are organized into numeric ranges of 100, and the 200 range indicates success. The server's
response code is validated for being in the 200 range by calling connection.getResponseCode( )
and dividing by 100. If the value of this division is 2, then the connection was successful.
Next, run( ) gets the content length by calling connection.getContentLength( ). The
content length represents the number of bytes in the requested file. If the content length is
less than 1, the error( ) method is called. The error( ) method updates the download's status
to ERROR, and then calls stateChanged( ). The stateChanged( ) method will be described
in detail later.
After getting the content length, the following code checks to see if it has already been
assigned to the size variable:
/* Set the size for this download if it
hasn't been already set. */
if (size == -1) {
size = contentLength;
stateChanged();
}
As you can see, instead of assigning the content length to the size variable unconditionally,
it only gets assigned if it hasn't already been given a value. The reason for this is because
the content length reflects how many bytes the server will be sending. If anything other than
a 0-based start range is specified, the content length will only represent a portion of the file's
size. The size variable has to be set to the complete size of the download's file.
The next few lines of code shown here create a new RandomAccessFile using the filename
portion of the download's URL that is retrieved with a call to the getFileName( ) method:
// Open file and seek to the end of it.
file = new RandomAccessFile(getFileName(url), "rw");
file.seek(downloaded);
The RandomAccessFile is opened in "rw" mode, which specifies that the file can be
written to and read from. Once the file is open, run( ) seeks to the end of the file by calling
the file.seek( ) method, passing in the downloaded variable. This tells the file to position
itself at the number of bytes that have been downloaded--in other words, at the end. It's
necessary to position the file at the end in case a download has been resumed. If a download
is resumed, the newly downloaded bytes are appended to the file and they don't overwrite
any previously downloaded bytes. After preparing the output file, a network stream handle
to the open server connection is obtained by calling connection.getInputStream( ), as
shown here:
stream = connection.getInputStream();
The heart of all the action begins next with a while loop:
while (status == DOWNLOADING) {
/* Size buffer according to how much of the
file is left to download. */
byte buffer[];
if (size - downloaded > MAX_BUFFER_SIZE) {
buffer = new byte[MAX_BUFFER_SIZE];
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