Showing failure problem with with concurrent access.

This results in one thread getting stuck in a loop.

src/main/java/org/rometools/feed/module/cc/types/License.java

@@ -128,6 +128,13 @@ public class License {
         return found;
     }
 
+    /**
+     * This is just useful for testing to allow clearing of the looked up licenses.
+     */
+    static void clear() {
+        lookupLicense.clear();
+    }
+
     public Behaviour[] getPermits() {
         return permits;
     }

src/test/java/org/rometools/feed/module/cc/types/LicenseTest.java

@@ -0,0 +1,72 @@
+package org.rometools.feed.module.cc.types;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.Random;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicLong;
+
+import static org.junit.Assert.assertFalse;
+
+/**
+ * @author Matthew Buckett
+ */
+public class LicenseTest {
+
+    @Before
+    public void setUp() {
+        // As the looked up licenses are held statically we need to clear this between tests.
+        License.clear();
+    }
+
+    @Test(timeout = 500)
+    public void testConcurrent() throws InterruptedException {
+        final AtomicBoolean run = new AtomicBoolean(true);
+        final AtomicLong type = new AtomicLong(0);
+        final AtomicBoolean hadProblem = new AtomicBoolean(false);
+
+        // This thread keeps on adding new licenses (not very realistic but shows the bug)
+        Thread addNew = new Thread(){
+            @Override
+            public void run() {
+                while(run.get()) {
+                    License license = License.findByValue("http://creativecommons.org/licenses/"+
+                            type.incrementAndGet()+ "/1");
+                    if (license == null) {
+                        hadProblem.set(true);
+                    }
+                }
+            }
+        };
+
+        // This thread attempts to get ones we know have already been put in.
+        Thread getExisting = new Thread() {
+            @Override
+            public void run() {
+                Random rnd = new Random();
+                while(run.get()) {
+                    License license = License.findByValue("http://creativecommons.org/licenses/"+
+                            rnd.nextInt(type.intValue())+"/1");
+                    if (license == null) {
+                        hadProblem.set(true);
+                    }
+                }
+            }
+        };
+
+        addNew.start();
+        getExisting.start();
+        // Let them do some stuff.
+        Thread.sleep(100);
+        // Get them to both stop.
+        run.set(false);
+        // Allow them a little time to stop.
+        addNew.join(20);
+        getExisting.join(20);
+        // Check we didn't have any problems and they have both stopped.
+        assertFalse(hadProblem.get());
+        assertFalse(addNew.isAlive());
+        assertFalse(getExisting.isAlive());
+    }
+}