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90-character pass for repeat.adoc

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I removed all the extraneous characters and line breaks from non-code lines and fitted all the non-code lines to 90 characters.

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Jay Bryant
2017-10-13 15:05:28 -05:00
committed by Glenn Renfro
parent 11fed07d16
commit 3a3abdc28a
1 changed files with 101 additions and 164 deletions
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spring-batch-docs/asciidoc/repeat.adoc
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@@ -1,4 +1,4 @@
:batch-asciidoc: http://docs.spring.io/spring-batch/reference/html/
:batch-asciidoc: ./
:toc: left
:toclevels: 4
@@ -8,15 +8,12 @@
[[repeatTemplate]]
=== 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:
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:
[source, java]
----
@@ -28,8 +25,7 @@ public interface RepeatOperations {
----
The callback is an interface, shown in the following definition, that lets you insert
some business logic to be repeated:
some business logic to be repeated:
[source, java]
----
@@ -40,22 +36,17 @@ public interface RepeatCallback {
}
----
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 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:
The simplest general purpose implementation of `RepeatOperations` is `RepeatTemplate`, as
shown in the following example:
[source, java]
@@ -74,42 +65,33 @@ template.iterate(new RepeatCallback() {
});
----
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.
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]]
==== 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.
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.
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
`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:
`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:
.RepeatStatus Properties
@@ -120,54 +102,37 @@ If there is a nested iteration in progress, a `RepeatContext` has a parent conte
|===============
`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`.
`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`.
[[completionPolicies]]
=== 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.
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).
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.
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.
[[repeatExceptionHandling]]
=== 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.
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:
@@ -181,41 +146,32 @@ public interface ExceptionHandler {
}
----
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.
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).
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).
[[repeatListeners]]
=== 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.
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:
[source, java]
----
public interface RepeatListener {
@@ -227,55 +183,40 @@ public interface RepeatListener {
}
----
The `open` and `close` callbacks come before and after the entire
iteration. `before`, `after`,
and `onError` apply to the individual
`RepeatCallback` calls.
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.
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.
[[repeatParallelProcessing]]
=== 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`).
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`).
[[declarativeIteration]]
=== 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):
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):
[source, xml]
----
@@ -289,19 +230,15 @@ The following example shows declarative iteration using the Spring AOP
<bean id="retryAdvice" class="org.spr...RepeatOperationsInterceptor"/>
----
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.
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`.
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`.