apacheGH-2787: Deadlock in JenaSystem.init()

- Added a fail-fast mechanism: now throwing an `ExceptionInInitializerError` when a thread attempts to enter `JenaSystem.init()` while another thread is in the initialization process.
- Refactored `JenaSystem.init()` using the "Initialization-on-demand holder idiom" for simpler and safer initialization.
- Implemented integration tests for `JenaSystem.init()`, including parallel execution cases to ensure thread safety.

jena-core/src/main/java/org/apache/jena/sys/JenaSystem.java

@@ -66,7 +66,8 @@ public JenaSystem() { }
      * to avoid the risk of recursive initialization.
      */
     public static boolean DEBUG_INIT = false ;
-    private static volatile boolean initialized = false ;
+    private static volatile boolean IS_INITIALIZING = false;
+    private static volatile long ID_OF_INIT_THREAD = -1;
 
     /** Output a debugging message if DEBUG_INIT is set */
     public static void logLifecycle(String fmt, Object ...args) {
@@ -76,22 +77,34 @@ public static void logLifecycle(String fmt, Object ...args) {
         System.err.println() ;
     }
 
+    /**
+     * Initialization-on-demand holder idiom
+     * @see <a href="http://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom">Initialization-on-demand holder idiom</a>
+     *
+     */
+    private static class LazyHolder {
+        static final boolean initialized = initialize();
+    }
+
     public static void init() {
-        // Once jena is initialized, all calls are an immediate return.
-        if ( initialized )
-            return ;
-        // Overlapping attempts to perform initialization will block on the synchronized.
-        synchronized(JenaSystem.class) {
-            if ( initialized )
-                return ;
-            setup();
-            if ( DEBUG_INIT )
-                singleton.debug(DEBUG_INIT);
-            singleton.initialize();
-            singleton.debug(false);
-            // Last so overlapping initialization waits on the synchronized
-            initialized = true;
+        if(IS_INITIALIZING && ID_OF_INIT_THREAD != Thread.currentThread().getId()) {
+            throw new ExceptionInInitializerError("Jena is already is being initialized by another thread. Please ensure that JenaSystem.init() is called before working with multiple threads.");
         }
+
+        // Access the initialized flag to trigger class loading
+        boolean init = LazyHolder.initialized;
+    }
+
+    private static boolean initialize() {
+        IS_INITIALIZING = true;
+        ID_OF_INIT_THREAD = Thread.currentThread().getId();
+        setup();
+        if ( DEBUG_INIT )
+            singleton.debug(DEBUG_INIT);
+        singleton.initialize();
+        singleton.debug(false);
+        IS_INITIALIZING = false;
+        return true;
     }
 
     public static void shutdown() { singleton.shutdown(); }

jena-integration-tests/src/test/java/org/apache/jena/sys/TS_Sys.java

@@ -0,0 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.jena.sys;
+
+import org.apache.jena.sparql.exec.http.*;
+import org.junit.runner.RunWith;
+import org.junit.runners.Suite;
+
+@RunWith(Suite.class)
+@Suite.SuiteClasses(
+    { TestJenaSystem.class
+    })
+
+public class TS_Sys { }

jena-integration-tests/src/test/java/org/apache/jena/sys/TestJenaSystem.java

@@ -0,0 +1,69 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.jena.sys;
+
+import org.apache.jena.rdf.model.ModelFactory;
+import org.junit.Test;
+
+
+import java.time.Duration;
+import java.time.temporal.ChronoUnit;
+import java.util.HashSet;
+import java.util.concurrent.Executors;
+import java.util.stream.IntStream;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class TestJenaSystem {
+
+    @Test
+    public void testInit() {
+        assertDoesNotThrow(() -> JenaSystem.init());
+    }
+
+    @Test
+    public void testInitParallel() {
+
+        // it is mandatory to init JenaSystem before running anything in parallel
+        JenaSystem.init();
+
+        var pool = Executors.newFixedThreadPool(8);
+
+        var futures = IntStream.range(0, 16)
+                .mapToObj(i -> pool.submit(() -> {
+                    if (i % 2 == 0)
+                        ModelFactory.createDefaultModel();
+                    else
+                        JenaSystem.init();
+
+                    return i;
+                }))
+                .toList();
+
+        var intSet = new HashSet<Integer>();
+        assertTimeoutPreemptively(
+                Duration.of(5, ChronoUnit.SECONDS),
+                () -> {
+                    for (var future : futures) {
+                        intSet.add(future.get());
+                    }
+                });
+
+        assertEquals(16, intSet.size());
+    }
+}