Support extensible slot chain builder using SPI mechanism (#145)

- Support extensible `SlotChainBuilder` using SPI mechanism
- Add a `SlotChainProvider` to load slot chain builder and create new slot chains

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

sentinel-core/src/main/java/com/alibaba/csp/sentinel/CtSph.java

@@ -28,6 +28,7 @@ import com.alibaba.csp.sentinel.slotchain.MethodResourceWrapper;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlot;
 import com.alibaba.csp.sentinel.slotchain.ProcessorSlotChain;
 import com.alibaba.csp.sentinel.slotchain.ResourceWrapper;
+import com.alibaba.csp.sentinel.slotchain.SlotChainProvider;
 import com.alibaba.csp.sentinel.slotchain.StringResourceWrapper;
 import com.alibaba.csp.sentinel.slots.block.BlockException;
 import com.alibaba.csp.sentinel.slots.block.Rule;
@@ -133,7 +134,7 @@ public class CtSph implements Sph {
                         return null;
                     }
 
-                    chain = Env.slotsChainbuilder.build();
+                    chain = SlotChainProvider.newSlotChain();
                     Map<ResourceWrapper, ProcessorSlotChain> newMap = new HashMap<ResourceWrapper, ProcessorSlotChain>(
                         chainMap.size() + 1);
                     newMap.putAll(chainMap);

sentinel-core/src/main/java/com/alibaba/csp/sentinel/Env.java

@@ -18,15 +18,12 @@ package com.alibaba.csp.sentinel;
 import com.alibaba.csp.sentinel.init.InitExecutor;
 import com.alibaba.csp.sentinel.node.DefaultNodeBuilder;
 import com.alibaba.csp.sentinel.node.NodeBuilder;
-import com.alibaba.csp.sentinel.slots.DefaultSlotsChainBuilder;
-import com.alibaba.csp.sentinel.slots.SlotsChainBuilder;
 
 /**
  * @author jialiang.linjl
  */
 public class Env {
 
-    public static final SlotsChainBuilder slotsChainbuilder = new DefaultSlotsChainBuilder();
     public static final NodeBuilder nodeBuilder = new DefaultNodeBuilder();
     public static final Sph sph = new CtSph();
 

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slotchain/SlotChainBuilder.java

@@ -13,20 +13,21 @@
  * 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.ProcessorSlotChain;
+package com.alibaba.csp.sentinel.slotchain;
 
 /**
+ * The builder for processor slot chain.
+ *
  * @author qinan.qn
  * @author leyou
+ * @author Eric Zhao
  */
-public interface SlotsChainBuilder {
+public interface SlotChainBuilder {
 
     /**
-     * Helper method to create processor slot chain.
+     * Build the processor slot chain.
      *
-     * @return a processor slot that chain some slots together.
+     * @return a processor slot that chain some slots together
      */
     ProcessorSlotChain build();
 }

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slotchain/SlotChainProvider.java

@@ -0,0 +1,78 @@
+/*
+ * 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.slotchain;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.ServiceLoader;
+
+import com.alibaba.csp.sentinel.log.RecordLog;
+import com.alibaba.csp.sentinel.slots.DefaultSlotChainBuilder;
+
+/**
+ * A provider for creating slot chains via resolved slot chain builder SPI.
+ *
+ * @author Eric Zhao
+ * @since 0.2.0
+ */
+public final class SlotChainProvider {
+
+    private static volatile SlotChainBuilder builder = null;
+
+    private static final ServiceLoader<SlotChainBuilder> LOADER = ServiceLoader.load(SlotChainBuilder.class);
+
+    /**
+     * The load and pick process is not thread-safe, but it's okay since the method should be only invoked
+     * via {@code lookProcessChain} in {@link com.alibaba.csp.sentinel.CtSph} under lock.
+     *
+     * @return new created slot chain
+     */
+    public static ProcessorSlotChain newSlotChain() {
+        if (builder != null) {
+            return builder.build();
+        }
+
+        resolveSlotChainBuilder();
+
+        if (builder == null) {
+            RecordLog.warn("[SlotChainProvider] Wrong state when resolving slot chain builder, using default");
+            builder = new DefaultSlotChainBuilder();
+        }
+        return builder.build();
+    }
+
+    private static void resolveSlotChainBuilder() {
+        List<SlotChainBuilder> list = new ArrayList<SlotChainBuilder>();
+        boolean hasOther = false;
+        for (SlotChainBuilder builder : LOADER) {
+            if (builder.getClass() != DefaultSlotChainBuilder.class) {
+                hasOther = true;
+                list.add(builder);
+            }
+        }
+        if (hasOther) {
+            builder = list.get(0);
+        } else {
+            // No custom builder, using default.
+            builder = new DefaultSlotChainBuilder();
+        }
+
+        RecordLog.info("[SlotChainProvider] Global slot chain builder resolved: "
+            + builder.getClass().getCanonicalName());
+    }
+
+    private SlotChainProvider() {}
+}

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

@@ -17,6 +17,7 @@ package com.alibaba.csp.sentinel.slots;
 
 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.block.authority.AuthoritySlot;
 import com.alibaba.csp.sentinel.slots.block.degrade.DegradeSlot;
 import com.alibaba.csp.sentinel.slots.block.flow.FlowSlot;
@@ -32,7 +33,7 @@ import com.alibaba.csp.sentinel.slots.system.SystemSlot;
  * @author qinan.qn
  * @author leyou
  */
-public class DefaultSlotsChainBuilder implements SlotsChainBuilder {
+public class DefaultSlotChainBuilder implements SlotChainBuilder {
 
     @Override
     public ProcessorSlotChain build() {

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

@@ -0,0 +1,2 @@
+# Default slot chain builder
+com.alibaba.csp.sentinel.slots.DefaultSlotChainBuilder