Set values directly for no-arg constructors (plan item 3.2)

core/src/main/java/dev/morphia/mapping/InstanceCreatorFactoryImpl.java

@@ -12,17 +12,12 @@
 
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
 
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
 /**
  * @hidden
  * @morphia.internal
  */
 @MorphiaInternal
 public class InstanceCreatorFactoryImpl implements InstanceCreatorFactory {
-    private static final Logger LOG = LoggerFactory.getLogger(InstanceCreatorFactoryImpl.class);
-
     private final EntityModel model;
     private Function<Conversions, MorphiaInstanceCreator> creator;
 
@@ -45,15 +40,10 @@ public MorphiaInstanceCreator create(Conversions conversions) {
                     ConstructorCreator.Plan plan = new ConstructorCreator.Plan(model, constructor);
                     creator = (c) -> new ConstructorCreator(plan, c);
                 } else {
-                    LOG.info("using old creator approach: " + model.getType().getName());
-                    try {
-                        Constructor<?> declared = model.getType().getDeclaredConstructor();
-                        creator = (c) -> new NoArgCreator(declared);
-                    } catch (NoSuchMethodException e) {
-                        Constructor<?> full = ConstructorCreator.getFullConstructor(model);
-                        ConstructorCreator.Plan plan = new ConstructorCreator.Plan(model, full);
-                        creator = (c) -> new ConstructorCreator(plan, c);
-                    }
+                    // bestConstructor already picks a no-arg constructor when there is one
+                    Constructor<?> full = ConstructorCreator.getFullConstructor(model);
+                    ConstructorCreator.Plan plan = new ConstructorCreator.Plan(model, full);
+                    creator = (c) -> new ConstructorCreator(plan, c);
                 }
             }
 

core/src/main/java/dev/morphia/mapping/internal/ConstructorCreator.java

@@ -8,6 +8,7 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.TreeMap;
+import java.util.regex.Pattern;
 
 import com.mongodb.lang.Nullable;
 
@@ -39,11 +40,17 @@
  */
 @MorphiaInternal
 public class ConstructorCreator implements MorphiaInstanceCreator {
+    private static final Pattern UNNAMED_PARAMETER = Pattern.compile("arg[0-9]+");
+    private static final Object[] NO_PARAMETERS = new Object[0];
+
     private final Plan plan;
     private final Conversions conversions;
     private final Object[] parameters;
-    private final List<PropertyModel> pendingModels = new ArrayList<>();
-    private final List<Object> pendingValues = new ArrayList<>();
+    // properties the constructor doesn't take, set once it has run; allocated only if a value arrives before then
+    @Nullable
+    private List<PropertyModel> pendingModels;
+    @Nullable
+    private List<Object> pendingValues;
     private Object instance;
 
     /**
@@ -63,7 +70,7 @@ public ConstructorCreator(EntityModel model, Constructor<?> constructor, Convers
     public ConstructorCreator(Plan plan, Conversions conversions) {
         this.plan = plan;
         this.conversions = conversions;
-        this.parameters = plan.zeroValues.clone();
+        this.parameters = plan.types.length == 0 ? NO_PARAMETERS : plan.zeroValues.clone();
     }
 
     /**
@@ -93,7 +100,7 @@ public Plan(EntityModel model, Constructor<?> constructor) {
                 types[i] = parameter.getType();
                 zeroValues[i] = zeroValue(parameter);
                 String name = getParameterName(parameter);
-                if (name.matches("arg[0-9]+")) {
+                if (UNNAMED_PARAMETER.matcher(name).matches()) {
                     throw new MappingException(Sofia.unnamedConstructorParameter(model.getType().getName()));
                 }
                 if (positions.put(name, i) != null) {
@@ -138,8 +145,10 @@ public Object getInstance() {
         if (instance == null) {
             try {
                 instance = plan.constructor.newInstance(parameters);
-                for (int i = 0; i < pendingModels.size(); i++) {
-                    pendingModels.get(i).setValue(instance, pendingValues.get(i));
+                if (pendingModels != null) {
+                    for (int i = 0; i < pendingModels.size(); i++) {
+                        pendingModels.get(i).setValue(instance, pendingValues.get(i));
+                    }
                 }
             } catch (Exception e) {
                 throw new MappingException(Sofia.cannotInstantiate(plan.model.getType().getName(), e.getMessage()), e);
@@ -193,13 +202,21 @@ private static boolean namesMatchProperties(EntityModel model, Constructor<?> co
     public void set(@Nullable Object value, PropertyModel model) {
         if (instance != null) {
             model.setValue(instance, value);
+        } else if (plan.types.length == 0) {
+            // a no-arg constructor needs none of the values, so construct now and set each value as it arrives instead
+            // of buffering them all
+            model.setValue(getInstance(), value);
         } else {
             Integer position = plan.positions.get(model.getName());
             if (position != null) {
                 parameters[position] = conversions.convert(value, plan.types[position]);
             } else {
                 // only properties the constructor doesn't take need setting after construction. re-setting the others would
                 // overwrite whatever the constructor did with the value and, for final fields, write them reflectively (JEP 500).
+                if (pendingModels == null) {
+                    pendingModels = new ArrayList<>();
+                    pendingValues = new ArrayList<>();
+                }
                 pendingModels.add(model);
                 pendingValues.add(value);
             }

core/src/test/java/dev/morphia/test/mapping/internal/ConstructorCreatorTest.java

@@ -95,6 +95,22 @@ public void testBestConstructor() {
         Assertions.assertEquals(0, constructor.getParameterCount());
     }
 
+    @Test
+    public void noArgConstructorSetsValuesAsTheyArrive() {
+        EntityModel model = getMapper().map(Default.class).get(0);
+        MorphiaInstanceCreator creator = model.getInstanceCreator(getMapper().getConversions());
+        Assertions.assertTrue(creator.isEagerInstanceSafe());
+
+        creator.set("named", model.getProperty("name"));
+        Default instance = (Default) creator.getInstance();
+        Assertions.assertEquals("named", instance.name);
+
+        // a value that needs converting still goes through Conversions on this path
+        creator.set(42.0, model.getProperty("count"));
+        Assertions.assertSame(instance, creator.getInstance());
+        Assertions.assertEquals(42, instance.count);
+    }
+
     @Test
     public void creatorsDoNotShareState() {
         EntityModel model = getMapper().map(Address.class).get(0);

design/mapping-game-plan.md

@@ -210,7 +210,7 @@ JMH numbers on both mappers.
 - Optional: send a known-rejected type straight to the fallback, avoiding a throw and stack trace for each drifted
   field.
 
-### 3.2 No-arg creator fast path (#2, A2/B4)
+### ~~3.2 No-arg creator fast path (#2, A2/B4)~~
 - When the chosen constructor has no parameters, construct the instance first and set values through
   `model.setValue`, which keeps `Conversions`. Allocate pending lists lazily.
 - Delete the dead `NoArgCreator` branch and its `LOG.info`. Delete `NoArgCreator` itself, which also skips