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Garbage collector

Automatic memory release: object reachable from the roots, reclaimable object, mark and sweep, reference counting and cycles, leaks through a reference kept by accident.

  • 29 cards
  • 5 topics
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  • Garbage collector: what is it?

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    The runtime mechanism that automatically frees the memory of objects the program can no longer use.

    In Java and Python, you write no free. The timing of a pass is up to the virtual machine: System.gc() is only a suggestion, with no guarantee.

  • After this code, which objects are reclaimable?

    ```java
    Noeud a = new Noeud("A");
    Noeud b = new Noeud("B");
    Noeud c = new Noeud("C");
    a = b;
    c = null;
    `

    • Objects A and C
    • Object C only, since A is still referred to
    • Object A only, since C was set to null
    • Objects B and C, since B was copied
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    Objects A and C

    After a = b, variables a and b both refer to object B, and nothing refers to object A any more. c = null cuts the only reference to object C. B stays reachable.

  • An object is … when no path of references links it to a … any more.

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    An object is reclaimable when no path of references links it to a root any more.

    That is the only criterion: it does not matter whether the object still has references to other objects, or whether it is referred to by an object that is itself unreachable.

  • Root: what is it?

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    A starting point of the program where a reference is directly usable: local variables of the running methods, static attributes.

    Everything you can reach by following references from a root is alive. An object no root can reach is unusable.

  • One mark-and-sweep pass: which step comes after "Find the program's roots"?

    • Mark the objects the roots refer to
    • Follow the marked objects' references and mark what they reach
    • Go through the heap looking for unmarked objects
    • Free the unmarked objects
    Show the answer

    Mark the objects the roots refer to

    Sweeping can only begin once marking is finished: while references remain to follow, an unmarked object might still be alive.

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