Java to JavaScript Compiler Discussion (2)

First, there are no clever ways for ClassLoader.
Second, in my conception, there are another term “asynchronous programming” besides “synchronous programming”.
Third, Java2Script compiler does perform very poorly at loading huge arsenal of SWT classes.

In the implementation of Java2Script ClassLoader (not full APIs implemented), it mainly supports three modes:
1. Asynchronous over SCRIPT tags (mainly in used)
2. Asynchronous over XMLHttpRequest (not used a lot)
3. Synchronous over XMLHttpRequest (seldom used),

Up until now, I seldom use ClassLoader.loadClass in synchronous XHR mode, as the browser may be locked. Instead, an asynchronous ClassLoader wrapper is introduced:

public class AClass {
// Java2Script compiler will generate different scripts without Thread for this method body
public static void load(final String clazzName, final Runnable afterLoaded) {
new Thread(new Runnable() {
public void run() {
try {
Class clz = Class.forName(clazzName);
if (afterLoaded instanceof ARunnable) {
ARunnable runnable = (ARunnable) afterLoaded;
} catch (ClassNotFoundException e) {
return ;

Java2Script recommends using this kind of asynchronous ClassLoader, in which the compiler will modify AClass.load with JavaScript codes without java.lang.Thread but with asynchronous XMLHttpRequest or SCRIPT-tag class loading codes.

And now it comes to something about “Asynchronous Programming” v.s. “Synchronous Programming”, if we use asynchronous class loading.

We know that JavaScript does not support Thread like Java. But when you use Java, lots of Thread things will be introduced. For example, when you use GUI toolkit to open a dialog, there are threads running? already. But when we introduce UI widgets in JavaScript, we have no Threads. What we can use are asynchronous callbacks. And we do use use asynchronous callbacks a lot in XMLHttpRequest. So if we take asynchronous callbacks for granted, we should always considering “Asynchronous Programming” pattern. That is to say, when we know some calls are *time-consuming*, we should put following codes into a callback and let that method call to call back later. So RPC and Class.forName call should be wrapped into a callback by developer intentionally.

Here is an example for scenario of using asynchronous class loader. There is a page, with many tabs. And each tab will present a different thing (a different class). We need not to load all those tab classes in starting up. We just need to load that tab class only when user switches to that tab. When user switches, here asynchronous class loader should be used to load the tab page class:

tabFolder.addSelectionListener(new SelectionAdapter() {
public void widgetSelected(SelectionEvent e) {
Object data = item.getData(); // class name actually
if (data != null) {
// ASWTClass#shellLoad is actually similar to AClass#load
ASWTClass.shellLoad(tabFolder.getShell(), (String) data, new ARunnable() {
public void run() {
try {
Constructor constructor = getClazz().getConstructor(new Class[] {ControlExample.class});
Object inst = constructor.newInstance(new Object[] {ControlExample.this});
Tab tab = (Tab) inst;
// ... do the following layout job
} catch (Throwable e) {
throw (Error) e;

Here when asynchronous class loader is loading classes, show some texts saying “fetching data …” should be fine.

The problem of *huge* number of HTTP round-trips is a *huge* problem of Java2Script compiler. So packed *.z.js, which is similar to *.jar, is introduced. First ClassLoader is designed to load class in per class mode but is also designed to load all related classes when one class is required. For example, when instantiating class A must require instantiating class B, then ClassLoader will try to load class B when class A is required, and only marks class A as loaded after class B or other classes are loaded. This may require algorithm to calculate class dependencies, which may be similar to GWT’s call graph. So if class A and class B have their relationship, we can pack A.js and B.js together to reduce the number of HTTP round-trips. In practical implementation of J2S’ SWT, ToolBar and ToolItem are packed as ToolBar.z.js, Shell.js, Decorations.js and Display.js are packed as Shell.z.js. This is a way Java2Script compiler trying to reduce *huge* number of HTTP round-trips. But as mentioned, the loading performance is not good enough. And the packing algorithm may require professional design besides compiler’s capabilities.

Actually, the original design was to pack most of? library *.js into some (less than 15 or 20) *.z.js files. And those *.z.js are static with different release versions. Other pages (or other websites’ pages) may load these static libraries when necessary. As different pages have common *.z.js URLs. There are no needs to download them again once they are already downloaded by other visited pages (Just like Flash is required to install once). And if the *.js is distributed on a fast network (like Google network or other distributed publishing network). In fact, maybe these library *.z.js are not too huge for such publishing and not small enough to be efficient libraries, as this design is not yet proved.

GWT’s *.js seems to be project-dependent. And different projects only share a bootstrap library and not share some big *.js libraries (But do share in Java source level). Am I right?

Currently Java2Script’s ClassLoader just ignore “Class not found” silently when SCRIPT-tag gets nothing. And this should be fixed. “Method not found” exceptions existed already before Java reflection was implemented.

As mentioned in my last post, I admired GWT team for the motivation of “to be as small and as efficient as possible” and “excellent 100k ~ 200k final *.js files with very good performance”. And I learned that two separate goals:
(1) Create highly optimized JS output
(2) Optionally, publish selected parts of your code with a JS-compatible API
For such design, without ClassLoader and Java reflection should be OK for me. But I am still wondering whether it’s a must feature or not for enterprise edition of GWT projects. OK, I am just wondering from my lazy perspective.

Maybe I should learn something from GWT’s motivation and goals. Maybe it’s direction for JavaScript, also for Java2Script. And may be it’s not. No matter what, I learned that my early (in May) understanding of GWT compiler was totally wrong. And thanks for Bruce, and thanks for this thread and all participators,? I have a better understanding of GWT compiler now.

By the way, about “Asynchronous Programming” v.s. “Synchronous Programming”, I think there are should be something inside. And I are wondering Java to JavaScript compiler should consider this seriously or not.

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Java to JavaScript Compiler Discussion (1)

Last year, I joined in a discussion about Java to JavaScript compiler in Google Web Toolkit (GWT) group. I think it is worthy to re-post my opinions here for a better understanding of Java2Script. Or later I may use the discussion to clarify my opinions.


“To be as small and as efficient as possible”!? I admire GWT team for their excellent 100k ~ 200k final *.js files with very good performance.

Java2Script (, as mentioned,? is a similar project for Java to JavaScript code generation. But the motivation in Java2Script is something different: “To provide same familiar Java APIs in JavaScript”.

I am just a lazy developer, lazy to learn new APIs for some same functions. For example, Swing and SWT are similar GUI toolkit. And using SWT can do all the things that Swing can do and vice versa. I would be lazy to learn one of them only and not to learn the other. And I prefer to SWT. And as now I am familiar with SWT, and if all things done by using GWT can be done by using SWT, I would be lazy not to learn the new GWT APIs but use my familiar SWT. So this lazy programmer will use Java2Script and its Java2Script SWT library.

In order to provide the same familiar Java APIs in JavaScript, Java2Script has to keep all fields, all methods, all inheritances and all polymorphic information of a class in the JavaScript codes so developer will always be happy and feel comfortable with those familiar APIs. Java2Script is actually providing compiler that compiles Java sources into static JavaScript libraries.

And following, I will just try to figure out my understanding of GWT’s Java to JavaScript compiling procedure. If I am wrong, please be kind to correct me.

GWT’s compiling procedure is somewhat similar to the procedure of compiling C sources into an executable file. In C compiling, C source is first compiled into an *.obj.? And then comes a linker, which will link *.obj with other *.lib files into an *.exe. In the linking procedure, not all method calls in those *.lib are linked into *.exe file. Only those which are used in the *.obj will be kept. Others may be discarded so the final *.exe is small and efficient. In order to find out which methods must be kept in linking, some recursive searching algorithm may be required.

In GWT compiling, there are no middle *.obj actually. GWT compiler tracks down those method calls which are related to the application. And if the other methods have no relationship with the application, they will be discarded even they are marked “public”. So the final *.js is very small and efficient. Those class meta information discarded may include class inheritances, overriding, polymorphisms and other OO information. So after compiling, there are actually not OO JavaScript. So OO concepts like “this” keyword or reflection calls can not be used. In the whole compiling, the key technology of GWT may be the algorithm to flat those super calls and polymorphic method calls into static calls correctly.

And I admire GWT team for their excellent 100k ~ 200k final *.js files with very good performance.

Even basing on the same JDT toolkit, compilers with different motivations result in different JavaScripts. In a comparison, Java2Script generates 500k+ *.js files, and? has its bottleneck on long waiting on loading those bundles of huge *.js files and poor performance on spending most of its CPU time in searching for polymorphic method calls. In fact, Java2Script has many optimizations on reducing the generated *.js file size and improving JavaScript performance, like minimizing variable name identities, generating smart scripts to avoid polymorphic method calls and others. But it seems that Java2Script is still far behind GWT in file size and performance.

But GWT, as the above compiling process described, may have its difficulties in supporting Java refection and dynamical class loading. But Java2Script already has its ClassLoader and is loading classes lazily. And it also has its early Java reflection implementation on JUnit tests (Reusing JUnit tests directly). But the way, GWT APIs and Java2Script SWT APIs is far different for comparison.

In some simple words, GWT compiler is an excellent Java to JavaScript *runtime compiler*, while Java2Script is a Java to JavaScript *library compiler*.

Maybe GWT would be kind to support Java reflection and dynamic class loading, if more developers vote for such features.

Posted in Architecture | 561 Comments

IE Hack: JavaScript Memory Leaks

The following is an article that I posted about one and a half years ago. I just repost it here as this is a hack to build up Java2Script Pacemaker.


Yes, I knew there was JavaScript memory leak in IE. I developed JavaScript codes mainly by Firefox/Mozilla, so I won’t notice the codes were leaking memory. These days I was developing J2S application in Eclipse, and found that the J2S applications were runnig more and more slowly in the J2S Console, which embed IE Browser widget. Once I did not think that memory leaking will make such poor performance. The slowness that I couldn’t stand finally taught me the lesson and I decided to fix the leaking codes.

Searching the net, I found that lots of articles were talking about this problem. I mainly read Justin Rogers’ Understanding and Solving Internet Explorer Leak Patterns. And after finishing the articles, I monitored one J2S SWT application (SWT Tree Snippet) while tring to refresh the page. I found 5 times of refreshing will leaking the memory from about 22M to 31M! Then I knew that maybe about 100M+ of memories was leaked in developing J2S applications when I felt the slowness of J2S Console.

I added a handler to the “onunload” event to break those circular references, codes like following:

* Only IE need to release the resources so that no memory is leaked
if (window.attachEvent) {
window.attachEvent (“onunload”, function () {
try {
org.eclipse.swt.widgets.Display.releaseAllResources ();
} catch (e) {
return true;

In the static method org.eclipse.swt.widgets.Display.releaseAllResources, I set those elements’ on* handlers to null and set those Controls’ parent and children to null. Things did work much better. But it was not fixed compeletely. Once I refreshed for a couple of times, I noticed that the browser will leak some 10~100k gradually. That was to say it will take about 10~100 times of refreshing for about 1M leaking and users will only perceive the slowness after about 1000 times of refreshing. So it was considered as working better.
I tried other ways to solve this leaking but did not get the right entrance.The current Java2Script library is almost built on closures of JavaScript, which will leak memory easily and hide leadking codes deep inside the whole codes. Replacing all those codes that using closures? No. Without closures, it’s somewhat hard to implement the Java Class inheritance.

It’s as Justin Roger’s saying “not all memory leaks are easy to find”. Memory leaking is just awful enough! 🙁

Posted in Hacks | 33 Comments

Reusing: Spirit of Java2Script

The following is an article that I posted about one and a half years ago. I just repost it here, because I think Reusing Java codes and tools is my fundamental point to build Java2Script Pacemaker.


There are two advances of Java2Script technologies:

  1. Reusing existed Java codes
  2. Reusing existed Java tools

It’s totally about reusing. Reusing is the most important things in the programming world: source codes reusing, binary library reusing, model pattern reusing, framework reusing. Without reusing, it will mean that everyone should have to re-inhevent the wheel and then we will always stay in the stage of round wheels. Thanks to the reusing, we have great world now.

Java2Script Pacemaker is developed to give developers a toolkit of reusing existed codes and tools. And Java2Script Pacemaker is reusing other codes and tools:

  1. J2S is reusing Eclipse JDT (J2S won’t exist without JDT)
  2. J2S is reusing Java SDK’s java.* sources
  3. J2S is reusing SWT sources and models (org.eclipse.swt.*) and Visual Editor

Reusing does not mean that the sources or tools should be reused without modifications. In above reusings, I extend some extension points of JDT to given new functions, and I modify parts of the sources java.* so that the generated JavaScript is simple, and I have to reinvent the SWT Widgets inside Browser beside reusing the Events and Layouts of SWT.

Reusing already saves me a lot of time. Without JDT, I won’t have the abilities to parse the Java sources and building the DOM tree and binding. Without DOM tree and binding, I can’t generated the JavaScript codes. Without reusing java.* sources, I will have to spend times to re-write those complicated Map, Set and List implementations. Without org.eclipse.swt.layout.*, I will dedicate most of my time in tunning the layout! Without all these reusing, J2S and the related JavaScript version of SWT won’t come out in a short time. Thanks to the reusing.

Following is the questions and answers I am thinking these days:
Q: Does the Java2Script Pacemaker focus on reusing source codes only?
A: Always reusing source codes is the first of step of reusing others. But you should have already noticed that J2S is also reusing tools including Visual Editor and the Eclipse Platform.

Q: Is Java2Script Pacemaker have the abilities to reuse existed frameworks?
A: I think we should not make too much expectation on the JavaScript codes. But we can integrate the Java2Script Pacemaker with other existed frameworks, as Java2Script technology is still developing.

Q: Will Java2Script be hot in the future?
A: I don’t know. But I know, for sure, that reusing existed codes will save people’s efforts in the progress of converting desktop applications into web applications. And reusing will always be the spirit of programming.

Enjoy using Java2Script, enjoy reusing and keep thinking in reusing.

Posted in Articles | 1 Comment

“Inside Java2Script” is a Book

Hi, this blog is about a book “Inside Java2Script“.

I would like to write the book as documentation of Java2Script technical details. It is another challenge for me, besides opening source Java2Script project. Because I am not a native English speaker, I am not quite good at English. Writing this book, I have to learn a lot. Besides those technologies of Java2Script, I must learn more about English syntax rules, how a book should be arranged, and how a book is published. Lots of unknown for me. If you are familiar with writing a book or know how to write a successful tutorial or an article, pleas be kind to give me some advices or point out my mistakes. You can reach me by email: zhourenjian at gmail dot com.

The content of “Inside Java2Script” will focus on how Java2Script is built up, why Java2Script is designed into that pattern, key technologies of Java2Script’s core and things that kernel developers would like to know. How to use Java2Script or demo of Java2Script may not be this book’s major focus. I hope that by this book, developers may know inside out how Java2Script is working. And I hope that developers may develop the key spirits into other technologies or projects.

Just stay tuned for more articles. I always like to hear your opinions.

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About Inside Java2Script

Java2Script, after Java2Script was opened source for about one and a half year, documentation is becoming more and more important for its community, especially when Java2Script’s community is still very small. But currently, as lacking of developers and contributors, Java2Script is poor documented. The tutorials are out of date, and API documents are out of date. There are no forces to push documentation forward.

After some deep thoughts about Java2Script, I decided to write a book for Java2Script, called “Inside Java2Script”. This book, in original goal, should be a book to be sold. But it may result in an e-book without being published in papers. But I try to work out this book. At least it will be a valuable document for Java2Script technology.

Writing a book is not an easy job for me. Yet, I have never written a book in Chinese, my mother tongue yet. And now I am going to write a book in English, my foreign language. It may cost me times. If you would like to help me or give me advices, you are kind to contact me by email: zhourenjian at gmail dot com.

If you would like to donating your money for my efforts on open source Java2Script project, you can send money by PayPal to my account: zhourenjian at gmail dot com.

Posted in Uncategorized | 77 Comments

About Zhou Renjian

Zhou Renjian, the author of this blog, is also the initial contributor of open source Java2Script project. He is from Shanghai, China. He has years of Java and JavaScript experiences. He began programming about 10 years ago, back to 1997.

Posted in Uncategorized | 1 Comment