NativeScript Performance – Part 2

The last two weeks I was busy with measuring and optimizing the performance of NativeScript for Android. My main focus was the application startup time and I would like to share some good news.


Let’s first see the results and then I will dig into the details. As in the previous tests I uses the same test devices:

  • Device1 – Nexus 5, Android 4.4.1, build KOT49E
  • Device2 – Nexus 6, Android 5.0.1, build LRX22C

I used the same application as well. Here are the results:

  • For Device1 the first startup time was reduced from average 3.1419 seconds to average 2.8262 seconds (10% improvement) [*]
  • For Device2 the first startup time was reduced from average 3.541 seconds to average 3.3147 seconds (6% improvement) [*]


Before I dig into the details, I would like to give you a quick reminder how I measured the times. As in the previous tests I used the built-in time/perf info that Android ActivityManager provides. It is not the best measuring tool but it is good enough for our purposes.

After detailed profiling with DDMS and NDK profilers I identified two areas for improvements:

  • asset extraction
  • proxy property access


The old implementation for asset extraction was based on AssetManager. While its API is very convenient, it is not well suited for optimal memory allocation. As a result using AssetManager along with* classes generates a lot of temporary objects which triggers the GC quite often. The solution we chose is to use libzip C++ library. It is fast and more importantly it doesn’t mess with the GC.

For applications with size similar to the test app using libzip doesn’t help much. The actual improvement is around 30-40 milliseconds. However, for big apps (e.g. 500+ files) libzip really shines. You can easily get improvement of 300-500ms, and in some scenarios more than a second. This was a good reason to reimplement the Java code into C++ and give NativeScript the ability to scale really well.

Java Object Wrappers

Proxies are an experimental ECMAScript 6 feature. In V8 (and for the matter of fact in any other JavaScript engine), direct property access is much faster than direct proxy access. This is easily understandable when you think how the JIT compiler emits the code to access traditional properties. Also, while proxies are good for scripting simple object access they don’t scale in more complex scenarios. With the time it becomes harder to implement the correct dispatch logic.

I am glad to say that we now use plain JavaScript objects to wrap Java objects. We also build the correct prototype chain to map Java class hierarchy. This give us an excellent opportunity to cache runtime objects at more granular level. And as we are going to see, caching changes everything.

While using libzip helped a little bit, it is easy to do the math and see that using prototype chains is the main factor for the improved startup time.

Let’s see how the new caches impact other scenarios. Take a look at the following code fragment.

var JavaDate = java.util.Date;
var start = new Date();
for (var i=0; i<10000; i++) {
    var d1 = new JavaDate();
    var d2 = new JavaDate();
var end = new Date();
console.log("time=" + (end.getTime() - start.getTime()));

This is not a real world scenario. I wrote this code for sole test purposes. My intent here is to exercise some Java intensive code. Also, note that using JavaScript Date.getTime is not the best way to measure time, but as we are going to see it is good enough for our purposes.

Here are the results.

  • On Device1 – using proxy objects it takes more than 12.5 seconds, using prototype chain it takes less than 2.6 seconds
  • On Device2 – using proxy objects it takes more than 11.6 seconds, using prototype chain it takes less than 2.2 seconds

In my opinion, there is no need for any further or more precise benchmarks. Simply put, using prototype chains along with proper caching is much faster than proxy objects.

Further Improvements

So far, we saw that the first startup of a simple application like CutenessIO takes around 3 seconds. Can we make it faster?

First, we have to set some reasonable expectations. Let’s see how fast HelloWorld applications written in Java and NativeScript start up. For the Java version I used the standard Eclipse project template (which is very similar to the one in Android Studio). I stripped all things like menus and fancy themes. My main goal was the make it as simple as possible (which is not much different from the standard empty project). I did the same for the NativeScript project.

Here are the results.

  • On Device1 – Java 200 milliseconds[*], NativeScript 641.5 milliseconds[*]
  • On Device2 – Java 333.5 milliseconds[*], NativeScript 875.3 milliseconds[*]

So, we have to investigate where the difference comes from. For the purpose of this article, I am going to pick Device1 (the analysis for Device2 is the same).

Let’s analyze a particular run.

  • Time for loading libNativeScript library: 7ms
  • Time for extracting assets: 30ms
  • Time for V8 initialization: 150ms
  • Time for calling Application.onCreate in JavaScript: 60ms
  • Time for calling Activity.onCreate in JavaScript: 100ms
  • Time from Application object initialization to Activity initialization: 510ms
  • Time to display main activity: 658ms

As we can see, the total time of asset extraction and V8 initialization is 180ms which is roughly the time needed for pure Java application to start. So far, it seems unlikely to reduce this time.

The total time spent in running JavaScript 160ms. This is a bit surprising. I would love to see the time spent in V8 to be, say, 400ms because this would mean that running JavaScript is 78% (400/510) of all time. High percentage of time spent inside in V8 is a good thing because this will give us an opportunity to optimize the performance. However, this would not be the case for most applications. We can think of NativeScript as a way to command Java world from JavaScript. Hence, most of the work is done in Java. That’s the nature of NativeScript.

So, we spent 160ms running a few lines of JavaScript. Can we do better? A careful analysis showed that most of this time is spent in JNI infrastructure calls and data marshalling. It seems hard to reduce it, but not unlikely. A possible option is to tweak V8 engine and/or use libffi to generate thunks.

Another 200ms is spent in some run-once pluming code. With a little effort, we could refactor the runtime to support components/modules and gain some performance. Finally, some time is spent inside the Java GC.

In closing, I would say that currently NativeScript for Android is performing well. There are no major performance issues. The current implementation is approaching the point where no big performance wins can be easily achieved. But easy is not interesting 😉 Stay tuned.

On NativeScript Performance


Last week NativeScript made it into public beta and just for a few days we got tremendous amount of feedback. One question that came up over and over again was, “How do NativeScript Apps Perform”?  In this post, I want to explain the details behind performance and share some great news with you about the upcoming release of NativeScript.

How it started

As other new projects NativeScript started from the idea to take a new look at the cross-platform mobile development with JavaScript. In the beginning, we had to determine if the concept of NativeScript was even feasible.  Should we translate JavaScript into Java?  What about Objective-C back into JavaScript?  During this exploratory phase, we learned that the answer was actually much simpler than this thanks to the JavaScript bridge that exists for both iOS and Android.  Well, thanks to Android fragmentation, this is only partially true.  Let me explain…


Working on a project like NativeScript is anything but easy. There are many challenges imposed by working with two very different runtimes like Dalvik and V8. Add the restricted environment in Android and you will get the idea. Controlling object lifetime when you have two garbage collectors, efficient type marshalling, lack of 64bit integers in JavaScript, correctly working with different UTF-8 encodings, and overloaded method resolution, just to name a few. All these are nontrivial problems.

Statically Generated Bindings

One specific problem is the extending/subclassing of Java types from JavaScript. It is astonishing how a simple task like working with a UI widget becomes a challenging technical problem. It takes no longer to look than the Button documentation and its seemingly innocent example.

button.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
        // Perform action on click

While the Java compiler is there for you to generate an anonymous class that implements View.OnClickListener interface there is no such facility in JavaScript. We solved this problem by generating proxy classes (bindings). Basically we generated *.java source files, compiled them to *.class files which in turn were compiled to *.dex files. You can find these *.dex files in assets/bindings folder of every NativeScript for Android project. The total size of these files is more than 12MB which is quite a lot.

Here begins the interesting part. Android 5 comes with a new runtime (ART). One of major changes in ART is the ahead-of-time (AOT) compiler. Now you can imagine what happens when the AOT compiler has to compile more than 12MB *.dex files on the very first run of any NativeScript for Android application. That’s right, it takes a long time. The problem is less apparent in Android 4.x but it is still there.

Dynamically Generated Bindings

The solution is obvious. We simply need to generate bindings in runtime instead of compile time. The immediate advantages are that we will generate bindings only for those classes that we actually extend in JavaScript. Lesser the bindings, lesser the work for the AOT compiler.

We started working on the new binding generator right after the first private beta. We were almost done for the public beta. However, almost doesn’t count. We decided to play safe and release the first beta with statically generated bindings. The good news is that the new binding generator is already merged in the master branch (only two days after the public beta announcement).

Today I ran some basic performance tests on the following devices:

  • Device1 – Nexus 5, Android 4.4.1, build KOT49E
  • Device2 – Nexus 6, Android 5.0.1, build LRX22C

For the tests I used the built-in time/perf info that Android OS provides. You probably have seen similar information in your logcat console.

I/ActivityManager(770): START u0 {act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000 cmp=com.tns/.NativeScriptActivity} from pid 1030
I/ActivityManager(770): Displayed com.tns/.NativeScriptActivity: +3s614ms

Here are the results:

  • For Device1 the first start-up time was reduced from average 60.761 seconds to average 3.1419 seconds
  • For Device2 the first start-up time was reduced from average 39.384 seconds to average 3.541 seconds

A consequential start-up time for both devices is ~2.5 or less seconds.

What’s next

There is a lot of room for performance improvement. Currently NativeScript for Android uses JavaScript proxy object to get a callback when Java field is accessed or Java method is invoked. The problem is that proxy objects (interceptors) are not fast. We plan to replace them with plain JavaScript objects that have properly constructed prototype chain with accessors instead of interceptors. Another benefit of using prototype chains with accessors is that we will support JavaScript instanceof operator.

Another area for improvement is the memory management. Currently, we generate a lot of temporary Java objects which may kick the Java GC unnecessary often. Moving some parts of the runtime from Java to C++ is a viable option that we are going to explore.


In closing, I would like to say that we are astounded by how popular NativeScript has become in such a short amount of time. We have learned so much in the building the NativeScript runtime, and our experience in that process helps us improve NativeScript every single day.  We’re looking forward to version 1. Building truly native mobile applications with native performance using JavaScript is the future, and the future is now.

Java Class Inheritance in NativeScript for Android

One of the more advanced scenarios in NativeScript for Android is the inheritance of Java classes. Let’s take a look at the following example.

// app.js
var MyButton = android.widget.Button.extend({
    setEnabled: function(enabled) {
        // do something
var btn = new MyButton(context);

(Please note that at the time of writing NativeScript is in private beta version and the syntax may change in the future.)

This is a minimalistic example how to define a new derived class with JavaScript. In this example we define new class MyButton that inherits from android.widget.Button and overrides setEnabled method. Behind the scenes NativeScript for Android will generate a new Java class as follows.

public class <runtime_generated_name> extends android.widget.Button {
    public void setEnabled(boolean enabled) {
        Object[] params = new Object[1];
        params[0] = enabled;
        NativeScriptRuntime.callJSMethod(this, "setEnabled", params);

This class serves as a simple proxy that calls the JavaScript implementation of setEnabled method. For the curious ones the method callJSMethod is defined as native and is used to dispatch the method invocation to the JavaScript engine.

public class NativeScriptRuntime {
    public static native Object callJSMethod(Object obj, String methodName, Object[] params);

So far we didn’t talk about the class’ <runtime_generated_name>. This name is generated from the following things:

  • the name of the base class
  • the name of the JavaScript file in which the class is defined
  • the line number
  • the column number

Let’s assume that MyButton is defined in app.js file on line 21 and column 38. In this case the generated class name may be something like <package_prefix>.android_widget_Button_app_L21_C38 where <package_prefix> is some hard-coded package name.

For more advanced scenarios NativeScript for Android allows you to overwrite the generated class name. You can do that using the full extend signature.

extend(<class_name>, implementationObject)

Usually you can omit class_name argument and let NativeScript to generate it for you.

There is one important difference between Java and JavaScript though. Java generates derived classes at compile time while NativeScript generates derived classes at runtime. As a consequence Java generates a derived class exactly once while NativeScript can execute extend function multiple times. This means that NativeScript must performs validation checks before it generates a class.

There are many approaches to this problem. As NativeScript for Android is in private beta we are still discussing what the validation checks should be. Here is a simplified diagram of one possible approach.


We basically can check whether extend is called with the same name and implementation object before and return the cached class. In this case it seems reasonable to freeze the implementation object on the first call in order to guarantee that multiple instances of a single extended class will have the same runtime behavior.

In closing, I would like to say that there is not much time until the official release of NativeScript. Today I showed you just one possible approach how to extend a Java class from JavaScript. There are other options as well. If you think there are better solutions please do not hesitate and leave a comment.