This MessageDigest class provides applications the functionality of a
message digest algorithm, such as MD5 or SHA.
Message digests are secure one-way hash functions that take arbitrary-sized
data and output a fixed-length hash value.
A MessageDigest object starts out initialized. The data is
processed through it using the update
methods. At any point reset
can be called
to reset the digest. Once all the data to be updated has been
updated, one of the digest
methods should
be called to complete the hash computation.
The digest
method can be called once for a given number
of updates. After digest
has been called, the MessageDigest
object is reset to its initialized state.
Implementations are free to implement the Cloneable interface.
Client applications can test cloneability by attempting cloning
and catching the CloneNotSupportedException:
MessageDigest md = MessageDigest.getInstance("SHA");
try {
md.update(toChapter1);
MessageDigest tc1 = md.clone();
byte[] toChapter1Digest = tc1.digest();
md.update(toChapter2);
...etc.
} catch (CloneNotSupportedException cnse) {
throw new DigestException("couldn't make digest of partial content");
}
Note that if a given implementation is not cloneable, it is
still possible to compute intermediate digests by instantiating
several instances, if the number of digests is known in advance.
Note that this class is abstract and extends from
MessageDigestSpi
for historical reasons.
Application developers should only take notice of the methods defined in
this MessageDigest
class; all the methods in
the superclass are intended for cryptographic service providers who wish to
supply their own implementations of message digest algorithms.