Make the slot as the SPI interface and add a @Order annotation for define slot order

sentinel-adapter/sentinel-api-gateway-adapter-common/src/main/java/com/alibaba/csp/sentinel/adapter/gateway/common/slot/GatewayFlowSlot.java

@@ -27,11 +27,13 @@
 import com.alibaba.csp.sentinel.slots.block.flow.param.ParamFlowException;
 import com.alibaba.csp.sentinel.slots.block.flow.param.ParamFlowRule;
 import com.alibaba.csp.sentinel.slots.block.flow.param.ParameterMetricStorage;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * @author Eric Zhao
  * @since 1.6.1
  */
+@SpiOrder(-4000)
 public class GatewayFlowSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override

sentinel-adapter/sentinel-api-gateway-adapter-common/src/main/java/com/alibaba/csp/sentinel/adapter/gateway/common/slot/GatewaySlotChainBuilder.java

@@ -17,7 +17,7 @@
 
 import com.alibaba.csp.sentinel.slotchain.DefaultProcessorSlotChain;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlotChain;
-import com.alibaba.csp.sentinel.slotchain.SlotChainBuilder;
+import com.alibaba.csp.sentinel.slots.DefaultSlotChainBuilder;
 import com.alibaba.csp.sentinel.slots.block.authority.AuthoritySlot;
 import com.alibaba.csp.sentinel.slots.block.degrade.DegradeSlot;
 import com.alibaba.csp.sentinel.slots.block.flow.FlowSlot;
@@ -31,8 +31,13 @@
 /**
  * @author Eric Zhao
  * @since 1.6.1
+ *
+ * @deprecated since 1.7.2, we can use @SpiOrder(-4000) to adjust the order of slots,
+ * this class is reserved for compatibility with older versions.
+ * @see {@link GatewayFlowSlot}
  */
-public class GatewaySlotChainBuilder implements SlotChainBuilder {
+@Deprecated
+public class GatewaySlotChainBuilder extends DefaultSlotChainBuilder {
 
     @Override
     public ProcessorSlotChain build() {

sentinel-adapter/sentinel-api-gateway-adapter-common/src/main/resources/META-INF/services/com.alibaba.csp.sentinel.slotchain.ProcessorSlot

@@ -0,0 +1 @@
+com.alibaba.csp.sentinel.adapter.gateway.common.slot.GatewayFlowSlot

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/DefaultSlotChainBuilder.java

@@ -15,17 +15,15 @@
  */
 package com.alibaba.csp.sentinel.slots;
 
+import com.alibaba.csp.sentinel.log.RecordLog;
+import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.DefaultProcessorSlotChain;
+import com.alibaba.csp.sentinel.slotchain.ProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlotChain;
 import com.alibaba.csp.sentinel.slotchain.SlotChainBuilder;
-import com.alibaba.csp.sentinel.slots.block.authority.AuthoritySlot;
-import com.alibaba.csp.sentinel.slots.block.degrade.DegradeSlot;
-import com.alibaba.csp.sentinel.slots.block.flow.FlowSlot;
-import com.alibaba.csp.sentinel.slots.clusterbuilder.ClusterBuilderSlot;
-import com.alibaba.csp.sentinel.slots.logger.LogSlot;
-import com.alibaba.csp.sentinel.slots.nodeselector.NodeSelectorSlot;
-import com.alibaba.csp.sentinel.slots.statistic.StatisticSlot;
-import com.alibaba.csp.sentinel.slots.system.SystemSlot;
+import com.alibaba.csp.sentinel.util.SpiLoader;
+
+import java.util.List;
 
 /**
  * Builder for a default {@link ProcessorSlotChain}.
@@ -38,16 +36,18 @@ public class DefaultSlotChainBuilder implements SlotChainBuilder {
     @Override
     public ProcessorSlotChain build() {
         ProcessorSlotChain chain = new DefaultProcessorSlotChain();
-        chain.addLast(new NodeSelectorSlot());
-        chain.addLast(new ClusterBuilderSlot());
-        chain.addLast(new LogSlot());
-        chain.addLast(new StatisticSlot());
-        chain.addLast(new AuthoritySlot());
-        chain.addLast(new SystemSlot());
-        chain.addLast(new FlowSlot());
-        chain.addLast(new DegradeSlot());
+
+        // Note: the instances of ProcessorSlot should be different, since they are not stateless.
+        List<ProcessorSlot> sortedSlotList = SpiLoader.loadDifferentInstanceListSorted(ProcessorSlot.class);
+        for (ProcessorSlot slot : sortedSlotList) {
+            if (!(slot instanceof AbstractLinkedProcessorSlot)) {
+                RecordLog.warn("The ProcessorSlot(" + slot.getClass().getCanonicalName() + ") is not an instance of AbstractLinkedProcessorSlot, can't be added into ProcessorSlotChain");
+                continue;
+            }
+
+            chain.addLast((AbstractLinkedProcessorSlot<?>) slot);
+        }
 
         return chain;
     }
-
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/authority/AuthoritySlot.java

@@ -23,13 +23,15 @@
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * A {@link ProcessorSlot} that dedicates to {@link AuthorityRule} checking.
  *
  * @author leyou
  * @author Eric Zhao
  */
+@SpiOrder(-6000)
 public class AuthoritySlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/degrade/DegradeSlot.java

@@ -20,12 +20,14 @@
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * A {@link ProcessorSlot} dedicates to {@link DegradeRule} checking.
  *
  * @author leyou
  */
+@SpiOrder(-1000)
 public class DegradeSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/block/flow/FlowSlot.java

@@ -15,18 +15,19 @@
  */
 package com.alibaba.csp.sentinel.slots.block.flow;
 
-import java.util.Collection;
-import java.util.List;
-import java.util.Map;
-
 import com.alibaba.csp.sentinel.context.Context;
 import com.alibaba.csp.sentinel.node.DefaultNode;
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
 import com.alibaba.csp.sentinel.slots.block.BlockException;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 import com.alibaba.csp.sentinel.util.AssertUtil;
 import com.alibaba.csp.sentinel.util.function.Function;
 
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+
 /**
  * <p>
  * Combined the runtime statistics collected from the previous
@@ -136,6 +137,7 @@
  * @author jialiang.linjl
  * @author Eric Zhao
  */
+@SpiOrder(-2000)
 public class FlowSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     private final FlowRuleChecker checker;

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/clusterbuilder/ClusterBuilderSlot.java

@@ -30,6 +30,7 @@
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlotChain;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
 import com.alibaba.csp.sentinel.slotchain.StringResourceWrapper;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * <p>
@@ -44,6 +45,7 @@
  *
  * @author jialiang.linjl
  */
+@SpiOrder(-9000)
 public class ClusterBuilderSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     /**

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/logger/LogSlot.java

@@ -21,11 +21,13 @@
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
 import com.alibaba.csp.sentinel.slots.block.BlockException;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * A {@link com.alibaba.csp.sentinel.slotchain.ProcessorSlot} that is response for logging block exceptions
  * to provide concrete logs for troubleshooting.
  */
+@SpiOrder(-8000)
 public class LogSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/nodeselector/NodeSelectorSlot.java

@@ -22,6 +22,7 @@
 import com.alibaba.csp.sentinel.node.EntranceNode;
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -122,6 +123,7 @@
  * @see EntranceNode
  * @see ContextUtil
  */
+@SpiOrder(-10000)
 public class NodeSelectorSlot extends AbstractLinkedProcessorSlot<Object> {
 
     /**

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/statistic/StatisticSlot.java

@@ -21,6 +21,7 @@
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlotEntryCallback;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlotExitCallback;
 import com.alibaba.csp.sentinel.slots.block.flow.PriorityWaitException;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 import com.alibaba.csp.sentinel.util.TimeUtil;
 import com.alibaba.csp.sentinel.Constants;
 import com.alibaba.csp.sentinel.EntryType;
@@ -47,6 +48,7 @@
  * @author jialiang.linjl
  * @author Eric Zhao
  */
+@SpiOrder(-7000)
 public class StatisticSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/system/SystemSlot.java

@@ -20,13 +20,15 @@
 import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
  * A {@link ProcessorSlot} that dedicates to {@link SystemRule} checking.
  *
  * @author jialiang.linjl
  * @author leyou
  */
+@SpiOrder(-5000)
 public class SystemSlot extends AbstractLinkedProcessorSlot<DefaultNode> {
 
     @Override

sentinel-core/src/main/java/com/alibaba/csp/sentinel/util/SpiLoader.java

@@ -129,6 +129,7 @@ public static <T> T loadHighestPriorityInstance(Class<T> clazz) {
     }
 
     /**
+     * Load and sorted SPI instance list.
      * Load the SPI instance list for provided SPI interface.
      *
      * @param clazz class of the SPI
@@ -161,6 +162,8 @@ public static <T> List<T> loadInstanceList(Class<T> clazz) {
     /**
      * Load the sorted SPI instance list for provided SPI interface.
      *
+     * Note: each call return new instances.
+     *
      * @param clazz class of the SPI
      * @param <T>   SPI type
      * @return sorted SPI instance list
@@ -196,6 +199,41 @@ public static <T> List<T> loadInstanceListSorted(Class<T> clazz) {
         }
     }
 
+    /**
+     * Load the sorted and different SPI instance list for provided SPI interface.
+     *
+     * Note: each call return new instances.
+     *
+     * @param clazz class of the SPI
+     * @param <T>   SPI type
+     * @return sorted and different SPI instance list
+     * @since 1.7.2
+     */
+    public static <T> List<T> loadDifferentInstanceListSorted(Class<T> clazz) {
+        try {
+            // Not use SERVICE_LOADER_MAP, to make sure the instances loaded are different.
+            ServiceLoader<T> serviceLoader = ServiceLoaderUtil.getServiceLoader(clazz);
+
+            List<SpiOrderWrapper<T>> orderWrappers = new ArrayList<>();
+            for (T spi : serviceLoader) {
+                int order = SpiOrderResolver.resolveOrder(spi);
+                // Since SPI is lazy initialized in ServiceLoader, we use online sort algorithm here.
+                SpiOrderResolver.insertSorted(orderWrappers, spi, order);
+                RecordLog.info("[SpiLoader] Found {0} SPI: {1} with order " + order, clazz.getSimpleName(),
+                        spi.getClass().getCanonicalName());
+            }
+            List<T> list = new ArrayList<>();
+            for (int i = 0; i < orderWrappers.size(); i++) {
+                list.add(i, orderWrappers.get(i).spi);
+            }
+            return list;
+        } catch (Throwable t) {
+            RecordLog.warn("[SpiLoader] ERROR: loadDifferentInstanceListSorted failed", t);
+            t.printStackTrace();
+            return new ArrayList<>();
+        }
+    }
+
     private static class SpiOrderResolver {
         private static <T> void insertSorted(List<SpiOrderWrapper<T>> list, T spi, int order) {
             int idx = 0;

sentinel-core/src/main/resources/META-INF/services/com.alibaba.csp.sentinel.slotchain.ProcessorSlot

@@ -0,0 +1,9 @@
+# Sentinel default ProcessorSlots
+com.alibaba.csp.sentinel.slots.nodeselector.NodeSelectorSlot
+com.alibaba.csp.sentinel.slots.clusterbuilder.ClusterBuilderSlot
+com.alibaba.csp.sentinel.slots.logger.LogSlot
+com.alibaba.csp.sentinel.slots.statistic.StatisticSlot
+com.alibaba.csp.sentinel.slots.system.SystemSlot
+com.alibaba.csp.sentinel.slots.block.authority.AuthoritySlot
+com.alibaba.csp.sentinel.slots.block.flow.FlowSlot
+com.alibaba.csp.sentinel.slots.block.degrade.DegradeSlot

sentinel-core/src/test/java/com/alibaba/csp/sentinel/slots/DefaultSlotChainBuilderTest.java

@@ -0,0 +1,89 @@
+/*
+ * 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.slots;
+
+import com.alibaba.csp.sentinel.slotchain.AbstractLinkedProcessorSlot;
+import com.alibaba.csp.sentinel.slotchain.ProcessorSlotChain;
+import com.alibaba.csp.sentinel.slots.block.authority.AuthoritySlot;
+import com.alibaba.csp.sentinel.slots.block.degrade.DegradeSlot;
+import com.alibaba.csp.sentinel.slots.block.flow.FlowSlot;
+import com.alibaba.csp.sentinel.slots.clusterbuilder.ClusterBuilderSlot;
+import com.alibaba.csp.sentinel.slots.logger.LogSlot;
+import com.alibaba.csp.sentinel.slots.nodeselector.NodeSelectorSlot;
+import com.alibaba.csp.sentinel.slots.statistic.StatisticSlot;
+import com.alibaba.csp.sentinel.slots.system.SystemSlot;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+/**
+ * Test cases for {@link DefaultSlotChainBuilder}.
+ *
+ * @author cdfive
+ */
+public class DefaultSlotChainBuilderTest {
+
+    @Test
+    public void testBuild() {
+        DefaultSlotChainBuilder builder = new DefaultSlotChainBuilder();
+        ProcessorSlotChain slotChain = builder.build();
+        assertNotNull(slotChain);
+
+        // Verify the order of slot
+        AbstractLinkedProcessorSlot<?> next = slotChain.getNext();
+        assertTrue(next instanceof NodeSelectorSlot);
+
+        // Store the first NodeSelectorSlot instance
+        NodeSelectorSlot nodeSelectorSlot = (NodeSelectorSlot) next;
+
+        next = next.getNext();
+        assertTrue(next instanceof ClusterBuilderSlot);
+
+        next = next.getNext();
+        assertTrue(next instanceof LogSlot);
+
+        next = next.getNext();
+        assertTrue(next instanceof StatisticSlot);
+
+        next = next.getNext();
+        assertTrue(next instanceof AuthoritySlot);
+
+        next = next.getNext();
+        assertTrue(next instanceof SystemSlot);
+
+        next = next.getNext();
+        assertTrue(next instanceof FlowSlot);
+
+        next = next.getNext();
+        assertTrue(next instanceof DegradeSlot);
+
+        next = next.getNext();
+        assertNull(next);
+
+        // Build again to verify different instances
+        ProcessorSlotChain slotChain2 = builder.build();
+        assertNotNull(slotChain2);
+        // Verify the two ProcessorSlotChain instances are different
+        assertNotSame(slotChain, slotChain2);
+
+        next = slotChain2.getNext();
+        assertTrue(next instanceof NodeSelectorSlot);
+        // Store the second NodeSelectorSlot instance
+        NodeSelectorSlot nodeSelectorSlot2 = (NodeSelectorSlot) next;
+        // Verify the two NodeSelectorSlot instances are different
+        assertNotSame(nodeSelectorSlot, nodeSelectorSlot2);
+    }
+}