Add basic pagination to /groups/[uuid]/epersons endpoint

dspace-api/src/main/java/org/dspace/eperson/EPersonServiceImpl.java

@@ -567,14 +567,29 @@ public class EPersonServiceImpl extends DSpaceObjectServiceImpl<EPerson> impleme
 
     @Override
     public List<EPerson> findByGroups(Context c, Set<Group> groups) throws SQLException {
+        return findByGroups(c, groups, -1, -1);
+    }
+
+    @Override
+    public List<EPerson> findByGroups(Context c, Set<Group> groups, int pageSize, int offset) throws SQLException {
         //Make sure we at least have one group, if not don't even bother searching.
         if (CollectionUtils.isNotEmpty(groups)) {
-            return ePersonDAO.findByGroups(c, groups);
+            return ePersonDAO.findByGroups(c, groups, pageSize, offset);
         } else {
             return new ArrayList<>();
         }
     }
 
+    @Override
+    public int countByGroups(Context c, Set<Group> groups) throws SQLException {
+        //Make sure we at least have one group, if not don't even bother counting.
+        if (CollectionUtils.isNotEmpty(groups)) {
+            return ePersonDAO.countByGroups(c, groups);
+        } else {
+            return 0;
+        }
+    }
+
     @Override
     public List<EPerson> findEPeopleWithSubscription(Context context) throws SQLException {
         return ePersonDAO.findAllSubscribers(context);

dspace-api/src/main/java/org/dspace/eperson/dao/EPersonDAO.java

@@ -38,7 +38,29 @@ public interface EPersonDAO extends DSpaceObjectDAO<EPerson>, DSpaceObjectLegacy
 
     public int searchResultCount(Context context, String query, List<MetadataField> queryFields) throws SQLException;
 
-    public List<EPerson> findByGroups(Context context, Set<Group> groups) throws SQLException;
+    /**
+     * Find all EPersons who are a member of one or more of the listed groups in a paginated fashion. Order is
+     * indeterminate.
+     *
+     * @param context current Context
+     * @param groups Set of group(s) to check membership in
+     * @param pageSize number of EPerson objects to load at one time. Set to <=0 to disable pagination
+     * @param offset number of page to load (starting with 1). Set to <=0 to disable pagination
+     * @return List of all EPersons who are a member of one or more groups.
+     * @throws SQLException
+     */
+    List<EPerson> findByGroups(Context context, Set<Group> groups, int pageSize, int offset) throws SQLException;
+
+    /**
+     * Count total number of EPersons who are a member of one or more of the listed groups. This provides the total
+     * number of results to expect from corresponding findByGroups() for pagination purposes.
+     *
+     * @param context current Context
+     * @param groups Set of group(s) to check membership in
+     * @return total number of (unique) EPersons who are a member of one or more groups.
+     * @throws SQLException
+     */
+    int countByGroups(Context context, Set<Group> groups) throws SQLException;
 
     public List<EPerson> findWithPasswordWithoutDigestAlgorithm(Context context) throws SQLException;
 

dspace-api/src/main/java/org/dspace/eperson/dao/impl/EPersonDAOImpl.java

@@ -112,7 +112,7 @@ public class EPersonDAOImpl extends AbstractHibernateDSODAO<EPerson> implements
     }
 
     @Override
-    public List<EPerson> findByGroups(Context context, Set<Group> groups) throws SQLException {
+    public List<EPerson> findByGroups(Context context, Set<Group> groups, int pageSize, int offset) throws SQLException {
         Query query = createQuery(context,
                                   "SELECT DISTINCT e FROM EPerson e " +
                                       "JOIN e.groups g " +
@@ -125,7 +125,24 @@ public class EPersonDAOImpl extends AbstractHibernateDSODAO<EPerson> implements
 
         query.setParameter("idList", idList);
 
-        return list(query);
+        return list(query, pageSize, offset);
+    }
+
+    @Override
+    public int countByGroups(Context context, Set<Group> groups) throws SQLException {
+        Query query = createQuery(context,
+                                  "SELECT count(DISTINCT e) FROM EPerson e " +
+                                      "JOIN e.groups g " +
+                                      "WHERE g.id IN (:idList) ");
+
+        List<UUID> idList = new ArrayList<>(groups.size());
+        for (Group group : groups) {
+            idList.add(group.getID());
+        }
+
+        query.setParameter("idList", idList);
+
+        return count(query);
     }
 
     @Override

dspace-api/src/main/java/org/dspace/eperson/service/EPersonService.java

@@ -252,14 +252,43 @@ public interface EPersonService extends DSpaceObjectService<EPerson>, DSpaceObje
     public List<String> getDeleteConstraints(Context context, EPerson ePerson) throws SQLException;
 
     /**
-     * Retrieve all accounts which belong to at least one of the specified groups.
+     * Retrieve all EPerson accounts which belong to at least one of the specified groups.
+     * <P>
+     * WARNING: This method should be used sparingly, as it could have performance issues for Groups with very large
+     * lists of members. In that situation, a very large number of EPerson objects will be loaded into memory.
+     * See https://github.com/DSpace/DSpace/issues/9052
+     * <P>
+     * For better performance, use the paginated version of this method.
      *
      * @param c      The relevant DSpace Context.
      * @param groups set of eperson groups
      * @return a list of epeople
      * @throws SQLException An exception that provides information on a database access error or other errors.
      */
-    public List<EPerson> findByGroups(Context c, Set<Group> groups) throws SQLException;
+    List<EPerson> findByGroups(Context c, Set<Group> groups) throws SQLException;
+
+    /**
+     * Retrieve all EPerson accounts which belong to at least one of the specified groups, in a paginated fashion.
+     *
+     * @param c      The relevant DSpace Context.
+     * @param groups Set of group(s) to check membership in
+     * @param pageSize number of EPerson objects to load at one time. Set to <=0 to disable pagination
+     * @param offset number of page to load (starting with 1). Set to <=0 to disable pagination
+     * @return a list of epeople
+     * @throws SQLException An exception that provides information on a database access error or other errors.
+     */
+    List<EPerson> findByGroups(Context c, Set<Group> groups, int pageSize, int offset) throws SQLException;
+
+    /**
+     * Count all EPerson accounts which belong to at least one of the specified groups. This provides the total
+     * number of results to expect from corresponding findByGroups() for pagination purposes.
+     *
+     * @param c      The relevant DSpace Context.
+     * @param groups Set of group(s) to check membership in
+     * @return total number of (unique) EPersons who are a member of one or more groups.
+     * @throws SQLException An exception that provides information on a database access error or other errors.
+     */
+    int countByGroups(Context c, Set<Group> groups) throws SQLException;
 
     /**
      * Retrieve all accounts which are subscribed to receive information about new items.

dspace-server-webapp/src/main/java/org/dspace/app/rest/repository/GroupEPersonLinkRepository.java

@@ -8,6 +8,8 @@
 package org.dspace.app.rest.repository;
 
 import java.sql.SQLException;
+import java.util.List;
+import java.util.Set;
 import java.util.UUID;
 import javax.annotation.Nullable;
 import javax.servlet.http.HttpServletRequest;
@@ -15,7 +17,9 @@ import javax.servlet.http.HttpServletRequest;
 import org.dspace.app.rest.model.GroupRest;
 import org.dspace.app.rest.projection.Projection;
 import org.dspace.core.Context;
+import org.dspace.eperson.EPerson;
 import org.dspace.eperson.Group;
+import org.dspace.eperson.service.EPersonService;
 import org.dspace.eperson.service.GroupService;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.data.domain.Page;
@@ -31,6 +35,9 @@ import org.springframework.stereotype.Component;
 public class GroupEPersonLinkRepository extends AbstractDSpaceRestRepository
         implements LinkRestRepository {
 
+    @Autowired
+    EPersonService epersonService;
+
     @Autowired
     GroupService groupService;
 
@@ -45,7 +52,11 @@ public class GroupEPersonLinkRepository extends AbstractDSpaceRestRepository
             if (group == null) {
                 throw new ResourceNotFoundException("No such group: " + groupId);
             }
-            return converter.toRestPage(group.getMembers(), optionalPageable, projection);
+            int total = epersonService.countByGroups(context, Set.of(group));
+            Pageable pageable = utils.getPageable(optionalPageable);
+            List<EPerson> members = epersonService.findByGroups(context, Set.of(group), pageable.getPageSize(),
+                                                                Math.toIntExact(pageable.getOffset()));
+            return converter.toRestPage(members, pageable, total, projection);
         } catch (SQLException e) {
             throw new RuntimeException(e);
         }