legacy/web/src/main/java/org/jboss/as/web/WebMigrateOperation.java
private void createWelcomeContentHandler(Map<PathAddress, ModelNode> migrationOperations) {

        PathAddress address = pathAddress(pathElement(SUBSYSTEM, UndertowExtension.SUBSYSTEM_NAME), pathElement(Constants.CONFIGURATION, Constants.HANDLER));
        migrationOperations.put(address, createAddOperation(address));

        address = pathAddress(pathElement(SUBSYSTEM, UndertowExtension.SUBSYSTEM_NAME), pathElement(Constants.CONFIGURATION, Constants.HANDLER), pathElement(Constants.FILE, "welcome-content"));
        final ModelNode add = createAddOperation(address);
        add.get(Constants.PATH).set(new ModelNode(new ValueExpression("${jboss.home.dir}/welcome-content")));
        migrationOperations.put(address, add);
    }
private void createWelcomeContentHandler(Map<PathAddress, ModelNode> migrationOperations) {

        PathAddress address = pathAddress(pathElement(SUBSYSTEM, UndertowExtension.SUBSYSTEM_NAME), pathElement(Constants.CONFIGURATION, Constants.HANDLER));
        migrationOperations.put(address, createAddOperation(address));

        address = pathAddress(pathElement(SUBSYSTEM, UndertowExtension.SUBSYSTEM_NAME), pathElement(Constants.CONFIGURATION, Constants.HANDLER), pathElement(Constants.FILE, "welcome-content"));
        final ModelNode add = createAddOperation(address);
        add.get(Constants.PATH).set(new ModelNode(new ValueExpression("${jboss.home.dir}/welcome-content")));
        migrationOperations.put(address, add);
    }
create a handler for serving welcome content