Some suggestions from PR Review 1

    * Fix documentation punctuation.
    * Add HelperNotFoundException
    * Rename template files

jni/README.md

@@ -16,7 +16,7 @@ This library contains:
 
 This is intended for one-off / debugging uses of JNI, as well as providing a base library for code generated by jni_gen. 
 
-__If you're looking to interface a complete java library, look forward for `jni_gen`.__
+__To interface a complete java library, look forward for `jni_gen`.__
 
 ## Platform support
 The focus of this project is Android, since it already has a JVM and JNI allows interop with existing code and Platform APIs. Experimental desktop support exists for Linux using the JVM invocation API.

jni/lib/jni.dart

@@ -28,12 +28,12 @@
 /// the wrapper libraries with the executable. Thus it needs to be explicitly
 /// placed in a accessible directory and provided as an argument to Jni.spawn.
 ///
-/// This module depends on a shared library written in C, when using dart
-/// standalone, it's your responsibility to:
+/// This module depends on a shared library written in C. Therefore on dart
+/// standalone:
 ///
-///  * Build the library `libdartjni.so` in src/ directory of this plugin
-///  * Bundle it appropriately with dart application
-///  * Pass the path to library as a parameter to `Jni.spawn()`
+///  * Build the library `libdartjni.so` in src/ directory of this plugin.
+///  * Bundle it appropriately with dart application.
+///  * Pass the path to library as a parameter to `Jni.spawn()`.
 ///
 /// __JNIEnv:__
 /// The types `JNIEnv` and `JavaVM` in JNI are available as `JniEnv` and
@@ -47,16 +47,15 @@
 /// * On desktop platforms you can use JniEnv.ExceptionDescribe to print any
 /// pending exception to stdout.
 /// * On Android, things are slightly easier since CheckJNI is usually enabled
-/// in debug builds.
-/// * If you are not getting clear stack traces on JNI errors, check the
-/// Android NDK page on how to enable CheckJNI using ADB.
+/// in debug builds. If you are not getting clear stack traces on JNI errors,
+/// check the Android NDK page on how to enable CheckJNI using ADB.
 /// * As a rule of thumb, when there's a NoClassDefFound / NoMethodFound error,
 /// first check your class and method signatures for typos.
 ///
 
-/// This library exports the minimum foundations of JNI.
+/// This file exports the minimum foundations of JNI.
 ///
-/// For a higher level API, import jni_object.dart.
+/// For a higher level API, import `'package:jni/jni_object.dart'`.
 library jni;
 
 export 'src/third_party/jni_bindings_generated.dart'; // currently export all

jni/lib/src/jni.dart

@@ -7,6 +7,7 @@ import 'package:path/path.dart';
 import 'third_party/jni_bindings_generated.dart';
 import 'extensions.dart';
 import 'jvalues.dart';
+import 'jni_exceptions.dart';
 
 import 'jni_object.dart';
 import 'jni_class.dart';
@@ -34,6 +35,9 @@ DynamicLibrary _loadJniHelpersLibrary(
     {String? dir, String baseName = "dartjni"}) {
   final fileName = _getLibraryFileName(baseName);
   final libPath = (dir != null) ? join(dir, fileName) : fileName;
+  if (!File(libPath).existsSync()) {
+    throw HelperNotFoundException(libPath);
+  }
   final dylib = DynamicLibrary.open(libPath);
   return dylib;
 }

jni/lib/src/jni_exceptions.dart

@@ -37,3 +37,13 @@ class JniException implements Exception {
 
   void deleteIn(Pointer<JniEnv> env) => env.DeleteLocalRef(err);
 }
+
+class HelperNotFoundException implements Exception {
+  HelperNotFoundException(this.path);
+  final String path;
+
+  @override
+  String toString() => "Lookup for helper library $path failed.\n"
+      "Please ensure that `dartjni` shared library is built.\n"
+      "If the library is already built, double check the path.";
+}

jni/test/exception_test.dart

@@ -7,7 +7,21 @@ import 'package:jni/jni_object.dart';
 
 void main() {
   if (!Platform.isAndroid) {
-    Jni.spawn(helperDir: "src/build");
+    bool caught = false;
+    try {
+      // If library does not exist, a helpful exception should be thrown.
+      // we can't test this directly because
+      // `test` schedules functions asynchronously
+      Jni.spawn(helperDir: "wrong_dir");
+    } on HelperNotFoundException catch (_) {
+      // stderr.write("\n$_\n");
+      Jni.spawn(helperDir: "src/build");
+      caught = true;
+    }
+    if (!caught) {
+      throw "Expected HelperNotFoundException\n"
+          "Read exception_test.dart for details.";
+    }
   }
   final jni = Jni.getInstance();
 

jni/tool/gen_aux_methods.dart

@@ -3,7 +3,7 @@
 import 'dart:io' as io;
 import 'package:path/path.dart';
 
-var targetTypes = {
+final targetTypes = {
   "String": "String",
   "Object": "JniObject",
   "Boolean": "bool",
@@ -17,7 +17,7 @@ var targetTypes = {
   "Void": "void"
 };
 
-var resultConverters = {
+final resultConverters = {
   "String": (String resultVar) => "final str = _env.asDartString($resultVar);"
       "_env.DeleteLocalRef($resultVar);"
       "return str",
@@ -26,7 +26,7 @@ var resultConverters = {
   "Boolean": (String resultVar) => "return $resultVar != 0",
 };
 
-var invokeResultConverters = {
+final invokeResultConverters = {
   "String": (String resultVar) => "final str = env.asDartString($resultVar);"
       "env.DeleteLocalRef($resultVar);"
       "env.DeleteLocalRef(cls);"
@@ -43,21 +43,21 @@ void main(List<String> args) {
     return io.File(join(scriptDir, 'templates', name)).readAsStringSync();
   }
 
-  final methodTemplates = getTemplate('JniObject_MethodCalls');
-  final fieldTemplates = getTemplate('JniObject_Fields');
-  final invokeTemplates = getTemplate('Invoke_Static_Methods');
-  final retrieveTemplates = getTemplate('Retrieve_Static_Fields');
+  final methodTemplates = getTemplate('jni_object_methods.dart.tmpl');
+  final fieldTemplates = getTemplate('jni_object_fields.dart.tmpl');
+  final invokeTemplates = getTemplate('invoke_static_methods.dart.tmpl');
+  final retrieveTemplates = getTemplate('retrieve_static_fields.dart.tmpl');
 
-  var outputDir = join("lib", "src");
-  var sInst = StringBuffer();
-  var sStatic = StringBuffer();
-  var sInvoke = StringBuffer();
-  var outPutPaths = {
+  final outputDir = join("lib", "src");
+  final sInst = StringBuffer();
+  final sStatic = StringBuffer();
+  final sInvoke = StringBuffer();
+  final outPutPaths = {
     sInst: join(outputDir, "jni_object_methods_generated.dart"),
     sStatic: join(outputDir, "jni_class_methods_generated.dart"),
     sInvoke: join(outputDir, "direct_methods_generated.dart")
   };
-  for (var s in [sInst, sStatic, sInvoke]) {
+  for (final s in [sInst, sStatic, sInvoke]) {
     s.write("// Autogenerated; DO NOT EDIT\n"
         "// Generated by running the script in tool/gen_aux_methods.dart\n");
   }
@@ -70,7 +70,7 @@ void main(List<String> args) {
   sStatic.write("extension JniClassCallMethods on JniClass {");
   sInvoke.write("extension JniInvokeMethods on Jni {");
 
-  for (var t in targetTypes.keys) {
+  for (final t in targetTypes.keys) {
     void write(String template) {
       final resultConverter =
           resultConverters[t] ?? (resultVar) => "return $resultVar";
@@ -91,7 +91,7 @@ void main(List<String> args) {
     if (t != "Void") {
       write(fieldTemplates);
     }
-    var invokeResultConverter =
+    final invokeResultConverter =
         (invokeResultConverters[t] ?? (String r) => "return $r");
     void writeI(String template) {
       final replaced = template
@@ -115,8 +115,8 @@ void main(List<String> args) {
   sInst.write("}");
   sStatic.write("}");
   sInvoke.write("}");
-  for (var s in [sInst, sStatic, sInvoke]) {
-    var outputFile = io.File(outPutPaths[s]!);
+  for (final s in [sInst, sStatic, sInvoke]) {
+    final outputFile = io.File(outPutPaths[s]!);
     outputFile.writeAsStringSync(s.toString(), flush: true);
   }
   io.stderr.write("Running dart format..\n");