Java IO.pdf - Nguyen Dang Binh

public void setMessageDigest(MessageDigest digest)
You can retrieve the message digest at any time by calling getMessageDigest():
public MessageDigest getMessageDigest()
Java I/O
After you invoke getMessageDigest(), the digest field contains the digest of all the data
written by the stream up to that point. However, it has not been finished. It is still necessary to
invoke digest() to complete the calculation. For example:
MessageDigest md = dout.getMessageDigest();
md.digest();
On rare occasions, you may only want to digest part of a stream. For instance, you might want
to calculate the digest of the body of an email message while ignoring the headers. You can
turn digesting off at any point by passing false to the on() method:
public void on(boolean on)
You can turn digesting back on by passing true to on(). When digest output streams are
created, they are on by default.
Finally, there's a toString() method, which is a little unusual in output streams. It simply
returns "[Digest Output Stream]" plus the string representation of the digest.
public String toString()
Example 10.3 is a FileDigestOutputStream class that reads data from a specified URL and
copies it into a file on the local system. As the file is written, its SHA digest is calculated.
When the file is closed, the digest is printed.
Example 10.3. FileDigestOutputStream
import java.net.*;
import java.io.*;
import java.security.*;
public class FileDigest {
public static void main(String[] args) {
if (args.length != 2) {
System.err.println("Usage: java FileDigest url filename");
return;
}
try {
URL u = new URL(args[0]);
FileOutputStream out = new FileOutputStream(args[1]);
copyFileWithDigest(u.openStream(), out);
out.close();
}
catch (MalformedURLException e) {
System.err.println(args[0] + " is not a URL");
}
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