This section describes how a custom pagination strategy can be implemented in AeroGear Controller.
Depending on if you need a completely different strategy, or if you simply want to change/add/rename the returned HTTP response headers, you have two options:
- Extend AbstractPaginationStrategy
- Implement PaginationStrategy
You would extend AbstractPaginationStrategy if you simply want to change/add/rename the returned HTTP response headers.
public class MyStrategy extends AbstractPaginationStrategy {
@Override
public void setResponseHeaders(PaginationMetadata md,
HttpServletResponse response,
int resultSize) {
for (Entry<String, String> entry : md.getHeaders(resultSize).entrySet()) {
response.setHeader(entry.getKey(), entry.getValue());
}
}
}This implementation actually does exactly what the default implementation does. But you can of course modify this to suite your own requirements. You have access to all the metadata for the pagination, like the raw links etc, so you could assemble headers in any way you see fit.
This will give you more control over the pagination strategy. The interface you have to implement looks like this:
/**
* A strategy for implementing pagination in AeroGear Controller.
*/
public interface PaginationStrategy {
/**
* Creates a PaginationInfo instance.
* How this information is gathered, be it from an Annotation on the target
* endpoint method, or by using separate request parameters is up to the
* concrete implementation.
*
* @param routeContext the {@link RouteContext} of the route being processed.
* @param arguments the extracted arguments from the current request.
* @return {@link PaginationInfo} the information requred for paging.
*/
PaginationInfo createPaginationInfo(RouteContext routeContext,
Map<String, Object> arguments);
/**
* Called before the target endpoint method has been invoked and enables a
* concrete strategy to manipulate the arguments that will be passed to
* the target endpoint method.
*
* @param pagingInfo the {@link PaginationInfo} instance created by
* this strategy.
* @param arguments the extracted arguments from the current request.
*
* @return {@code Object[]} the arguments that will be passed to the target
* endpoint method.
*/
Object[] preInvocation(PaginationInfo pagingInfo, Map<String, Object> arguments);
/**
* Called after the target endpoint method has been invoked and
* allows the strategy to set HTTP Response headers.
*
* @param result the result returned from the target endpoint method.
* @param routeContext the {@link RouteContext}.
* @param pagingInfo the {@link PaginationInfo} instance created by this strategy.
*
* @return {@code Object} Either the unchanged result or a modified result
* depending on the underlying implementation.
*/
Object postInvocation(Object result,
RouteContext routeContext,
PaginationInfo pagingInfo);
}The createPaginationInfo method is called by AeroGear Controller prior to invoking the target endpoint method. The arguments extracted from the current request are made available to this method. The implementation can choose whatever way it likes to match information from the request that are related to pagination. This could be using an annotation on the target endpoint method (which is what the default implementation does) or it could simply pick known parameters from the request.
The preInvocation methods gives the strategy a chance to modify the actual arguments that will be used to invoke the target endpoint method. Depending on the concrete implementation there might not be anything to be done here other than simply returning the value of the arguments map. But, for example with the default strategy, an endpoint has the option to accept a PaginationInfo type a parameter. In this case we have to add this instance to the arguments before calling the endpoint.
The postInvocation method is called after the target endpoint method has been invoked allows the strategy to set the HTTP Response headers. This method has access to the results of the invocation which is can use to decide on what links should be returned, for example one might not want to return a previous next link header if there is no more data (the number of items in the results is less than the limit).
Configuration can be done by adding a CDI Producer, for example:
public class PaginationStrategyProducer {
@Produces
public PaginationStrategy createPagingStrategy() {
return new AbstractPaginationStrategy() {
@Override
public void setResponseHeaders(PaginationMetadata md,
HttpServletResponse response,
int resultSize) {
Set<Entry<String, String>> hds = md.getHeaders(resultSize).entrySet();
for (Entry<String, String> header : hds) {
response.setHeader(header.getKey(), header.getValue());
}
}
};
}
}Or, if you prefer to implement the strategy as a standalone class:
public class CustomPaginationStrategy extends AbstractPaginationStrategy {
@Override
public void setResponseHeaders(PaginationMetadata metadata, HttpServletResponse response, int resultSize) {
final Map<String, String> headers = metadata.getHeaders(resultSize);
Set<Entry<String, String>> entrySet = headers.entrySet();
for (Entry<String, String> entry : entrySet) {
response.setHeader(entry.getKey(), entry.getValue());
}
}
}