This is the fix for issue #723. I have made the static GroovyBackend instance a ThreadLocal so it can be run in parallel in the same JVM.

groovy/src/main/code_generator/I18n.groovy.txt

@@ -8,11 +8,11 @@ import java.util.regex.Pattern;
 public class ${i18n.underscoredIsoCode.toUpperCase()} {
 <% i18n.codeKeywords.each { kw -> %>
     public static void ${kw}(Pattern regexp, Closure body) throws Throwable {
-        GroovyBackend.instance.addStepDefinition(regexp, 0, body);
+        GroovyBackend.getInstance().addStepDefinition(regexp, 0, body);
     }
 
     public static void ${java.text.Normalizer.normalize(kw, java.text.Normalizer.Form.NFC)}(Pattern regexp, long timeoutMillis, Closure body) throws Throwable {
-        GroovyBackend.instance.addStepDefinition(regexp, timeoutMillis, body);
+        GroovyBackend.getInstance().addStepDefinition(regexp, timeoutMillis, body);
     }
 <% } %>
 }

groovy/src/main/java/cucumber/api/groovy/Hooks.java

@@ -9,7 +9,7 @@
 
 public class Hooks {
     public static void World(Closure body) throws Throwable {
-        GroovyBackend.instance.registerWorld(body);
+        GroovyBackend.getInstance().registerWorld(body);
     }
 
     public static void Before(Object... args) throws Throwable {
@@ -37,9 +37,9 @@ private static void addHook(Object[] tagsExpressionsAndBody, boolean before) {
 
         TagExpression tagExpression = new TagExpression(tagExpressions);
         if (before) {
-            GroovyBackend.instance.addBeforeHook(tagExpression, timeoutMillis, body);
+            GroovyBackend.getInstance().addBeforeHook(tagExpression, timeoutMillis, body);
         } else {
-            GroovyBackend.instance.addAfterHook(tagExpression, timeoutMillis, body);
+            GroovyBackend.getInstance().addAfterHook(tagExpression, timeoutMillis, body);
         }
     }
 }

groovy/src/main/java/cucumber/runtime/groovy/GroovyBackend.java

@@ -30,7 +30,7 @@
 import static cucumber.runtime.io.MultiLoader.packageName;
 
 public class GroovyBackend implements Backend {
-    public static GroovyBackend instance;
+    public static ThreadLocal<GroovyBackend> instanceThreadLocal = new ThreadLocal<GroovyBackend>();
     private final Set<Class> scripts = new HashSet<Class>();
     private final SnippetGenerator snippetGenerator = new SnippetGenerator(new GroovySnippet());
     private final ResourceLoader resourceLoader;
@@ -41,6 +41,10 @@ public class GroovyBackend implements Backend {
     private Object world;
     private Glue glue;
 
+    public static GroovyBackend getInstance(){
+        return instanceThreadLocal.get();
+    }
+
     private static GroovyShell createShell() {
         CompilerConfiguration compilerConfig = new CompilerConfiguration();
         // Probably not needed:
@@ -55,7 +59,7 @@ public GroovyBackend(ResourceLoader resourceLoader) {
     public GroovyBackend(GroovyShell shell, ResourceLoader resourceLoader) {
         this.shell = shell;
         this.resourceLoader = resourceLoader;
-        instance = this;
+        instanceThreadLocal.set(this);
         classFinder = new ResourceLoaderClassFinder(resourceLoader, shell.getClassLoader());
     }
 

groovy/src/test/java/cucumber/runtime/groovy/ParallelTest.java

@@ -0,0 +1,51 @@
+package cucumber.runtime.groovy;
+
+import cucumber.runtime.io.ResourceLoader;
+import groovy.lang.Closure;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.runners.MockitoJUnitRunner;
+
+
+@RunWith(MockitoJUnitRunner.class)
+public class ParallelTest {
+    @Mock
+    ResourceLoader resourceLoader;
+    @Mock
+    Closure closure;
+
+    @Test(expected = RuntimeException.class)
+    public void exception_throw_when_world_already_set_on_same_thread() {
+        GroovyBackend groovyBackend = new GroovyBackend(resourceLoader);
+        groovyBackend.registerWorld(closure);
+        groovyBackend.registerWorld(closure);
+    }
+
+    @Test
+    public void can_have_a_new_backend_on_a_different_thread() {
+        new GroovyBackend(resourceLoader);
+        Thread interactWithBackendThread = new Thread(new Runnable(){
+            @Override
+            public void run() {
+                try {
+                    GroovyBackend.getInstance().registerWorld(closure);
+                } catch (NullPointerException e){
+                    // This is what we want as there should be no GroovyBackend on this thread
+                }
+            }
+        });
+        runAndWait(interactWithBackendThread);
+        GroovyBackend.getInstance().registerWorld(closure);
+    }
+
+    private void runAndWait(Thread interactWithBackendThread) {
+        interactWithBackendThread.start();
+        try {
+            interactWithBackendThread.join();
+        } catch (InterruptedException e) {
+            throw new RuntimeException("Doh");
+        }
+    }
+
+}