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Persisting an application's data

Keeping state between two launches: text file, CSV or JSON, object serialisation, preferences, embedded database; when to load and when to save; a model that ignores its storage.

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  • Persistence: what is it?

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    The property of data that outlives the program's shutdown because it is written to durable storage.

    A variable or an object exists only in working memory: on exit, everything disappears. A file, preferences or a database keep the state from one launch to the next.

  • What is the exact content of the file taches.json after this program runs?

    ```python
    import json

    taches = [{"titre": "Acheter du pain", "faite": False}, {"titre": "Appeler Ada", "faite": True}]
    with open("taches.json", "w") as f:
    json.dump(taches, f)
    `

    • [{"titre": "Acheter du pain", "faite": false}, {"titre": "Appeler Ada", "faite": true}]
    • [{"titre": "Acheter du pain", "faite": "false"}, {"titre": "Appeler Ada", "faite": "true"}]
    • [{'titre': 'Acheter du pain', 'faite': false}, {'titre': 'Appeler Ada', 'faite': true}]
    • {"titre": "Acheter du pain", "faite": false}, {"titre": "Appeler Ada", "faite": true}
    Show the answer

    [{"titre": "Acheter du pain", "faite": false}, {"titre": "Appeler Ada", "faite": true}]

    json.dump translates Python types into JSON: False becomes false, True becomes true, keys and strings get double quotes, and the list becomes an array in square brackets, with no line break by default.

  • State is loaded at … and saved as soon as it changes, not only on …: a crash or a sudden kill would prevent that last save.

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    State is loaded at startup and saved as soon as it changes, not only on exit: a crash or a sudden kill would prevent that last save.

    Saving at each significant change, or at regular intervals, limits what is lost in an incident.

  • Serialisation: what is it?

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    Turning an object into a sequence of bytes or text that can be written to a file or sent over a network.

    It picks a format: JSON is readable and language-independent; native Java serialisation produces bytes tied to the class.

  • A user's preferences are saved with Java serialisation, but their theme field, marked transient, comes back as null at every launch. What must be done to keep it?

    • Remove transient from the field, since that is what forbids writing it
    • Declare the field static, so it is shared between launches
    • Call the class constructor after the file is read back
    • Make the class implement Serializable, which it does not yet
    Show the answer

    Remove transient from the field, since that is what forbids writing it

    A transient field is deliberately skipped by serialisation. A static field does not belong to the object and is not written either; and the class is already serialised, since the rest is read back.

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