Fixes cases where the getEngine method in the EngineProvider class returns null when called concurrently.

api/src/main/java/ai/djl/engine/Engine.java

@@ -59,7 +59,7 @@ public abstract class Engine {
 
     private static final Map<String, EngineProvider> ALL_ENGINES = new ConcurrentHashMap<>();
 
-    private static String defaultEngine = initEngine();
+    private static final String DEFAULT_ENGINE = initEngine();
     private static final Pattern PATTERN =
             Pattern.compile("KEY|TOKEN|PASSWORD", Pattern.CASE_INSENSITIVE);
 
@@ -69,10 +69,6 @@ public abstract class Engine {
     private Integer seed;
 
     private static synchronized String initEngine() {
-        if (Boolean.parseBoolean(Utils.getenv("DJL_ENGINE_MANUAL_INIT"))) {
-            return null;
-        }
-
         ServiceLoader<EngineProvider> loaders = ServiceLoader.load(EngineProvider.class);
         for (EngineProvider provider : loaders) {
             registerEngine(provider);
@@ -84,21 +80,21 @@ private static synchronized String initEngine() {
         }
 
         String def = System.getProperty("ai.djl.default_engine");
-        String newDefaultEngine = Utils.getenv("DJL_DEFAULT_ENGINE", def);
-        if (newDefaultEngine == null || newDefaultEngine.isEmpty()) {
+        String defaultEngine = Utils.getenv("DJL_DEFAULT_ENGINE", def);
+        if (defaultEngine == null || defaultEngine.isEmpty()) {
             int rank = Integer.MAX_VALUE;
             for (EngineProvider provider : ALL_ENGINES.values()) {
                 if (provider.getEngineRank() < rank) {
-                    newDefaultEngine = provider.getEngineName();
+                    defaultEngine = provider.getEngineName();
                     rank = provider.getEngineRank();
                 }
             }
-        } else if (!ALL_ENGINES.containsKey(newDefaultEngine)) {
-            throw new EngineException("Unknown default engine: " + newDefaultEngine);
+        } else if (!ALL_ENGINES.containsKey(defaultEngine)) {
+            throw new EngineException("Unknown default engine: " + defaultEngine);
         }
-        logger.debug("Found default engine: {}", newDefaultEngine);
-        Ec2Utils.callHome(newDefaultEngine);
-        return newDefaultEngine;
+        logger.debug("Found default engine: {}", defaultEngine);
+        Ec2Utils.callHome(defaultEngine);
+        return defaultEngine;
     }
 
     /**
@@ -128,7 +124,7 @@ private static synchronized String initEngine() {
      * @return the default Engine name
      */
     public static String getDefaultEngineName() {
-        return System.getProperty("ai.djl.default_engine", defaultEngine);
+        return System.getProperty("ai.djl.default_engine", DEFAULT_ENGINE);
     }
 
     /**
@@ -138,7 +134,7 @@ public static String getDefaultEngineName() {
      * @see EngineProvider
      */
     public static Engine getInstance() {
-        if (defaultEngine == null) {
+        if (DEFAULT_ENGINE == null) {
             throw new EngineException(
                     "No deep learning engine found."
                             + System.lineSeparator()
@@ -167,29 +163,7 @@ public static boolean hasEngine(String engineName) {
      */
     public static void registerEngine(EngineProvider provider) {
         logger.debug("Registering EngineProvider: {}", provider.getEngineName());
-        ALL_ENGINES.put(provider.getEngineName(), provider);
-    }
-
-    /**
-     * Returns the default engine.
-     *
-     * @return the default engine
-     */
-    public static String getDefaultEngine() {
-        return defaultEngine;
-    }
-
-    /**
-     * Sets the default engine returned by {@link #getInstance()}.
-     *
-     * @param engineName the new default engine's name
-     */
-    public static void setDefaultEngine(String engineName) {
-        // Requires an engine to be loaded (without exception) before being the default
-        getEngine(engineName);
-
-        logger.debug("Setting new default engine: {}", engineName);
-        defaultEngine = engineName;
+        ALL_ENGINES.putIfAbsent(provider.getEngineName(), provider);
     }
 
     /**
@@ -213,12 +187,7 @@ public static Engine getEngine(String engineName) {
         if (provider == null) {
             throw new IllegalArgumentException("Deep learning engine not found: " + engineName);
         }
-        Engine engine = provider.getEngine();
-        if (engine == null) {
-            throw new IllegalStateException(
-                    "The engine " + engineName + " was not able to initialize");
-        }
-        return engine;
+        return provider.getEngine();
     }
 
     /**

docs/development/troubleshooting.md

@@ -105,11 +105,6 @@ For more information, please refer to [DJL Cache Management](cache_management.md
 It happened when you had a wrong version with DJL and Deep Engines. 
 You can check the combination [here](dependency_management.md) and use DJL BOM to solve the issue.
 
-### 1.6 Manual initialization
-
-If you are using manual engine initialization, you must both register an engine and set it as the default.
-This can be done with `Engine.registerEngine(..)` and `Engine.setDefaultEngine(..)`.
-
 ## 2. IntelliJ throws the `No Log4j 2 configuration file found.` exception.
 The following exception may appear after running the `./gradlew clean` command:
 

engines/ml/lightgbm/src/main/java/ai/djl/ml/lightgbm/LgbmEngineProvider.java

@@ -18,9 +18,6 @@
 /** {@code LgbmEngineProvider} is the LightGBM implementation of {@link EngineProvider}. */
 public class LgbmEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -36,14 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
-            synchronized (LgbmEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
-                    engine = LgbmEngine.newInstance();
-                }
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = LgbmEngine.newInstance();
     }
 }

engines/ml/xgboost/src/main/java/ai/djl/ml/xgboost/XgbEngineProvider.java

@@ -18,9 +18,6 @@
 /** {@code XgbEngineProvider} is the XGBoost implementation of {@link EngineProvider}. */
 public class XgbEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -36,14 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
-            synchronized (XgbEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
-                    engine = XgbEngine.newInstance();
-                }
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = XgbEngine.newInstance();
     }
 }

engines/mxnet/mxnet-engine/src/main/java/ai/djl/mxnet/engine/MxEngineProvider.java

@@ -18,9 +18,6 @@
 /** {@code MxEngineProvider} is the MXNet implementation of {@link EngineProvider}. */
 public class MxEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -36,14 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
-            synchronized (MxEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
-                    engine = MxEngine.newInstance();
-                }
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = MxEngine.newInstance();
     }
 }

engines/onnxruntime/onnxruntime-engine/src/main/java/ai/djl/onnxruntime/engine/OrtEngineProvider.java

@@ -18,9 +18,6 @@
 /** {@code OrtEngineProvider} is the ONNX Runtime implementation of {@link EngineProvider}. */
 public class OrtEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -36,14 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
-            synchronized (OrtEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
-                    engine = OrtEngine.newInstance();
-                }
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = OrtEngine.newInstance();
     }
 }

engines/paddlepaddle/paddlepaddle-engine/src/main/java/ai/djl/paddlepaddle/engine/PpEngineProvider.java

@@ -18,9 +18,6 @@
 /** {@code PpEngineProvider} is the PaddlePaddle implementation of {@link EngineProvider}. */
 public class PpEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -36,14 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
-            synchronized (PpEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
-                    engine = PpEngine.newInstance();
-                }
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = PpEngine.newInstance();
     }
 }

engines/pytorch/pytorch-engine/src/main/java/ai/djl/pytorch/engine/PtEngineProvider.java

@@ -18,8 +18,7 @@
 /** {@code PtEngineProvider} is the PyTorch implementation of {@link EngineProvider}. */
 public class PtEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
+    private static volatile Engine engine; // NOPMD
 
     /** {@inheritDoc} */
     @Override
@@ -36,10 +35,9 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
+        if (engine == null) {
             synchronized (PtEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
+                if (engine == null) {
                     engine = PtEngine.newInstance();
                 }
             }

engines/tensorflow/tensorflow-engine/src/main/java/ai/djl/tensorflow/engine/TfEngineProvider.java

@@ -18,8 +18,7 @@
 /** {@code TfEngineProvider} is the TensorFlow implementation of {@link EngineProvider}. */
 public class TfEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
+    private static volatile Engine engine; // NOPMD
 
     /** {@inheritDoc} */
     @Override
@@ -36,10 +35,9 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
+        if (engine == null) {
             synchronized (TfEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
+                if (engine == null) {
                     engine = TfEngine.newInstance();
                 }
             }

engines/tensorrt/src/main/java/ai/djl/tensorrt/engine/TrtEngineProvider.java

@@ -18,9 +18,6 @@
 /** {@code TrtEngineProvider} is the TensorRT implementation of {@link EngineProvider}. */
 public class TrtEngineProvider implements EngineProvider {
 
-    private volatile Engine engine; // NOPMD
-    private volatile boolean initialized; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -36,14 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (!initialized) {
-            synchronized (TrtEngineProvider.class) {
-                if (!initialized) {
-                    initialized = true;
-                    engine = TrtEngine.newInstance();
-                }
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = TrtEngine.newInstance();
     }
 }

engines/tensorrt/src/test/java/ai/djl/tensorrt/engine/TrtEngineTest.java

@@ -26,7 +26,7 @@ public void getVersion() {
         try {
             Engine engine = Engine.getEngine("TensorRT");
             version = engine.getVersion();
-        } catch (Exception ignore) {
+        } catch (Throwable ignore) {
             throw new SkipException("Your os configuration doesn't support TensorRT.");
         }
         Assert.assertEquals(version, "8.4.1");

engines/tensorrt/src/test/java/ai/djl/tensorrt/engine/TrtNDManagerTest.java

@@ -28,7 +28,7 @@ public void testNDArray() {
         Engine engine;
         try {
             engine = Engine.getEngine("TensorRT");
-        } catch (Exception ignore) {
+        } catch (Throwable ignore) {
             throw new SkipException("Your os configuration doesn't support TensorRT.");
         }
         if (!engine.defaultDevice().isGpu()) {

engines/tensorrt/src/test/java/ai/djl/tensorrt/integration/TrtTest.java

@@ -49,7 +49,7 @@ public void testTrtOnnx() throws ModelException, IOException, TranslateException
         Engine engine;
         try {
             engine = Engine.getEngine("TensorRT");
-        } catch (Exception ignore) {
+        } catch (Throwable ignore) {
             throw new SkipException("Your os configuration doesn't support TensorRT.");
         }
         if (!engine.defaultDevice().isGpu()) {
@@ -75,7 +75,7 @@ public void testTrtUff() throws ModelException, IOException, TranslateException
         Engine engine;
         try {
             engine = Engine.getEngine("TensorRT");
-        } catch (Exception ignore) {
+        } catch (Throwable ignore) {
             throw new SkipException("Your os configuration doesn't support TensorRT.");
         }
         if (!engine.defaultDevice().isGpu()) {
@@ -112,7 +112,7 @@ public void testSerializedEngine() throws ModelException, IOException, Translate
         Engine engine;
         try {
             engine = Engine.getEngine("TensorRT");
-        } catch (Exception ignore) {
+        } catch (Throwable ignore) {
             throw new SkipException("Your os configuration doesn't support TensorRT.");
         }
         Device device = engine.defaultDevice();