Attempt to reduce code 500 errors (#156)

* Avoid sending null message to HttpServletResponse

* Failback to localhost if unable to reach request local server

Author: russgold

README.md

@@ -53,6 +53,8 @@ Note that there are two parts to the configuration. The optional top portion def
 | `domainQualifier` | If true, the domain name will be included as a qualifier for all metrics. Defaults to false. |
 | `restPort` | Optional, used in the web application only. Overrides the port on which the exporter should contact the REST API. Needed if the exporter cannot find the REST API. The most common case is running on a system with the administration port enabled. In that case, you must specify the administration port in this field and access the exporter by using the SSL port. |
 
+Note that if unable to contact the REST API using the inferred host and port, the exporter will try the local host name and, if the REST port is specified, the local port. 
+
 The `query` field is more complex. Each query consists of a hierarchy of the [MBeans](https://docs.oracle.com/middleware/12213/wls/WLMBR/core/index.html), starting relative to `ServerRuntimes`.
 Within each section, there are a number of options:
 
@@ -178,7 +180,7 @@ Setting | Default | Property
 ------------ | ------------- | -------------
 Domain name | (use WLS definition) | `DOMAIN`
 Exporter port | `8080` | `EXPORTER_PORT`
-WebLogic host | `localhost` | `WLS_HOST`
+WebLogic host name | `localhost` | `WLS_HOST`
 WebLogic port | `7001` | `WLS_PORT`
 Use https | `false` | `WLS_SECURE`
 
@@ -234,7 +236,7 @@ vulnerability disclosure process.
 
 ## License
 
-Copyright (c) 2019, 2021 Oracle and/or its affiliates.
+Copyright (c) 2019, 2022 Oracle and/or its affiliates.
 
 Released under the Universal Permissive License v1.0 as shown at
 <https://oss.oracle.com/licenses/upl/>.

wls-exporter-core/src/main/java/com/oracle/wls/exporter/AuthenticatedCall.java

@@ -1,10 +1,11 @@
-// Copyright (c) 2017, 2021, Oracle and/or its affiliates.
+// Copyright (c) 2017, 2022, Oracle and/or its affiliates.
 // Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl.
 
 package com.oracle.wls.exporter;
 
 import java.io.IOException;
 import java.io.PrintStream;
+import java.util.List;
 
 import com.oracle.wls.exporter.domain.MBeanSelector;
 import com.oracle.wls.exporter.domain.QueryType;
@@ -77,12 +78,21 @@ public void doWithAuthentication() throws IOException {
             context.sendError(status, e.getMessage());
         } catch (RestPortConnectionException e) {
             context.setStatus(HTTP_INTERNAL_ERROR);
-            reportUnableToContactRestApi(e.getUri());
+            reportUnableToContactRestApiPort(getFailedHosts());
         } finally {
             context.close();
         }
     }
 
+    private String getFailedHosts() {
+        final List<String> hosts = urlBuilder.getFailedHosts();
+        if (hosts.size() < 3) {
+            return String.join(" or ", hosts);
+        } else {
+            return String.join(", ", hosts.subList(0, hosts.size()-1)) + " or " + hosts.get(hosts.size() - 1);
+        }
+    }
+
     /**
      * Returns a name for the instance for which the exporter is reporting metrics.
      */
@@ -104,15 +114,17 @@ private void performRequest(WebClient webClient) throws IOException {
         } while (webClient.isRetryNeeded());
     }
 
-    private void reportUnableToContactRestApi(String uri) throws IOException {
+    private void reportUnableToContactRestApiPort(String uri) throws IOException {
         try (PrintStream out = context.getResponseStream()) {
-            out.println("# Unable to contact the REST API at " + uri + ". May be using the wrong port.");
+            out.println("# Unable to contact the REST API at " + uri + ". May be using the wrong host name or port.");
             out.println("#");
-            out.println("# This most commonly occurs when the exporter is accessed via a load balancer");
-            out.println("# configured on a different port than the managed server.");
+            out.println("# This can happen when the exporter is accessed via a load balancer");
+            out.println("# configured on a different port than the server's REST API,");
+            out.println("# or when the remote server DNS name is not resolvable from the server itself.");
             out.println("#");
             out.println("# You can correct this by giving the exporter WAR an initial configuration with the");
-            out.println("# restPort field set to the managed server's plain text port.");
+            out.println("# restPort field set to the managed server's plain text port and/or");
+            out.println("# restHostName field set to the managed server's locally resolvable name.");
         }
     }
 

wls-exporter-core/src/main/java/com/oracle/wls/exporter/DemoInputs.java

@@ -1,4 +1,4 @@
-// Copyright (c) 2017, 2020, Oracle and/or its affiliates.
+// Copyright (c) 2017, 2022, Oracle and/or its affiliates.
 // Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl.
 
 package com.oracle.wls.exporter;
@@ -10,8 +10,6 @@ class DemoInputs {
     @SuppressWarnings("unused")
     static final String YAML_STRING = "---\n" +
             "startDelaySeconds: 5\n" +
-            "host: localhost\n" +
-            "port: 7001\n" +
             "username: system\n" +
             "password: gumby1234\n" +
             "\n" +
@@ -35,8 +33,6 @@ class DemoInputs {
     @SuppressWarnings("unused")
     static final String YAML_STRING2 = "---\n" +
             "startDelaySeconds: 5\n" +
-            "host: localhost\n" +
-            "port: 7001\n" +
             "username: system\n" +
             "password: gumby1234\n" +
             "queries:\n" +

wls-exporter-core/src/main/java/com/oracle/wls/exporter/LiveConfiguration.java

@@ -1,4 +1,4 @@
-// Copyright (c) 2017, 2021, Oracle and/or its affiliates.
+// Copyright (c) 2017, 2022, Oracle and/or its affiliates.
 // Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl.
 
 package com.oracle.wls.exporter;
@@ -124,7 +124,7 @@ private static void installUpdater(QuerySyncConfiguration syncConfiguration) {
      * @return a human readable representation of the configuration
      */
     public static String asString() {
-        return "host: " + serverName + '\n' +
+        return "hostName: " + serverName + '\n' +
                "port: " + serverPort + '\n' + getConfig();
     }
 

wls-exporter-core/src/main/java/com/oracle/wls/exporter/ServerErrorException.java

@@ -1,8 +1,10 @@
-// Copyright 2020, 2021, Oracle and/or its affiliates.
+// Copyright 2020, 2022, Oracle and/or its affiliates.
 // Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl.
 
 package com.oracle.wls.exporter;
 
+import java.util.Optional;
+
 /**
  * An exception thrown when a 5xx status code is received from the server.
  */
@@ -22,4 +24,11 @@ public ServerErrorException(int status, String message) {
   public int getStatus() {
     return status;
   }
+
+  @Override
+  public String getMessage() {
+    StringBuilder sb = new StringBuilder("Received status ").append(status);
+    Optional.ofNullable(super.getMessage()).ifPresent(m -> sb.append(" - ").append(m));
+    return sb.toString();
+  }
 }

wls-exporter-core/src/main/java/com/oracle/wls/exporter/ServletInvocationContext.java

@@ -1,11 +1,13 @@
-// Copyright (c) 2021, Oracle and/or its affiliates.
+// Copyright (c) 2021, 2022, Oracle and/or its affiliates.
 // Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl.
 
 package com.oracle.wls.exporter;
 
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.PrintStream;
+import java.net.InetAddress;
+import java.net.UnknownHostException;
 import java.util.Optional;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
@@ -18,6 +20,7 @@ public class ServletInvocationContext implements InvocationContext {
 
   private final HttpServletRequest request;
   private final HttpServletResponse response;
+  private final String localHostName = getLocalHostName();
 
   public ServletInvocationContext(HttpServletRequest request, HttpServletResponse response) {
     this.request = request;
@@ -31,11 +34,21 @@ public void close() {
 
   @Override
   public UrlBuilder createUrlBuilder() {
-    return UrlBuilder.create(request.getLocalName(), request.isSecure())
+    return UrlBuilder.create(request.isSecure())
+          .withHostName(request.getLocalName())
+          .withHostName(localHostName)
           .withPort(LiveConfiguration.getConfiguredRestPort())
           .withPort(request.getLocalPort());
   }
 
+  static String getLocalHostName() {
+    try {
+      return InetAddress.getLocalHost().getHostName();
+    } catch (UnknownHostException e) {
+      return "localhost";
+    }
+  }
+
   @Override
   public String getApplicationContext() {
     return request.getContextPath();
@@ -68,7 +81,11 @@ public PrintStream getResponseStream() throws IOException {
 
   @Override
   public void sendError(int status, String msg) throws IOException {
+    if (msg == null) {
+      response.sendError(status);
+    } else {
       response.sendError(status, msg);
+    }
   }
 
   @Override