Add a CommandCenterProvider to resolve CommandCenter API and cache the instance

sentinel-transport/sentinel-transport-common/src/main/java/com/alibaba/csp/sentinel/command/CommandCenterProvider.java

@@ -0,0 +1,37 @@
+package com.alibaba.csp.sentinel.command;
+
+import com.alibaba.csp.sentinel.log.RecordLog;
+import com.alibaba.csp.sentinel.transport.CommandCenter;
+import com.alibaba.csp.sentinel.util.SpiLoader;
+
+/**
+ * Provider for a universal {@link CommandCenter} instance.
+ *
+ * @author cdfive
+ * @date 2019-01-11
+ */
+public class CommandCenterProvider {
+
+    private static CommandCenter commandCenter = null;
+
+    static {
+        resolveInstance();
+    }
+
+    private static void resolveInstance() {
+        CommandCenter resolveCommandCenter = SpiLoader.loadFirstInstance(CommandCenter.class);
+
+        if (resolveCommandCenter == null) {
+            RecordLog.warn("[CommandCenterProvider] No existing CommandCenter, resolve failed");
+        } else {
+            commandCenter = resolveCommandCenter;
+            RecordLog.info("[CommandCenterProvider] CommandCenter resolved: " + resolveCommandCenter.getClass().getCanonicalName());
+        }
+    }
+
+    public static CommandCenter getCommandCenter() {
+        return commandCenter;
+    }
+
+    private CommandCenterProvider() {}
+}

sentinel-transport/sentinel-transport-common/src/main/java/com/alibaba/csp/sentinel/transport/init/CommandCenterInitFunc.java

@@ -15,9 +15,7 @@
  */
 package com.alibaba.csp.sentinel.transport.init;
 
-import java.util.Iterator;
-import java.util.ServiceLoader;
-
+import com.alibaba.csp.sentinel.command.CommandCenterProvider;
 import com.alibaba.csp.sentinel.init.InitFunc;
 import com.alibaba.csp.sentinel.init.InitOrder;
 import com.alibaba.csp.sentinel.log.RecordLog;
@@ -31,18 +29,16 @@ public class CommandCenterInitFunc implements InitFunc {
 
     @Override
     public void init() throws Exception {
-        ServiceLoader<CommandCenter> loader = ServiceLoader.load(CommandCenter.class);
-        Iterator<CommandCenter> iterator = loader.iterator();
-        if (iterator.hasNext()) {
-            CommandCenter commandCenter = iterator.next();
-            if (iterator.hasNext()) {
-                throw new IllegalStateException("Only single command center can be started");
-            } else {
-                commandCenter.beforeStart();
-                commandCenter.start();
-                RecordLog.info("[CommandCenterInit] Starting command center: "
-                    + commandCenter.getClass().getCanonicalName());
-            }
+        CommandCenter commandCenter = CommandCenterProvider.getCommandCenter();
+
+        if (commandCenter == null) {
+            RecordLog.warn("[CommandCenterInitFunc] Cannot resolve CommandCenter");
+            return;
         }
+
+        commandCenter.beforeStart();
+        commandCenter.start();
+        RecordLog.info("[CommandCenterInit] Starting command center: "
+                + commandCenter.getClass().getCanonicalName());
     }
 }