Java IO.pdf - Nguyen Dang Binh

}
}
catch (Exception e) {System.err.println(e);}
public static void copyFileWithDigest(InputStream in, OutputStream out)
throws IOException, NoSuchAlgorithmException {
}
MessageDigest sha = MessageDigest.getInstance("SHA");
DigestOutputStream dout = new DigestOutputStream(out, sha);
byte[] data = new byte[128];
while (true) {
int bytesRead = in.read(data);
if (bytesRead < 0) break;
dout.write(data, 0, bytesRead);
}
dout.flush();
byte[] result = dout.getMessageDigest().digest();
for (int i = 0; i < result.length; i++) {
System.out.print(result[i] + " ");
}
System.out.println();
A sample run looks like this:
% java FileDigest http://www.oreilly.com/ oreilly.html
10 -10 103 -27 -110 3 -2 -115 8 -112 13 19 25 76 -120 31 51 116 -94 -58
Java I/O
To be perfectly honest, I'm not sure if DigestOutputStream is all that useful. You still need
to construct the MessageDigest object, pass it to the DigestOutputStream() constructor,
retrieve the MessageDigest object from the output stream, invoke its digest() method, and
retrieve the digest data from that object. The only real reason I can think of to use
DigestOutputStream would be if you needed a digest in the middle of a chain of filter
streams. For instance, you could write data onto a data output stream chained to a gzip output
stream chained to a file output stream. When you had finished writing the data onto the data
output stream, you could calculate the digest and write that directly onto the file output
stream. When the data was read back in, you could use a digest input stream chained to a data
input stream to check that the file had not been corrupted in the meantime. If the digest
calculated by the digest input stream matched the digest stored in the file, you'd know the data
was OK.
I would prefer the DigestOutputStream to completely hide the MessageDigest object. You
could pass the name of the digest algorithm to the constructor rather than an actual
MessageDigest object. The digest would only be made available after the stream was closed,
and then only through its data, not through the actual object. Example 10.4 demonstrates how
such a class might be implemented.
Example 10.4. EasyDigestOutputStream
package com.macfaq.security;
import java.io.*;
import java.security.*;
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