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Execute JavaScript in Android without WebView

05 Apr 2015

For some reason, I need to execute JavaScript code in an Android project. Because the JavaScript code was running in a background thread(in doInBackground part in an AsyncTask), I don't want to use WebView.

Why not WebView

First of all, WebView is heavy. WebView is slow, especially in a large/huge project. Plus, you need to add a lot of WebView related settings to just execute one code snippet. It's just simply not worth it.

Secondly, WebView is UI related. A WebView should be created in UI thread. Sure, you can create it in UI thread, load JavaScript code in a background thread, then pass the result back to UI thread. It's workable, but it's not safe.

If you call methods on WebView from any thread other than your app's UI thread, it can cause unexpected results. [source]

Last but not least, WebView is fragmented. After Android 4.4(API level 19), Android is using WebView baseed on Chromuim. Although this is toally a good thing, the WebView in Android 4.3 and below suffers the old version. You should code two version of WebView related code to just make it work. A lot of code should be re-write according to this article.

Luckily, the good people in Mozilla make a project called Rhino. It is typically embedded into Java applications to provide scripting to end users.

How to use Rhino

Rhino is a open source project, you can get the source code using this command:

git clone https://github.com/mozilla/rhino.git

Or fork their project on GitHub.

Of course, you don't have to get the source code. If you want to use it in your Android project. Download Rhino first, unzip it, put the js.jar file under libs folder. It is very small, so you don't need to worry your apk file will be ridiculously large because of this one external jar.

Here is some simple code to execute JavaScript code.

Object[] params = new Object[] { "javaScriptParam" };

// Every Rhino VM begins with the enter()
// This Context is not Android's Context
Context rhino = Context.enter();

// Turn off optimization to make Rhino Android compatible
rhino.setOptimizationLevel(-1);
try {
    Scriptable scope = rhino.initStandardObjects();

    // Note the forth argument is 1, which means the JavaScript source has
    // been compressed to only one line using something like YUI
    rhino.evaluateString(scope, javaScriptCode, "JavaScript", 1, null);

    // Get the functionName defined in JavaScriptCode
    Object obj = scope.get(functionNameInJavaScriptCode, scope);

    if (obj instanceof Function) {
        Function jsFunction = (Function) obj;

        // Call the function with params
        Object jsResult = jsFunction.call(rhino, scope, scope, params);
        // Parse the jsResult object to a String
        String result = Context.toString(jsResult);
    }
} finally {
    Context.exit();
}

As you can see, it is quite easy. You can dig up more in Rhino document.

Proguard & Obfuscation

You can set up Rhino's proguard setting by adding this following line into your proguard-android.txt file.

-keep class org.mozilla.javascript.** { *; }

When you run the assembleRelease task, with minifyEnabled true of course, you still can see a lot of warning like these:

Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class javax.swing.JMenu
Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class javax.swing.JMenu
Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class javax.swing.JMenu
Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class javax.swing.ButtonGroup
Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class javax.swing.JScrollPane
Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class java.awt.event.ActionEvent
Warning: org.mozilla.javascript.tools.shell.JSConsole: can't find referenced class java.awt.event.ActionEvent
Warning: org.mozilla.javascript.tools.shell.JSConsole$1: can't find referenced class javax.swing.filechooser.FileFilter
Warning: org.mozilla.javascript.tools.shell.JSConsole$2: can't find referenced class java.awt.event.WindowAdapter
Warning: org.mozilla.javascript.tools.shell.JSConsole$2: can't find referenced class java.awt.event.WindowEvent
Warning: org.mozilla.javascript.tools.shell.JSConsole$2: can't find referenced class java.awt.event.WindowEvent
Warning: org.mozilla.javascript.xml.impl.xmlbeans.LogicalEquality: can't find referenced class org.apache.xmlbeans.XmlCursor
Warning: org.mozilla.javascript.xml.impl.xmlbeans.LogicalEquality: can't find referenced class org.apache.xmlbeans.XmlCursor
Warning: org.mozilla.javascript.xml.impl.xmlbeans.LogicalEquality: can't find referenced class org.apache.xmlbeans.XmlCursor

Luckily, evan with these warnings, the app runs fine. You can ignore them(that's what I do), or adding this line inot your proguard-android.txt file.

-dontwarn org.mozilla.javascript.**

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