From:

Ident.: DSC-25-11-10-00001006.0001001 / 09 OCT 12
Applicable to: MSN 2649-3685, 4670-5367

A forward-opening hinge door separates the cockpit from the passenger compartment. It has three
electric locking strikes, controlled by the flight crew. In normal conditions, when the door is closed,
they remain locked. When there is a request to enter the cockpit, the flight crew can authorize entry
by unlocking the door, that remains closed until it is pushed open.
When the flight crew does not respond to requests for entry, the door can also be unlocked by
the cabin crew, by entering a two to seven-digit code (programmed by the airline) on the keypad,

installed on the lateral side of the Forward Attendant Panel (FAP).

More:

http://nicmosis.as.arizona.edu:8000/ECLIPSE_WEB/TSE2015/A320_DOCUMENTS/A320_Operating_Manual_PDF_N_FCOM_RJA_TF_N_EU__20130329_DSC_25.pdf

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They cannot do that if the plane computer was taken over by the hijack unit. In that case pilot has no control over the plane, radio com or any part of the plane, including the door.

There were 2 military jets flying by Germanwings until it crashed into the Alps after a long, even, computer guided, descend. Both had the hardware to drive Germanwings w/o pilot.
That pilot that banged on the door, knowing that he is about to die.

IZAKOVIC