Update token client interface in sentinel-core

- Add `start` and `stop` method for automatic control
- Update TokenClientProvider using SpiLoader

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

sentinel-core/src/main/java/com/alibaba/csp/sentinel/cluster/client/ClusterTokenClient.java

@@ -13,7 +13,10 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package com.alibaba.csp.sentinel.cluster;
+package com.alibaba.csp.sentinel.cluster.client;
+
+import com.alibaba.csp.sentinel.cluster.TokenServerDescriptor;
+import com.alibaba.csp.sentinel.cluster.TokenService;
 
 /**
  * Token client interface for distributed flow control.
@@ -29,4 +32,18 @@ public interface ClusterTokenClient extends TokenService {
      * @return current token server if connected, otherwise null
      */
     TokenServerDescriptor currentServer();
+
+    /**
+     * Start the token client.
+     *
+     * @throws Exception some error occurs
+     */
+    void start() throws Exception;
+
+    /**
+     * Stop the token client.
+     *
+     * @throws Exception some error occurs
+     */
+    void stop() throws Exception;
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/cluster/client/TokenClientProvider.java

@@ -13,13 +13,10 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package com.alibaba.csp.sentinel.cluster;
+package com.alibaba.csp.sentinel.cluster.client;
-
-import java.util.ArrayList;
-import java.util.List;
-import java.util.ServiceLoader;
 
 import com.alibaba.csp.sentinel.log.RecordLog;
+import com.alibaba.csp.sentinel.util.SpiLoader;
 
 /**
  * Provider for a universal {@link ClusterTokenClient} instance.
@@ -31,8 +28,6 @@ public final class TokenClientProvider {
 
     private static ClusterTokenClient client = null;
 
-    private static final ServiceLoader<ClusterTokenClient> LOADER = ServiceLoader.load(ClusterTokenClient.class);
-
     static {
         // Not strictly thread-safe, but it's OK since it will be resolved only once.
         resolveTokenClientInstance();
@@ -43,17 +38,14 @@ public final class TokenClientProvider {
     }
 
     private static void resolveTokenClientInstance() {
-        List<ClusterTokenClient> clients = new ArrayList<ClusterTokenClient>();
+        ClusterTokenClient resolvedClient = SpiLoader.loadFirstInstance(ClusterTokenClient.class);
-        for (ClusterTokenClient client : LOADER) {
+        if (resolvedClient == null) {
-            clients.add(client);
+            RecordLog.info(
-        }
+                "[TokenClientProvider] No existing cluster token client, cluster client mode will not be activated");
-
-        if (!clients.isEmpty()) {
-            // Get first.
-            client = clients.get(0);
-            RecordLog.info("[TokenClientProvider] Token client resolved: " + client.getClass().getCanonicalName());
         } else {
-            RecordLog.warn("[TokenClientProvider] No existing token client, resolve failed");
+            client = resolvedClient;
+            RecordLog.info(
+                "[TokenClientProvider] Cluster token client resolved: " + client.getClass().getCanonicalName());
         }
     }