Make it possible to inject a glue when creating a Runtime

To aid testing, make it possible to inject a glue when creating a
Runtime. Also add a test specify that a CucumberException is thrown
if no backends are found (or an emtpy collection of backends is
injected).

Author: brasmusson

core/src/main/java/cucumber/runtime/Runtime.java

@@ -51,19 +51,24 @@ public Runtime(ResourceLoader resourceLoader, ClassLoader classLoader, RuntimeOp
     }
 
     public Runtime(ResourceLoader resourceLoader, ClassLoader classLoader, Collection<? extends Backend> backends, RuntimeOptions runtimeOptions) {
-        this.resourceLoader = resourceLoader;
-        this.classLoader = classLoader;
+        this(resourceLoader, classLoader, backends, runtimeOptions, null);
+    }
+
+    public Runtime(ResourceLoader resourceLoader, ClassLoader classLoader, Collection<? extends Backend> backends,
+            RuntimeOptions runtimeOptions, RuntimeGlue optionalGlue) {
         if (backends.isEmpty()) {
             throw new CucumberException("No backends were found. Please make sure you have a backend module on your CLASSPATH.");
         }
+        this.resourceLoader = resourceLoader;
+        this.classLoader = classLoader;
         this.backends = backends;
-        glue = new RuntimeGlue(undefinedStepsTracker, new LocalizedXStreams(classLoader));
+        this.runtimeOptions = runtimeOptions;
+        this.glue = optionalGlue != null ? optionalGlue : new RuntimeGlue(undefinedStepsTracker, new LocalizedXStreams(classLoader));
 
         for (Backend backend : backends) {
             backend.loadGlue(glue, runtimeOptions.getGlue());
             backend.setUnreportedStepExecutor(this);
         }
-        this.runtimeOptions = runtimeOptions;
     }
 
     private static Collection<? extends Backend> loadBackends(ResourceLoader resourceLoader, ClassLoader classLoader) {

core/src/test/java/cucumber/runtime/RuntimeTest.java

@@ -13,12 +13,14 @@
 
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.Properties;
 
 import static cucumber.runtime.TestHelper.feature;
 import static java.util.Arrays.asList;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
 import static org.mockito.Mockito.mock;
 
 public class RuntimeTest {
@@ -163,6 +165,18 @@ public void strict_with_errors() {
         assertEquals(0x1, runtime.exitStatus());
     }
 
+    @Test
+    public void should_throw_cucumer_exception_if_no_backends_are_found() throws Exception {
+        try {
+            ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
+            new Runtime(new ClasspathResourceLoader(classLoader), classLoader, Collections.<Backend>emptyList(),
+                    new RuntimeOptions(new Properties()));
+            fail("A CucumberException should have been thrown");
+        } catch (CucumberException e) {
+            assertEquals("No backends were found. Please make sure you have a backend module on your CLASSPATH.", e.getMessage());
+        }
+    }
+
     private Runtime createStrictRuntime() {
         return createRuntime("-g", "anything", "--strict");
     }