Add cluster token server interface to core and support embedded mode

- Add a EmbeddedClusterTokenServer interface

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/cluster/server/ClusterTokenServer.java

@@ -23,7 +23,17 @@ package com.alibaba.csp.sentinel.cluster.server;
  */
 public interface ClusterTokenServer {
 
+    /**
+     * Start the Sentinel cluster server.
+     *
+     * @throws Exception if any error occurs
+     */
     void start() throws Exception;
 
+    /**
+     * Stop the Sentinel cluster server.
+     *
+     * @throws Exception if any error occurs
+     */
     void stop() throws Exception;
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/cluster/server/EmbeddedClusterTokenServer.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright 1999-2018 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.cluster.server;
+
+import com.alibaba.csp.sentinel.cluster.TokenService;
+
+/**
+ * Embedded token server interface that can work in embedded mode.
+ *
+ * @author Eric Zhao
+ * @since 1.4.0
+ */
+public interface EmbeddedClusterTokenServer extends ClusterTokenServer, TokenService {
+}

sentinel-core/src/main/java/com/alibaba/csp/sentinel/cluster/server/EmbeddedClusterTokenServerProvider.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 1999-2018 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.csp.sentinel.cluster.server;
+
+import com.alibaba.csp.sentinel.log.RecordLog;
+import com.alibaba.csp.sentinel.util.SpiLoader;
+
+/**
+ * @author Eric Zhao
+ * @since 1.4.0
+ */
+public final class EmbeddedClusterTokenServerProvider {
+
+    private static EmbeddedClusterTokenServer server = null;
+
+    static {
+        resolveInstance();
+    }
+
+    private static void resolveInstance() {
+        EmbeddedClusterTokenServer s = SpiLoader.loadFirstInstance(EmbeddedClusterTokenServer.class);
+        if (s == null) {
+            RecordLog.warn("[EmbeddedClusterTokenServerProvider] No existing cluster token server, cluster server mode will not be activated");
+        } else {
+            server = s;
+            RecordLog.info("[EmbeddedClusterTokenServerProvider] Cluster token server resolved: " + server.getClass().getCanonicalName());
+        }
+    }
+
+    public static EmbeddedClusterTokenServer getServer() {
+        return server;
+    }
+
+    private EmbeddedClusterTokenServerProvider() {}
+}