Introduce SmallRyeConfig.subset

implementation/pom.xml

@@ -82,6 +82,11 @@
       <groupId>io.smallrye.testing</groupId>
       <artifactId>smallrye-testing-utilities</artifactId>
     </dependency>
+    <dependency>
+      <groupId>org.assertj</groupId>
+      <artifactId>assertj-core</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>

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

@@ -223,7 +223,6 @@ public <K, V> Map<K, V> getValuesAsMap(String name, Converter<K> keyConverter, C
     }
 
     /**
-     *
      * This method handles calls from both {@link Config#getValue} and {@link Config#getOptionalValue}.<br>
      */
     @SuppressWarnings("unchecked")
@@ -459,6 +458,37 @@ public Optional<ConfigSource> getConfigSource(final String name) {
         return Optional.empty();
     }
 
+    /**
+     * Returns a {@link Config} containing every key from the current {@link Config} that starts with the specified
+     * prefix. The prefix is removed from the keys in the subset. For example, if the configuration contains the following
+     * properties:
+     *
+     * <pre>
+     *    prefix.number = 1
+     *    prefix.string = Hello
+     *    prefixed.foo = bar
+     *    prefix = World
+     * </pre>
+     * <p>
+     * the Configuration returned by {@code subset("prefix")} will contain the properties:
+     *
+     * <pre>
+     *    number = 1
+     *    string = Hello
+     *    = World
+     * </pre>
+     * <p>
+     * (The key for the value "World" is an empty string)
+     * <p>
+     *
+     * @param prefix The prefix used to select the properties.
+     * @return a subset configuration
+     */
+    @Experimental("Return a subset of the configuration")
+    public Config subset(final String prefix) {
+        return new SmallRyeSubsetConfig(prefix, this);
+    }
+
     public <T> T convert(String value, Class<T> asType) {
         return value != null ? requireConverter(asType).convert(value) : null;
     }

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

@@ -0,0 +1,93 @@
+package io.smallrye.config;
+
+import java.util.List;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.stream.Collectors;
+import java.util.stream.StreamSupport;
+
+import org.eclipse.microprofile.config.Config;
+import org.eclipse.microprofile.config.ConfigValue;
+import org.eclipse.microprofile.config.spi.ConfigSource;
+import org.eclipse.microprofile.config.spi.Converter;
+
+/**
+ * @author George Gastaldi
+ */
+class SmallRyeSubsetConfig implements Config {
+
+    private final String prefix;
+
+    private final Config delegate;
+
+    public SmallRyeSubsetConfig(String prefix, Config delegate) {
+        this.prefix = prefix;
+        this.delegate = delegate;
+    }
+
+    @Override
+    public <T> T getValue(String propertyName, Class<T> propertyType) {
+        return delegate.getValue(toSubsetPropertyName(propertyName), propertyType);
+    }
+
+    @Override
+    public ConfigValue getConfigValue(String propertyName) {
+        return delegate.getConfigValue(toSubsetPropertyName(propertyName));
+    }
+
+    @Override
+    public <T> List<T> getValues(String propertyName, Class<T> propertyType) {
+        return delegate.getValues(toSubsetPropertyName(propertyName), propertyType);
+    }
+
+    @Override
+    public <T> Optional<T> getOptionalValue(String propertyName, Class<T> propertyType) {
+        return delegate.getOptionalValue(toSubsetPropertyName(propertyName), propertyType);
+    }
+
+    @Override
+    public <T> Optional<List<T>> getOptionalValues(String propertyName, Class<T> propertyType) {
+        return delegate.getOptionalValues(toSubsetPropertyName(propertyName), propertyType);
+    }
+
+    @Override
+    public Iterable<String> getPropertyNames() {
+        return StreamSupport.stream(delegate.getPropertyNames().spliterator(), false)
+                .map(this::chopSubsetPropertyName)
+                .filter(Objects::nonNull)
+                .collect(Collectors.toSet());
+    }
+
+    @Override
+    public Iterable<ConfigSource> getConfigSources() {
+        return delegate.getConfigSources();
+    }
+
+    @Override
+    public <T> Optional<Converter<T>> getConverter(Class<T> forType) {
+        return delegate.getConverter(forType);
+    }
+
+    @Override
+    public <T> T unwrap(Class<T> type) {
+        throw new UnsupportedOperationException("Cannot unwrap a subset configuration");
+    }
+
+    private String toSubsetPropertyName(String propertyName) {
+        if (propertyName.isBlank()) {
+            return prefix;
+        } else {
+            return prefix + "." + propertyName;
+        }
+    }
+
+    private String chopSubsetPropertyName(String propertyName) {
+        if (propertyName.equalsIgnoreCase(prefix)) {
+            return "";
+        } else if (propertyName.startsWith(prefix + '.')) {
+            return propertyName.substring(prefix.length() + 1);
+        } else {
+            return null;
+        }
+    }
+}

implementation/src/test/java/io/smallrye/config/SmallRyeConfigTest.java

@@ -6,6 +6,8 @@
 import static java.util.Collections.singletonMap;
 import static java.util.stream.Collectors.toSet;
 import static java.util.stream.StreamSupport.stream;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
@@ -405,4 +407,21 @@ void emptyPropertyNames() {
 
         assertEquals("value", config.getRawValue(""));
     }
+
+    @Test
+    void subset() {
+        SmallRyeConfig config = new SmallRyeConfigBuilder()
+                .withSources(config(
+                        "app.foo", "bar",
+                        "app.foo.user", "guest",
+                        "app.foo.password", "apassword",
+                        "app.fooed.user", "wrong"))
+                .build();
+        Config subset = config.subset("app.foo");
+        assertEquals("bar", subset.getValue("", String.class));
+        assertEquals("guest", subset.getValue("user", String.class));
+        assertEquals("apassword", subset.getValue("password", String.class));
+        assertThat(subset.getPropertyNames()).containsExactlyInAnyOrder("", "user", "password");
+        assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(() -> subset.unwrap(SmallRyeConfig.class));
+    }
 }