[api] Replace double-check singlton with lazy initialization (#2826)

Author: frankfliu

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

@@ -18,8 +18,6 @@
 /** {@code LgbmEngineProvider} is the LightGBM implementation of {@link EngineProvider}. */
 public class LgbmEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (LgbmEngineProvider.class) {
-                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

@@ -18,8 +18,6 @@
 /** {@code XgbEngineProvider} is the XGBoost implementation of {@link EngineProvider}. */
 public class XgbEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (XgbEngineProvider.class) {
-                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

@@ -18,8 +18,6 @@
 /** {@code MxEngineProvider} is the MXNet implementation of {@link EngineProvider}. */
 public class MxEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (MxEngineProvider.class) {
-                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

@@ -18,8 +18,6 @@
 /** {@code OrtEngineProvider} is the ONNX Runtime implementation of {@link EngineProvider}. */
 public class OrtEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (OrtEngineProvider.class) {
-                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

@@ -18,8 +18,6 @@
 /** {@code PpEngineProvider} is the PaddlePaddle implementation of {@link EngineProvider}. */
 public class PpEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (PpEngineProvider.class) {
-                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 +18,6 @@
 /** {@code PtEngineProvider} is the PyTorch implementation of {@link EngineProvider}. */
 public class PtEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (PtEngineProvider.class) {
-                engine = PtEngine.newInstance();
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = PtEngine.newInstance();
     }
 }

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

@@ -18,8 +18,6 @@
 /** {@code TfEngineProvider} is the TensorFlow implementation of {@link EngineProvider}. */
 public class TfEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (TfEngineProvider.class) {
-                engine = TfEngine.newInstance();
-            }
-        }
-        return engine;
+        return InstanceHolder.INSTANCE;
+    }
+
+    private static class InstanceHolder {
+        static final Engine INSTANCE = TfEngine.newInstance();
     }
 }

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

@@ -18,8 +18,6 @@
 /** {@code TrtEngineProvider} is the TensorRT implementation of {@link EngineProvider}. */
 public class TrtEngineProvider implements EngineProvider {
 
-    private static volatile Engine engine; // NOPMD
-
     /** {@inheritDoc} */
     @Override
     public String getEngineName() {
@@ -35,11 +33,10 @@ public int getEngineRank() {
     /** {@inheritDoc} */
     @Override
     public Engine getEngine() {
-        if (engine == null) {
-            synchronized (TrtEngineProvider.class) {
-                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()) {