Avoid load default engine if not being used

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

@@ -101,6 +101,15 @@ private static synchronized String initEngine() {
      */
     public abstract int getRank();
 
+    /**
+     * Returns the default Engine name.
+     *
+     * @return the default Engine name
+     */
+    public static String getDefaultEngineName() {
+        return DEFAULT_ENGINE;
+    }
+
     /**
      * Returns the default Engine.
      *

api/src/main/java/ai/djl/modality/cv/util/NDImageUtils.java

@@ -12,7 +12,6 @@
  */
 package ai.djl.modality.cv.util;
 
-import ai.djl.engine.Engine;
 import ai.djl.modality.cv.Image;
 import ai.djl.ndarray.NDArray;
 import ai.djl.ndarray.types.Shape;
@@ -109,7 +108,7 @@ public static NDArray normalize(NDArray input, float mean, float std) {
      */
     public static NDArray normalize(NDArray input, float[] mean, float[] std) {
         boolean chw = isCHW(input.getShape());
-        boolean tf = "TensorFlow".equals(Engine.getInstance().getEngineName());
+        boolean tf = "TensorFlow".equals(input.getManager().getEngine().getEngineName());
         if ((chw && tf) || (!chw && !tf)) {
             throw new IllegalArgumentException(
                     "normalize requires CHW format. TensorFlow requires HWC");

api/src/main/java/ai/djl/repository/zoo/BaseModelLoader.java

@@ -117,7 +117,7 @@ public <I, O> ZooModel<I, O> loadModel(Criteria<I, O> criteria)
             // Otherwise check the modelzoo supported engine and grab a random engine in the list.
             // Otherwise if none of them is specified or model zoo is null, go to default engine.
             if (engine == null && modelZoo != null) {
-                String defaultEngine = Engine.getInstance().getEngineName();
+                String defaultEngine = Engine.getDefaultEngineName();
                 for (String supportedEngine : modelZoo.getSupportedEngines()) {
                     if (supportedEngine.equals(defaultEngine)) {
                         engine = supportedEngine;

api/src/main/java/ai/djl/training/listener/LoggingTrainingListener.java

@@ -158,8 +158,9 @@ public void onTrainingBegin(Trainer trainer) {
         logger.info("Training on: {}.", devicesMsg);
 
         long init = System.nanoTime();
-        String engineName = Engine.getInstance().getEngineName();
-        String version = Engine.getInstance().getVersion();
+        Engine engine = trainer.getManager().getEngine();
+        String engineName = engine.getEngineName();
+        String version = engine.getVersion();
         long loaded = System.nanoTime();
         logger.info(
                 String.format(

extensions/benchmark/src/main/java/ai/djl/benchmark/Arguments.java

@@ -76,7 +76,7 @@ public class Arguments {
         if (cmd.hasOption("engine")) {
             engine = cmd.getOptionValue("engine");
         } else {
-            engine = Engine.getInstance().getEngineName();
+            engine = Engine.getDefaultEngineName();
         }
 
         if (cmd.hasOption("duration")) {

extensions/fasttext/src/main/java/ai/djl/fasttext/zoo/FtModelZoo.java

@@ -50,6 +50,6 @@ public String getGroupId() {
     /** {@inheritDoc} */
     @Override
     public Set<String> getSupportedEngines() {
-        return Collections.singleton(Engine.getInstance().getEngineName());
+        return Collections.singleton(Engine.getDefaultEngineName());
     }
 }