Add ServiceLoaderUtil to improve SPI classloader mechanism and update SpiLoader

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

sentinel-core/src/main/java/com/alibaba/csp/sentinel/config/SentinelConfig.java

@@ -48,6 +48,7 @@ public class SentinelConfig {
     public static final String TOTAL_METRIC_FILE_COUNT = "csp.sentinel.metric.file.total.count";
     public static final String COLD_FACTOR = "csp.sentinel.flow.cold.factor";
     public static final String STATISTIC_MAX_RT = "csp.sentinel.statistic.max.rt";
+    public static final String SPI_CLASSLOADER = "csp.sentinel.spi.classloader";
 
     static final String DEFAULT_CHARSET = "UTF-8";
     static final long DEFAULT_SINGLE_METRIC_FILE_SIZE = 1024 * 1024 * 50;

sentinel-core/src/main/java/com/alibaba/csp/sentinel/spi/ServiceLoaderUtil.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright 1999-2019 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.spi;
+
+import java.util.ServiceLoader;
+
+import com.alibaba.csp.sentinel.config.SentinelConfig;
+
+/**
+ * @author Eric Zhao
+ * @since 1.7.0
+ */
+public final class ServiceLoaderUtil {
+
+    private static final String CLASSLOADER_DEFAULT = "default";
+    private static final String CLASSLOADER_CONTEXT = "context";
+
+    public static <S> ServiceLoader<S> getServiceLoader(Class<S> clazz) {
+        if (shouldUseContextClassloader()) {
+            return ServiceLoader.load(clazz);
+        } else {
+            return ServiceLoader.load(clazz, clazz.getClassLoader());
+        }
+    }
+
+    public static boolean shouldUseContextClassloader() {
+        String classloaderConf = SentinelConfig.getConfig(SentinelConfig.SPI_CLASSLOADER);
+        return CLASSLOADER_CONTEXT.equalsIgnoreCase(classloaderConf);
+    }
+
+    private ServiceLoaderUtil() {}
+}

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

@@ -23,6 +23,7 @@ import java.util.ServiceLoader;
 import java.util.concurrent.ConcurrentHashMap;
 
 import com.alibaba.csp.sentinel.log.RecordLog;
+import com.alibaba.csp.sentinel.spi.ServiceLoaderUtil;
 import com.alibaba.csp.sentinel.spi.SpiOrder;
 
 /**
@@ -34,12 +35,13 @@ public final class SpiLoader {
     private static final Map<String, ServiceLoader> SERVICE_LOADER_MAP = new ConcurrentHashMap<String, ServiceLoader>();
 
     public static <T> T loadFirstInstance(Class<T> clazz) {
+        AssertUtil.notNull(clazz, "SPI class cannot be null");
         try {
             String key = clazz.getName();
             // Not thread-safe, as it's expected to be resolved in a thread-safe context.
             ServiceLoader<T> serviceLoader = SERVICE_LOADER_MAP.get(key);
             if (serviceLoader == null) {
-                serviceLoader = ServiceLoader.load(clazz);
+                serviceLoader = ServiceLoaderUtil.getServiceLoader(clazz);
                 SERVICE_LOADER_MAP.put(key, serviceLoader);
             }
 
@@ -56,6 +58,41 @@ public final class SpiLoader {
         }
     }
 
+    /**
+     * Load the first-found specific SPI instance (excluding provided default SPI class).
+     * If no other SPI implementation found, then create a default SPI instance.
+     *
+     * @param clazz        class of the SPI interface
+     * @param defaultClass class of the default SPI implementation (if no other implementation found)
+     * @param <T>          SPI type
+     * @return the first specific SPI instance if exists, or else the default SPI instance
+     * @since 1.7.0
+     */
+    public static <T> T loadFirstInstanceOrDefault(Class<T> clazz, Class<? extends T> defaultClass) {
+        AssertUtil.notNull(clazz, "SPI class cannot be null");
+        AssertUtil.notNull(defaultClass, "default SPI class cannot be null");
+        try {
+            String key = clazz.getName();
+            // Not thread-safe, as it's expected to be resolved in a thread-safe context.
+            ServiceLoader<T> serviceLoader = SERVICE_LOADER_MAP.get(key);
+            if (serviceLoader == null) {
+                serviceLoader = ServiceLoaderUtil.getServiceLoader(clazz);
+                SERVICE_LOADER_MAP.put(key, serviceLoader);
+            }
+
+            for (T instance : serviceLoader) {
+                if (instance.getClass() != defaultClass) {
+                    return instance;
+                }
+            }
+            return defaultClass.newInstance();
+        } catch (Throwable t) {
+            RecordLog.warn("[SpiLoader] ERROR: loadFirstInstanceOrDefault failed", t);
+            t.printStackTrace();
+            return null;
+        }
+    }
+
     /**
      * Load the SPI instance with highest priority.
      *
@@ -70,7 +107,7 @@ public final class SpiLoader {
             // Not thread-safe, as it's expected to be resolved in a thread-safe context.
             ServiceLoader<T> serviceLoader = SERVICE_LOADER_MAP.get(key);
             if (serviceLoader == null) {
-                serviceLoader = ServiceLoader.load(clazz);
+                serviceLoader = ServiceLoaderUtil.getServiceLoader(clazz);
                 SERVICE_LOADER_MAP.put(key, serviceLoader);
             }
 
@@ -105,7 +142,7 @@ public final class SpiLoader {
             // Not thread-safe, as it's expected to be resolved in a thread-safe context.
             ServiceLoader<T> serviceLoader = SERVICE_LOADER_MAP.get(key);
             if (serviceLoader == null) {
-                serviceLoader = ServiceLoader.load(clazz);
+                serviceLoader = ServiceLoaderUtil.getServiceLoader(clazz);
                 SERVICE_LOADER_MAP.put(key, serviceLoader);
             }
 
@@ -135,7 +172,7 @@ public final class SpiLoader {
             // Not thread-safe, as it's expected to be resolved in a thread-safe context.
             ServiceLoader<T> serviceLoader = SERVICE_LOADER_MAP.get(key);
             if (serviceLoader == null) {
-                serviceLoader = ServiceLoader.load(clazz);
+                serviceLoader = ServiceLoaderUtil.getServiceLoader(clazz);
                 SERVICE_LOADER_MAP.put(key, serviceLoader);
             }