JSR-352 Support

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As of Spring Batch 3.0 support for JSR-352 has been fully implemented. This section is not a replacement for the spec itself and instead, intends to explain how the JSR-352 specific concepts apply to Spring Batch. Additional information on JSR-352 can be found via the JCP here: https://jcp.org/en/jsr/detail?id=352

General Notes about Spring Batch and JSR-352

Spring Batch and JSR-352 are structurally the same. They both have jobs that are made up of steps. They both have readers, processors, writers, and listeners. However, their interactions are subtly different. For example, the org.springframework.batch.core.SkipListener#onSkipInWrite(S item, Throwable t) within Spring Batch receives two parameters: the item that was skipped and the Exception that caused the skip. The JSR-352 version of the same method (javax.batch.api.chunk.listener.SkipWriteListener#onSkipWriteItem(List<Object> items, Exception ex)) also receives two parameters. However the first one is a List of all the items within the current chunk with the second being the Exception that caused the skip. Because of these differences, it is important to note that there are two paths to execute a job within Spring Batch: either a traditional Spring Batch job or a JSR-352 based job. While the use of Spring Batch artifacts (readers, writers, etc) will work within a job configured with JSR-352’s JSL and executed with the JsrJobOperator, they will behave according to the rules of JSR-352. It is also important to note that batch artifacts that have been developed against the JSR-352 interfaces will not work within a traditional Spring Batch job.

Setup

Application Contexts

All JSR-352 based jobs within Spring Batch consist of two application contexts. A parent context, that contains beans related to the infrastructure of Spring Batch such as the JobRepository, PlatformTransactionManager, etc and a child context that consists of the configuration of the job to be run. The parent context is defined via the jsrBaseContext.xml provided by the framework. This context may be overridden by setting the JSR-352-BASE-CONTEXT system property.

The base context is not processed by the JSR-352 processors for things like property injection so no components requiring that additional processing should be configured there.

Launching a JSR-352 based job

JSR-352 requires a very simple path to executing a batch job. The following code is all that is needed to execute your first batch job:

JobOperator operator = BatchRuntime.getJobOperator();
jobOperator.start("myJob", new Properties());

While that is convenient for developers, the devil is in the details. Spring Batch bootstraps a bit of infrastructure behind the scenes that a developer may want to override. The following is bootstrapped the first time BatchRuntime.getJobOperator() is called:

Bean Name

Default Configuration

Notes

dataSource

Apache DBCP BasicDataSource with configured values.

By default, HSQLDB is bootstrapped.

transactionManager

org.springframework.jdbc.datasource.DataSourceTransactionManager

References the dataSource bean defined above.

A Datasource initializer

This is configured to execute the scripts configured via the batch.drop.script and batch.schema.script properties. By default, the schema scripts for HSQLDB are executed. This behavior can be disabled by setting the batch.data.source.init property.

jobRepository

A JDBC based SimpleJobRepository.

This JobRepository uses the previously mentioned data source and transaction manager. The schema’s table prefix is configurable (defaults to BATCH_) via the batch.table.prefix property.

jobLauncher

org.springframework.batch.core.launch.support.SimpleJobLauncher

Used to launch jobs.

batchJobOperator

org.springframework.batch.core.launch.support.SimpleJobOperator

The JsrJobOperator wraps this to provide most of it’s functionality.

jobExplorer

org.springframework.batch.core.explore.support.JobExplorerFactoryBean

Used to address lookup functionality provided by the JsrJobOperator.

jobParametersConverter

org.springframework.batch.core.jsr.JsrJobParametersConverter

JSR-352 specific implementation of the JobParametersConverter.

jobRegistry

org.springframework.batch.core.configuration.support.MapJobRegistry

Used by the SimpleJobOperator.

placeholderProperties

org.springframework.beans.factory.config.PropertyPlaceholderConfigure

Loads the properties file batch-${ENVIRONMENT:hsql}.properties to configure the properties mentioned above. ENVIRONMENT is a System property (defaults to hsql) that can be used to specify any of the supported databases Spring Batch currently supports.

None of the above beans are optional for executing JSR-352 based jobs. All may be overridden to provide customized functionality as needed.

Dependency Injection

JSR-352 is based heavily on the Spring Batch programming model. As such, while not explicitly requiring a formal dependency injection implementation, DI of some kind implied. Spring Batch supports all three methods for loading batch artifacts defined by JSR-352:

Implementation Specific Loader: Spring Batch is built upon Spring and so supports Spring dependency injection within JSR-352 batch jobs.
Archive Loader: JSR-352 defines the existing of a batch.xml file that provides mappings between a logical name and a class name. This file must be found within the /META-INF/ directory if it is used.
Thread Context Class Loader: JSR-352 allows configurations to specify batch artifact implementations in their JSL by providing the fully qualified class name inline. Spring Batch supports this as well in JSR-352 configured jobs.

To use Spring dependency injection within a JSR-352 based batch job consists of configuring batch artifacts using a Spring application context as beans. Once the beans have been defined, a job can refer to them as it would any bean defined within the batch.xml file.

The following example shows how to use Spring dependency injection within a JSR-352 based batch job in XML:

XML Configuration

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.springframework.org/schema/beans
                              https://www.springframework.org/schema/beans/spring-beans.xsd
                              http://xmlns.jcp.org/xml/ns/javaee
                              https://xmlns.jcp.org/xml/ns/javaee/jobXML_1_0.xsd">

    <!-- javax.batch.api.Batchlet implementation -->
    <bean id="fooBatchlet" class="io.spring.FooBatchlet">
            <property name="prop" value="bar"/>
    </bean>

    <!-- Job is defined using the JSL schema provided in JSR-352 -->
    <job id="fooJob" xmlns="http://xmlns.jcp.org/xml/ns/javaee" version="1.0">
        <step id="step1">
            <batchlet ref="fooBatchlet"/>
        </step>
    </job>
</beans>

The following example shows how to use Spring dependency injection within a JSR-352 based batch job in Java:

Java Configuration

@Configuration
public class BatchConfiguration {

    @Bean
    public Batchlet fooBatchlet() {
        FooBatchlet batchlet = new FooBatchlet();
        batchlet.setProp("bar");
        return batchlet;
       }
}


<?xml version="1.0" encoding="UTF-8"?>
<job id="fooJob" xmlns="http://xmlns.jcp.org/xml/ns/javaee" version="1.0">
    <step id="step1" >
        <batchlet ref="fooBatchlet" />
    </step>
</job>

The assembly of Spring contexts (imports, etc) works with JSR-352 jobs just as it would with any other Spring based application. The only difference with a JSR-352 based job is that the entry point for the context definition will be the job definition found in /META-INF/batch-jobs/.

To use the thread context class loader approach, all you need to do is provide the fully qualified class name as the ref. It is important to note that when using this approach or the batch.xml approach, the class referenced requires a no argument constructor which will be used to create the bean.

<?xml version="1.0" encoding="UTF-8"?>
<job id="fooJob" xmlns="http://xmlns.jcp.org/xml/ns/javaee" version="1.0">
    <step id="step1" >
        <batchlet ref="io.spring.FooBatchlet" />
    </step>
</job>

Batch Properties

Property Support

JSR-352 allows for properties to be defined at the Job, Step and batch artifact level by way of configuration in the JSL. Batch properties are configured at each level in the following way:

<properties>
    <property name="propertyName1" value="propertyValue1"/>
    <property name="propertyName2" value="propertyValue2"/>
</properties>

Properties may be configured on any batch artifact.

@BatchProperty annotation

Properties are referenced in batch artifacts by annotating class fields with the @BatchProperty and @Inject annotations (both annotations are required by the spec). As defined by JSR-352, fields for properties must be String typed. Any type conversion is up to the implementing developer to perform.

An javax.batch.api.chunk.ItemReader artifact could be configured with a properties block such as the one described above and accessed as such:

public class MyItemReader extends AbstractItemReader {
    @Inject
    @BatchProperty
    private String propertyName1;

    ...
}

The value of the field "propertyName1" will be "propertyValue1"

Property Substitution

Property substitution is provided by way of operators and simple conditional expressions. The general usage is #{operator['key']}.

Supported operators:

jobParameters: access job parameter values that the job was started/restarted with.
jobProperties: access properties configured at the job level of the JSL.
systemProperties: access named system properties.
partitionPlan: access named property from the partition plan of a partitioned step.

#{jobParameters['unresolving.prop']}?:#{systemProperties['file.separator']}

The left hand side of the assignment is the expected value, the right hand side is the default value. In the preceding example, the result will resolve to a value of the system property file.separator as #{jobParameters['unresolving.prop']} is assumed to not be resolvable. If neither expressions can be resolved, an empty String will be returned. Multiple conditions can be used, which are separated by a ';'.

Processing Models

JSR-352 provides the same two basic processing models that Spring Batch does:

Item based processing - Using an javax.batch.api.chunk.ItemReader, an optional javax.batch.api.chunk.ItemProcessor, and an javax.batch.api.chunk.ItemWriter.
Task based processing - Using a javax.batch.api.Batchlet implementation. This processing model is the same as the org.springframework.batch.core.step.tasklet.Tasklet based processing currently available.

Item based processing

Item based processing in this context is a chunk size being set by the number of items read by an ItemReader. To configure a step this way, specify the item-count (which defaults to 10) and optionally configure the checkpoint-policy as item (this is the default).

...
<step id="step1">
    <chunk checkpoint-policy="item" item-count="3">
        <reader ref="fooReader"/>
        <processor ref="fooProcessor"/>
        <writer ref="fooWriter"/>
    </chunk>
</step>
...

If item-based checkpointing is chosen, an additional attribute time-limit is supported. This sets a time limit for how long the number of items specified has to be processed. If the timeout is reached, the chunk will complete with however many items have been read by then regardless of what the item-count is configured to be.

Custom checkpointing

JSR-352 calls the process around the commit interval within a step "checkpointing". Item-based checkpointing is one approach as mentioned above. However, this is not robust enough in many cases. Because of this, the spec allows for the implementation of a custom checkpointing algorithm by implementing the javax.batch.api.chunk.CheckpointAlgorithm interface. This functionality is functionally the same as Spring Batch’s custom completion policy. To use an implementation of CheckpointAlgorithm, configure your step with the custom checkpoint-policy as shown below where fooCheckpointer refers to an implementation of CheckpointAlgorithm.

...
<step id="step1">
    <chunk checkpoint-policy="custom">
        <checkpoint-algorithm ref="fooCheckpointer"/>
        <reader ref="fooReader"/>
        <processor ref="fooProcessor"/>
        <writer ref="fooWriter"/>
    </chunk>
</step>
...

Running a job

The entrance to executing a JSR-352 based job is through the javax.batch.operations.JobOperator. Spring Batch provides its own implementation of this interface (org.springframework.batch.core.jsr.launch.JsrJobOperator). This implementation is loaded via the javax.batch.runtime.BatchRuntime. Launching a JSR-352 based batch job is implemented as follows:

JobOperator jobOperator = BatchRuntime.getJobOperator();
long jobExecutionId = jobOperator.start("fooJob", new Properties());

The above code does the following:

Bootstraps a base ApplicationContext: In order to provide batch functionality, the framework needs some infrastructure bootstrapped. This occurs once per JVM. The components that are bootstrapped are similar to those provided by @EnableBatchProcessing. Specific details can be found in the javadoc for the JsrJobOperator.
Loads an ApplicationContext for the job requested: In the example above, the framework looks in /META-INF/batch-jobs for a file named fooJob.xml and load a context that is a child of the shared context mentioned previously.
Launch the job: The job defined within the context will be executed asynchronously. The JobExecution’s ID will be returned.

All JSR-352 based batch jobs are executed asynchronously.

When JobOperator#start is called using SimpleJobOperator, Spring Batch determines if the call is an initial run or a retry of a previously executed run. Using the JSR-352 based JobOperator#start(String jobXMLName, Properties jobParameters), the framework will always create a new JobInstance (JSR-352 job parameters are non-identifying). In order to restart a job, a call to JobOperator#restart(long executionId, Properties restartParameters) is required.

Contexts

JSR-352 defines two context objects that are used to interact with the meta-data of a job or step from within a batch artifact: javax.batch.runtime.context.JobContext and javax.batch.runtime.context.StepContext. Both of these are available in any step level artifact (Batchlet, ItemReader, etc) with the JobContext being available to job level artifacts as well (JobListener for example).

To obtain a reference to the JobContext or StepContext within the current scope, simply use the @Inject annotation:

@Inject
JobContext jobContext;

@Autowire for JSR-352 contexts

Using Spring’s @Autowire is not supported for the injection of these contexts.

In Spring Batch, the JobContext and StepContext wrap their corresponding execution objects (JobExecution and StepExecution respectively). Data stored through StepContext#setPersistentUserData(Serializable data) is stored in the Spring Batch StepExecution#executionContext.

Step Flow

Within a JSR-352 based job, the flow of steps works similarly as it does within Spring Batch. However, there are a few subtle differences:

Decision’s are steps - In a regular Spring Batch job, a decision is a state that does not have an independent StepExecution or any of the rights and responsibilities that go along with being a full step.. However, with JSR-352, a decision is a step just like any other and will behave just as any other steps (transactionality, it gets a StepExecution, etc). This means that they are treated the same as any other step on restarts as well.
next attribute and step transitions - In a regular job, these are allowed to appear together in the same step. JSR-352 allows them to both be used in the same step with the next attribute taking precedence in evaluation.
Transition element ordering - In a standard Spring Batch job, transition elements are sorted from most specific to least specific and evaluated in that order. JSR-352 jobs evaluate transition elements in the order they are specified in the XML.

Scaling a JSR-352 batch job

Traditional Spring Batch jobs have four ways of scaling (the last two capable of being executed across multiple JVMs):

Split - Running multiple steps in parallel.
Multiple threads - Executing a single step via multiple threads.
Partitioning - Dividing the data up for parallel processing (manager/worker).
Remote Chunking - Executing the processor piece of logic remotely.

JSR-352 provides two options for scaling batch jobs. Both options support only a single JVM:

Split - Same as Spring Batch
Partitioning - Conceptually the same as Spring Batch however implemented slightly different.

Partitioning

Conceptually, partitioning in JSR-352 is the same as it is in Spring Batch. Meta-data is provided to each worker to identify the input to be processed, with the workers reporting back to the manager the results upon completion. However, there are some important differences:

Partitioned Batchlet - This will run multiple instances of the configured Batchlet on multiple threads. Each instance will have it’s own set of properties as provided by the JSL or the PartitionPlan
PartitionPlan - With Spring Batch’s partitioning, an ExecutionContext is provided for each partition. With JSR-352, a single javax.batch.api.partition.PartitionPlan is provided with an array of Properties providing the meta-data for each partition.
PartitionMapper - JSR-352 provides two ways to generate partition meta-data. One is via the JSL (partition properties). The second is via an implementation of the javax.batch.api.partition.PartitionMapper interface. Functionally, this interface is similar to the org.springframework.batch.core.partition.support.Partitioner interface provided by Spring Batch in that it provides a way to programmatically generate meta-data for partitioning.
StepExecutions - In Spring Batch, partitioned steps are run as manager/worker. Within JSR-352, the same configuration occurs. However, the worker steps do not get official StepExecutions. Because of that, calls to JsrJobOperator#getStepExecutions(long jobExecutionId) will only return the StepExecution for the manager.

The child StepExecutions still exist in the job repository and are available through the JobExplorer.

Compensating logic - Since Spring Batch implements the manager/worker logic of partitioning using steps, StepExecutionListeners can be used to handle compensating logic if something goes wrong. However, since the workers JSR-352 provides a collection of other components for the ability to provide compensating logic when errors occur and to dynamically set the exit status. These components include the following:

Artifact Interface

Description

javax.batch.api.partition.PartitionCollector

Provides a way for worker steps to send information back to the manager. There is one instance per worker thread.

javax.batch.api.partition.PartitionAnalyzer

End point that receives the information collected by the PartitionCollector as well as the resulting statuses from a completed partition.

javax.batch.api.partition.PartitionReducer

Provides the ability to provide compensating logic for a partitioned step.

Testing

Since all JSR-352 based jobs are executed asynchronously, it can be difficult to determine when a job has completed. To help with testing, Spring Batch provides the org.springframework.batch.test.JsrTestUtils. This utility class provides the ability to start a job and restart a job and wait for it to complete. Once the job completes, the associated JobExecution is returned.