Create MessageDigest with Guice

Question

I'm trying to find a way to initialize MessageDigest with Guice. Currently I have this:

public class MyClass {

    MessageDigest md;
    
    public MyClass() {
        try {
            md = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
    }
....
}

I'm trying to see if it can written as:

@Inject
MessageDigest md;

But how do I make Guice call MessageDigest.getInstance("MD5") ?

Answer 1

MessageDigest is a very special class. All its instances are one-use only. Therefore it must be injected very carefully.

First, you'll have to tell Guice to create multiple instances of the class. This is done as below in your Module class.

@Provides
MessageDigest provideMD5() throws NoSuchAlgorithmException {
  return MessageDigest.getInstance("MD5");
}

Then, once you have your provider that can create instances everytime, it's injected, you might not actually want only one instance per injected instance. So you will probably have to do the following:

@Inject
Provider<MessageDigest> md5Provider;

...

// Later in the class
MessageDigest md5 = md5Provider.get();