Fix stop watch thread safety

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

@@ -9,17 +9,17 @@ public interface StopWatch {
     long stop();
 
     StopWatch SYSTEM = new StopWatch() {
-        public Long start;
+        public ThreadLocal<Long> start = new ThreadLocal<Long>();
 
         @Override
         public void start() {
-            start = System.nanoTime();
+            start.set(System.nanoTime());
         }
 
         @Override
         public long stop() {
-            Long duration = System.nanoTime() - start;
-            start = null;
+            Long duration = System.nanoTime() - start.get();
+            start.set(null);
             return duration;
         }
     };

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

@@ -0,0 +1,49 @@
+package cucumber.runtime;
+
+import static org.junit.Assert.assertNull;
+
+import org.junit.Test;
+
+public class StopWatchTest {
+    private final StopWatch stopWatch = StopWatch.SYSTEM;
+    private Throwable exception;
+
+    @Test
+    public void should_be_thread_safe() {
+        try {
+            Thread timerThreadOne = new TimerThread(500L);
+            Thread timerThreadTwo = new TimerThread(750L);
+
+            timerThreadOne.start();
+            timerThreadTwo.start();
+
+            timerThreadOne.join();
+            timerThreadTwo.join();
+
+            assertNull("null_pointer_exception", exception);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
+
+    class TimerThread extends Thread {
+        private final long timeoutMillis;
+
+        public TimerThread(long timeoutMillis) {
+            this.timeoutMillis = timeoutMillis;
+        }
+
+        @Override
+        public void run() {
+            try {
+                stopWatch.start();
+                Thread.sleep(timeoutMillis);
+                stopWatch.stop();
+            } catch (NullPointerException e) {
+                exception = e;
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+            }
+        }
+    }
+}