Make getProjectId() supports GAE (#1726)

APPENGINE.md

@@ -0,0 +1,33 @@
+Use Google Cloud Java Client with Google App Engine
+=================================================
+
+Quickstart
+----------
+This file provides extra instructions to set up Google Cloud Java Client with Google App Engine.
+
+See [Google Cloud JavaQuickstart](./README.md#quickstart) for general instructions on using Google Cloud Java.
+
+See [Quickstart for App Engine Standard Environment for Java](https://cloud.google.com/appengine/docs/standard/java/quickstart) for extra Google App Engine instructions.
+
+If you are using Maven, add this to your pom.xml file
+```xml
+<dependency>
+    <groupId>com.google.appengine</groupId>
+    <artifactId>appengine-api-1.0-sdk</artifactId>
+    <version>1.9.50</version>
+</dependency>
+```
+
+If you are using Gradle, add this to your dependencies
+```Groovy
+compile 'com.google.appengine:appengine-api-1.0-sdk:1.9.50'
+```
+If you are using SBT, add this to your dependencies
+```Scala
+libraryDependencies += "com.google.appengine" % "appengine-api-1.0-sdk" % "1.9.50"
+```
+
+To test with local dev server, run
+```
+mvn appengine:devserver
+```

README.md

@@ -57,6 +57,8 @@ If you are using SBT, add this to your dependencies
 libraryDependencies += "com.google.cloud" % "google-cloud" % "0.9.4-alpha"
 ```
 
+For running on Google App Engine, see [more instructions here](./APPENGINE.md).
+
 Example Applications
 --------------------
 

google-cloud-core/src/main/java/com/google/cloud/ServiceOptions.java

@@ -394,8 +394,17 @@ protected static String getAppEngineProjectId() {
       String serviceAccountName = (String) method.invoke(appIdentityService);
       int indexOfAtSign = serviceAccountName.indexOf('@');
       return serviceAccountName.substring(0, indexOfAtSign);
+    } catch (ClassNotFoundException exception) {
+      if (System.getProperty("com.google.appengine.runtime.version") != null) {
+        // Could not resolve appengine classes under GAE environment.
+        throw new RuntimeException("Google App Engine runtime detected "
+            + "(the environment variable \"com.google.appengine.runtime.version\" is set), "
+            + "but unable to resolve appengine-sdk classes. "
+            + "For more details see "
+            + "https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/APPENGINE.md");
+      }
+      return null;
     } catch (Exception ignore) {
-      // return null if can't determine
       return null;
     }
   }