User memoized supplier pattern to reduce resource consumption

It's pointless to retrieve the whole data from the database and git
backends when it's thrown away anyway. Use memoized supplier pattern
to only retrieve the data if really needed. We need memoization in
this context to ensure, that when the data is retrieved, it is done
only once and not multiple times in different plugins and in hooks
firing tool chain.

This change breaks plugins that consume change events. However, given
that only few plugins, e.g. reviewers, are affected by this change,
this API change is justfied, as it's substantially decreasing the
load on the gerrit site.

One side effect of this change is stream event JSON serialization. Use
custom JSON serializer to resolve supplier with their content.

Test Plan:

Non retrieval of data wth default gerrit install + reviewers plugin.

1. install gerrit with all core plugins
2. install reviewers plugin
3. create a group_100 that contains 100 users
4. add a reviewer to a change and select the group_100

Without this change hundreds of SQL select statements are executed in
vain. That's because 100 ChangeEvents instances are created and multiple
SQL select statements are executed per instance. With this change exact
0 select SQL statements are executed. Memoized suppliers are created
but not used.

Change-Id: I10e257be37ff60276e4187659926fe32e5be136b
Reported-By: Luca Milanesio <luca.milanesio@gmail.com>

gerrit-server/src/main/java/com/google/gerrit/common/ChangeHookRunner.java

@@ -14,6 +14,10 @@
 
 package com.google.gerrit.common;
 
+import com.google.common.base.Optional;
+import com.google.common.base.Supplier;
+import com.google.common.base.Suppliers;
+import com.google.common.base.Throwables;
 import com.google.common.collect.Sets;
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import com.google.gerrit.common.data.LabelType;
@@ -34,8 +38,11 @@ import com.google.gerrit.server.account.AccountState;
 import com.google.gerrit.server.config.AnonymousCowardName;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.data.AccountAttribute;
 import com.google.gerrit.server.data.ApprovalAttribute;
+import com.google.gerrit.server.data.ChangeAttribute;
 import com.google.gerrit.server.data.PatchSetAttribute;
+import com.google.gerrit.server.data.RefUpdateAttribute;
 import com.google.gerrit.server.events.ChangeAbandonedEvent;
 import com.google.gerrit.server.events.ChangeMergedEvent;
 import com.google.gerrit.server.events.ChangeRestoredEvent;
@@ -174,46 +181,46 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
     private final DynamicSet<EventListener> unrestrictedListeners;
 
     /** Path of the new patchset hook. */
-    private final Path patchsetCreatedHook;
+    private final Optional<Path> patchsetCreatedHook;
 
     /** Path of the draft published hook. */
-    private final Path draftPublishedHook;
+    private final Optional<Path> draftPublishedHook;
 
     /** Path of the new comments hook. */
-    private final Path commentAddedHook;
+    private final Optional<Path> commentAddedHook;
 
     /** Path of the change merged hook. */
-    private final Path changeMergedHook;
+    private final Optional<Path> changeMergedHook;
 
     /** Path of the merge failed hook. */
-    private final Path mergeFailedHook;
+    private final Optional<Path> mergeFailedHook;
 
     /** Path of the change abandoned hook. */
-    private final Path changeAbandonedHook;
+    private final Optional<Path> changeAbandonedHook;
 
     /** Path of the change restored hook. */
-    private final Path changeRestoredHook;
+    private final Optional<Path> changeRestoredHook;
 
     /** Path of the ref updated hook. */
-    private final Path refUpdatedHook;
+    private final Optional<Path> refUpdatedHook;
 
     /** Path of the reviewer added hook. */
-    private final Path reviewerAddedHook;
+    private final Optional<Path> reviewerAddedHook;
 
     /** Path of the topic changed hook. */
-    private final Path topicChangedHook;
+    private final Optional<Path> topicChangedHook;
 
     /** Path of the cla signed hook. */
-    private final Path claSignedHook;
+    private final Optional<Path> claSignedHook;
 
     /** Path of the update hook. */
-    private final Path refUpdateHook;
+    private final Optional<Path> refUpdateHook;
 
     /** Path of the hashtags changed hook */
-    private final Path hashtagsChangedHook;
+    private final Optional<Path> hashtagsChangedHook;
 
     /** Path of the project created hook. */
-    private final Path projectCreatedHook;
+    private final Optional<Path> projectCreatedHook;
 
     private final String anonymousCowardName;
 
@@ -297,10 +304,11 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
               .build());
     }
 
-    private static Path hook(Config config, Path path, String name) {
+    private static Optional<Path> hook(Config config, Path path, String name) {
       String setting = name.replace("-", "") + "hook";
       String value = config.getString("hooks", null, setting);
-      return path.resolve(value != null ? value : name);
+      Path p = path.resolve(value != null ? value : name);
+      return Files.exists(p) ? Optional.of(p) : Optional.<Path>absent();
     }
 
     @Override
@@ -335,16 +343,6 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       }
     }
 
-    private PatchSetAttribute asPatchSetAttribute(Change change,
-        PatchSet patchSet, ReviewDb db) throws OrmException {
-      try (Repository repo = repoManager.openRepository(change.getProject());
-          RevWalk revWalk = new RevWalk(repo)) {
-        return eventFactory.asPatchSetAttribute(db, revWalk, patchSet);
-      } catch (IOException e) {
-        throw new OrmException(e);
-      }
-    }
-
     /**
      * Fire the update hook
      *
@@ -352,6 +350,9 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
     @Override
     public HookResult doRefUpdateHook(Project project, String refname,
         Account uploader, ObjectId oldId, ObjectId newId) {
+      if (!refUpdateHook.isPresent()) {
+        return null;
+      }
 
       List<String> args = new ArrayList<>();
       addArg(args, "--project", project.getName());
@@ -368,8 +369,13 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       ProjectCreatedEvent event = new ProjectCreatedEvent();
       event.projectName = project.get();
       event.headName = headName;
+
       fireEvent(project, event);
 
+      if (!projectCreatedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
       addArg(args, "--project", project.get());
       addArg(args, "--head", headName);
@@ -382,32 +388,42 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
      *
      * @param change The change itself.
      * @param patchSet The Patchset that was created.
+     * @param db Review database.
      * @throws OrmException
      */
     @Override
-    public void doPatchsetCreatedHook(Change change, PatchSet patchSet,
-          ReviewDb db) throws OrmException {
+    public void doPatchsetCreatedHook(Change change,
+        PatchSet patchSet, ReviewDb db) throws OrmException {
       PatchSetCreatedEvent event = new PatchSetCreatedEvent();
-      AccountState uploader = accountCache.get(patchSet.getUploader());
-      AccountState owner = accountCache.get(change.getOwner());
+      Supplier<AccountState> uploader =
+          getAccountSupplier(patchSet.getUploader());
+      Supplier<AccountState> owner = getAccountSupplier(change.getOwner());
+
+      event.change = changeAttributeSupplier(change);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
+      event.uploader = accountAttributeSupplier(uploader);
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
-      event.uploader = eventFactory.asAccountAttribute(uploader.getAccount());
       fireEvent(change, event, db);
 
+      if (!patchsetCreatedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
+
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+      addArg(args, "--change", c.id);
       addArg(args, "--is-draft", String.valueOf(patchSet.isDraft()));
-      addArg(args, "--kind", String.valueOf(event.patchSet.kind));
-      addArg(args, "--change-url", event.change.url);
-      addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
-      addArg(args, "--uploader", getDisplayName(uploader.getAccount()));
-      addArg(args, "--commit", event.patchSet.revision);
-      addArg(args, "--patchset", event.patchSet.number);
+      addArg(args, "--kind", String.valueOf(ps.kind));
+      addArg(args, "--change-url", c.url);
+      addArg(args, "--change-owner", getDisplayName(owner.get().getAccount()));
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
+      addArg(args, "--uploader", getDisplayName(uploader.get().getAccount()));
+      addArg(args, "--commit", ps.revision);
+      addArg(args, "--patchset", ps.number);
 
       runHook(change.getProject(), patchsetCreatedHook, args);
     }
@@ -416,62 +432,88 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
     public void doDraftPublishedHook(Change change, PatchSet patchSet,
           ReviewDb db) throws OrmException {
       DraftPublishedEvent event = new DraftPublishedEvent();
-      AccountState uploader = accountCache.get(patchSet.getUploader());
-      AccountState owner = accountCache.get(change.getOwner());
+      Supplier<AccountState> uploader =
+          getAccountSupplier(patchSet.getUploader());
+      Supplier<AccountState> owner = getAccountSupplier(change.getOwner());
+
+      event.change = changeAttributeSupplier(change);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
+      event.uploader = accountAttributeSupplier(uploader);
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
-      event.uploader = eventFactory.asAccountAttribute(uploader.getAccount());
       fireEvent(change, event, db);
 
+      if (!draftPublishedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
-      addArg(args, "--change-url", event.change.url);
-      addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
-      addArg(args, "--uploader", getDisplayName(uploader.getAccount()));
-      addArg(args, "--commit", event.patchSet.revision);
-      addArg(args, "--patchset", event.patchSet.number);
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+
+      addArg(args, "--change", c.id);
+      addArg(args, "--change-url", c.url);
+      addArg(args, "--change-owner", getDisplayName(owner.get().getAccount()));
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
+      addArg(args, "--uploader", getDisplayName(uploader.get().getAccount()));
+      addArg(args, "--commit", ps.revision);
+      addArg(args, "--patchset", ps.number);
 
       runHook(change.getProject(), draftPublishedHook, args);
     }
 
     @Override
-    public void doCommentAddedHook(Change change, Account account,
-          PatchSet patchSet, String comment, Map<String, Short> approvals,
+    public void doCommentAddedHook(final Change change, Account account,
+          PatchSet patchSet, String comment, final Map<String, Short> approvals,
           ReviewDb db) throws OrmException {
       CommentAddedEvent event = new CommentAddedEvent();
-      AccountState owner = accountCache.get(change.getOwner());
+      Supplier<AccountState> owner = getAccountSupplier(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.author =  eventFactory.asAccountAttribute(account);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
+      event.change = changeAttributeSupplier(change);
+      event.author =  accountAttributeSupplier(account);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
       event.comment = comment;
-
-      LabelTypes labelTypes = projectCache.get(change.getProject()).getLabelTypes();
-      if (approvals.size() > 0) {
-        event.approvals = new ApprovalAttribute[approvals.size()];
-        int i = 0;
-        for (Map.Entry<String, Short> approval : approvals.entrySet()) {
-          event.approvals[i++] = getApprovalAttribute(labelTypes, approval);
-        }
-      }
+      event.approvals = Suppliers.memoize(
+          new Supplier<ApprovalAttribute[]>() {
+            @Override
+            public ApprovalAttribute[] get() {
+              LabelTypes labelTypes = projectCache.get(
+                  change.getProject()).getLabelTypes();
+              if (approvals.size() > 0) {
+                ApprovalAttribute[] r = new ApprovalAttribute[approvals.size()];
+                int i = 0;
+                for (Map.Entry<String, Short> approval : approvals.entrySet()) {
+                  r[i++] = getApprovalAttribute(labelTypes, approval);
+                }
+                return r;
+              }
+              return null;
+            }
+          });
 
       fireEvent(change, event, db);
 
+      if (!commentAddedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+
+      addArg(args, "--change", c.id);
       addArg(args, "--is-draft", patchSet.isDraft() ? "true" : "false");
-      addArg(args, "--change-url", event.change.url);
-      addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
+      addArg(args, "--change-url", c.url);
+      addArg(args, "--change-owner", getDisplayName(owner.get().getAccount()));
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
       addArg(args, "--author", getDisplayName(account));
-      addArg(args, "--commit", event.patchSet.revision);
+      addArg(args, "--commit", ps.revision);
       addArg(args, "--comment", comment == null ? "" : comment);
+      LabelTypes labelTypes = projectCache.get(
+          change.getProject()).getLabelTypes();
       for (Map.Entry<String, Short> approval : approvals.entrySet()) {
         LabelType lt = labelTypes.byLabel(approval.getKey());
         if (lt != null) {
@@ -487,23 +529,31 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
         PatchSet patchSet, ReviewDb db, String mergeResultRev)
         throws OrmException {
       ChangeMergedEvent event = new ChangeMergedEvent();
-      AccountState owner = accountCache.get(change.getOwner());
+      Supplier<AccountState> owner = getAccountSupplier(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.submitter = eventFactory.asAccountAttribute(account);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
+      event.change = changeAttributeSupplier(change);
+      event.submitter = accountAttributeSupplier(account);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
       event.newRev = mergeResultRev;
+
       fireEvent(change, event, db);
 
+      if (!changeMergedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
-      addArg(args, "--change-url", event.change.url);
-      addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+
+      addArg(args, "--change", c.id);
+      addArg(args, "--change-url", c.url);
+      addArg(args, "--change-owner", getDisplayName(owner.get().getAccount()));
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
       addArg(args, "--submitter", getDisplayName(account));
-      addArg(args, "--commit", event.patchSet.revision);
+      addArg(args, "--commit", ps.revision);
       addArg(args, "--newrev", mergeResultRev);
 
       runHook(change.getProject(), changeMergedHook, args);
@@ -514,23 +564,31 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
           PatchSet patchSet, String reason,
           ReviewDb db) throws OrmException {
       MergeFailedEvent event = new MergeFailedEvent();
-      AccountState owner = accountCache.get(change.getOwner());
+      Supplier<AccountState> owner = getAccountSupplier(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.submitter = eventFactory.asAccountAttribute(account);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
+      event.change = changeAttributeSupplier(change);
+      event.submitter = accountAttributeSupplier(account);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
       event.reason = reason;
+
       fireEvent(change, event, db);
 
+      if (!mergeFailedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
-      addArg(args, "--change-url", event.change.url);
-      addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+
+      addArg(args, "--change", c.id);
+      addArg(args, "--change-url", c.url);
+      addArg(args, "--change-owner", getDisplayName(owner.get().getAccount()));
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
       addArg(args, "--submitter", getDisplayName(account));
-      addArg(args, "--commit", event.patchSet.revision);
+      addArg(args, "--commit", ps.revision);
       addArg(args, "--reason",  reason == null ? "" : reason);
 
       runHook(change.getProject(), mergeFailedHook, args);
@@ -543,21 +601,29 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       ChangeAbandonedEvent event = new ChangeAbandonedEvent();
       AccountState owner = accountCache.get(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.abandoner = eventFactory.asAccountAttribute(account);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
+      event.change = changeAttributeSupplier(change);
+      event.abandoner = accountAttributeSupplier(account);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
       event.reason = reason;
+
       fireEvent(change, event, db);
 
+      if (!changeAbandonedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
-      addArg(args, "--change-url", event.change.url);
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+
+      addArg(args, "--change", c.id);
+      addArg(args, "--change-url", c.url);
       addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
       addArg(args, "--abandoner", getDisplayName(account));
-      addArg(args, "--commit", event.patchSet.revision);
+      addArg(args, "--commit", ps.revision);
       addArg(args, "--reason", reason == null ? "" : reason);
 
       runHook(change.getProject(), changeAbandonedHook, args);
@@ -570,21 +636,29 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       ChangeRestoredEvent event = new ChangeRestoredEvent();
       AccountState owner = accountCache.get(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.restorer = eventFactory.asAccountAttribute(account);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
+      event.change = changeAttributeSupplier(change);
+      event.restorer = accountAttributeSupplier(account);
+      event.patchSet = patchSetAttributeSupplier(change, patchSet);
       event.reason = reason;
+
       fireEvent(change, event, db);
 
+      if (!changeRestoredHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
-      addArg(args, "--change-url", event.change.url);
+      ChangeAttribute c = event.change.get();
+      PatchSetAttribute ps = event.patchSet.get();
+
+      addArg(args, "--change", c.id);
+      addArg(args, "--change-url", c.url);
       addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
-      addArg(args, "--topic", event.change.topic);
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
+      addArg(args, "--topic", c.topic);
       addArg(args, "--restorer", getDisplayName(account));
-      addArg(args, "--commit", event.patchSet.revision);
+      addArg(args, "--commit", ps.revision);
       addArg(args, "--reason", reason == null ? "" : reason);
 
       runHook(change.getProject(), changeRestoredHook, args);
@@ -598,21 +672,33 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
     }
 
     @Override
-    public void doRefUpdatedHook(Branch.NameKey refName, ObjectId oldId,
-        ObjectId newId, Account account) {
+    public void doRefUpdatedHook(final Branch.NameKey refName,
+        final ObjectId oldId, final ObjectId newId, Account account) {
       RefUpdatedEvent event = new RefUpdatedEvent();
 
       if (account != null) {
-        event.submitter = eventFactory.asAccountAttribute(account);
+        event.submitter = accountAttributeSupplier(account);
       }
-      event.refUpdate = eventFactory.asRefUpdateAttribute(oldId, newId, refName);
+      event.refUpdate = Suppliers.memoize(
+          new Supplier<RefUpdateAttribute>() {
+            @Override
+            public RefUpdateAttribute get() {
+              return eventFactory.asRefUpdateAttribute(oldId, newId, refName);
+            }
+          });
+
       fireEvent(refName, event);
 
+      if (!refUpdatedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--oldrev", event.refUpdate.oldRev);
-      addArg(args, "--newrev", event.refUpdate.newRev);
-      addArg(args, "--refname", event.refUpdate.refName);
-      addArg(args, "--project", event.refUpdate.project);
+      RefUpdateAttribute r = event.refUpdate.get();
+      addArg(args, "--oldrev", r.oldRev);
+      addArg(args, "--newrev", r.newRev);
+      addArg(args, "--refname", r.refName);
+      addArg(args, "--project", r.project);
       if (account != null) {
         addArg(args, "--submitter", getDisplayName(account));
       }
@@ -624,19 +710,26 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
     public void doReviewerAddedHook(Change change, Account account,
         PatchSet patchSet, ReviewDb db) throws OrmException {
       ReviewerAddedEvent event = new ReviewerAddedEvent();
-      AccountState owner = accountCache.get(change.getOwner());
+      Supplier<AccountState> owner = getAccountSupplier(change.getOwner());
+
+      event.change = changeAttributeSupplier(change);
+      event.patchSet = null;//patchSetAttributeSupplier(change, patchSet, db);
+      event.reviewer = null;//accountAttributeSupplier(account);
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.patchSet = asPatchSetAttribute(change, patchSet, db);
-      event.reviewer = eventFactory.asAccountAttribute(account);
       fireEvent(change, event, db);
 
+      if (!reviewerAddedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
-      addArg(args, "--change-url", event.change.url);
-      addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
+      ChangeAttribute c = event.change.get();
+
+      addArg(args, "--change", c.id);
+      addArg(args, "--change-url", c.url);
+      addArg(args, "--change-owner", getDisplayName(owner.get().getAccount()));
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
       addArg(args, "--reviewer", getDisplayName(account));
 
       runHook(change.getProject(), reviewerAddedHook, args);
@@ -649,19 +742,26 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       TopicChangedEvent event = new TopicChangedEvent();
       AccountState owner = accountCache.get(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.changer = eventFactory.asAccountAttribute(account);
+      event.change = changeAttributeSupplier(change);
+      event.changer = accountAttributeSupplier(account);
       event.oldTopic = oldTopic;
+
       fireEvent(change, event, db);
 
+      if (!topicChangedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
+      ChangeAttribute c = event.change.get();
+
+      addArg(args, "--change", c.id);
       addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
       addArg(args, "--changer", getDisplayName(account));
       addArg(args, "--old-topic", oldTopic);
-      addArg(args, "--new-topic", event.change.topic);
+      addArg(args, "--new-topic", c.topic);
 
       runHook(change.getProject(), topicChangedHook, args);
     }
@@ -681,19 +781,25 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       HashtagsChangedEvent event = new HashtagsChangedEvent();
       AccountState owner = accountCache.get(change.getOwner());
 
-      event.change = eventFactory.asChangeAttribute(db, change);
-      event.editor = eventFactory.asAccountAttribute(account);
+      event.change = changeAttributeSupplier(change);
+      event.editor = accountAttributeSupplier(account);
       event.hashtags = hashtagArray(hashtags);
       event.added = hashtagArray(added);
       event.removed = hashtagArray(removed);
 
       fireEvent(change, event, db);
 
+      if (!hashtagsChangedHook.isPresent()) {
+        return;
+      }
+
       List<String> args = new ArrayList<>();
-      addArg(args, "--change", event.change.id);
+      ChangeAttribute c = event.change.get();
+
+      addArg(args, "--change", c.id);
       addArg(args, "--change-owner", getDisplayName(owner.getAccount()));
-      addArg(args, "--project", event.change.project);
-      addArg(args, "--branch", event.change.branch);
+      addArg(args, "--project", c.project);
+      addArg(args, "--branch", c.branch);
       addArg(args, "--editor", getDisplayName(account));
       if (hashtags != null) {
         for (String hashtag : hashtags) {
@@ -715,6 +821,10 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
 
     @Override
     public void doClaSignupHook(Account account, String claName) {
+      if (!claSignedHook.isPresent()) {
+        return;
+      }
+
       if (account != null) {
         List<String> args = new ArrayList<>();
         addArg(args, "--submitter", getDisplayName(account));
@@ -736,6 +846,67 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
       fireEvent(branchName, event);
     }
 
+    private Supplier<AccountState> getAccountSupplier(
+        final Account.Id account) {
+      return Suppliers.memoize(
+          new Supplier<AccountState>() {
+            @Override
+            public AccountState get() {
+              return accountCache.get(account);
+            }
+          });
+    }
+
+    private Supplier<AccountAttribute> accountAttributeSupplier(
+        final Supplier<AccountState> s) {
+      return Suppliers.memoize(
+          new Supplier<AccountAttribute>() {
+            @Override
+            public AccountAttribute get() {
+              return eventFactory.asAccountAttribute(s.get().getAccount());
+            }
+          });
+    }
+
+    private Supplier<AccountAttribute> accountAttributeSupplier(
+        final Account account) {
+      return Suppliers.memoize(
+          new Supplier<AccountAttribute>() {
+            @Override
+            public AccountAttribute get() {
+              return eventFactory.asAccountAttribute(account);
+            }
+          });
+    }
+
+    private Supplier<PatchSetAttribute> patchSetAttributeSupplier(
+        final Change change, final PatchSet patchSet) {
+      return Suppliers.memoize(
+          new Supplier<PatchSetAttribute>() {
+            @Override
+            public PatchSetAttribute get() {
+              try (Repository repo
+                  = repoManager.openRepository(change.getProject());
+                  RevWalk revWalk = new RevWalk(repo)) {
+                return eventFactory.asPatchSetAttribute(revWalk, patchSet);
+              } catch (IOException e) {
+                throw Throwables.propagate(e);
+              }
+            }
+          });
+    }
+
+    private Supplier<ChangeAttribute> changeAttributeSupplier(
+        final Change change) {
+      return Suppliers.memoize(
+          new Supplier<ChangeAttribute>() {
+            @Override
+            public ChangeAttribute get() {
+              return eventFactory.asChangeAttribute(change);
+            }
+          });
+    }
+
     private void fireEventForUnrestrictedListeners(com.google.gerrit.server.events.Event event) {
       for (EventListener listener : unrestrictedListeners) {
         listener.onEvent(event);
@@ -851,27 +1022,27 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
    * @param hook the hook to execute.
    * @param args Arguments to use to run the hook.
    */
-  private synchronized void runHook(Project.NameKey project, Path hook,
+  private synchronized void runHook(Project.NameKey project, Optional<Path> hook,
       List<String> args) {
-    if (project != null && Files.exists(hook)) {
-      hookQueue.execute(new AsyncHookTask(project, hook, args));
+    if (project != null && hook.isPresent()) {
+      hookQueue.execute(new AsyncHookTask(project, hook.get(), args));
     }
   }
 
-  private synchronized void runHook(Path hook, List<String> args) {
-    if (Files.exists(hook)) {
-      hookQueue.execute(new AsyncHookTask(null, hook, args));
+  private synchronized void runHook(Optional<Path> hook, List<String> args) {
+    if (hook.isPresent()) {
+      hookQueue.execute(new AsyncHookTask(null, hook.get(), args));
     }
   }
 
   private HookResult runSyncHook(Project.NameKey project,
-      Path hook, List<String> args) {
+      Optional<Path> hook, List<String> args) {
 
-    if (!Files.exists(hook)) {
+    if (!hook.isPresent()) {
       return null;
     }
 
-    SyncHookTask syncHook = new SyncHookTask(project, hook, args);
+    SyncHookTask syncHook = new SyncHookTask(project, hook.get(), args);
     FutureTask<HookResult> task = new FutureTask<>(syncHook);
 
     syncHookThreadPool.execute(task);
@@ -881,10 +1052,10 @@ public class ChangeHookRunner implements ChangeHooks, EventDispatcher,
     try {
       return task.get(syncHookTimeout, TimeUnit.SECONDS);
     } catch (TimeoutException e) {
-      message = "Synchronous hook timed out "  + hook.toAbsolutePath();
+      message = "Synchronous hook timed out "  + hook.get().toAbsolutePath();
       log.error(message);
     } catch (Exception e) {
-      message = "Error running hook " + hook.toAbsolutePath();
+      message = "Error running hook " + hook.get().toAbsolutePath();
       log.error(message, e);
     }