implement the ability to plugin fallback values

avaje-config/src/main/java/io/avaje/config/ConfigServiceLoader.java

@@ -22,11 +22,13 @@ static ConfigServiceLoader get() {
   private final List<ConfigurationSource> sources = new ArrayList<>();
   private final List<ConfigurationPlugin> plugins = new ArrayList<>();
   private final Parsers parsers;
+  private final ConfigurationFallback fallback;
 
   ConfigServiceLoader() {
     ModificationEventRunner _eventRunner = null;
     ConfigurationLog _log = null;
     ResourceLoader _resourceLoader = null;
+    ConfigurationFallback _fallback = null;
     List<ConfigParser> otherParsers = new ArrayList<>();
 
     for (var spi : ServiceLoader.load(ConfigExtension.class)) {
@@ -42,13 +44,16 @@ static ConfigServiceLoader get() {
         _resourceLoader = (ResourceLoader) spi;
       } else if (spi instanceof ModificationEventRunner) {
         _eventRunner = (ModificationEventRunner) spi;
+      } else if (spi instanceof ConfigurationFallback) {
+        _fallback = (ConfigurationFallback) spi;
       }
     }
 
     this.log = _log == null ? new DefaultConfigurationLog() : _log;
     this.resourceLoader = _resourceLoader == null ? new DefaultResourceLoader() : _resourceLoader;
     this.eventRunner = _eventRunner == null ? new CoreConfiguration.ForegroundEventRunner() : _eventRunner;
     this.parsers = new Parsers(otherParsers);
+    this.fallback = _fallback == null ? new DefaultFallback() : _fallback;
   }
 
   Parsers parsers() {
@@ -67,6 +72,10 @@ ModificationEventRunner eventRunner() {
     return eventRunner;
   }
 
+  ConfigurationFallback fallback() {
+    return fallback;
+  }
+
   List<ConfigurationSource> sources() {
     return sources;
   }

avaje-config/src/main/java/io/avaje/config/Configuration.java

@@ -802,6 +802,13 @@ interface Builder {
      */
     Builder includeResourceLoading();
 
+    /**
+     * Defines the means by which the configuration is able to maybe fallback to a value if there is no configured value
+     * for a given key.
+     */
+
+    Builder fallback(ConfigurationFallback fallback);
+
     /**
      * Build and return the Configuration.
      * <p>
@@ -816,6 +823,13 @@ interface Builder {
    */
   interface Entry {
 
+    /**
+     * Return an entry given the value and source.
+     */
+    static Entry of(@Nullable String val, String source) {
+      return CoreEntry.of(val, source);
+    }
+
     /**
      * Return the source of the entry.
      */
@@ -827,5 +841,20 @@ interface Entry {
      */
     @Nullable
     String value();
+
+    /**
+     * Return true if the entry needs evaluation.
+     */
+    boolean needsEvaluation();
+
+    /**
+     * Return true if the entry represents a null.
+     */
+    boolean isNull();
+
+    /**
+     * Return the boolean value for the entry.
+     */
+    boolean boolValue();
   }
 }

avaje-config/src/main/java/io/avaje/config/ConfigurationFallback.java

@@ -0,0 +1,38 @@
+package io.avaje.config;
+
+import io.avaje.config.Configuration.Entry;
+
+import java.util.Optional;
+
+/**
+ * Implementations of this class are able to define a fallback value when there is no
+ * value available for a configuration key.
+ * <p>
+ * The default implementation uses System Properties and Environment variables.
+ */
+public interface ConfigurationFallback extends ConfigExtension {
+
+  /**
+   * Provides the ability to override the value that is going to be set.
+   * <p>
+   * By default, this just returns the value that is passed in and does not
+   * override the value.
+   *
+   * @param key    The property key
+   * @param value  The value that can be overridden by the returned value
+   * @param source The source of the key value pair
+   */
+  default Entry overrideValue(String key, String value, String source) {
+    return Entry.of(value, source);
+  }
+
+  /**
+   * Return a value for the supplied key or {@code null} if there is no value.
+   *
+   * @param key The configuration key to get a fallback value for.
+   */
+  default Optional<Entry> fallbackValue(String key) {
+    return Optional.empty();
+  }
+
+}

avaje-config/src/main/java/io/avaje/config/CoreComponents.java

@@ -8,15 +8,23 @@ final class CoreComponents {
   private final ModificationEventRunner runner;
   private final ConfigurationLog log;
   private final Parsers parsers;
+  private final ConfigurationFallback fallback;
   private final List<ConfigurationSource> sources;
   private final List<ConfigurationPlugin> plugins;
 
-  CoreComponents(ModificationEventRunner runner, ConfigurationLog log, Parsers parsers, List<ConfigurationSource> sources, List<ConfigurationPlugin> plugins) {
+  CoreComponents(
+          ModificationEventRunner runner,
+          ConfigurationLog log,
+          Parsers parsers,
+          ConfigurationFallback fallback,
+          List<ConfigurationSource> sources,
+          List<ConfigurationPlugin> plugins) {
     this.runner = runner;
     this.log = log;
     this.parsers = parsers;
     this.sources = sources;
     this.plugins = plugins;
+    this.fallback = fallback;
   }
 
   /** For testing only */
@@ -26,6 +34,7 @@ final class CoreComponents {
     this.parsers = new Parsers(Collections.emptyList());
     this.sources = Collections.emptyList();
     this.plugins = Collections.emptyList();
+    this.fallback = new DefaultFallback();
   }
 
   Parsers parsers() {
@@ -40,6 +49,10 @@ ModificationEventRunner runner() {
     return runner;
   }
 
+  ConfigurationFallback fallback() {
+    return fallback;
+  }
+
   List<ConfigurationSource> sources() {
     return sources;
   }

avaje-config/src/main/java/io/avaje/config/CoreConfiguration.java

@@ -1,6 +1,5 @@
 package io.avaje.config;
 
-import static io.avaje.config.Constants.SYSTEM_PROPS;
 import static io.avaje.config.Constants.USER_PROVIDED_DEFAULT;
 import static java.lang.System.Logger.Level.ERROR;
 import static java.util.Objects.requireNonNull;
@@ -50,7 +49,7 @@ final class CoreConfiguration implements Configuration {
     this.log = components.log();
     this.sources = components.sources();
     this.plugins = components.plugins();
-    this.properties = new ModifyAwareProperties(entries);
+    this.properties = new ModifyAwareProperties(entries, components.fallback());
     this.listValue = new CoreListValue(this);
     this.setValue = new CoreSetValue(this);
     this.pathPrefix = "";
@@ -62,7 +61,7 @@ final class CoreConfiguration implements Configuration {
     this.log = parent.log;
     this.sources = parent.sources;
     this.plugins = parent.plugins;
-    this.properties = new ModifyAwareProperties(entries);
+    this.properties = new ModifyAwareProperties(entries, parent.properties.fallback);
     this.listValue = new CoreListValue(this);
     this.setValue = new CoreSetValue(this);
     this.pathPrefix = prefix;
@@ -487,18 +486,16 @@ void fireOnChange(String value) {
     }
   }
 
-  static String toEnvKey(String key) {
-    return key.replace('.', '_').replace("-", "").toUpperCase();
-  }
-
   private static class ModifyAwareProperties {
 
     private final CoreEntry.CoreMap entries;
     private final Configuration.ExpressionEval eval;
+    private final ConfigurationFallback fallback;
 
-    ModifyAwareProperties(CoreEntry.CoreMap entries) {
+    ModifyAwareProperties(CoreEntry.CoreMap entries, ConfigurationFallback fallback) {
       this.entries = entries;
       this.eval = new CoreExpressionEval(entries);
+      this.fallback = fallback;
     }
 
     int size() {
@@ -512,7 +509,7 @@ String eval(String value) {
 
     @Nullable
     String valueOrNull(String key) {
-      CoreEntry entry = entries.get(key);
+      Entry entry = entries.get(key);
       return entry == null ? null : entry.value();
     }
 
@@ -525,11 +522,11 @@ boolean getBool(String key, boolean defaultValue) {
       return entry(key, String.valueOf(defaultValue)).boolValue();
     }
 
-    CoreEntry entry(String key) {
+    Entry entry(String key) {
       return _entry(key, null);
     }
 
-    CoreEntry entry(String key, @Nullable String defaultValue) {
+    Entry entry(String key, @Nullable String defaultValue) {
       return _entry(key, defaultValue);
     }
 
@@ -539,18 +536,18 @@ CoreEntry entry(String key, @Nullable String defaultValue) {
      */
     Optional<Entry> optionalEntry(String key) {
       return Optional.ofNullable(entries.get(key))
-        .filter(entry -> !entry.isNull())
-        .map(entry -> entry);
+        .filter(entry -> !entry.isNull());
     }
 
     /**
      * Get property with caching taking into account defaultValue and "null".
      */
-    private CoreEntry _entry(String key, @Nullable String defaultValue) {
-      CoreEntry value = entries.get(key);
+    private Entry _entry(String key, @Nullable String defaultValue) {
+      Entry value = entries.get(key);
       if (value == null) {
-        // defining property at runtime with System property/ENV backing
-        value = defaultEntry(defaultValue, systemValue(key));
+        value = fallback.fallbackValue(key)
+          .or(() -> asDefault(defaultValue))
+          .orElse(CoreEntry.NULL_ENTRY);
         entries.put(key, value);
       } else if (value.isNull() && defaultValue != null) {
         value = CoreEntry.of(defaultValue, USER_PROVIDED_DEFAULT);
@@ -559,20 +556,8 @@ private CoreEntry _entry(String key, @Nullable String defaultValue) {
       return value;
     }
 
-    private static CoreEntry defaultEntry(@Nullable String defaultValue, @Nullable String systemValue) {
-      if (systemValue != null) {
-        return CoreEntry.of(systemValue, SYSTEM_PROPS);
-      } else if (defaultValue != null) {
-        return CoreEntry.of(defaultValue, USER_PROVIDED_DEFAULT);
-      } else {
-        return CoreEntry.NULL_ENTRY;
-      }
-    }
-
-    @Nullable
-    private static String systemValue(String key) {
-      final String val = System.getProperty(key, System.getenv(key));
-      return val != null ? val : System.getenv(toEnvKey(key));
+    private Optional<Entry> asDefault(@Nullable String defaultValue) {
+      return defaultValue == null ? Optional.empty() : Optional.of(CoreEntry.of(defaultValue, USER_PROVIDED_DEFAULT));
     }
 
     void loadIntoSystemProperties(Set<String> excludedSet) {