URLConnections - Java Network Programming - page 194

byte [] data = new byte [ contentLength ];
int offset = 0 ;
while ( offset < contentLength ) {
int bytesRead = in . read ( data , offset , data . length - offset );
if ( bytesRead == - 1 ) break ;
offset += bytesRead ;
}
if ( offset != contentLength ) {
throw new IOException ( "Only read " + offset
+ " bytes; Expected " + contentLength + " bytes" );
}
String filename = u . getFile ();
filename = filename . substring ( filename . lastIndexOf ( '/' ) + 1 );
try ( FileOutputStream fout = new FileOutputStream ( filename )) {
fout . write ( data );
fout . flush ();
}
}
}
}
As usual, the main() method loops over the URLs entered on the command line, passing
each URL to the saveBinaryFile() method. saveBinaryFile() opens a URLConnec
tion uc to the URL . It puts the type into the variable contentType and the content length
into the variable contentLength . Next, an if statement checks whether the content type
is text or the Content-length field is missing or invalid ( contentLength == -1 ). If
either of these is true , an IOException is thrown. If these checks are both false , you
have a binary file of known length: that's what you want.
Now that you have a genuine binary file on your hands, you prepare to read it into an
array of bytes called data . data is initialized to the number of bytes required to hold the
binary object, contentLength . Ideally, you would like to fill data with a single call to
read() but you probably won't get all the bytes at once, so the read is placed in a loop.
The number of bytes read up to this point is accumulated into the offset variable, which
also keeps track of the location in the data array at which to start placing the data
retrieved by the next call to read() . The loop continues until offset equals or exceeds
contentLength ; that is, the array has been filled with the expected number of bytes. You
also break out of the while loop if read() returns -1, indicating an unexpected end of
stream. The offset variable now contains the total number of bytes read, which should
be equal to the content length. If they are not equal, an error has occurred, so saveBi
naryFile() throws an IOException . This is the general procedure for reading binary
files from HTTP connections.
Now you're ready to save the data in a file. saveBinaryFile() gets the filename from
the URL using the getFile() method and strips any path information by calling file
name.substring(theFile.lastIndexOf( / ) + 1) . A new FileOutputStream fout is