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+ Improves performance Is easily accessible from anywhere In many cases, this may indicate a design flaw (see Inline Singleton, 114) Is not useful when an object has state that can't be shared
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Mechanics
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Before you perform this refactoring, make sure the object you want to turn into a Singleton has no state or has state that is sharable Because most classes that end up becoming Singletons have one constructor, these mechanics assume you have one constructor on your class 1 Identify a multiple instance class, a class that gets instantiated more than once by one or more clients Apply the mechanics from Replace Constructors with Creation Methods (57) even though your class has only one constructor The return type for your new Creation Method should be the multiple instance class Compile and test
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2 Declare a singleton field, a private static field of type multiple instance class in the multiple instance class and, if possible, initialize it to an instance of the multiple instance class It may not be possible to initialize this field because to do so, you need arguments passed by a client at runtime In that case, simply define the field and don't initialize it Compile 3 Make your Creation Method return the value in the singleton field If it must be lazily instantiated, do that lazy instantiation in the Creation Method, based on whatever parameters are passed in Compile and test
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Example
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This example is based on the security code example found in the refactoring Replace State-Altering Conditionals with State (166) If you study the code produced after applying that refactoring, you'll find that each State instance is a Singleton However, these Singleton State instances weren't created for performance reasons; they resulted from performing the refactoring Replace Type Code with Class (286) When I initially refactored to the State pattern on the security code project, I did not apply Replace Type Code with Class (286) I wasn't yet aware of how much that refactoring simplifies the later steps in refactoring to the State pattern My earlier approach to the State refactoring involved instantiating Permission subclasses each time they were needed, paying no regard to the Singleton pattern On that project, my colleague and I profiled our code and found several places where it could be optimized One of those places involved the frequent instantiation of the state classes So, as part of an overall effort to improve performance, the code to repeatedly instantiate the Permission subclasses was refactored to use the Singleton pattern I describe the steps here
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1 There are six State classes, each of which is a multiple instance class because clients instantiate them multiple times In this example, I'll work with the PermissionRequested class, which looks like this:
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public class PermissionRequested extends Permission { public static final String NAME= "REQUESTED"; public String name() { return NAME; } public void claimedBy(SystemAdmin admin, SystemPermission permission) { permissionwillBeHandledBy(admin); permissionsetState(new PermissionClaimed()); } }
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PermissionRequested doesn't define a constructor because it uses Java's default constructor Because the first step in the mechanics is to convert its constructor(s) to Creation Methods, I define a Creation Method like so:
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public class PermissionRequested extends Permission
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public static Permission state() { return new PermissionRequested();
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You'll notice that I usePermission as the return type for this Creation Method I do that because I want all client code to interact with State subclasses via the interface of their superclass I also update all callers of the constructor to now call the Creation Method:
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public class SystemPermission private Permission state; public SystemPermission(SystemUser requestor, SystemProfile profile) { thisrequestor = requestor; thisprofile = profile;
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state = new PermissionRequested(); state = PermissionRequestedstate();
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}
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I compile and test to make sure that this trivial change didn't break anything 2 Now I create the singleton field, a private static field of typePermission in PermisionRequested, and initialize it to an instance of PermissionRequested:
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