Update Sentinel Web Servlet integration

- Add RequestOriginParser interface to extract request origin from the HTTP request
- Some code refinement

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

sentinel-adapter/sentinel-web-servlet/README.md

@@ -1,7 +1,16 @@
 # Sentinel Web Servlet Filter
 
-Sentinel provides Servlet filter integration. To use the filter,
-you can simply configure your `web.xml` with:
+Sentinel provides Servlet filter integration to enable flow control for web requests. Add the following dependency in `pom.xml` (if you are using Maven):
+
+```xml
+<dependency>
+    <groupId>com.alibaba.csp</groupId>
+    <artifactId>sentinel-web-servlet</artifactId>
+    <version>x.y.z</version>
+</dependency>
+```
+
+To use the filter, you can simply configure your `web.xml` with:
 
 ```xml
 <filter>
@@ -20,3 +29,9 @@ If customized block page is set (via `WebServletConfig.setBlockPage(blockPage)`
 the filter will redirect the request to provided URL. You can also implement your own
 block handler (the `UrlBlockHandler` interface) and register to `WebCallbackManager`.
 
+The `UrlCleaner` interface is designed for clean and unify the URL resource.
+For REST APIs, you have to clean the URL resource (e.g. `/foo/1` and `/foo/2` -> `/foo/:id`), or
+the amount of context and resources will exceed the threshold.
+
+`RequestOriginParser` interface is useful for extracting request origin (e.g. IP or appName from HTTP Header)
+from HTTP request. You can implement your own `RequestOriginParser` and register to `WebCallbackManager`.

sentinel-adapter/sentinel-web-servlet/src/main/java/com/alibaba/csp/sentinel/adapter/servlet/CommonFilter.java

@@ -30,15 +30,19 @@ import com.alibaba.csp.sentinel.Entry;
 import com.alibaba.csp.sentinel.EntryType;
 import com.alibaba.csp.sentinel.SphU;
 import com.alibaba.csp.sentinel.Tracer;
+import com.alibaba.csp.sentinel.adapter.servlet.callback.RequestOriginParser;
+import com.alibaba.csp.sentinel.adapter.servlet.callback.UrlCleaner;
 import com.alibaba.csp.sentinel.adapter.servlet.callback.WebCallbackManager;
 import com.alibaba.csp.sentinel.adapter.servlet.util.FilterUtil;
 import com.alibaba.csp.sentinel.context.ContextUtil;
 import com.alibaba.csp.sentinel.slots.block.BlockException;
+import com.alibaba.csp.sentinel.util.StringUtil;
 
 /***
  * Servlet filter that integrates with Sentinel.
  *
  * @author youji.zj
+ * @author Eric Zhao
  */
 public class CommonFilter implements Filter {
 
@@ -55,14 +59,24 @@ public class CommonFilter implements Filter {
 
         try {
             String target = FilterUtil.filterTarget(sRequest);
-            target = WebCallbackManager.getUrlCleaner().clean(target);
+            // Clean and unify the URL.
+            // For REST APIs, you have to clean the URL (e.g. `/foo/1` and `/foo/2` -> `/foo/:id`), or
+            // the amount of context and resources will exceed the threshold.
+            UrlCleaner urlCleaner = WebCallbackManager.getUrlCleaner();
+            if (urlCleaner != null) {
+                target = urlCleaner.clean(target);
+            }
+
+            // Parse the request origin using registered origin parser.
+            String origin = parseOrigin(sRequest);
 
-            ContextUtil.enter(target);
+            ContextUtil.enter(target, origin);
             entry = SphU.entry(target, EntryType.IN);
 
             chain.doFilter(request, response);
         } catch (BlockException e) {
             HttpServletResponse sResponse = (HttpServletResponse)response;
+            // Return the block page, or redirect to another URL.
             WebCallbackManager.getUrlBlockHandler().blocked(sRequest, sResponse);
         } catch (IOException e2) {
             Tracer.trace(e2);
@@ -81,8 +95,22 @@ public class CommonFilter implements Filter {
         }
     }
 
+    private String parseOrigin(HttpServletRequest request) {
+        RequestOriginParser originParser = WebCallbackManager.getRequestOriginParser();
+        String origin = EMPTY_ORIGIN;
+        if (originParser != null) {
+            origin = originParser.parseOrigin(request);
+            if (StringUtil.isEmpty(origin)) {
+                return EMPTY_ORIGIN;
+            }
+        }
+        return origin;
+    }
+
     @Override
     public void destroy() {
 
     }
+
+    private static final String EMPTY_ORIGIN = "";
 }

sentinel-adapter/sentinel-web-servlet/src/main/java/com/alibaba/csp/sentinel/adapter/servlet/callback/RequestOriginParser.java

@@ -0,0 +1,35 @@
+/*
+ * 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.adapter.servlet.callback;
+
+import javax.servlet.http.HttpServletRequest;
+
+/**
+ * The origin parser parses request origin (e.g. IP, user, appName) from HTTP request.
+ *
+ * @author Eric Zhao
+ * @since 0.2.0
+ */
+public interface RequestOriginParser {
+
+    /**
+     * Parse the origin from given HTTP request.
+     *
+     * @param request HTTP request
+     * @return parsed origin
+     */
+    String parseOrigin(HttpServletRequest request);
+}

sentinel-adapter/sentinel-web-servlet/src/main/java/com/alibaba/csp/sentinel/adapter/servlet/callback/WebCallbackManager.java

@@ -15,6 +15,8 @@
  */
 package com.alibaba.csp.sentinel.adapter.servlet.callback;
 
+import com.alibaba.csp.sentinel.util.AssertUtil;
+
 /**
  * Registry for URL cleaner and URL block handler.
  *
@@ -23,15 +25,17 @@ package com.alibaba.csp.sentinel.adapter.servlet.callback;
 public class WebCallbackManager {
 
     /**
-     * URL cleaner
+     * URL cleaner.
      */
     private static volatile UrlCleaner urlCleaner = new DefaultUrlCleaner();
 
     /**
-     * URL block handler
+     * URL block handler.
      */
     private static volatile UrlBlockHandler urlBlockHandler = new DefaultUrlBlockHandler();
 
+    private static volatile RequestOriginParser requestOriginParser = null;
+
     public static UrlCleaner getUrlCleaner() {
         return urlCleaner;
     }
@@ -45,6 +49,16 @@ public class WebCallbackManager {
     }
 
     public static void setUrlBlockHandler(UrlBlockHandler urlBlockHandler) {
+        AssertUtil.isTrue(urlBlockHandler != null, "URL block handler should not be null");
         WebCallbackManager.urlBlockHandler = urlBlockHandler;
     }
+
+    public static RequestOriginParser getRequestOriginParser() {
+        return requestOriginParser;
+    }
+
+    public static void setRequestOriginParser(
+        RequestOriginParser requestOriginParser) {
+        WebCallbackManager.requestOriginParser = requestOriginParser;
+    }
 }