Spring Batch - Reference Documentation

Spring Batch supports the following business scenarios:

Spring Batch has the following technical objectives:

The following key principles, guidelines, and general considerations should be considered when building a batch solution.

To help design and implement batch systems, basic batch application building blocks and patterns should be provided to the designers and programmers in the form of sample structure charts and code shells. When starting to design a batch job, the business logic should be decomposed into a series of steps that can be implemented by using the following standard building blocks:

Additionally, a basic application shell should be provided for business logic that cannot be built by using the previously mentioned building blocks.

In addition to the main building blocks, each application may use one or more standard utility steps, such as:

Batch applications can additionally be categorized by their input source:

The foundation of any batch system is the processing strategy. Factors affecting the selection of the strategy include: estimated batch system volume, concurrency with online systems or with other batch systems, available batch windows. (Note that, with more enterprises wanting to be up and running 24x7, clear batch windows are disappearing).

Typical processing options for batch are (in increasing order of implementation complexity):

Some or all of these options may be supported by a commercial scheduler.

The remainder of this section discusses these processing options in more detail. Note that, as a rule of thumb, the commit and locking strategy adopted by batch processes depends on the type of processing performed and that the online locking strategy should also use the same principles. Therefore, the batch architecture cannot be simply an afterthought when designing an overall architecture.

The locking strategy can be to use only normal database locks or to implement an additional custom locking service in the architecture. The locking service would track database locking (for example, by storing the necessary information in a dedicated database table) and give or deny permissions to the application programs requesting a database operation. Retry logic could also be implemented by this architecture to avoid aborting a batch job in case of a lock situation.

1. Normal processing in a batch window For simple batch processes running in a separate batch window where the data being updated is not required by online users or other batch processes, concurrency is not an issue and a single commit can be done at the end of the batch run.

In most cases, a more robust approach is more appropriate. Keep in mind that batch systems have a tendency to grow as time goes by, both in terms of complexity and the data volumes they handle. If no locking strategy is in place and the system still relies on a single commit point, modifying the batch programs can be painful. Therefore, even with the simplest batch systems, consider the need for commit logic for restart-recovery options as well as the information concerning the more complex cases described later in this section.

2. Concurrent batch or on-line processing Batch applications processing data that can be simultaneously updated by online users should not lock any data (either in the database or in files) that could be required by on-line users for more than a few seconds. Also, updates should be committed to the database at the end of every few transactions. Doing so minimizes the portion of data that is unavailable to other processes and the elapsed time the data is unavailable.

Another option to minimize physical locking is to have logical row-level locking implemented with either an optimistic locking pattern or a pessimistic locking pattern.

These patterns are not necessarily suitable for batch processing, but they might be used for concurrent batch and online processing (such as in cases where the database does not support row-level locking). As a general rule, optimistic locking is more suitable for online applications, while pessimistic locking is more suitable for batch applications. Whenever logical locking is used, the same scheme must be used for all applications that access the data entities protected by logical locks.

Note that both of these solutions only address locking a single record. Often, we may need to lock a logically related group of records. With physical locks, you have to manage these very carefully to avoid potential deadlocks. With logical locks, it is usually best to build a logical lock manager that understands the logical record groups you want to protect and that can ensure that locks are coherent and non-deadlocking. This logical lock manager usually uses its own tables for lock management, contention reporting, time-out mechanism, and other concerns.

3. Parallel Processing Parallel processing lets multiple batch runs or jobs run in parallel to minimize the total elapsed batch processing time. This is not a problem as long as the jobs are not sharing the same files, database tables, or index spaces. If they do, this service should be implemented by using partitioned data. Another option is to build an architecture module for maintaining interdependencies by using a control table. A control table should contain a row for each shared resource and whether it is in use by an application or not. The batch architecture or the application in a parallel job would then retrieve information from that table to determine whether it can get access to the resource it needs.

If the data access is not a problem, parallel processing can be implemented through the use of additional threads to process in parallel. In a mainframe environment, parallel job classes have traditionally been used, to ensure adequate CPU time for all the processes. Regardless, the solution has to be robust enough to ensure time slices for all the running processes.

Other key issues in parallel processing include load balancing and the availability of general system resources, such as files, database buffer pools, and so on. Also, note that the control table itself can easily become a critical resource.

4. Partitioning Using partitioning lets multiple versions of large batch applications run concurrently. The purpose of this is to reduce the elapsed time required to process long batch jobs. Processes that can be successfully partitioned are those where the input file can be split or the main database tables partitioned to let the application run against different sets of data.

In addition, processes that are partitioned must be designed to process only their assigned data set. A partitioning architecture has to be closely tied to the database design and the database partitioning strategy. Note that database partitioning does not necessarily mean physical partitioning of the database (although, in most cases, this is advisable). The following image illustrates the partitioning approach:

The architecture should be flexible enough to allow dynamic configuration of the number of partitions. You shoul consider both automatic and user controlled configuration. Automatic configuration may be based on such parameters as the input file size and the number of input records.

4.1 Partitioning Approaches Selecting a partitioning approach has to be done on a case-by-case basis. The following list describes some of the possible partitioning approaches:

1. Fixed and Even Break-Up of Record Set

This involves breaking the input record set into an even number of portions (for example, 10, where each portion has exactly 1/10th of the entire record set). Each portion is then processed by one instance of the batch/extract application.

To use this approach, preprocessing is required to split the record set up. The result of this split is a lower and upper bound placement number that you can use as input to the batch/extract application to restrict its processing to only its portion.

Preprocessing could be a large overhead, as it has to calculate and determine the bounds of each portion of the record set.

2. Break up by a Key Column

This involves breaking up the input record set by a key column, such as a location code, and assigning data from each key to a batch instance. To achieve this, column values can be either:

Under option 1, adding new values means a manual reconfiguration of the batch or extract to ensure that the new value is added to a particular instance.

Under option 2, this ensures that all values are covered by an instance of the batch job. However, the number of values processed by one instance is dependent on the distribution of column values (there may be a large number of locations in the 0000-0999 range and few in the 1000-1999 range). Under this option, the data range should be designed with partitioning in mind.

Under both options, the optimal even distribution of records to batch instances cannot be realized. There is no dynamic configuration of the number of batch instances used.

3. Breakup by Views

This approach is basically breakup by a key column but on the database level. It involves breaking up the record set into views. These views are used by each instance of the batch application during its processing. The breakup is done by grouping the data.

With this option, each instance of a batch application has to be configured to hit a particular view (instead of the main table). Also, with the addition of new data values, this new group of data has to be included into a view. There is no dynamic configuration capability, as a change in the number of instances results in a change to the views.

4. Addition of a Processing Indicator

This involves the addition of a new column to the input table, which acts as an indicator. As a preprocessing step, all indicators are marked as being non-processed. During the record fetch stage of the batch application, records are read on the condition that an individual record is marked as being non-processed, and, once it is read (with lock), it is marked as being in processing. When that record is completed, the indicator is updated to either complete or error. You can start many instances of a batch application without a change, as the additional column ensures that a record is only processed once.

With this option, I/O on the table increases dynamically. In the case of an updating batch application, this impact is reduced, as a write must occur anyway.

5. Extract Table to a Flat File

This approach involves the extraction of the table into a flat file. This file can then be split into multiple segments and used as input to the batch instances.

With this option, the additional overhead of extracting the table into a file and splitting it may cancel out the effect of multi-partitioning. Dynamic configuration can be achieved by changing the file splitting script.

6. Use of a Hashing Column

This scheme involves the addition of a hash column (key or index) to the database tables used to retrieve the driver record. This hash column has an indicator to determine which instance of the batch application processes this particular row. For example, if there are three batch instances to be started, an indicator of 'A' marks a row for processing by instance 1, an indicator of 'B' marks a row for processing by instance 2, and an indicator of 'C' marks a row for processing by instance 3.

The procedure used to retrieve the records would then have an additional WHERE clause to select all rows marked by a particular indicator. The inserts in this table would involve the addition of the marker field, which would be defaulted to one of the instances (such as 'A').

A simple batch application would be used to update the indicators, such as to redistribute the load between the different instances. When a sufficiently large number of new rows have been added, this batch can be run (anytime, except in the batch window) to redistribute the new rows to other instances.

Additional instances of the batch application require only the running of the batch application (as described in the preceding paragraphs) to redistribute the indicators to work with a new number of instances.

4.2 Database and Application Design Principles

An architecture that supports multi-partitioned applications that run against partitioned database tables and use the key column approach should include a central partition repository for storing partition parameters. This provides flexibility and ensures maintainability. The repository generally consists of a single table, known as the partition table.

Information stored in the partition table is static and, in general, should be maintained by the DBA. The table should consist of one row of information for each partition of a multi-partitioned application. The table should have columns for Program ID Code, Partition Number (the logical ID of the partition), Low Value of the database key column for this partition, and High Value of the database key column for this partition.

On program start-up, the program id and partition number should be passed to the application from the architecture (specifically, from the control processing tasklet). If a key column approach is used, these variables are used to read the partition table to determine what range of data the application is to process. In addition, the partition number must be used throughout the processing to:

4.3 Minimizing Deadlocks

When applications run in parallel or are partitioned, contention for database resources and deadlocks may occur. It is critical that the database design team eliminate potential contention situations as much as possible, as part of the database design.

Also, the developers must ensure that the database index tables are designed with deadlock prevention and performance in mind.

Deadlocks or hot spots often occur in administration or architecture tables, such as log tables, control tables, and lock tables. The implications of these should be taken into account as well. Realistic stress tests are crucial for identifying the possible bottlenecks in the architecture.

To minimize the impact of conflicts on data, the architecture should provide services (such as wait-and-retry intervals) when attaching to a database or when encountering a deadlock. This means a built-in mechanism to react to certain database return codes and, instead of issuing an immediate error, waiting a predetermined amount of time and retrying the database operation.

4.4 Parameter Passing and Validation

The partition architecture should be relatively transparent to application developers. The architecture should perform all tasks associated with running the application in a partitioned mode, including:

The validation should include checks to ensure that:

If the database is partitioned, some additional validation may be necessary to ensure that a single partition does not span database partitions.

Also, the architecture should take into consideration the consolidation of partitions. Key questions include:

This section shows the major highlights of Spring Batch 5. For more details, please refer to the migration guide.

Historically, Spring Batch provided a map-based job repository and job explorer implementations to work with an in-memory job repository. These implementations were deprecated in version 4 and completely removed in version 5. The recommended replacement is to use the JDBC-based implementations with an embedded database, such as H2, HSQL, and others.

In this release, the @EnableBatchProcessing annotation configures a JDBC-based JobRepository, which requires a DataSource and PlatformTransactionManager beans to be defined in the application context. The DataSource bean could refer to an embedded database to work with an in-memory job repository.

Until version 4.3, the @EnableBatchProcessing annotation exposed a transaction manager bean in the application context. While this was convenient in many cases, the unconditional exposure of a transaction manager could interfere with a user-defined transaction manager. In this release, @EnableBatchProcessing no longer exposes a transaction manager bean in the application context.

In this release, the @EnableBatchProcessing annotation provides new attributes to specify which components and parameters should be used to configure the Batch infrastructure beans. For example, it is now possible to specify which data source and transaction manager Spring Batch should configure in the job repository as follows:

In this example, batchDataSource and batchTransactionManager refer to beans in the application context, and which will be used to configure the job repository and job explorer. There is no need to define a custom BatchConfigurer anymore, which was removed in this release.

In this release, a new configuration class named DefaultBatchConfiguration can be used as an alternative to using @EnableBatchProcessing for the configuration of infrastructure beans. This class provides infrastructure beans with default configuration which can be customized as needed. The following snippet shows a typical usage of this class:

In this example, the JobRepository bean injected in the Job bean definition is defined in the DefaultBatchConfiguration class. Custom parameters can be specified by overriding the corresponding getter. For example, the following example shows how to override the default character encoding used in the job repository and job explorer:

This version adds support to use any type as a job parameter, and not only the 4 pre-defined types (long, double, string, date) as in v4. This change has an impact on how job parameters are persisted in the database (There are no more 4 distinct columns for each predefined type). Please check Column change in BATCH_JOB_EXECUTION_PARAMS for DDL changes. The fully qualified name of the type of the parameter is now persisted as a String, as well as the parameter value. String literals are converted to the parameter type with the standard Spring conversion service. The standard conversion service can be enriched with any required converter to convert user specific types to and from String literals.

The default notation of job parameters in v4 was specified as follows:

where parameterType is one of [string,long,double,date]. This notation is limited, constraining, does not play well with environment variables and is not friendly with Spring Boot.

In v5, there are two way to specify job parameters:

Up to version 4.3, the JobLauncherTestUtils and JobRepositoryTestUtils used to autowire the job under test as well as the test datasource to facilitate the testing infrastructure setup. While this was convenient for most use cases, it turned out to cause several issues for test contexts where multiple jobs or multiple data sources are defined.

In this release, we introduced a few changes to remove the autowiring of such dependencies in order to avoid any issues while importing those utilities either manually or through the @SpringBatchTest annotation.

In this release, the entire test suite of Spring Batch has been migrated to JUnit 5. While this does not impact end users directly, it helps the Batch team as well as community contributors to use the next generation of JUnit to write better tests.

As of version 4, the EnableBatchProcessing annotation provided all the basic infrastructure beans that are required to launch Spring Batch jobs. However, it did not register a job operator bean, which is the main entry point to stop, restart and abandon job executions.

While these utilities are not used as often as launching jobs, adding a job operator automatically in the application context can be useful to avoid a manual configuration of such a bean by end users.

The support for Java records as items in a chunk-oriented step has initially been introduced in v4.3, but that support was limited due to the fact that v4 has Java 8 as a baseline. The initial support was based on reflection tricks to create Java records and populate them with data, without having access to the java.lang.Record API that was finalised in Java 16.

Now that v5 has Java 17 as a baseline, we have improved records support in Spring Batch by leveraging the Record API in different parts of the framework. For example, the FlatFileItemReaderBuilder is now able to detect if the item type is a record or a regular class and configure the corresponding FieldSetMapper implementation accordingly (ie RecordFieldSetMapper for records and BeanWrapperFieldSetMapper for regular classes). The goal here is to make the configuration of the required FieldSetMapper type transparent to the user.

With the upgrade to Micrometer 1.10, you can now get Batch tracing in addition to Batch metrics. Spring Batch will create a span for each job and a span for each step within a job. This tracing meta-data can be collected and viewed on a dashboard like Zipkin for example.

Moreover, this release introduces new metrics like the currently active step, as well as the job launch count through the provided JobLauncher.

We took the opportunity of this major release to improve the code base with features from Java 8+, for example:

This release introduces the support of SAP HANA as an additional supported database for the job repository.

Up until v4.3, Spring Batch provided support for MariaDB by considering it as MySQL. In this release, MariaDB is treated as an independent product with its own DDL script and DataFieldMaxValueIncrementer.

This feature has been requested several times and is finally shipped in v5. It is now possible to use the newly added Maven BOM to import Spring Batch modules with a consistent version number.

Several issues related to characters encoding have been reported over the years in different areas of the framework, like inconsistent default encoding between file-based item readers and writers, serialization/deserialization issues when dealing with multi-byte characters in the execution context, etc.

In the same spirit as JEP 400 and following the UTF-8 manifesto, this release updates the default encoding to UTF-8 in all areas of the framework and ensures this default is configurable as needed.

The effort towards providing support to compile Spring Batch applications as native executables using the GraalVM native-image compiler has started in v4.2 and was shipped as experimental in v4.3.

In this release, the native support has been improved significantly by providing the necessary runtime hints to natively compile Spring Batch applications with GraalVM and is now considered out of beta.

In addition to what Spring Batch already persists in the execution context with regard to runtime information (like the step type, restart flag, etc), this release adds an important detail in the execution context which is the Spring Batch version that was used to serialize the context.

While this seems a detail, it has a huge added value when debugging upgrade issue with regard to execution context serialization and deserialization.

In this release, the documentation was updated to use the Spring Asciidoctor Backend. This backend ensures that all projects from the portfolio follow the same documentation style. For consistency with other projects, the reference documentation of Spring Batch was updated to use this backend in this release.

In this major release, all APIs that were deprecated in previous versions have been removed. Moreover, some APIs have been deprecated in v5.0 and are scheduled for removal in v5.2. Finally, some APIs have been moved or removed without deprecation for practical reasons.

Please refer to the migration guide for more details about these changes.

SqlFire has been announced to be EOL as of November 1st, 2014. The support of SQLFire as a job repository was deprecated in version v4.3 and removed in version v5.0.

Based on the [decision to discontinue](https://github.com/spring-projects/spring-data-geode#notice ) the support of Spring Data for Apache Geode, the support for Geode in Spring Batch was removed. The code was moved to the [spring-batch-extensions](https://github.com/spring-projects/spring-batch-extensions) repository as a community-driven effort.

Due to a lack of adoption, the implementation of JSR-352 has been discontinued in this release.

A JobInstance refers to the concept of a logical job run. Consider a batch job that should be run once at the end of the day, such as the EndOfDay Job from the preceding diagram. There is one EndOfDay job, but each individual run of the Job must be tracked separately. In the case of this job, there is one logical JobInstance per day. For example, there is a January 1st run, a January 2nd run, and so on. If the January 1st run fails the first time and is run again the next day, it is still the January 1st run. (Usually, this corresponds with the data it is processing as well, meaning the January 1st run processes data for January 1st). Therefore, each JobInstance can have multiple executions (JobExecution is discussed in more detail later in this chapter), and only one JobInstance (which corresponds to a particular Job and identifying JobParameters) can run at a given time.

The definition of a JobInstance has absolutely no bearing on the data to be loaded. It is entirely up to the ItemReader implementation to determine how data is loaded. For example, in the EndOfDay scenario, there may be a column on the data that indicates the effective date or schedule date to which the data belongs. So, the January 1st run would load only data from the 1st, and the January 2nd run would use only data from the 2nd. Because this determination is likely to be a business decision, it is left up to the ItemReader to decide. However, using the same JobInstance determines whether or not the “state” (that is, the ExecutionContext, which is discussed later in this chapter) from previous executions is used. Using a new JobInstance means “start from the beginning,” and using an existing instance generally means “start from where you left off”.

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. A JobParameters object holds a set of parameters used to start a batch job. They can be used for identification or even as reference data during the run, as the following image shows:

In the preceding example, where there are two instances, one for January 1st and another for January 2nd, there is really only one Job, but it has two JobParameter objects: one that was started with a job parameter of 01-01-2017 and another that was started with a parameter of 01-02-2017. 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.

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 is not considered to be complete unless the execution completes successfully. Using the EndOfDay Job described previously as an example, consider a JobInstance for 01-01-2017 that failed the first time it was run. If it is run again with the same identifying job parameters as the first run (01-01-2017), a new JobExecution is created. However, there is still only one JobInstance.

A Job defines what a job is and how it is to be executed, and a 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 contains many more properties that must be controlled and persisted, as the following table shows:

These properties are important because they are 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 are made in the batch metadata tables:

Now that the job has failed, assume that it took the entire night for the problem to be determined, so that the “batch window” is now closed. Further assuming that the window starts at 9:00 PM, the job is kicked off again for 01-01, starting where it left off and completing successfully at 9:30. Because it is now the next day, the 01-02 job must be run as well, and it 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 are separate JobInstances, Spring Batch makes no attempt to stop them from being run concurrently. (Attempting to run the same JobInstance while another is already running results 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, as shown in the following tables:

A StepExecution represents a single attempt to execute a Step. A new StepExecution is created each time a Step is run, similar to JobExecution. However, if a step fails to execute because the step before it fails, no execution is persisted for it. A StepExecution is created only when its Step is actually started.

Step executions are represented by objects of the StepExecution class. Each execution contains a reference to its corresponding step and JobExecution and transaction-related data, such as commit and rollback counts and start and end times. Additionally, each step execution contains an ExecutionContext, which contains any data a developer needs to have persisted across batch runs, such as statistics or state information needed to restart. The following table lists the properties for StepExecution:

One key issue when executing a batch job concerns the behavior of a Job when it is restarted. The launching of a Job is considered to be a “restart” if a JobExecution already exists for the particular JobInstance. Ideally, all jobs should be able to start up where they left off, but there are scenarios where this is not possible. In this scenario, it is entirely up to the developer to ensure that a new JobInstance is created. However, Spring Batch does provide some help. If a Job should never be restarted but should always be run as part of a new JobInstance, you can set the restartable property to false.

To phrase it another way, setting restartable to false means “this Job does not support being started again”. Restarting a Job that is not restartable causes a JobRestartException to be thrown. The following Junit code causes the exception to be thrown:

The first attempt to create a JobExecution for a non-restartable job causes no issues. However, the second attempt throws a JobRestartException.

During the course of the execution of a Job, it may be useful to be notified of various events in its lifecycle so that custom code can be run. SimpleJob allows for this by calling a JobListener at the appropriate time:

You can add JobListeners to a SimpleJob by setting listeners on the job.

Note that the afterJob method is called regardless of the success or failure of the Job. If you need to determine success or failure, you can get that information from the JobExecution:

The annotations corresponding to this interface are:

A job declared in the XML namespace or using any subclass of AbstractJob can optionally declare a validator for the job parameters at runtime. This is useful when, for instance, you need to assert that a job is started with all its mandatory parameters. There is a DefaultJobParametersValidator that you can use to constrain combinations of simple mandatory and optional parameters. For more complex constraints, you can implement the interface yourself.

If the namespace or the provided FactoryBean is used, transactional advice is automatically created around the repository. This is to ensure that the batch metadata, including state that is necessary for restarts after a failure, is persisted correctly. The behavior of the framework is not well defined if the repository methods are not transactional. The isolation level in the create* method attributes is specified separately to ensure that, when jobs are launched, if two processes try to launch the same job at the same time, only one succeeds. The default isolation level for that method is SERIALIZABLE, which is quite aggressive. READ_COMMITTED usually works equally well. READ_UNCOMMITTED is fine if two processes are not likely to collide in this way. However, since a call to the create* method is quite short, it is unlikely that SERIALIZED causes problems, as long as the database platform supports it. However, you can override this setting.

If the namespace is not used, you must also configure the transactional behavior of the repository by using AOP.

Another modifiable property of the JobRepository is the table prefix of the meta-data tables. By default, they are all prefaced with BATCH_. BATCH_JOB_EXECUTION and BATCH_STEP_EXECUTION are two examples. However, there are potential reasons to modify this prefix. If the schema names need to be prepended to the table names or if more than one set of metadata tables is needed within the same schema, the table prefix needs to be changed.

Given the preceding changes, every query to the metadata tables is prefixed with SYSTEM.TEST_. BATCH_JOB_EXECUTION is referred to as SYSTEM.TEST_JOB_EXECUTION.

If you use a database platform that is not in the list of supported platforms, you may be able to use one of the supported types, if the SQL variant is close enough. To do this, you can use the raw JobRepositoryFactoryBean instead of the namespace shortcut and use it to set the database type to the closest match.

If the database type is not specified, the JobRepositoryFactoryBean tries to auto-detect the database type from the DataSource. The major differences between platforms are mainly accounted for by the strategy for incrementing primary keys, so it is often necessary to override the incrementerFactory as well (by using one of the standard implementations from the Spring Framework).

If even that does not work or if you are not using an RDBMS, the only option may be to implement the various Dao interfaces that the SimpleJobRepository depends on and wire one up manually in the normal Spring way.

If you want to run your jobs from an enterprise scheduler, the command line is the primary interface. This is because most schedulers (with the exception of Quartz, unless using NativeJob) work directly with operating system processes, primarily kicked off with shell scripts. There are many ways to launch a Java process besides a shell script, such as Perl, Ruby, or even build tools, such as Ant or Maven. However, because most people are familiar with shell scripts, this example focuses on them.

Historically, offline processing (such as batch jobs) has been launched from the command-line, as described earlier. However, there are many cases where launching from an HttpRequest is a better option. Many such use cases include reporting, ad-hoc job running, and web application support. Because a batch job (by definition) is long running, the most important concern is to launch the job asynchronously:

The controller in this case is a Spring MVC controller. See the Spring Framework Reference Guide for more about Spring MVC. The controller launches a Job by using a JobLauncher that has been configured to launch asynchronously, which immediately returns a JobExecution. The Job is likely still running. However, this nonblocking behavior lets the controller return immediately, which is required when handling an HttpRequest. The following listing shows an example:

The most basic need before any advanced features is the ability to query the repository for existing executions. This functionality is provided by the JobExplorer interface:

As is evident from its method signatures, JobExplorer is a read-only version of the JobRepository, and, like the JobRepository, it can be easily configured by using a factory bean.

Earlier in this chapter, we noted that you can modify the table prefix of the JobRepository to allow for different versions or schemas. Because the JobExplorer works with the same tables, it also needs the ability to set a prefix.

A JobRegistry (and its parent interface, JobLocator) is not mandatory, but it can be useful if you want to keep track of which jobs are available in the context. It is also useful for collecting jobs centrally in an application context when they have been created elsewhere (for example, in child contexts). You can also use custom JobRegistry implementations to manipulate the names and other properties of the jobs that are registered. There is only one implementation provided by the framework and this is based on a simple map from job name to job instance.

You can populate a JobRegistry in either of two ways: by using a bean post processor or by using a registrar lifecycle component. The coming sections describe these two mechanisms.

As previously discussed, the JobRepository provides CRUD operations on the meta-data, and the JobExplorer provides read-only operations on the metadata. However, those operations are most useful when used together to perform common monitoring tasks such as stopping, restarting, or summarizing a Job, as is commonly done by batch operators. Spring Batch provides these types of operations in the JobOperator interface:

The preceding operations represent methods from many different interfaces, such as JobLauncher, JobRepository, JobExplorer, and JobRegistry. For this reason, the provided implementation of JobOperator (SimpleJobOperator) has many dependencies.

As of version 5.0, the @EnableBatchProcessing annotation automatically registers a job operator bean in the application context.

Most of the methods on JobOperator are self-explanatory, and you can find more detailed explanations in the Javadoc of the interface. However, the startNextInstance method is worth noting. This method always starts a new instance of a Job. This can be extremely useful if there are serious issues in a JobExecution and the Job needs to be started over again from the beginning. Unlike JobLauncher (which requires a new JobParameters object that triggers a new JobInstance), if the parameters are different from any previous set of parameters, the startNextInstance method uses the JobParametersIncrementer tied to the Job to force the Job to a new instance:

The contract of JobParametersIncrementer is that, given a JobParameters object, it returns the “next” JobParameters object by incrementing any necessary values it may contain. This strategy is useful because the framework has no way of knowing what changes to the JobParameters make it the “next” instance. For example, if the only value in JobParameters is a date and the next instance should be created, should that value be incremented by one day or one week (if the job is weekly, for instance)? The same can be said for any numerical values that help to identify the Job, as the following example shows:

In this example, the value with a key of run.id is used to discriminate between JobInstances. If the JobParameters passed in is null, it can be assumed that the Job has never been run before and, thus, its initial state can be returned. However, if not, the old value is obtained, incremented by one, and returned.

One of the most common use cases of JobOperator is gracefully stopping a Job:

The shutdown is not immediate, since there is no way to force immediate shutdown, especially if the execution is currently in developer code that the framework has no control over, such as a business service. However, as soon as control is returned back to the framework, it sets the status of the current StepExecution to BatchStatus.STOPPED, saves it, and does the same for the JobExecution before finishing.

A job execution that is FAILED can be restarted (if the Job is restartable). A job execution whose status is ABANDONED cannot be restarted by the framework. The ABANDONED status is also used in step executions to mark them as skippable in a restarted job execution. If a job is running and encounters a step that has been marked ABANDONED in the previous failed job execution, it moves on to the next step (as determined by the job flow definition and the step execution exit status).

If the process died (kill -9 or server failure), the job is, of course, not running, but the JobRepository has no way of knowing because no one told it before the process died. You have to tell it manually that you know that the execution either failed or should be considered aborted (change its status to FAILED or ABANDONED). This is a business decision, and there is no way to automate it. Change the status to FAILED only if it is restartable and you know that the restart data is valid.

Despite the relatively short list of required dependencies for a Step, it is an extremely complex class that can potentially contain many collaborators.

As mentioned previously, a step reads in and writes out items, periodically committing by using the supplied PlatformTransactionManager. With a commit-interval of 1, it commits after writing each individual item. This is less than ideal in many situations, since beginning and committing a transaction is expensive. Ideally, it is preferable to process as many items as possible in each transaction, which is completely dependent upon the type of data being processed and the resources with which the step is interacting. For this reason, you can configure the number of items that are processed within a commit.

In the preceding example, 10 items are processed within each transaction. At the beginning of processing, a transaction is begun. Also, each time read is called on the ItemReader, a counter is incremented. When it reaches 10, the list of aggregated items is passed to the ItemWriter, and the transaction is committed.

In the “Configuring and Running a Job” section , restarting a Job was discussed. Restart has numerous impacts on steps, and, consequently, may require some specific configuration.

There are many scenarios where errors encountered while processing should not result in Step failure but should be skipped instead. This is usually a decision that must be made by someone who understands the data itself and what meaning it has. Financial data, for example, may not be skippable because it results in money being transferred, which needs to be completely accurate. Loading a list of vendors, on the other hand, might allow for skips. If a vendor is not loaded because it was formatted incorrectly or was missing necessary information, there probably are not issues. Usually, these bad records are logged as well, which is covered later when discussing listeners.

In the preceding example, a FlatFileItemReader is used. If, at any point, a FlatFileParseException is thrown, the item is skipped and counted against the total skip limit of 10. Exceptions (and their subclasses) that are declared might be thrown during any phase of the chunk processing (read, process, or write). Separate counts are made of skips on read, process, and write inside the step execution, but the limit applies across all skips. Once the skip limit is reached, the next exception found causes the step to fail. In other words, the eleventh skip triggers the exception, not the tenth.

One problem with the preceding example is that any other exception besides a FlatFileParseException causes the Job to fail. In certain scenarios, this may be the correct behavior. However, in other scenarios, it may be easier to identify which exceptions should cause failure and skip everything else.

By identifying java.lang.Exception as a skippable exception class, the configuration indicates that all Exceptions are skippable. However, by “excluding” java.io.FileNotFoundException, the configuration refines the list of skippable exception classes to be all Exceptions except FileNotFoundException. Any excluded exception class is fatal if encountered (that is, they are not skipped).

For any exception encountered, the skippability is determined by the nearest superclass in the class hierarchy. Any unclassified exception is treated as 'fatal'.

In most cases, you want an exception to cause either a skip or a Step failure. However, not all exceptions are deterministic. If a FlatFileParseException is encountered while reading, it is always thrown for that record. Resetting the ItemReader does not help. However, for other exceptions (such as a DeadlockLoserDataAccessException, which indicates that the current process has attempted to update a record that another process holds a lock on), waiting and trying again might result in success.

The Step allows a limit for the number of times an individual item can be retried and a list of exceptions that are “retryable”. You can find more details on how retry works in retry.

By default, regardless of retry or skip, any exceptions thrown from the ItemWriter cause the transaction controlled by the Step to rollback. If skip is configured as described earlier, exceptions thrown from the ItemReader do not cause a rollback. However, there are many scenarios in which exceptions thrown from the ItemWriter should not cause a rollback, because no action has taken place to invalidate the transaction. For this reason, you can configure the Step with a list of exceptions that should not cause rollback.

You can use transaction attributes to control the isolation, propagation, and timeout settings. You can find more information on setting transaction attributes in the Spring core documentation.

The step has to take care of ItemStream callbacks at the necessary points in its lifecycle. (For more information on the ItemStream interface, see ItemStream). This is vital if a step fails and might need to be restarted, because the ItemStream interface is where the step gets the information it needs about persistent state between executions.

If the ItemReader, ItemProcessor, or ItemWriter itself implements the ItemStream interface, these are registered automatically. Any other streams need to be registered separately. This is often the case where indirect dependencies, such as delegates, are injected into the reader and writer. You can register a stream on the step through the stream element.

In the preceding example, the CompositeItemWriter is not an ItemStream, but both of its delegates are. Therefore, both delegate writers must be explicitly registered as streams for the framework to handle them correctly. The ItemReader does not need to be explicitly registered as a stream because it is a direct property of the Step. The step is now restartable, and the state of the reader and writer is correctly persisted in the event of a failure.

Just as with the Job, there are many events during the execution of a Step where a user may need to perform some functionality. For example, to write out to a flat file that requires a footer, the ItemWriter needs to be notified when the Step has been completed so that the footer can be written. This can be accomplished with one of many Step scoped listeners.

You can apply any class that implements one of the extensions of StepListener (but not that interface itself, since it is empty) to a step through the listeners element. The listeners element is valid inside a step, tasklet, or chunk declaration. We recommend that you declare the listeners at the level at which its function applies or, if it is multi-featured (such as StepExecutionListener and ItemReadListener), declare it at the most granular level where it applies.

An ItemReader, ItemWriter, or ItemProcessor that itself implements one of the StepListener interfaces is registered automatically with the Step if using the namespace <step> element or one of the *StepFactoryBean factories. This only applies to components directly injected into the Step. If the listener is nested inside another component, you need to explicitly register it (as described previously under Registering ItemStream with a Step).

In addition to the StepListener interfaces, annotations are provided to address the same concerns. Plain old Java objects can have methods with these annotations that are then converted into the corresponding StepListener type. It is also common to annotate custom implementations of chunk components, such as ItemReader or ItemWriter or Tasklet. The annotations are analyzed by the XML parser for the <listener/> elements as well as registered with the listener methods in the builders, so all you need to do is use the XML namespace or builders to register the listeners with a step.

As with other adapters for the ItemReader and ItemWriter interfaces, the Tasklet interface contains an implementation that allows for adapting itself to any pre-existing class: TaskletAdapter. An example where this may be useful is an existing DAO that is used to update a flag on a set of records. You can use the TaskletAdapter to call this class without having to write an adapter for the Tasklet interface.

Many batch jobs contain steps that must be done before the main processing begins, to set up various resources or after processing has completed to cleanup those resources. In the case of a job that works heavily with files, it is often necessary to delete certain files locally after they have been uploaded successfully to another location. The following example (taken from the Spring Batch samples project) is a Tasklet implementation with just such a responsibility:

The preceding tasklet implementation deletes all files within a given directory. It should be noted that the execute method is called only once. All that is left is to reference the tasklet from the step.

The simplest flow scenario is a job where all of the steps execute sequentially, as the following image shows:

This can be achieved by using next in a step.

In the scenario above, stepA runs first because it is the first Step listed. If stepA completes normally, stepB runs, and so on. However, if step A fails, the entire Job fails and stepB does not execute.

In the preceding example, there are only two possibilities:

In many cases, this may be sufficient. However, what about a scenario in which the failure of a step should trigger a different step, rather than causing failure? The following image shows such a flow:

Only two special characters are allowed in the pattern:

For example, c*t matches cat and count, while c?t matches cat but not count.

While there is no limit to the number of transition elements on a Step, if the Step execution results in an ExitStatus that is not covered by an element, the framework throws an exception and the Job fails. The framework automatically orders transitions from most specific to least specific. This means that, even if the ordering were swapped for stepA in the preceding example, an ExitStatus of FAILED would still go to stepC.

After the discussion of BatchStatus and ExitStatus, one might wonder how the BatchStatus and ExitStatus are determined for the Job. While these statuses are determined for the Step by the code that is executed, the statuses for the Job are determined based on the configuration.

So far, all of the job configurations discussed have had at least one final Step with no transitions.

If no transitions are defined for a Step, the status of the Job is defined as follows:

While this method of terminating a batch job is sufficient for some batch jobs, such as a simple sequential step job, custom defined job-stopping scenarios may be required. For this purpose, Spring Batch provides three transition elements to stop a Job (in addition to the next element that we discussed previously). Each of these stopping elements stops a Job with a particular BatchStatus. It is important to note that the stop transition elements have no effect on either the BatchStatus or ExitStatus of any Steps in the Job. These elements affect only the final statuses of the Job. For example, it is possible for every step in a job to have a status of FAILED but for the job to have a status of COMPLETED.

In some situations, more information than the ExitStatus may be required to decide which step to execute next. In this case, a JobExecutionDecider can be used to assist in the decision, as the following example shows:

Every scenario described so far has involved a Job that executes its steps one at a time in a linear fashion. In addition to this typical style, Spring Batch also allows for a job to be configured with parallel flows.

Part of the flow in a job can be externalized as a separate bean definition and then re-used. There are two ways to do so. The first is to declare the flow as a reference to one defined elsewhere.

The effect of defining an external flow, as shown in the preceding example, is to insert the steps from the external flow into the job as if they had been declared inline. In this way, many jobs can refer to the same template flow and compose such templates into different logical flows. This is also a good way to separate the integration testing of the individual flows.

The other form of an externalized flow is to use a JobStep. A JobStep is similar to a FlowStep but actually creates and launches a separate job execution for the steps in the flow specified.

The job parameters extractor is a strategy that determines how the ExecutionContext for the Step is converted into JobParameters for the Job that is run. The JobStep is useful when you want to have some more granular options for monitoring and reporting on jobs and steps. Using JobStep is also often a good answer to the question: “How do I create dependencies between jobs?” It is a good way to break up a large system into smaller modules and control the flow of jobs.

All of the late binding examples shown earlier have a scope of step declared on the bean definition.

Using a scope of Step is required to use late binding, because the bean cannot actually be instantiated until the Step starts, to let the attributes be found. Because it is not part of the Spring container by default, the scope must be added explicitly, by using the batch namespace, by including a bean definition explicitly for the StepScope, or by using the @EnableBatchProcessing annotation. Use only one of those methods. The following example uses the batch namespace:

The following example includes the bean definition explicitly:

Job scope, introduced in Spring Batch 3.0, is similar to Step scope in configuration but is a scope for the Job context, so that there is only one instance of such a bean per running job. Additionally, support is provided for late binding of references accessible from the JobContext by using #{..} placeholders. Using this feature, you can pull bean properties from the job or job execution context and the job parameters.

Because it is not part of the Spring container by default, the scope must be added explicitly, by using the batch namespace, by including a bean definition explicitly for the JobScope, or by using the @EnableBatchProcessing annotation (choose only one approach). The following example uses the batch namespace:

The following example includes a bean that explicitly defines the JobScope:

When using the Java configuration style to define job or step scoped ItemStream beans, the return type of the bean definition method should be at least ItemStream. This is required so that Spring Batch correctly creates a proxy that implements this interface, and therefore honors its contract by calling open, update and close methods as expected.

It is recommended to make the bean definition method of such beans return the most specific known implementation, as shown in the following example:

When working with flat files in Spring Batch, regardless of whether it is for input or output, one of the most important classes is the FieldSet. Many architectures and libraries contain abstractions for helping you read in from a file, but they usually return a String or an array of String objects. This really only gets you halfway there. A FieldSet is Spring Batch’s abstraction for enabling the binding of fields from a file resource. It allows developers to work with file input in much the same way as they would work with database input. A FieldSet is conceptually similar to a JDBC ResultSet. A FieldSet requires only one argument: a String array of tokens. Optionally, you can also configure the names of the fields so that the fields may be accessed either by index or name as patterned after ResultSet, as shown in the following example:

There are many more options on the FieldSet interface, such as Date, long, BigDecimal, and so on. The biggest advantage of the FieldSet is that it provides consistent parsing of flat file input. Rather than each batch job parsing differently in potentially unexpected ways, it can be consistent, both when handling errors caused by a format exception, or when doing simple data conversions.

A flat file is any type of file that contains at most two-dimensional (tabular) data. Reading flat files in the Spring Batch framework is facilitated by the class called FlatFileItemReader, which provides basic functionality for reading and parsing flat files. The two most important required dependencies of FlatFileItemReader are Resource and LineMapper. The LineMapper interface is explored more in the next sections. The resource property represents a Spring Core Resource. Documentation explaining how to create beans of this type can be found in Spring Framework, Chapter 5. Resources. Therefore, this guide does not go into the details of creating Resource objects beyond showing the following simple example:

In complex batch environments, the directory structures are often managed by the Enterprise Application Integration (EAI) infrastructure, where drop zones for external interfaces are established for moving files from FTP locations to batch processing locations and vice versa. File moving utilities are beyond the scope of the Spring Batch architecture, but it is not unusual for batch job streams to include file moving utilities as steps in the job stream. The batch architecture only needs to know how to locate the files to be processed. Spring Batch begins the process of feeding the data into the pipe from this starting point. However, Spring Integration provides many of these types of services.

The other properties in FlatFileItemReader let you further specify how your data is interpreted, as described in the following table:

Writing out to flat files has the same problems and issues that reading in from a file must overcome. A step must be able to write either delimited or fixed length formats in a transactional manner.

The StaxEventItemReader configuration provides a typical setup for the processing of records from an XML input stream. First, consider the following set of XML records that the StaxEventItemReader can process:

To be able to process the XML records, the following is needed:

Note that, in this example, we have chosen to use an XStreamMarshaller, which accepts an alias passed in as a map with the first key and value being the name of the fragment (that is, a root element) and the object type to bind. Then, similar to a FieldSet, the names of the other elements that map to fields within the object type are described as key/value pairs in the map. In the configuration file, we can use a Spring configuration utility to describe the required alias.

On input, the reader reads the XML resource until it recognizes that a new fragment is about to start. By default, the reader matches the element name to recognize that a new fragment is about to start. The reader creates a standalone XML document from the fragment and passes the document to a deserializer (typically a wrapper around a Spring OXM Unmarshaller) to map the XML to a Java object.

In summary, this procedure is analogous to the following Java code, which uses the injection provided by the Spring configuration:

Output works symmetrically to input. The StaxEventItemWriter needs a Resource, a marshaller, and a rootTagName. A Java object is passed to a marshaller (typically a standard Spring OXM Marshaller) which writes to a Resource by using a custom event writer that filters the StartDocument and EndDocument events produced for each fragment by the OXM tools.

The preceding configuration sets up the three required properties and sets the optional overwriteOutput=true attrbute, mentioned earlier in this chapter for specifying whether an existing file can be overwritten.

To summarize with a Java example, the following code illustrates all of the points discussed, demonstrating the programmatic setup of the required properties:

The JsonItemReader delegates JSON parsing and binding to implementations of the org.springframework.batch.item.json.JsonObjectReader interface. This interface is intended to be implemented by using a streaming API to read JSON objects in chunks. Two implementations are currently provided:

To be able to process JSON records, the following is needed:

The following example shows how to define a JsonItemReader that works with the previous JSON resource org/springframework/batch/item/json/trades.json and a JsonObjectReader based on Jackson:

The JsonFileItemWriter delegates the marshalling of items to the org.springframework.batch.item.json.JsonObjectMarshaller interface. The contract of this interface is to take an object and marshall it to a JSON String. Two implementations are currently provided:

To be able to write JSON records, the following is needed:

The following example shows how to define a JsonFileItemWriter:

Using a database cursor is generally the default approach of most batch developers, because it is the database’s solution to the problem of 'streaming' relational data. The Java ResultSet class is essentially an object oriented mechanism for manipulating a cursor. A ResultSet maintains a cursor to the current row of data. Calling next on a ResultSet moves this cursor to the next row. The Spring Batch cursor-based ItemReader implementation opens a cursor on initialization and moves the cursor forward one row for every call to read, returning a mapped object that can be used for processing. The close method is then called to ensure all resources are freed up. The Spring core JdbcTemplate gets around this problem by using the callback pattern to completely map all rows in a ResultSet and close before returning control back to the method caller. However, in batch, this must wait until the step is complete. The following image shows a generic diagram of how a cursor-based ItemReader works. Note that, while the example uses SQL (because SQL is so widely known), any technology could implement the basic approach.

This example illustrates the basic pattern. Given a 'FOO' table, which has three columns: ID, NAME, and BAR, select all rows with an ID greater than 1 but less than 7. This puts the beginning of the cursor (row 1) on ID 2. The result of this row should be a completely mapped Foo object. Calling read() again moves the cursor to the next row, which is the Foo with an ID of 3. The results of these reads are written out after each read, allowing the objects to be garbage collected (assuming no instance variables are maintaining references to them).

An alternative to using a database cursor is running multiple queries where each query fetches a portion of the results. We refer to this portion as a page. Each query must specify the starting row number and the number of rows that we want returned in the page.

While both flat files and XML files have a specific ItemWriter instance, there is no exact equivalent in the database world. This is because transactions provide all the needed functionality. ItemWriter implementations are necessary for files because they must act as if they’re transactional, keeping track of written items and flushing or clearing at the appropriate times. Databases have no need for this functionality, since the write is already contained in a transaction. Users can create their own DAOs that implement the ItemWriter interface or use one from a custom ItemWriter that’s written for generic processing concerns. Either way, they should work without any issues. One thing to look out for is the performance and error handling capabilities that are provided by batching the outputs. This is most common when using hibernate as an ItemWriter but could have the same issues when using JDBC batch mode. Batching database output does not have any inherent flaws, assuming we are careful to flush and there are no errors in the data. However, any errors while writing can cause confusion, because there is no way to know which individual item caused an exception or even if any individual item was responsible, as illustrated in the following image:

If items are buffered before being written, any errors are not thrown until the buffer is flushed just before a commit. For example, assume that 20 items are written per chunk, and the 15th item throws a DataIntegrityViolationException. As far as the Step is concerned, all 20 item are written successfully, since there is no way to know that an error occurs until they are actually written. Once Session#flush() is called, the buffer is emptied and the exception is hit. At this point, there is nothing the Step can do. The transaction must be rolled back. Normally, this exception might cause the item to be skipped (depending upon the skip/retry policies), and then it is not written again. However, in the batched scenario, there is no way to know which item caused the issue. The whole buffer was being written when the failure happened. The only way to solve this issue is to flush after each item, as shown in the following image:

This is a common use case, especially when using Hibernate, and the simple guideline for implementations of ItemWriter is to flush on each call to write(). Doing so allows for items to be skipped reliably, with Spring Batch internally taking care of the granularity of the calls to ItemWriter after an error.

For the purpose of this example, we create a simple ItemReader implementation that reads from a provided list. We start by implementing the most basic contract of ItemReader, the read method, as shown in the following code:

The preceding class takes a list of items and returns them one at a time, removing each from the list. When the list is empty, it returns null, thus satisfying the most basic requirements of an ItemReader, as illustrated in the following test code:

Implementing a Custom ItemWriter is similar in many ways to the ItemReader example above but differs in enough ways as to warrant its own example. However, adding restartability is essentially the same, so it is not covered in this example. As with the ItemReader example, a List is used in order to keep the example as simple as possible:

In some cases, a user needs specialized behavior to be appended to a pre-existing ItemReader. Spring Batch offers some out of the box decorators that can add additional behavior to to your ItemReader and ItemWriter implementations.

Spring Batch includes the following decorators:

Spring Batch offers the following readers and writers for commonly used messaging systems:

Spring Batch offers the following database readers:

Spring Batch offers the following database writers:

Spring Batch offers the following specialized readers:

Spring Batch offers the following specialized writers:

Spring Batch offers the following specialized processors:

PartitionHandler is the component that knows about the fabric of the remoting or grid environment. It is able to send StepExecution requests to the remote Step instances, wrapped in some fabric-specific format, like a DTO. It does not have to know how to split the input data or how to aggregate the result of multiple Step executions. Generally speaking, it probably also does not need to know about resilience or failover, since those are features of the fabric in many cases. In any case, Spring Batch always provides restartability independent of the fabric. A failed Job can always be restarted, and, in that case, only the failed Steps are re-executed.

The PartitionHandler interface can have specialized implementations for a variety of fabric types, including simple RMI remoting, EJB remoting, custom web service, JMS, Java Spaces, shared memory grids (such as Terracotta or Coherence), and grid execution fabrics (such as GridGain). Spring Batch does not contain implementations for any proprietary grid or remoting fabrics.

Spring Batch does, however, provide a useful implementation of PartitionHandler that executes Step instances locally in separate threads of execution, using the TaskExecutor strategy from Spring. The implementation is called TaskExecutorPartitionHandler.

The gridSize attribute determines the number of separate step executions to create, so it can be matched to the size of the thread pool in the TaskExecutor. Alternatively, it can be set to be larger than the number of threads available, which makes the blocks of work smaller.

The TaskExecutorPartitionHandler is useful for IO-intensive Step instances, such as copying large numbers of files or replicating filesystems into content management systems. It can also be used for remote execution by providing a Step implementation that is a proxy for a remote invocation (such as using Spring Remoting).

The Partitioner has a simpler responsibility: to generate execution contexts as input parameters for new step executions only (no need to worry about restarts). It has a single method, as the following interface definition shows:

The return value from this method associates a unique name for each step execution (the String) with input parameters in the form of an ExecutionContext. The names show up later in the Batch metadata as the step name in the partitioned StepExecutions. The ExecutionContext is just a bag of name-value pairs, so it might contain a range of primary keys, line numbers, or the location of an input file. The remote Step then normally binds to the context input by using #{…​} placeholders (late binding in step scope), as shown in the next section.

The names of the step executions (the keys in the Map returned by Partitioner) need to be unique amongst the step executions of a Job but do not have any other specific requirements. The easiest way to do this (and to make the names meaningful for users) is to use a prefix+suffix naming convention, where the prefix is the name of the step that is being executed (which itself is unique in the Job) and the suffix is just a counter. There is a SimplePartitioner in the framework that uses this convention.

You can use an optional interface called PartitionNameProvider to provide the partition names separately from the partitions themselves. If a Partitioner implements this interface, only the names are queried on a restart. If partitioning is expensive, this can be a useful optimization. The names provided by the PartitionNameProvider must match those provided by the Partitioner.

It is very efficient for the steps that are executed by the PartitionHandler to have identical configuration and for their input parameters to be bound at runtime from the ExecutionContext. This is easy to do with the StepScope feature of Spring Batch (covered in more detail in the section on Late Binding). For example, if the Partitioner creates ExecutionContext instances with an attribute key called fileName, pointing to a different file (or directory) for each step invocation, the Partitioner output might resemble the content of the following table:

Then the file name can be bound to a step by using late binding to the execution context.

The method parameter for the RepeatCallback is a RepeatContext. Many callbacks ignore the context. However, if necessary, you can use it as an attribute bag to store transient data for the duration of the iteration. After the iterate method returns, the context no longer exists.

If there is a nested iteration in progress, a RepeatContext has a parent context. The parent context is occasionally useful for storing data that need to be shared between calls to iterate. This is the case, for instance, if you want to count the number of occurrences of an event in the iteration and remember it across subsequent calls.

RepeatStatus is an enumeration used by Spring Batch to indicate whether processing has finished. It has two possible RepeatStatus values: