatchStatus.STARTED, if it fails, it's BatchStatus.FAILED, and if it finishes successfully, it's BatchStatus.COMPLETED
startTime	A java.util.Date representing the current system time when the execution was started.
endTime	A java.util.Date representing the current system time when the execution finished, regardless of whether or not it was successful.
exitStatus	The ExitStatus indicating the result of the run. It is most important because it contains an exit code that will be returned to the caller. See chapter 5 for more details.
createTime	A java.util.Date representing the current system time when the JobExecution was first persisted. The job may not have been started yet (and thus has no start time), but it will always have a createTime, which is required by the framework for managing job level ExecutionContexts.
lastUpdated	A java.util.Date representing the last time a JobExecution was persisted.
executionContext	The 'property bag' containing any user data that needs to be persisted between executions.
failureExceptions	The list of exceptions encountered during the execution of a Job. These can be useful if more than one exception is encountered during the failure of a Job.

These properties are important because they will be persisted and can be used to completely determine the status of an execution. For example, if the EndOfDay job for 01-01 is executed at 9:00 PM, and fails at 9:30, the following entries will be made in the batch meta data tables:

Table 3.2. BATCH_JOB_INSTANCE

JOB_INST_ID	JOB_NAME
1	EndOfDayJob

Table 3.3. BATCH_JOB_EXECUTION_PARAMS

JOB_EXECUTION_ID	TYPE_CD	KEY_NAME	DATE_VAL	IDENTIFYING
1	DATE	schedule.Date	2008-01-01	TRUE

Table 3.4. BATCH_JOB_EXECUTION

JOB_EXEC_ID	JOB_INST_ID	START_TIME	END_TIME	STATUS
1	1	2008-01-01 21:00	2008-01-01 21:30	FAILED

	Note
	column names may have been abbreviated or removed for clarity and formatting

Now that the job has failed, let's assume that it took the entire course of the night for the problem to be determined, so that the 'batch window' is now closed. Assuming the window starts at 9:00 PM, the job will be kicked off again for 01-01, starting where it left off and completing successfully at 9:30. Because it's now the next day, the 01-02 job must be run as well, which is kicked off just afterwards at 9:31, and completes in its normal one hour time at 10:30. There is no requirement that one JobInstance be kicked off after another, unless there is potential for the two jobs to attempt to access the same data, causing issues with locking at the database level. It is entirely up to the scheduler to determine when a Job should be run. Since they're separate JobInstances, Spring Batch will make no attempt to stop them from being run concurrently. (Attempting to run the same JobInstance while another is already running will result in a JobExecutionAlreadyRunningException being thrown). There should now be an extra entry in both the JobInstance and JobParameters tables, and two extra entries in the JobExecution table:

Table 3.5. BATCH_JOB_INSTANCE

JOB_INST_ID	JOB_NAME
1	EndOfDayJob
2	EndOfDayJob

Table 3.6. BATCH_JOB_EXECUTION_PARAMS

JOB_EXECUTION_ID	TYPE_CD	KEY_NAME	DATE_VAL	IDENTIFYING
1	DATE	schedule.Date	2008-01-01 00:00:00	TRUE
2	DATE	schedule.Date	2008-01-01 00:00:00	TRUE
3	DATE	schedule.Date	2008-01-02 00:00:00	TRUE

Table 3.7. BATCH_JOB_EXECUTION

JOB_EXEC_ID	JOB_INST_ID	START_TIME	END_TIME	STATUS
1	1	2008-01-01 21:00	2008-01-01 21:30	FAILED
2	1	2008-01-02 21:00	2008-01-02 21:30	COMPLETED
3	2	2008-01-02 21:31	2008-01-02 22:29	COMPLETED

	Note
	column names may have been abbreviated or removed for clarity and formatting

A Step is a domain object that encapsulates an independent, sequential phase of a batch job. Therefore, every Job is composed entirely of one or more steps. A Step contains all of the information necessary to define and control the actual batch processing. This is a necessarily vague description because the contents of any given Step are at the discretion of the developer writing a Job. A Step can be as simple or complex as the developer desires. A simple Step might load data from a file into the database, requiring little or no code. (depending upon the implementations used) A more complex Step may have complicated business rules that are applied as part of the processing. As with Job, a Step has an individual StepExecution that corresponds with a unique JobExecution:

An ExecutionContext represents a collection of key/value pairs that are persisted and controlled by the framework in order to allow developers a place to store persistent state that is scoped to a StepExecution or JobExecution. For those familiar with Quartz, it is very similar to JobDataMap. The best usage example is to facilitate restart. Using flat file input as an example, while processing individual lines, the framework periodically persists the ExecutionContext at commit points. This allows the ItemReader to store its state in case a fatal error occurs during the run, or even if the power goes out. All that is needed is to put the current number of lines read into the context, and the framework will do the rest:

executionContext.putLong(getKey(LINES_READ_COUNT), reader.getPosition());

Using the EndOfDay example from the Job Stereotypes section as an example, assume there's one step: 'loadData', that loads a file into the database. After the first failed run, the meta data tables would look like the following:

In this case, the Step ran for 30 minutes and processed 40,321 'pieces', which would represent lines in a file in this scenario. This value will be updated just before each commit by the framework, and can contain multiple rows corresponding to entries within the ExecutionContext. Being notified before a commit requires one of the various StepListeners, or an ItemStream, which are discussed in more detail later in this guide. As with the previous example, it is assumed that the Job is restarted the next day. When it is restarted, the values from the ExecutionContext of the last run are reconstituted from the database, and when the ItemReader is opened, it can check to see if it has any stored state in the context, and initialize itself from there:

if (executionContext.containsKey(getKey(LINES_READ_COUNT))) {
    log.debug("Initializing for restart. Restart data is: " + executionContext);

    long lineCount = executionContext.getLong(getKey(LINES_READ_COUNT));

    LineReader reader = getReader();

    Object record = "";
    while (reader.getPosition() < lineCount && record != null) {
        record = readLine();
    }
}

In this case, after the above code is executed, the current line will be 40,322, allowing the Step to start again from where it left off. The ExecutionContext can also be used for statistics that need to be persisted about the run itself. For example, if a flat file contains orders for processing that exist across multiple lines, it may be necessary to store how many orders have been processed (which is much different from than the number of lines read) so that an email can be sent at the end of the Step with the total orders processed in the body. The framework handles storing this for the developer, in order to correctly scope it with an individual JobInstance. It can be very difficult to know whether an existing ExecutionContext should be used or not. For example, using the 'EndOfDay' example from above, when the 01-01 run starts again for the second time, the framework recognizes that it is the same JobInstance and on an individual Step basis, pulls the ExecutionContext out of the database and hands it as part of the StepExecution to the Step itself. Conversely, for the 01-02 run the framework recognizes that it is a different instance, so an empty context must be handed to the Step. There are many of these types of determinations that the framework makes for the developer to ensure the state is given to them at the correct time. It is also important to note that exactly one ExecutionContext exists per StepExecution at any given time. Clients of the ExecutionContext should be careful because this creates a shared keyspace, so care should be taken when putting values in to ensure no data is overwritten. However, the Step stores absolutely no data in the context, so there is no way to adversely affect the framework.

It is also important to note that there is at least one ExecutionContext per JobExecution, and one for every StepExecution. For example, consider the following code snippet:

ExecutionContext ecStep = stepExecution.getExecutionContext();
ExecutionContext ecJob = jobExecution.getExecutionContext();
//ecStep does not equal ecJob

As noted in the comment, ecStep will not equal ecJob; they are two different ExecutionContexts. The one scoped to the Step will be saved at every commit point in the Step, whereas the one scoped to the Job will be saved in between every Step execution.

JobRepository is the persistence mechanism for all of the Stereotypes mentioned above. It provides CRUD operations for JobLauncher, Job, and Step implementations. When a Job is first launched, a JobExecution is obtained from the repository, and during the course of execution StepExecution and JobExecution implementations are persisted by passing them to the repository:

<job-repository id="jobRepository"/>

JobLauncher represents a simple interface for launching a Job with a given set of JobParameters:

public interface JobLauncher {

    public JobExecution run(Job job, JobParameters jobParameters)
                throws JobExecutionAlreadyRunningException, JobRestartException;
}

It is expected that implementations will obtain a valid JobExecution from the JobRepository and execute the Job.

ItemReader is an abstraction that represents the retrieval of input for a Step, one item at a time. When the ItemReader has exhausted the items it can provide, it will indicate this by returning null. More details about the ItemReader interface and its various implementations can be found in Chapter 6, ItemReaders and ItemWriters.

ItemWriter is an abstraction that represents the output of a Step, one batch or chunk of items at a time. Generally, an item writer has no knowledge of the input it will receive next, only the item that was passed in its current invocation. More details about the ItemWriter interface and its various implementations can be found in Chapter 6, ItemReaders and ItemWriters.

ItemProcessor is an abstraction that represents the business processing of an item. While the ItemReader reads one item, and the ItemWriter writes them, the ItemProcessor provides access to transform or apply other business processing. If, while processing the item, it is determined that the item is not valid, returning null indicates that the item should not be written out. More details about the ItemProcessor interface can be found in Chapter 6, ItemReaders and ItemWriters.

Many of the domain concepts listed above need to be configured in a Spring ApplicationContext. While there are implementations of the interfaces above that can be used in a standard bean definition, a namespace has been provided for ease of configuration:

<beans:beans xmlns="http://www.springframework.org/schema/batch"
     xmlns:beans="http://www.springframework.org/schema/beans"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="
           http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans.xsd
           http://www.springframework.org/schema/batch
           http://www.springframework.org/schema/batch/spring-batch-2.2.xsd">

    <job id="ioSampleJob">
        <step id="step1">
            <tasklet>
                <chunk reader="itemReader" writer="itemWriter" commit-interval="2"/>
            </tasklet>
        </step>
    </job>

</beans:beans>

As long as the batch namespace has been declared, any of its elements can be used. More information on configuring a Job can be found in Chapter 4, Configuring and Running a Job. More information on configuring a Step can be found in Chapter 5, Configuring a Step.