Ensure compatibility for elegant shutdown under servlet container. ap…

…ache#1998

dubbo-config/dubbo-config-spring/src/main/java/org/apache/dubbo/config/spring/initializer/DubboContextLoaderListener.java

@@ -0,0 +1,58 @@
+package org.apache.dubbo.config.spring.initializer;
+
+import org.springframework.web.context.ContextLoaderListener;
+import org.springframework.web.context.WebApplicationContext;
+import org.springframework.web.context.support.XmlWebApplicationContext;
+
+import javax.servlet.ServletContext;
+
+/**
+ * This class is introduced mainly for compatibility.
+ * Consider the following case:
+ * 1. listener class {@link ContextLoaderListener} is defined in web.xml
+ *    servlet container will initialized {@link ContextLoaderListener} before this class.
+ *    Therefore Dubbo's elegant shutdown feature will not be enabled.
+ *    To enable it, you need to either:
+ *    1) remove listener class {@link ContextLoaderListener} in your web.xml
+ *    2) change listener class to {@link DubboContextLoaderListener} in your web.xml
+ * 2. listener class {@link DubboContextLoaderListener} is defined in web.xml
+ *    This is automatically enable Dubbo's elegant shutdown feature,
+ *    even for a servlet container which is not 3.0 compatible
+ * 3. no listener class defined in web.xml
+ *    Dubbo's elegant shutdown feature will be automatically enabled
+ *    if Dubbo is running under a servlet 3.0+ compatible container
+ *    see {@link DubboWebApplicationInitializer} for more details.
+ */
+public class DubboContextLoaderListener extends ContextLoaderListener {
+
+    /**
+     * The root WebApplicationContext instance that this loader manages.
+     */
+    private WebApplicationContext context;
+
+    /**
+     * This is used for xml configuration, which does not require servlet version to be 3.0+
+     */
+    public DubboContextLoaderListener(){
+        super();
+        this.context = new XmlWebApplicationContext();
+        ((XmlWebApplicationContext)context).addApplicationListener(new DubboApplicationListener());
+    }
+
+    /**
+     * This is used for programmatic API, which requires servlet version to be 3.0+
+     * @param context the web application context
+     */
+    public DubboContextLoaderListener(WebApplicationContext context) {
+        this.context = context;
+    }
+
+    @Override
+    public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
+        if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
+            // if the root web application has already been created, just ignore.
+            return this.context;
+        }
+        return super.initWebApplicationContext(servletContext);
+    }
+}

dubbo-config/dubbo-config-spring/src/main/java/org/apache/dubbo/config/spring/initializer/DubboWebApplicationInitializer.java

@@ -20,6 +20,8 @@
 import org.springframework.web.context.WebApplicationContext;
 import org.springframework.web.context.support.XmlWebApplicationContext;
 
+import javax.servlet.ServletContext;
+
 /**
  * An initializer to register {@link DubboApplicationListener}
  * to the ApplicationContext seamlessly.
@@ -37,4 +39,21 @@ protected WebApplicationContext createRootApplicationContext() {
         webApplicationContext.addApplicationListener(new DubboApplicationListener());
         return webApplicationContext;
     }
+
+    /**
+     * Register {@link DubboContextLoaderListener} to application context.
+     * @param servletContext
+     */
+    protected void registerContextLoaderListener(ServletContext servletContext) {
+        WebApplicationContext rootAppContext = createRootApplicationContext();
+        if (rootAppContext != null) {
+            DubboContextLoaderListener listener = new DubboContextLoaderListener(rootAppContext);
+            listener.setContextInitializers(getRootApplicationContextInitializers());
+            servletContext.addListener(listener);
+        }
+        else {
+            logger.debug("No ContextLoaderListener registered, as " +
+                    "createRootApplicationContext() did not return an application context");
+        }
+    }
 }