Jochen
I conducted extensive tests with various Base64 encoding libraries:
Test code:
public static void main(String [] args) throws IOException { File file = new File("./resources/so.png"); BufferedInputStream bufRead = new BufferedInputStream(new FileInputStream(file)); ByteBuffer buffer = ByteBuffer.allocate(30*1024) ;
Conclusions:
(gwt-base64) Doesn't work at all. This failed: java.lang.StringIndexOutOfBoundsException: String index out of range on every image I tried. Please note that I commented on the code.
(google) The Base64 encoding that it creates cannot be understood by the browser.
(apache) . Works with this constructor: new Base64 (-1) = no output lines, no URLs.
Credit point . You can do the work of implementing google if you change the last 2 characters of the character map: '$', '_' to '+', '/'.
My main conclusion is that the Base64 library you are using is an error. I suggest that when searching for alternative implementations try to keep the byte stream between the image and the base64 encoder in binary format (byte []). A string can work if the same encoding / decoding is used, but it is risky if encoding and decoding are performed in two different places (for example, client / server).
Good luck
PS: Try the following :-)
<img src="data:unknown;base64,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"/>
Edit: Further research
I spent more time than reasonably trying to find a solution. The GWT wrapper will not allow you to get raw data, but you can use JSNI to force the browser to get binary data:
native String getBinaryResource(String url) /*-{ // ...implemented with JavaScript var req = new XMLHttpRequest(); req.open("GET", url, false); // The last parameter determines whether the request is asynchronous -> this case is sync. req.overrideMimeType('text/plain; charset=x-user-defined'); req.send(null); if (req.status == 200) { return req.responseText; } else return null }-*/;
Love to crack a hard nut, but I applied all this, but, however, base64 encoding does not work. JS and Java wrappers still have an encoding problem, and I can't get the string to decode back to the correct byte []. Tried all possible coding combinations. A possible way would be to use base64 req.responseText using your own javascript library and return String to a java copy.
So far for your initial question.
Now, looking at some alternative ideas and requirements behind your question: during my research, I saw that Base64 is very often used for embedded images on the server side , to avoid the additional HTTP overhead to get images on the client. This seems to be a popular alternative to CSS-inlining.
In the context of this question, the code works with the client (browser), and these reasons do not apply. Using the basic XMLHTTPRequest to retrieve the binary image file creates an additional HTTP request from your browser to the server. Given that in the client context, you obviously have the image URL (passed to IoProvider.get().makeRequest(*url*,...) ), there is a reason why we could not allow the browser to do the job with a simple dynamic update if the image object:
(this may not be the best code sample, but it gives you an image (sorry for the pun ;-))
void setImage(String url) { final HTML imageHolder = new HTML(); String imgTag = "<IMG src='"+url+"' />'"; imageHolder.setHTML(imgTag); RootPanel.get("imageContainer").add(imageHolder);
Edit: nut cracked
Finally, I found the last missing piece of the puzzle: how to correctly decode binary data from a JavaScript string. Note some caveats: it will not work in IE if they do not support the overrideMimeType method in native XMLHTTPRequest.
native String getBinaryResource(String url) /*-{ var req = new XMLHttpRequest(); req.open("GET", url, false); // The last parameter determines whether the request is asynchronous. req.overrideMimeType('text/plain; charset=x-user-defined'); req.send(null); if (req.status == 200) { return req.responseText; } else return null }-*/; private void sendRequestBinary() { String url = URL.encode("/Computer_File_030.gif"); String data = getBinaryResource(url); if (data != null) { // The secret sauce: Method to decode the binary data in the response string byte[] binData = new byte[data.length()]; for (int i=0;i<data.length();i++) { binData[i] = (byte)(data.charAt(i) & 0xff); } final HTML imageHolder = new HTML(); String base64=Base64Utils.toBase64(binData); String imgTag = "<IMG src='data:image/gif;base64,"+base64+"' />'"; imageHolder.setHTML(imgTag); RootPanel.get("imageContainer").add(imageHolder); errorLabel.setText("Base64:"); } else { errorLabel.setText("Another error :-("); } }