Read File Contents (Blobs) in GWT and Gears

What do you do if you want to read the contents of a file in your GWT application? One possible solution is to use Google Gears.

Gears offers a method to allow a user to select file(s). You can then use the returned "getFiles" method of the "OpenFilesEvent" object to find out what file(s) the user selected. Each "File" class has a method called "getBlob" that returns a "Blob" object. However, each blob object doesn't provide any direct way to access the blobs contents. Many people haven't found a way to read the blob contents.

I wrote a class to access the contents of the blob. The class is called "BlobReader" and is very simple to use. The trick lies in the fact that in Javascript we can access the "getBytes" method of the blob.

Here is an example reading line by line of a text file:

try {
    Desktop dt = Factory.getInstance().createDesktop();
    dt.openFiles(new OpenFilesHandler(){
        public void onOpenFiles(OpenFilesEvent event) {
            File[] files = event.getFiles();
            File file = files[0];
            Blob data = file.getBlob();

            BlobReader br = new BlobReader(data);
            while (!br.endOfBlob())
        }, true);
} catch (Exception ex){

Here is

package com.yourpackage.client;

public class BlobReader {

    private final int CHUNK_SIZE = 1024;

    public BlobReader(Blob blob){
        this.blob = blob;

    private Blob blob;
    public Blob getBlob(){ return this.blob; }

    private int blobPointer = 0;
    public int getBlobPointer(){ return this.blobPointer; }

    public boolean endOfBlob(){
        return (blobPointer >= blob.getLength());

    public byte[] readAllBytes(){
        return getBlobBytes(this.blob);

    public String readAllText(){
        int size = this.blob.getLength();
        StringBuilder sb = new StringBuilder(size);

        JsArrayInteger bytes = getBlobBytes(this.blob, 0, size);
        for (int x = 0; x < size; x++)

        return sb.toString();

    public String readLine(){
        int size = CHUNK_SIZE;
        boolean foundEndOfLine = false;

        JsArrayInteger bytes;
        StringBuilder sb = new StringBuilder(CHUNK_SIZE);

        while (!foundEndOfLine && size > 0){
            int dif = this.blob.getLength() - this.blobPointer; //remainder of the data
            if (dif < CHUNK_SIZE)
                size = dif;
            bytes = getBlobBytes(this.blob, this.blobPointer, size);

            for (int x = 0; x < size; x++){
                int b = bytes.get(x);
                if (b == 10){ //newline
                    foundEndOfLine = true;

                if (this.getBlobPointer() % 1000000 == 0)
                    Window.alert("" + this.getBlobPointer());

        if (sb.substring(sb.length() - 1) == "\r")
            sb.deleteCharAt(sb.length() - 1);

        return sb.toString();

       public void reset(){
        this.blobPointer = 0;

    private byte[] getBlobBytes(Blob b){
        JsArrayInteger array = getBlobBytes(b, 0, b.getLength());
        byte[] bytes = new byte[array.length()];
        for (int x = 0; x < bytes.length; x++)
            bytes[x] = (byte) array.get(x); //in google docs we are told these are from 0-255

        return bytes;

    private native JsArrayInteger getBlobBytes(Blob b, int offset, int length) /*-{
        return b.getBytes(offset, length);

Notes: 1) This hasn't been thoroughly tested/debugged. 2) At the time of this writing, I'm using Gears 0.5 with the GWT-Gears 1.2.1. In the current GWT-Gears svn trunk you can see upcoming support for the GWT methods "getNativeBytes" and "getBytes"

If you made it this far, you should follow me on Twitter.


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