add Config.resolveWith()

This allows replacing substitutions with values which
are not merged into the Config.

Fixes #95

config/src/main/java/com/typesafe/config/Config.java

@@ -184,6 +184,46 @@ public interface Config extends ConfigMergeable {
      */
     boolean isResolved();
 
+    /**
+     * Like {@link Config#resolve()} except that substitution values are looked
+     * up in the given source, rather than in this instance. This is a
+     * special-purpose method which doesn't make sense to use in most cases;
+     * it's only needed if you're constructing some sort of app-specific custom
+     * approach to configuration. The more usual approach if you have a source
+     * of substitution values would be to merge that source into your config
+     * stack using {@link Config#withFallback} and then resolve.
+     * <p>
+     * Note that this method does NOT look in this instance for substitution
+     * values. If you want to do that, you could either merge this instance into
+     * your value source using {@link Config#withFallback}, or you could resolve
+     * multiple times with multiple sources (using
+     * {@link ConfigResolveOptions#setAllowUnresolved(boolean)} so the partial
+     * resolves don't fail).
+     * 
+     * @param source
+     *            configuration to pull values from
+     * @return an immutable object with substitutions resolved
+     * @throws ConfigException.UnresolvedSubstitution
+     *             if any substitutions refer to paths which are not in the
+     *             source
+     * @throws ConfigException
+     *             some other config exception if there are other problems
+     */
+    Config resolveWith(Config source);
+
+    /**
+     * Like {@link Config#resolveWith(Config)} but allows you to specify
+     * non-default options.
+     * 
+     * @param source
+     *            source configuration to pull values from
+     * @param options
+     *            resolve options
+     * @return the resolved <code>Config</code> (may be only partially resolved
+     *         if options are set to allow unresolved)
+     */
+    Config resolveWith(Config source, ConfigResolveOptions options);
+
     /**
      * Validates this config against a reference config, throwing an exception
      * if it is invalid. The purpose of this method is to "fail early" with a

config/src/main/java/com/typesafe/config/impl/SimpleConfig.java

@@ -57,7 +57,17 @@ final class SimpleConfig implements Config, MergeableValue, Serializable {
 
     @Override
     public SimpleConfig resolve(ConfigResolveOptions options) {
-        AbstractConfigValue resolved = ResolveContext.resolve(object, object, options);
+        return resolveWith(this, options);
+    }
+
+    @Override
+    public SimpleConfig resolveWith(Config source) {
+        return resolveWith(source, ConfigResolveOptions.defaults());
+    }
+
+    @Override
+    public SimpleConfig resolveWith(Config source, ConfigResolveOptions options) {
+        AbstractConfigValue resolved = ResolveContext.resolve(object, ((SimpleConfig) source).object, options);
 
         if (resolved == object)
             return this;
@@ -65,7 +75,6 @@ final class SimpleConfig implements Config, MergeableValue, Serializable {
             return new SimpleConfig((AbstractConfigObject) resolved);
     }
 
-
     @Override
     public boolean hasPath(String pathExpression) {
         Path path = Path.newPath(pathExpression);

config/src/test/scala/com/typesafe/config/impl/ConfigTest.scala

@@ -1138,11 +1138,34 @@ class ConfigTest extends TestUtils {
         }
         // and the partially-resolved thing is not resolved
         assertFalse("partially-resolved object is not resolved", allowedUnresolved.isResolved)
-        // and given the values for the resolve, we should be able to
-        val resolved = allowedUnresolved.withFallback(values).resolve()
-        for (kv <- Seq("a" -> 1, "b" -> 2, "c.x" -> 3, "c.y" -> 4)) {
-            assertEquals(kv._2, resolved.getInt(kv._1))
+
+        // scope "val resolved"
+        {
+            // and given the values for the resolve, we should be able to
+            val resolved = allowedUnresolved.withFallback(values).resolve()
+            for (kv <- Seq("a" -> 1, "b" -> 2, "c.x" -> 3, "c.y" -> 4)) {
+                assertEquals(kv._2, resolved.getInt(kv._1))
+            }
+            assertTrue("fully resolved object is resolved", resolved.isResolved)
         }
-        assertTrue("fully resolved object is resolved", resolved.isResolved)
+
+        // we should also be able to use resolveWith
+        {
+            val resolved = allowedUnresolved.resolveWith(values)
+            for (kv <- Seq("a" -> 1, "b" -> 2, "c.x" -> 3, "c.y" -> 4)) {
+                assertEquals(kv._2, resolved.getInt(kv._1))
+            }
+            assertTrue("fully resolved object is resolved", resolved.isResolved)
+        }
+    }
+
+    @Test
+    def resolveWithWorks(): Unit = {
+        // the a=42 is present here to be sure it gets ignored when we resolveWith
+        val unresolved = ConfigFactory.parseString("foo = ${a}, a = 42")
+        assertEquals(42, unresolved.resolve().getInt("foo"))
+        val source = ConfigFactory.parseString("a = 43")
+        val resolved = unresolved.resolveWith(source)
+        assertEquals(43, resolved.getInt("foo"))
     }
 }