test(server): enable run single unit test (#2940)

* test(server-test): enable run single unit test

* fix: thread-safe graph() method

* fix: more clear error handling in graph() method

Author: JisoLya

hugegraph-server/hugegraph-test/src/main/java/org/apache/hugegraph/core/CoreTestSuite.java

@@ -26,7 +26,6 @@
 import org.apache.hugegraph.testutil.Utils;
 import org.apache.hugegraph.util.Log;
 import org.junit.AfterClass;
-import org.junit.Assert;
 import org.junit.BeforeClass;
 import org.junit.runner.RunWith;
 import org.junit.runners.Suite;
@@ -52,11 +51,29 @@
 public class CoreTestSuite {
 
     private static boolean registered = false;
-    private static HugeGraph graph = null;
+    private static volatile HugeGraph graph = null;
 
     public static HugeGraph graph() {
-        Assert.assertNotNull(graph);
-        //Assert.assertFalse(graph.closed());
+        if (graph == null) {
+            synchronized (CoreTestSuite.class) {
+                if (graph == null) {
+                    try {
+                        initEnv();
+                        init();
+                    } catch (Throwable e) {
+                        LOG.error("Failed to initialize HugeGraph instance", e);
+                        graph = null;
+                        throw new RuntimeException("Failed to initialize HugeGraph instance", e);
+                    }
+                    if (graph == null) {
+                        String msg = "HugeGraph instance is null after initialization. " +
+                                     "Please check Utils.open() configuration.";
+                        LOG.error(msg);
+                        throw new IllegalStateException(msg);
+                    }
+                }
+            }
+        }
         return graph;
     }