Remove and forbid use of the type-unsafe empty Collections fields

This commit removes and now forbids all uses of the type-unsafe empty
Collections fields Collections#EMPTY_LIST, Collections#EMPTY_MAP, and
Collections#EMPTY_SET. The type-safe methods Collections#emptyList,
Collections#emptyMap, and Collections#emptySet should be used instead.

Relates elastic/elasticsearchelastic/elasticsearch#15187

Original commit: elastic/x-pack-elasticsearch@99f6fdd3a6

shield/src/main/java/org/elasticsearch/shield/transport/filter/IPFilter.java

@@ -76,7 +76,7 @@ public class IPFilter extends AbstractLifecycleComponent<IPFilter> {
     private final Transport transport;
     private final ShieldLicenseState licenseState;
     private final boolean alwaysAllowBoundAddresses;
-    private Map<String, ShieldIpFilterRule[]> rules = Collections.EMPTY_MAP;
+    private Map<String, ShieldIpFilterRule[]> rules = Collections.emptyMap();
     private HttpServerTransport httpServerTransport = null;
 
     @Inject
@@ -156,7 +156,7 @@ public class IPFilter extends AbstractLifecycleComponent<IPFilter> {
         boolean isHttpFilterEnabled = settings.getAsBoolean(IP_FILTER_ENABLED_HTTP_SETTING, isIpFilterEnabled);
 
         if (!isIpFilterEnabled && !isHttpFilterEnabled) {
-            return Collections.EMPTY_MAP;
+            return Collections.emptyMap();
         }
 
         Map<String, ShieldIpFilterRule[]> profileRules = new HashMap<>();

watcher/src/main/java/org/elasticsearch/watcher/execution/TriggeredWatchStore.java

@@ -132,7 +132,7 @@ public class TriggeredWatchStore extends AbstractComponent {
     public void putAll(final List<TriggeredWatch> triggeredWatches, final ActionListener<List<Integer>> listener) throws Exception {
 
         if (triggeredWatches.isEmpty()) {
-            listener.onResponse(Collections.EMPTY_LIST);
+            listener.onResponse(Collections.emptyList());
             return;
         }
 

watcher/src/main/java/org/elasticsearch/watcher/support/Script.java

@@ -56,7 +56,7 @@ public class Script implements ToXContent {
     }
 
     public Map<String, Object> params() {
-        return params != null ? params : Collections.EMPTY_MAP;
+        return params != null ? params : Collections.emptyMap();
     }
 
     @Override

watcher/src/main/java/org/elasticsearch/watcher/support/text/TextTemplate.java

@@ -51,7 +51,7 @@ public class TextTemplate implements ToXContent {
     }
 
     public Map<String, Object> getParams() {
-        return params != null ? params : Collections.EMPTY_MAP;
+        return params != null ? params : Collections.emptyMap();
     }
 
     @Override

watcher/src/main/java/org/elasticsearch/watcher/watch/Watch.java

@@ -230,7 +230,7 @@ public class Watch implements TriggerEngine.Job, ToXContent {
             this.secretService = secretService;
             this.defaultInput = new ExecutableNoneInput(logger);
             this.defaultCondition = new ExecutableAlwaysCondition(logger);
-            this.defaultActions = new ExecutableActions(Collections.EMPTY_LIST);
+            this.defaultActions = new ExecutableActions(Collections.emptyList());
             this.clock = clock;
         }
 

watcher/src/test/java/org/elasticsearch/watcher/actions/slack/service/message/SlackMessageTests.java

@@ -450,7 +450,7 @@ public class SlackMessageTests extends ESTestCase {
 
         SlackMessage.Template template = templateBuilder.build();
 
-        SlackMessage message = template.render("_w1", "_a1", engine, Collections.EMPTY_MAP, defaults);
+        SlackMessage message = template.render("_w1", "_a1", engine, Collections.emptyMap(), defaults);
         assertThat(message, notNullValue());
         if (template.from != null) {
             assertThat(message.from, is(template.from.getTemplate()));

watcher/src/test/java/org/elasticsearch/watcher/watch/WatchTests.java

@@ -260,7 +260,7 @@ public class WatchTests extends ESTestCase {
         ConditionRegistry conditionRegistry = registry(new ExecutableAlwaysCondition(logger));
         InputRegistry inputRegistry = registry(new ExecutableNoneInput(logger));
         TransformRegistry transformRegistry = transformRegistry();
-        ExecutableActions actions =  new ExecutableActions(Collections.EMPTY_LIST);
+        ExecutableActions actions =  new ExecutableActions(Collections.emptyList());
         ActionRegistry actionRegistry = registry(actions, transformRegistry);
 
         XContentBuilder builder = XContentFactory.jsonBuilder();