Use the config dir app.d for application mode if present

application-mode/build.gradle

@@ -13,6 +13,8 @@ dependencies {
 
     api project(':proto:proto-backplane-grpc')
 
+    implementation project(':Configuration')
+
     Classpaths.inheritJUnitPlatform(project)
     Classpaths.inheritAssertJ(project)
     testImplementation 'org.junit.jupiter:junit-jupiter'

application-mode/src/main/java/io/deephaven/appmode/ApplicationConfig.java

@@ -5,38 +5,40 @@
 
 import java.io.IOException;
 import java.nio.file.Files;
-import java.nio.file.InvalidPathException;
 import java.nio.file.LinkOption;
 import java.nio.file.Path;
-import java.nio.file.Paths;
 import java.util.List;
+import java.util.Optional;
 
 public interface ApplicationConfig {
 
+    /**
+     * Returns true if the configuration property {@value ApplicationConfigImpl#APPLICATION_DIR_PROP} is set, or if the
+     * {@value ApplicationConfigImpl#DEFAULT_APP_DIRNAME} exists in the configuration directory.
+     *
+     * @return true if custom application mode is enabled
+     */
     static boolean isCustomApplicationModeEnabled() {
-        return ApplicationConfigImpl.APPLICATION_DIR != null;
+        return ApplicationConfigImpl.applicationDir().isPresent();
     }
 
     /**
-     * The custom application directory. Custom application mode must be enabled.
+     * The custom application directory. Returns the configuration property
+     * {@value ApplicationConfigImpl#APPLICATION_DIR_PROP} if it set, otherwise it returns the
+     * {@value ApplicationConfigImpl#DEFAULT_APP_DIRNAME} from the configuration directory if it exists, otherwise it
+     * throws an {@link IllegalStateException}.
      *
      * @return the application dir
      * @see #isCustomApplicationModeEnabled()
      */
     static Path customApplicationDir() {
-        if (!isCustomApplicationModeEnabled()) {
+        final Optional<Path> applicationDir = ApplicationConfigImpl.applicationDir();
+        if (applicationDir.isEmpty()) {
             throw new IllegalStateException(
-                    String.format("Custom application mode is not enabled, please set system property '%s'",
+                    String.format("Custom application mode is not enabled, please set the configuration property '%s'",
                             ApplicationConfigImpl.APPLICATION_DIR_PROP));
         }
-        final Path applicationDir;
-        try {
-            applicationDir = Paths.get(ApplicationConfigImpl.APPLICATION_DIR);
-        } catch (InvalidPathException e) {
-            throw new IllegalArgumentException(String.format("Invalid application directory '%s'",
-                    ApplicationConfigImpl.APPLICATION_DIR));
-        }
-        return applicationDir;
+        return applicationDir.get();
     }
 
     /**

application-mode/src/main/java/io/deephaven/appmode/ApplicationConfigImpl.java

@@ -3,6 +3,9 @@
  */
 package io.deephaven.appmode;
 
+import io.deephaven.configuration.ConfigDir;
+import io.deephaven.configuration.Configuration;
+
 import java.io.BufferedReader;
 import java.io.IOException;
 import java.nio.file.Files;
@@ -11,14 +14,22 @@
 import java.util.ArrayList;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Optional;
 import java.util.Properties;
 import java.util.stream.Stream;
 
 public class ApplicationConfigImpl {
 
     public static final String APPLICATION_DIR_PROP = "deephaven.application.dir";
+    public static final String DEFAULT_APP_DIRNAME = "app.d";
 
-    public static final String APPLICATION_DIR = System.getProperty(APPLICATION_DIR_PROP, null);
+    public static Optional<Path> applicationDir() {
+        final String explicitDir = Configuration.getInstance().getStringWithDefault(APPLICATION_DIR_PROP, null);
+        if (explicitDir != null) {
+            return Optional.of(Path.of(explicitDir));
+        }
+        return ConfigDir.get().map(ApplicationConfigImpl::defaultAppDir).filter(Files::isDirectory);
+    }
 
     public static List<ApplicationConfig> find(Path dir) throws IOException, ClassNotFoundException {
         try (Stream<Path> stream =
@@ -32,6 +43,10 @@ public static List<ApplicationConfig> find(Path dir) throws IOException, ClassNo
         }
     }
 
+    private static Path defaultAppDir(Path configDir) {
+        return configDir.resolve(DEFAULT_APP_DIRNAME);
+    }
+
     private static boolean isAppFile(Path path) {
         return path.getFileName().toString().endsWith(".app") && Files.isReadable(path)
                 && Files.isRegularFile(path, LinkOption.NOFOLLOW_LINKS);