Extract compareTo impl to Router interface and concrete Router only responsible for provide priority.

dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/Router.java

@@ -85,4 +85,19 @@ default <T> void notify(List<Invoker<T>> invokers) {
      * @return router's priority
      */
     int getPriority();
+
+    @Override
+    default int compareTo(Router o) {
+        if (o == null) {
+            throw new IllegalArgumentException();
+        }
+        if (this.getPriority() == o.getPriority()) {
+            if (o.getUrl() == null) {
+                return -1;
+            }
+            return getUrl().toFullString().compareTo(o.getUrl().toFullString());
+        } else {
+            return getPriority() > o.getPriority() ? 1 : -1;
+        }
+    }
 }

dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/router/AbstractRouter.java

@@ -20,9 +20,6 @@
 import org.apache.dubbo.configcenter.DynamicConfiguration;
 import org.apache.dubbo.rpc.cluster.Router;
 
-/**
- * TODO Extract more code to here if necessary
- */
 public abstract class AbstractRouter implements Router {
     protected int priority;
     protected boolean force = false;
@@ -65,11 +62,6 @@ public void setForce(boolean force) {
         this.force = force;
     }
 
-    @Override
-    public int compareTo(Router o) {
-        return (this.getPriority() >= o.getPriority()) ? 1 : -1;
-    }
-
     public int getPriority() {
         return priority;
     }

dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/router/mock/MockInvokersSelector.java

@@ -22,7 +22,6 @@
 import org.apache.dubbo.rpc.Invocation;
 import org.apache.dubbo.rpc.Invoker;
 import org.apache.dubbo.rpc.RpcException;
-import org.apache.dubbo.rpc.cluster.Router;
 import org.apache.dubbo.rpc.cluster.router.AbstractRouter;
 
 import java.util.ArrayList;
@@ -95,15 +94,9 @@ private <T> boolean hasMockProviders(final List<Invoker<T>> invokers) {
         return hasMockProvider;
     }
 
-    /**
-     * Always stay on the top of the list
-     *
-     * @param o
-     * @return
-     */
     @Override
-    public int compareTo(Router o) {
-        return 1;
+    public int getPriority() {
+        return Integer.MAX_VALUE;
     }
 
 }

dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/router/script/ScriptRouter.java

@@ -24,6 +24,7 @@
 import org.apache.dubbo.rpc.Invoker;
 import org.apache.dubbo.rpc.RpcContext;
 import org.apache.dubbo.rpc.RpcException;
+import org.apache.dubbo.rpc.cluster.Router;
 import org.apache.dubbo.rpc.cluster.router.AbstractRouter;
 
 import javax.script.Bindings;
@@ -120,4 +121,13 @@ public boolean isRuntime() {
     public boolean isForce() {
         return url.getParameter(Constants.FORCE_KEY, false);
     }
+
+    @Override
+    public int compareTo(Router o) {
+        if (o == null || o.getClass() != ScriptRouter.class) {
+            return 1;
+        }
+        ScriptRouter c = (ScriptRouter) o;
+        return this.priority == c.priority ? rule.compareTo(c.rule) : (this.priority > c.priority ? 1 : -1);
+    }
 }

dubbo-compatible/src/main/java/com/alibaba/dubbo/rpc/cluster/Router.java

@@ -26,7 +26,7 @@
 import java.util.stream.Collectors;
 
 @Deprecated
-public interface Router extends org.apache.dubbo.rpc.cluster.Router {
+public interface Router extends org.apache.dubbo.rpc.cluster.Router{
 
     @Override
     com.alibaba.dubbo.common.URL getUrl();
@@ -36,7 +36,7 @@ <T> List<com.alibaba.dubbo.rpc.Invoker<T>> route(List<com.alibaba.dubbo.rpc.Invo
                                                      com.alibaba.dubbo.rpc.Invocation invocation)
             throws com.alibaba.dubbo.rpc.RpcException;
 
-    int compareTo(com.alibaba.dubbo.rpc.cluster.Router o);
+    int compareTo(Router o);
 
     // Add since 2.7.0
     @Override
@@ -65,7 +65,11 @@ default int getPriority() {
     }
 
     @Override
-    default int compareTo(org.apache.dubbo.rpc.cluster.Router o) {
-        return compareTo((Router) o);
+    default int compareTo (org.apache.dubbo.rpc.cluster.Router o) {
+        if (!(o instanceof Router)) {
+            return 1;
+        }
+
+        return this.compareTo((Router)o);
     }
 }

dubbo-compatible/src/test/java/org/apache/dubbo/rpc/cluster/CompatibleRouter.java

@@ -28,6 +28,7 @@
  *
  */
 public class CompatibleRouter implements Router {
+
     @Override
     public URL getUrl() {
         return null;

dubbo-compatible/src/test/java/org/apache/dubbo/rpc/cluster/CompatibleRouter2.java

@@ -0,0 +1,45 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.dubbo.rpc.cluster;
+
+import com.alibaba.dubbo.common.URL;
+import com.alibaba.dubbo.rpc.Invocation;
+import com.alibaba.dubbo.rpc.Invoker;
+import com.alibaba.dubbo.rpc.RpcException;
+import com.alibaba.dubbo.rpc.cluster.Router;
+
+import java.util.List;
+
+/**
+ *
+ */
+public class CompatibleRouter2 implements Router {
+    @Override
+    public URL getUrl() {
+        return null;
+    }
+
+    @Override
+    public <T> List<Invoker<T>> route(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException {
+        return null;
+    }
+
+    @Override
+    public int compareTo(Router o) {
+        return 0;
+    }
+}

dubbo-compatible/src/test/java/org/apache/dubbo/rpc/cluster/NewRouter.java

@@ -0,0 +1,54 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.dubbo.rpc.cluster;
+
+import org.apache.dubbo.common.URL;
+import org.apache.dubbo.rpc.Invocation;
+import org.apache.dubbo.rpc.Invoker;
+import org.apache.dubbo.rpc.RpcException;
+
+import java.util.List;
+
+/**
+ *
+ */
+public class NewRouter implements Router {
+    @Override
+    public URL getUrl() {
+        return null;
+    }
+
+    @Override
+    public <T> List<Invoker<T>> route(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException {
+        return null;
+    }
+
+    @Override
+    public boolean isRuntime() {
+        return false;
+    }
+
+    @Override
+    public boolean isForce() {
+        return false;
+    }
+
+    @Override
+    public int getPriority() {
+        return 0;
+    }
+}

dubbo-compatible/src/test/java/org/apache/dubbo/rpc/cluster/RouterTest.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 org.apache.dubbo.rpc.cluster;
+
+import org.junit.Assert;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ *
+ */
+public class RouterTest {
+
+    private static List<Router> routers = new ArrayList<>();
+
+    @BeforeClass
+    public static void setUp () {
+        CompatibleRouter compatibleRouter = new CompatibleRouter();
+        routers.add(compatibleRouter);
+        CompatibleRouter2 compatibleRouter2 = new CompatibleRouter2();
+        routers.add(compatibleRouter2);
+        NewRouter newRouter = new NewRouter();
+        routers.add(newRouter);
+    }
+
+    @Test
+    public void testCompareTo () {
+        try {
+            Collections.sort(routers);
+            Assert.assertTrue(true);
+        } catch (Exception e) {
+            Assert.assertFalse(false);
+        }
+    }
+}