Repeat
Repeat
RepeatTemplate
Batch processing is about repetitive actions, either as a simple optimization or as part
of a job. To strategize and generalize the repetition and to provide what amounts to an
iterator framework, Spring Batch has the RepeatOperations interface. The
RepeatOperations interface has the following definition:
public interface RepeatOperations {
RepeatStatus iterate(RepeatCallback callback) throws RepeatException;
}
The callback is an interface, shown in the following definition, that lets you insert some business logic to be repeated:
public interface RepeatCallback {
RepeatStatus doInIteration(RepeatContext context) throws Exception;
}
The callback is executed repeatedly until the implementation determines that the
iteration should end. The return value in these interfaces is an enumeration that can
either be RepeatStatus.CONTINUABLE or RepeatStatus.FINISHED. A RepeatStatus
enumeration conveys information to the caller of the repeat operations about whether
there is any more work to do. Generally speaking, implementations of RepeatOperations
should inspect the RepeatStatus and use it as part of the decision to end the
iteration. Any callback that wishes to signal to the caller that there is no more work to
do can return RepeatStatus.FINISHED.
The simplest general purpose implementation of RepeatOperations is RepeatTemplate, as
shown in the following example:
RepeatTemplate template = new RepeatTemplate();
template.setCompletionPolicy(new SimpleCompletionPolicy(2));
template.iterate(new RepeatCallback() {
public RepeatStatus doInIteration(RepeatContext context) {
// Do stuff in batch...
return RepeatStatus.CONTINUABLE;
}
});
In the preceding example, we return RepeatStatus.CONTINUABLE, to show that there is
more work to do. The callback can also return RepeatStatus.FINISHED, to signal to the
caller that there is no more work to do. Some iterations can be terminated by
considerations intrinsic to the work being done in the callback. Others are effectively
infinite loops as far as the callback is concerned and the completion decision is
delegated to an external policy, as in the case shown in the preceding example.
RepeatContext
The method parameter for the RepeatCallback is a RepeatContext. Many callbacks ignore
the context. However, if necessary, it can be used 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
RepeatStatus is an enumeration used by Spring Batch to indicate whether processing has
finished. It has two possible RepeatStatus values, described in the following table:
Value |
Description |
CONTINUABLE |
There is more work to do. |
FINISHED |
No more repetitions should take place. |
RepeatStatus values can also be combined with a logical AND operation by using the
and() method in RepeatStatus. The effect of this is to do a logical AND on the
continuable flag. In other words, if either status is FINISHED, then the result is
FINISHED.
Completion Policies
Inside a RepeatTemplate, the termination of the loop in the iterate method is
determined by a CompletionPolicy, which is also a factory for the RepeatContext. The
RepeatTemplate has the responsibility to use the current policy to create a
RepeatContext and pass that in to the RepeatCallback at every stage in the iteration.
After a callback completes its doInIteration, the RepeatTemplate has to make a call
to the CompletionPolicy to ask it to update its state (which will be stored in the
RepeatContext). Then it asks the policy if the iteration is complete.
Spring Batch provides some simple general purpose implementations of CompletionPolicy.
SimpleCompletionPolicy allows execution up to a fixed number of times (with
RepeatStatus.FINISHED forcing early completion at any time).
Users might need to implement their own completion policies for more complicated decisions. For example, a batch processing window that prevents batch jobs from executing once the online systems are in use would require a custom policy.
Exception Handling
If there is an exception thrown inside a RepeatCallback, the RepeatTemplate consults
an ExceptionHandler, which can decide whether or not to re-throw the exception.
The following listing shows the ExceptionHandler interface definition:
public interface ExceptionHandler {
void handleException(RepeatContext context, Throwable throwable)
throws Throwable;
}
A common use case is to count the number of exceptions of a given type and fail when a
limit is reached. For this purpose, Spring Batch provides the
SimpleLimitExceptionHandler and a slightly more flexible
RethrowOnThresholdExceptionHandler. The SimpleLimitExceptionHandler has a limit
property and an exception type that should be compared with the current exception. All
subclasses of the provided type are also counted. Exceptions of the given type are
ignored until the limit is reached, and then they are rethrown. Exceptions of other types
are always rethrown.
An important optional property of the SimpleLimitExceptionHandler is the boolean flag
called useParent. It is false by default, so the limit is only accounted for in the
current RepeatContext. When set to true, the limit is kept across sibling contexts in
a nested iteration (such as a set of chunks inside a step).
Listeners
Often, it is useful to be able to receive additional callbacks for cross-cutting concerns
across a number of different iterations. For this purpose, Spring Batch provides the
RepeatListener interface. The RepeatTemplate lets users register RepeatListener
implementations, and they are given callbacks with the RepeatContext and RepeatStatus
where available during the iteration.
The RepeatListener interface has the following definition:
public interface RepeatListener {
void before(RepeatContext context);
void after(RepeatContext context, RepeatStatus result);
void open(RepeatContext context);
void onError(RepeatContext context, Throwable e);
void close(RepeatContext context);
}
The open and close callbacks come before and after the entire iteration. before,
after, and onError apply to the individual RepeatCallback calls.
Note that, when there is more than one listener, they are in a list, so there is an
order. In this case, open and before are called in the same order while after,
onError, and close are called in reverse order.
Parallel Processing
Implementations of RepeatOperations are not restricted to executing the callback
sequentially. It is quite important that some implementations are able to execute their
callbacks in parallel. To this end, Spring Batch provides the
TaskExecutorRepeatTemplate, which uses the Spring TaskExecutor strategy to run the
RepeatCallback. The default is to use a SynchronousTaskExecutor, which has the effect
of executing the whole iteration in the same thread (the same as a normal
RepeatTemplate).
Declarative Iteration
Sometimes there is some business processing that you know you want to repeat every time
it happens. The classic example of this is the optimization of a message pipeline. It is
more efficient to process a batch of messages, if they are arriving frequently, than to
bear the cost of a separate transaction for every message. Spring Batch provides an AOP
interceptor that wraps a method call in a RepeatOperations object for just this
purpose. The RepeatOperationsInterceptor executes the intercepted method and repeats
according to the CompletionPolicy in the provided RepeatTemplate.
The following example shows declarative iteration using the Spring AOP namespace to
repeat a service call to a method called processMessage (for more detail on how to
configure AOP interceptors, see the Spring User Guide):
<aop:config>
<aop:pointcut id="transactional"
expression="execution(* com..*Service.processMessage(..))" />
<aop:advisor pointcut-ref="transactional"
advice-ref="retryAdvice" order="-1"/>
</aop:config>
<bean id="retryAdvice" class="org.spr...RepeatOperationsInterceptor"/>The following example demonstrates using Java configuration to
repeat a service call to a method called processMessage (for more detail on how to
configure AOP interceptors, see the Spring User Guide):
@Bean
public MyService myService() {
ProxyFactory factory = new ProxyFactory(RepeatOperations.class.getClassLoader());
factory.setInterfaces(MyService.class);
factory.setTarget(new MyService());
MyService service = (MyService) factory.getProxy();
JdkRegexpMethodPointcut pointcut = new JdkRegexpMethodPointcut();
pointcut.setPatterns(".*processMessage.*");
RepeatOperationsInterceptor interceptor = new RepeatOperationsInterceptor();
((Advised) service).addAdvisor(new DefaultPointcutAdvisor(pointcut, interceptor));
return service;
}
The preceding example uses a default RepeatTemplate inside the interceptor. To change
the policies, listeners, and other details, you can inject an instance of
RepeatTemplate into the interceptor.
If the intercepted method returns void, then the interceptor always returns
RepeatStatus.CONTINUABLE (so there is a danger of an infinite loop if the
CompletionPolicy does not have a finite end point). Otherwise, it returns
RepeatStatus.CONTINUABLE until the return value from the intercepted method is null,
at which point it returns RepeatStatus.FINISHED. Consequently, the business logic
inside the target method can signal that there is no more work to do by returning null
or by throwing an exception that is re-thrown by the ExceptionHandler in the provided
RepeatTemplate.