Copy-on-Write in Java

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244 Copy-on-Write
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When a set of fields comprising the state of an object must maintain a set of interrelated invariants, you can isolate these fields in another object that preserves the intended semantic guarantees
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A good way to go about this is to rely on immutable representation objects that at all times maintain consistent snapshots of legal object states Relying on immutability eliminates the need to otherwise coordinate separate readings of related attributes It also normally eliminates the need to hide these representations from clients For example, in 1111, we had to take special precautions involving block synchronization in order to guarantee that consistent (x, y) coordinates of Particles were always displayed correctly And the Shape classes described in 242 do not even provide a mechanism for doing this One solution is to employ a separate ImmutablePoint class that maintains location information that is at all times consistent:
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class ImmutablePoint { private final int x; private final int y; public ImmutablePoint(int initX, int initY) { x = initX; y = initY; } public int x() { return x; } public int y() { return y; } } ImmutablePoints could be used in the following Dot class that is otherwise similar to the Particle class in 1111 This class illustrates the general techniques surrounding copy-on-write
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updates, in which state changes do not directly update fields, but instead construct and attach new representation objects Note that synchronization of some form is required here Even though the point representation objects are immutable, the loc reference is mutable While synchronization of the accessor method location might be loosened in accord with the considerations in 241, the shiftX method must be synchronized (or perhaps otherwise modified) in order to preclude multiple concurrent executions in which different versions of loc are obtained when accessing locx() and locy()
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class Dot { protected ImmutablePoint loc; public Dot(int x, int y) { loc = new ImmutablePoint(x, y); } public synchronized ImmutablePoint location() { return loc; } protected synchronized void updateLoc(ImmutablePoint newLoc) { loc = newLoc; }
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public void moveTo(int x, int y) { updateLoc(new ImmutablePoint(x, y)); } public synchronized void shiftX(int delta) { updateLoc(new ImmutablePoint(locx() + delta, locy())); } }
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2441 Internal copy-on-write When state representations are strictly internal to an object, there is no compelling reason to create new classes just to enforce immutable access Copy-on-write can be applied whenever the need to obtain consistent representations quickly and effortlessly overwhelms construction costs It requires at most one synchronized operation to access all of the state held by an immutable representation object Additionally, in some contexts, it is convenient to obtain a single snapshot rather than one that reflects any state modifications made during the use of that snapshot For example, copy-on-write collection objects can be very useful for maintaining collections of listeners in event and multicast frameworks (see 41) Here objects maintain lists of listeners or handlers that must receive notifications of state changes or other events of interest These lists rarely change, but may be traversed very frequently Also, when objects receiving notifications make changes in the list of notifyees, they are almost always intended to take effect the next time a notification is issued, not in the current round While there are other good choices for the underlying data structure (including the special-purpose tree-based structure used in javaawtEventMulticaster, and more elaborate structures maintaining edit-records from a common base), an array-based copy-on-write collection class is suitable for most applications Traversal via iterators is not only fast but also avoids ConcurrentModificationExceptions that can occur in some other approaches to traversal (see 223)
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class CopyOnWriteArrayList { protected Object[] array = new Object[0];
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