Added two more unit tests, one of which requires @VisibleForTesting.

rskj-core/src/main/java/co/rsk/net/discovery/upnp/UpnpService.java

@@ -19,6 +19,7 @@
 package co.rsk.net.discovery.upnp;
 
 import co.rsk.config.InternalService;
+import com.google.common.annotations.VisibleForTesting;
 import org.bitlet.weupnp.GatewayDevice;
 import org.bitlet.weupnp.GatewayDiscover;
 import org.ethereum.config.SystemProperties;
@@ -206,4 +207,9 @@ private synchronized boolean isValidGateway(GatewayDevice gateway) {
             return false;
         }
     }
+
+    @VisibleForTesting
+    Map<InetAddress, UpnpGatewayManager> getCachedGatewayManagers() {
+        return cachedGatewayManagers;
+    }
 }

rskj-core/src/test/java/co/rsk/net/discovery/upnp/UpnpServiceTest.java

@@ -43,6 +43,7 @@ public final class UpnpServiceTest {
     private static final String LOCAL_ADDRESS_VALID_1 = "192.168.0.100";
     private static final String LOCAL_ADDRESS_VALID_2 = "192.168.1.100";
     private static final String LOCAL_ADDRESS_INVALID = "192.168.0.101";
+    private static final String LOCAL_ADDRESS_UNKNOWN_HOST = "[" + LOCAL_ADDRESS_VALID_1 + "]";
     private static final String VALID_GATEWAY_NAME_1 = "Valid Mock Gateway 1";
     private static final String VALID_GATEWAY_NAME_2 = "Valid Mock Gateway 2";
     private static final String INVALID_GATEWAY_NAME = "Invalid Mock Gateway";
@@ -156,6 +157,32 @@ private static UpnpService createAndStartUpnpService(GatewayDiscover query) {
         return upnpService;
     }
 
+    @Test
+    public void testExceptionDuringStop() {
+        UpnpService testService = createAndStartUpnpService(mockWildcardDiscoverValid);
+        testService.findGateway();
+
+        // force a NPE during stop()'s Executor task construction
+        testService.getCachedGatewayManagers().clear();
+        testService.getCachedGatewayManagers().put(wildcardAddress, null);
+
+        try {
+            testService.stop();
+        } catch (Exception e) {
+            Assert.fail("Should not throw Exception when stop()'s cleanup fails.");
+        }
+    }
+
+    @Test
+    public void testStringAddressThrowsUnknownHost() {
+        UpnpService testService = createAndStartUpnpService(mockWildcardDiscoverValid);
+        Optional<UpnpGatewayManager> gdm = testService.findGateway(LOCAL_ADDRESS_UNKNOWN_HOST);
+        Assert.assertFalse(
+                "An unresolvable hostname should not return a gateway manager",
+                gdm.isPresent()
+        );
+    }
+
     @Test
     public void testStopWithoutValidSearchResults() {
         try {