Support a fixed list of Map keys statically @WithKeys

documentation/src/main/docs/config/mappings.md

@@ -469,6 +469,13 @@ keys.
 
     A `Map` mapping is backed by an `HashMap`.
 
+When populating a `Map`, `SmallRyeConfig` requires the configuration names listed in 
+`SmallRyeConfig#getPropertyNames` to find the `Map` keys. If a `ConfigSource` does not support 
+`getPropertyNames` (empty), the names must be provided by another `ConfigSource` that can do so. After retrieving the 
+map keys, `SmallRyeConfig` performs the lookup of the values with the regular `ConfigSource` ordinal ordering. Even if 
+a `ConfigSource` does not provide `getPropertyNames` it can provide the value by having the name listed in another 
+capable `ConfigSource`.
+
 For collection types, the key requires the indexed format. The configuration name `server.aliases.localhost[0].name` 
 maps to the `Map<String, List<Alias>> aliases()` member, where `localhost` is the `Map` key, `[0]` is the index of the 
 `List<Alias>` collection where the `Alias` element will be stored, containing the name `prod`.
@@ -511,6 +518,44 @@ Map<String, Alias> localhost = server.aliases.get("localhost");
 
      If the unnamed key (in this case `localhost`) is explicitly set in a property name, the mapping will throw an error.
 
+### `@WithKeys`
+
+The `io.smallrye.config.WithKeys` annotation allows to define which `Map` keys must be loaded by 
+the configuration: 
+
+```java
+@ConfigMapping(prefix = "server")
+public interface Server {
+    @WithKeys(KeysProvider.class)
+    Map<String, Alias> aliases();
+
+    interface Alias {
+        String name();
+    }
+
+    class KeysProvider implements Supplier<Iterable<String>> {
+        @Override
+        public Iterable<String> get() {
+            return List.of("dev", "test", "prod");
+        }
+    }
+}
+```
+
+In this case, `SmallRyeConfig` will look for the map keys `dev`, `test` and `prod` instead of discovering the keys 
+with `SmallRyeConfig#getPropertyNames`:
+
+```properties
+servers.alias.dev.name=dev
+servers.alias.test.name=test
+servers.alias.prod.name=prod
+```
+
+The provided list will effectively substitute the lookup in `SmallRyeConfig#getPropertyNames`, thus enabling a
+`ConfigSource` that does not list its properties, to contribute configuration to the `Map`.  Each key must exist in 
+the final configuration (relative to the `Map` path segment), or the mapping will fail with a 
+`ConfigValidationException`.
+
 ### `@WithDefaults`
 
 The `io.smallrye.config.WithDefaults` is a marker annotation to use only in a `Map` to return the default value for

implementation/src/main/java/io/smallrye/config/ConfigMappingGenerator.java

@@ -61,6 +61,7 @@
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.Supplier;
 import java.util.regex.Pattern;
 
 import org.eclipse.microprofile.config.inject.ConfigProperties;
@@ -94,15 +95,16 @@ public class ConfigMappingGenerator {
     }
 
     private static final String I_CLASS = getInternalName(Class.class);
+    private static final String I_FIELD = getInternalName(Field.class);
     private static final String I_CONFIGURATION_OBJECT = getInternalName(ConfigMappingObject.class);
     private static final String I_MAPPING_CONTEXT = getInternalName(ConfigMappingContext.class);
+    private static final String I_NAMING_STRATEGY = getInternalName(NamingStrategy.class);
     private static final String I_OBJECT_CREATOR = getInternalName(ConfigMappingContext.ObjectCreator.class);
     private static final String I_OBJECT = getInternalName(Object.class);
     private static final String I_RUNTIME_EXCEPTION = getInternalName(RuntimeException.class);
     private static final String I_STRING_BUILDER = getInternalName(StringBuilder.class);
     private static final String I_STRING = getInternalName(String.class);
-    private static final String I_NAMING_STRATEGY = getInternalName(NamingStrategy.class);
-    private static final String I_FIELD = getInternalName(Field.class);
+    private static final String I_ITERABLE = getInternalName(Iterable.class);
 
     private static final int V_THIS = 0;
     private static final int V_MAPPING_CONTEXT = 1;
@@ -519,13 +521,18 @@ private static void generateProperty(final MethodVisitor ctor, final Property pr
                 } else {
                     ctor.visitInsn(ACONST_NULL);
                 }
+                if (mapProperty.hasKeyProvider()) {
+                    generateMapKeysProvider(ctor, mapProperty.getKeysProvider());
+                } else {
+                    ctor.visitInsn(ACONST_NULL);
+                }
                 if (mapProperty.hasDefaultValue() && mapProperty.getDefaultValue() != null) {
                     ctor.visitLdcInsn(mapProperty.getDefaultValue());
                 } else {
                     ctor.visitInsn(ACONST_NULL);
                 }
                 ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "values", "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_CLASS
-                        + ";L" + I_CLASS + ";L" + I_STRING + ";)L" + I_OBJECT_CREATOR + ";", false);
+                        + ";L" + I_CLASS + ";L" + I_ITERABLE + ";L" + I_STRING + ";)L" + I_OBJECT_CREATOR + ";", false);
             } else if (valueProperty.isGroup()) {
                 ctor.visitLdcInsn(getType(mapProperty.getKeyRawType()));
                 if (mapProperty.hasKeyConvertWith()) {
@@ -538,17 +545,28 @@ private static void generateProperty(final MethodVisitor ctor, final Property pr
                 } else {
                     ctor.visitInsn(ACONST_NULL);
                 }
+                if (mapProperty.hasKeyProvider()) {
+                    generateMapKeysProvider(ctor, mapProperty.getKeysProvider());
+                } else {
+                    ctor.visitInsn(ACONST_NULL);
+                }
                 if (mapProperty.hasDefaultValue()) {
                     ctor.visitLdcInsn(getType(valueProperty.asGroup().getGroupType().getInterfaceType()));
                 } else {
                     ctor.visitInsn(ACONST_NULL);
                 }
                 ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "map",
-                        "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_STRING + ";L" + I_CLASS + ";)L" + I_OBJECT_CREATOR + ";",
+                        "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_STRING + ";L" + I_ITERABLE + ";L" + I_CLASS + ";)L"
+                                + I_OBJECT_CREATOR + ";",
                         false);
                 ctor.visitLdcInsn(getType(valueProperty.asGroup().getGroupType().getInterfaceType()));
-                ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "lazyGroup",
-                        "(L" + I_CLASS + ";)L" + I_OBJECT_CREATOR + ";", false);
+                if (mapProperty.hasKeyProvider()) {
+                    ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "group",
+                            "(L" + I_CLASS + ";)L" + I_OBJECT_CREATOR + ";", false);
+                } else {
+                    ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "lazyGroup",
+                            "(L" + I_CLASS + ";)L" + I_OBJECT_CREATOR + ";", false);
+                }
             } else if (valueProperty.isCollection() && valueProperty.asCollection().getElement().isLeaf()) {
                 ctor.visitLdcInsn(getType(mapProperty.getKeyRawType()));
                 if (mapProperty.hasKeyConvertWith()) {
@@ -564,13 +582,21 @@ private static void generateProperty(final MethodVisitor ctor, final Property pr
                     ctor.visitInsn(ACONST_NULL);
                 }
                 ctor.visitLdcInsn(getType(mapProperty.getValueProperty().asCollection().getCollectionRawType()));
+                if (mapProperty.hasKeyProvider()) {
+                    generateMapKeysProvider(ctor, mapProperty.getKeysProvider());
+                } else {
+                    ctor.visitInsn(ACONST_NULL);
+                }
                 if (mapProperty.hasDefaultValue()) {
                     ctor.visitLdcInsn(mapProperty.getDefaultValue());
                 } else {
                     ctor.visitInsn(ACONST_NULL);
                 }
-                ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "values", "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_CLASS
-                        + ";L" + I_CLASS + ";L" + I_CLASS + ";L" + I_STRING + ";)L" + I_OBJECT_CREATOR + ";", false);
+                ctor.visitMethodInsn(
+                        INVOKEVIRTUAL, I_OBJECT_CREATOR, "values", "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_CLASS + ";L"
+                                + I_CLASS + ";L" + I_CLASS + ";L" + I_ITERABLE + ";L" + I_STRING + ";)L" + I_OBJECT_CREATOR
+                                + ";",
+                        false);
             } else {
                 unwrapProperty(ctor, property);
             }
@@ -635,8 +661,14 @@ private static void unwrapProperty(final MethodVisitor ctor, final Property prop
             } else {
                 ctor.visitInsn(ACONST_NULL);
             }
+            if (mapProperty.hasKeyProvider()) {
+                generateMapKeysProvider(ctor, mapProperty.getKeysProvider());
+            } else {
+                ctor.visitInsn(ACONST_NULL);
+            }
             ctor.visitMethodInsn(INVOKEVIRTUAL, I_OBJECT_CREATOR, "map",
-                    "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_STRING + ";)L" + I_OBJECT_CREATOR + ";", false);
+                    "(L" + I_CLASS + ";L" + I_CLASS + ";L" + I_STRING + ";L" + I_ITERABLE + ";)L" + I_OBJECT_CREATOR + ";",
+                    false);
             generateProperty(ctor, mapProperty.getValueProperty());
         } else if (property.isCollection()) {
             CollectionProperty collectionProperty = property.asCollection();
@@ -649,6 +681,15 @@ private static void unwrapProperty(final MethodVisitor ctor, final Property prop
         }
     }
 
+    private static void generateMapKeysProvider(final MethodVisitor ctor,
+            final Class<? extends Supplier<Iterable<String>>> mapKeysProvider) {
+        String provider = getInternalName(mapKeysProvider);
+        ctor.visitTypeInsn(NEW, provider);
+        ctor.visitInsn(DUP);
+        ctor.visitMethodInsn(INVOKESPECIAL, provider, "<init>", "()V", false);
+        ctor.visitMethodInsn(INVOKEVIRTUAL, provider, "get", "()L" + I_ITERABLE + ";", false);
+    }
+
     private static void appendPropertyName(final MethodVisitor ctor, final Property property) {
         if (property.isParentPropertyName()) {
             return;

implementation/src/main/java/io/smallrye/config/ConfigMappingInterface.java

@@ -19,6 +19,7 @@
 import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
+import java.util.function.Supplier;
 
 import org.eclipse.microprofile.config.spi.Converter;
 
@@ -32,7 +33,7 @@
 public final class ConfigMappingInterface implements ConfigMappingMetadata {
     static final ConfigMappingInterface[] NO_TYPES = new ConfigMappingInterface[0];
     static final Property[] NO_PROPERTIES = new Property[0];
-    static final ClassValue<ConfigMappingInterface> cv = new ClassValue<ConfigMappingInterface>() {
+    static final ClassValue<ConfigMappingInterface> cv = new ClassValue<>() {
         protected ConfigMappingInterface computeValue(final Class<?> type) {
             return createConfigurationInterface(type);
         }
@@ -594,6 +595,7 @@ public LeafProperty asLeaf() {
     public static final class MapProperty extends Property {
         private final Type keyType;
         private final String keyUnnamed;
+        private final Class<? extends Supplier<Iterable<String>>> keysProvider;
         private final Class<? extends Converter<?>> keyConvertWith;
         private final Property valueProperty;
         private final boolean hasDefault;
@@ -604,6 +606,7 @@ public static final class MapProperty extends Property {
                 final String propertyName,
                 final Type keyType,
                 final String keyUnnamed,
+                final Class<? extends Supplier<Iterable<String>>> keysProvider,
                 final Class<? extends Converter<?>> keyConvertWith,
                 final Property valueProperty,
                 final boolean hasDefault,
@@ -612,6 +615,7 @@ public static final class MapProperty extends Property {
             super(method, propertyName);
             this.keyType = keyType;
             this.keyUnnamed = keyUnnamed;
+            this.keysProvider = keysProvider;
             this.keyConvertWith = keyConvertWith;
             this.valueProperty = valueProperty;
             this.hasDefault = hasDefault;
@@ -634,6 +638,14 @@ public boolean hasKeyUnnamed() {
             return keyUnnamed != null;
         }
 
+        public Class<? extends Supplier<Iterable<String>>> getKeysProvider() {
+            return Assert.checkNotNullParam("keyProvider", keysProvider);
+        }
+
+        public boolean hasKeyProvider() {
+            return keysProvider != null;
+        }
+
         public Class<? extends Converter<?>> getKeyConvertWith() {
             return Assert.checkNotNullParam("keyConvertWith", keyConvertWith);
         }
@@ -846,9 +858,14 @@ private static Property getPropertyDef(Method method, AnnotatedType type) {
                 AnnotatedType keyType = typeOfParameter(type, 0);
                 AnnotatedType valueType = typeOfParameter(type, 1);
                 String defaultValue = getDefaultValue(method);
-                return new MapProperty(method, propertyName, keyType.getType(), getUnnamedKey(keyType, method),
-                        getConverter(keyType, method), getPropertyDef(method, valueType),
-                        defaultValue != null || hasDefaults(method), defaultValue);
+                return new MapProperty(method,
+                        propertyName, keyType.getType(),
+                        getUnnamedKey(keyType, method),
+                        getKeysProvider(keyType, method),
+                        getConverter(keyType, method),
+                        getPropertyDef(method, valueType),
+                        defaultValue != null || hasDefaults(method),
+                        defaultValue);
             }
             if (rawType == List.class || rawType == Set.class) {
                 AnnotatedType elementType = typeOfParameter(type, 0);
@@ -940,6 +957,14 @@ private static String getUnnamedKey(final AnnotatedType type, final Method metho
         return annotation != null ? annotation.value() : null;
     }
 
+    private static Class<? extends Supplier<Iterable<String>>> getKeysProvider(final AnnotatedType type, final Method method) {
+        WithKeys annotation = type.getAnnotation(WithKeys.class);
+        if (annotation == null) {
+            annotation = method.getAnnotation(WithKeys.class);
+        }
+        return annotation != null ? annotation.value() : null;
+    }
+
     private static Class<? extends Converter<?>> getConverter(final AnnotatedType type, final Method method) {
         WithConverter annotation = type.getAnnotation(WithConverter.class);
         // fallback to method

implementation/src/main/java/io/smallrye/config/NameIterator.java

@@ -329,12 +329,6 @@ public void next() {
         pos = getNextEnd();
     }
 
-    public void next(int segments) {
-        for (int i = 0; i < segments; i++) {
-            next();
-        }
-    }
-
     public void previous() {
         pos = getPreviousStart() - 1;
     }