Introduce SmallRyeConfig.subset

- Fixes smallrye#981

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")
@@ -246,17 +245,17 @@ public <T> T getValue(String name, Converter<T> converter) {
     /**
      * This method handles converting values for both CDI injections and programatical calls.<br>
      * <br>
-     *
+     * <p>
      * Calls for converting non-optional values ({@link Config#getValue} and "Injecting Native Values")
      * should throw an {@link Exception} for each of the following:<br>
-     *
+     * <p>
      * 1. {@link IllegalArgumentException} - if the property cannot be converted by the {@link Converter} to the specified type
      * <br>
      * 2. {@link NoSuchElementException} - if the property is not defined <br>
      * 3. {@link NoSuchElementException} - if the property is defined as an empty string <br>
      * 4. {@link NoSuchElementException} - if the {@link Converter} returns {@code null} <br>
      * <br>
-     *
+     * <p>
      * Calls for converting optional values ({@link Config#getOptionalValue} and "Injecting Optional Values")
      * should only throw an {@link Exception} for #1 ({@link IllegalArgumentException} when the property cannot be converted to
      * the specified type).
@@ -459,6 +458,11 @@ public Optional<ConfigSource> getConfigSource(final String name) {
         return Optional.empty();
     }
 
+    @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 +1,85 @@
+package io.smallrye.config;
+
+import java.util.List;
+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
+ */
+public 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)
+                .filter(propertyName -> propertyName.startsWith(prefix))
+                .map(this::chopSubsetPropertyName)
+                .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) {
+        return delegate.unwrap(type);
+    }
+
+    private String toSubsetPropertyName(String propertyName) {
+        if (propertyName.isBlank()) {
+            return prefix;
+        }
+        return prefix + "." + propertyName;
+    }
+
+    private String chopSubsetPropertyName(String propertyName) {
+        return propertyName.substring(prefix.length() + 1);
+    }
+}

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

@@ -405,4 +405,16 @@ void emptyPropertyNames() {
 
         assertEquals("value", config.getRawValue(""));
     }
+
+    @Test
+    void subset() {
+        SmallRyeConfig config = new SmallRyeConfigBuilder()
+                .withSources(config(
+                        "app.foo.user", "guest",
+                        "app.foo.password", "apassword"))
+                .build();
+        Config subset = config.subset("app.foo");
+        assertEquals("guest", subset.getValue("user", String.class));
+        assertEquals("apassword", subset.getValue("password", String.class));
+    }
 }