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Editorial Team
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Asked: 2026-06-11T15:10:51+00:00

In GWT web/production mode, Java code is complied into Javascript code that is rendered

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In GWT web/production mode, Java code is complied into Javascript code that is rendered in the browser.

Also,I have always thought that in GWT development mode, GWT developer plugin compiles my Java code into JavaScript to render it in the browser. But after reading on some site, I came to know that there’s no compiling of code to JavaScript to view it in the browser in development mode.

So, I wonder: What are all these widgets I see in the browser during this mode if they aren’t JavaScript code?. I don’t understand it.

Please help understanding this.

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1 Answer

  1. Editorial Team

    The crux of the Dev Mode is that your code runs in Java. This is a prerequisite if you can use a standard Java debugger. You’ll find a high-level overview in the GWT documentation.

    The magic happens with JSNI methods and overlay types: when a class is loaded, all its JSNI methods are extracted and their JS body is sent to the browser, ready to be executed (as JavaScript then), and the class is rewritten on the fly to reimplement the JSNI method to make a call to the browser (via the Dev Plugin you installed there and is triggered by ?gwt.codesvr= in the URL) to execute the corresponding JS function. This is the reason why Java objects are seen as opaque handles in JSNI methods; they’re assigned a numeric ID to pair the Java object with a dummy JS object on the server-side. A similar though more complex rewriting is done for overlay types, and the same ID mapping is used when JS objects are passed to Java code (as overlay types).

    BTW, Super Dev Mode compiles to JavaScript (almost) on the fly.

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