Handle empty and duplicate namespace definitions

buildtools/src/main/resources/checkstyle/checkstyle.xml

@@ -60,10 +60,7 @@
       <property name="severity" value="error"/>
     </module>
 
-    <!-- Check that finds import statements that use the * notation. -->
-    <module name="AvoidStarImport"/>
     <!-- Checks for long lines. -->
-
     <module name="LineLength">
       <property name="max" value="120"/>
     </module>

db/src/main/java/org/trellisldp/ext/db/DBNamespaceService.java

@@ -13,6 +13,8 @@
  */
 package org.trellisldp.ext.db;
 
+import static org.slf4j.LoggerFactory.getLogger;
+
 import java.util.AbstractMap.SimpleImmutableEntry;
 import java.util.HashMap;
 import java.util.Map;
@@ -21,13 +23,17 @@
 import javax.sql.DataSource;
 
 import org.jdbi.v3.core.Jdbi;
+import org.jdbi.v3.core.statement.StatementException;
+import org.slf4j.Logger;
 import org.trellisldp.api.NamespaceService;
 
 /**
  * A namespace service that stores data in a database.
  */
 public class DBNamespaceService implements NamespaceService {
 
+    private static final Logger LOGGER = getLogger(DBNamespaceService.class);
+
     private final Jdbi jdbi;
 
     /**
@@ -59,8 +65,15 @@ public Map<String, String> getNamespaces() {
 
     @Override
     public boolean setPrefix(final String prefix, final String namespace) {
-        jdbi.useHandle(handle ->
-            handle.execute("INSERT INTO namespaces (prefix, namespace) VALUES (?, ?)", prefix, namespace));
-        return true;
+        if (!prefix.isEmpty()) {
+            try {
+                jdbi.useHandle(handle ->
+                    handle.execute("INSERT INTO namespaces (prefix, namespace) VALUES (?, ?)", prefix, namespace));
+                return true;
+            } catch (final StatementException ex) {
+                LOGGER.warn("Could not save prefix {} with namespace {}: {}", prefix, namespace, ex.getMessage());
+            }
+        }
+        return false;
     }
 }

db/src/test/java/org/trellisldp/ext/db/DBNamespaceServiceTest.java

@@ -14,8 +14,7 @@
 package org.trellisldp.ext.db;
 
 import static java.io.File.separator;
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.*;
 import static org.junit.jupiter.api.condition.OS.WINDOWS;
 import static org.slf4j.LoggerFactory.getLogger;
 
@@ -75,6 +74,8 @@ public void testNamespaceService() {
         assertTrue(svc.setPrefix("ex", "http://example.com/"));
         assertEquals(size + 1, svc.getNamespaces().size());
         assertEquals("http://example.com/", svc.getNamespaces().get("ex"));
+        assertFalse(svc.setPrefix("ex", "http://example.com/Other/"));
+        assertFalse(svc.setPrefix("", "http://example.com/Resource/"));
     }
 }