support -Dconfig.resource, -Dconfig.file, -Dconfig.url system props

These override application.{conf,json,properties} for ConfigFactory.load()

README.md

@@ -86,6 +86,25 @@ to provide a custom `Config` object to be used instead of the
 default, in case the application needs multiple configurations in
 one JVM or wants to load extra config files from somewhere.
 
+For applications using `application.{conf,json,properties}`,
+system properties can be used to force a different config source:
+
+ - `config.resource` specifies a resource name - not a
+   basename, i.e. `application.conf` not `application`
+ - `config.file` specifies a filesystem path, again
+   it should include the extension, not be a basename
+ - `config.url` specifies a URL
+
+These system properties specify a _replacement_ for
+`application.{conf,json,properties}`, not an addition. They only
+affect apps using the default `ConfigFactory.load()`
+configuration. In the replacement config file, you can use
+`include "application"` to include the original default config
+file; after the include statement you could go on to override
+certain settings. (Caveat: right now includes are relative to the
+including entity, so `include "application"` would only work from
+another classpath resource, not from a file or URL.)
+
 ## JSON Superset
 
 Tentatively called "Human-Optimized Config Object Notation" or

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

@@ -5,6 +5,7 @@ package com.typesafe.config;
 
 import java.io.File;
 import java.io.Reader;
+import java.net.MalformedURLException;
 import java.net.URL;
 import java.util.Map;
 import java.util.Properties;
@@ -102,15 +103,72 @@ public final class ConfigFactory {
     }
 
     private static class DefaultConfigHolder {
-        static final Config defaultConfig = load("application");
+
+        private static Config loadDefaultConfig() {
+            int specified = 0;
+
+            // override application.conf with config.file, config.resource,
+            // config.url if requested.
+            String resource = System.getProperty("config.resource");
+            if (resource != null)
+                specified += 1;
+            String file = System.getProperty("config.file");
+            if (file != null)
+                specified += 1;
+            String url = System.getProperty("config.url");
+            if (url != null)
+                specified += 1;
+
+            if (specified == 0) {
+                return load("application");
+            } else if (specified > 1) {
+                throw new ConfigException(
+                        "You set more than one system property from config.file, config.url, config.resource; don't know which one to use!");
+            } else {
+                if (resource != null) {
+                    // this deliberately does not parseResourcesAnySyntax; if
+                    // people want that they can use an include statement.
+                    return load(parseResources(ConfigFactory.class, resource));
+                } else if (file != null) {
+                    return load(parseFile(new File(file)));
+                } else {
+                    try {
+                        return load(parseURL(new URL(url)));
+                    } catch (MalformedURLException e) {
+                        throw new ConfigException("Bad URL in config.url system property: '" + url
+                                + "'", e);
+                    }
+                }
+            }
+        }
+
+        static final Config defaultConfig = loadDefaultConfig();
     }
 
     /**
      * Loads a default configuration, equivalent to {@link #load(String)
-     * load("application")}. This configuration should be used by libraries and
-     * frameworks unless an application provides a different one.
+     * load("application")} in most cases. This configuration should be used by
+     * libraries and frameworks unless an application provides a different one.
      * <p>
      * This method may return a cached singleton.
+     * <p>
+     * If the system properties <code>config.resource</code>,
+     * <code>config.file</code>, or <code>config.url</code> are set, then the
+     * classpath resource, file, or URL specified in those properties will be
+     * used rather than the default
+     * <code>application.{conf,json,properties}</code> classpath resources.
+     * These system properties should not be set in code (after all, you can
+     * just parse whatever you want manually and then use {@link #load(Config)
+     * if you don't want to use <code>application.conf</code>}). The properties
+     * are intended for use by the person or script launching the application.
+     * For example someone might have a <code>production.conf</code> that
+     * include <code>application.conf</code> but then change a couple of values.
+     * When launching the app they could specify
+     * <code>-Dconfig.resource=production.conf</code> to get production mode.
+     * <p>
+     * If no system properties are set to change the location of the default
+     * configuration, <code>ConfigFactory.load()</code> is equivalent to
+     * <code>ConfigFactory.load("application")</code>.
      *
      * @return configuration for an application
      */