apacheGH-2787: Deadlock in JenaSystem.init()

- reinstated early return from JenaSystem.init() from before apache@8654c07
- simplified init using "initialization-on-demand holder idiom"
- added integration tests for JenaSystem.init(), also for the parallel case.

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

@@ -78,22 +78,34 @@ public static void logLifecycle(String fmt, Object ...args) {
 
     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 ;
+        if(initialized) {
+            return;
+        }
+
+        // Access the initialized flag to trigger class loading
+        initialized = LazyHolder.IS_INITIALIZED;
+    }
+
+    /**
+     * 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 {
+        public static final boolean IS_INITIALIZED = initialize();
+
+        private static boolean initialize() {
+            initialized = true; // Set early to avoid blocking on static initialization.
             setup();
             if ( DEBUG_INIT )
                 singleton.debug(DEBUG_INIT);
             singleton.initialize();
             singleton.debug(false);
-            // Last so overlapping initialization waits on the synchronized
-            initialized = true;
+            return true;
         }
     }
 
+
     public static void shutdown() { singleton.shutdown(); }
 
     /** The level 0 subsystem - inserted without using the Registry load function.

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 +1,79 @@
+/*
+ * 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 that the first initialization of JenaSystem is successful.
+     * The name is chosen so that it is executed first.
+     */
+    @Test
+    public void testFirstInitParallel() {
+
+        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());
+    }
+
+    /**
+     * Test that the first initialization of JenaSystem is successful.
+     * This test is placed in the integration tests module to ensure that the initialization
+     * is successful when multiple modules are loaded.
+     */
+    @Test
+    public void testLaterInit() {
+        assertDoesNotThrow(() -> JenaSystem.init());
+    }
+
+
+}