Day One Android has the option to enable a passcode or use a fingerprint sensor to unlock the app.
Tap Settings > Passcode to get started
Create a new 4-digit passcode
Now you will see the customizable options for Passcode and Biometrics. The options are:
Change Passcode - Change the passcode to a different 4-digit passcode
Require After... which allows you to set when the passcode will be required
Fingerprint - If your device has a fingerprint sensor and you have a fingerprint registered, you can use this option to unlock your journal