Item 35: Prefer interfaces to reflection in Java

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Item 35: Prefer interfaces to reflection
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The reflection facility, javalangreflect, offers programmatic access to information about loaded classes Given a Class instance, you can obtain Constructor, Method, and Field instances representing the constructors, methods, and fields of the class represented by the
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Effective Java: Programming Language Guide Class instance These objects provide programmatic access to the class's member names, field types, method signatures, and so on
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Moreover, Constructor, Method, and Field instances let you manipulate their underlying counterparts reflectively: You can construct instances, invoke methods, and access fields of the underlying class by invoking methods on the Constructor, Field, and Method instances For example, Methodinvoke lets you invoke any method on any object of any class (subject to the usual security constraints) Reflection allows one class to use another, even if the latter class did not exist when the former was compiled This power, however, comes at a price:
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You lose all the benefits of compile-time type checking, including exception checking If a program attempts to invoke a nonexistent or inaccessible method reflectively, it will fail at run time unless you've taken special precautions The code required to perform reflective access is clumsy and verbose It is tedious to write and difficult to read Performance suffers As of release 13, reflective method invocation was forty times slower on my machine than normal method invocation Reflection was rearchitected in release 14 for greatly improved performance, but it is still twice as slow as normal access, and the gap is unlikely to narrow
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The reflection facility was originally designed for component-based application builder tools Such tools generally load classes on demand and use reflection to find out what methods and constructors they support The tools let their users interactively construct applications that access these classes, but the generated applications access the classes normally, not reflectively Reflection is used only at design time As a rule, objects should not be accessed reflectively in normal applications at run time There are a few sophisticated applications that demand the use of reflection Examples include class browsers, object inspectors, code analysis tools, and interpretive embedded systems Reflection is also appropriate for use in RPC systems to eliminate the need for stub compilers If you have any doubts as to whether your application falls into one of these categories, it probably doesn't You can obtain many of the benefits of reflection while incurring few of its costs by using it only in a very limited form For many programs that must use a class unavailable at compile time, there exists at compile time an appropriate interface or superclass by which to refer to the class (Item 34) If this is the case, you can create instances reflectively and access them normally via their interface or superclass If the appropriate constructor has no parameters, as is usually the case, then you don't even need to use the javalangreflect package; the ClassnewInstance method provides the required functionality For example, here's a program that creates a Set instance whose class is specified by the first command line argument The program inserts the remaining command line arguments into the set and prints it Regardless of the first argument, the program prints the remaining arguments with duplicates eliminated The order in which these arguments are printed depends on the class specified in the first argument If you specify javautilHashSet, they're printed in apparently random order; if you specify javautilTreeSet, they're printed in alphabetical order, as the elements in a TreeSet are sorted:
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Effective Java: Programming Language Guide // Reflective instantiation with interface access public static void main(String[] args) { // Translate the class name into a class object Class cl = null; try { cl = ClassforName(args[0]); } catch(ClassNotFoundException e) { Systemerrprintln("Class not found"); Systemexit(1); } // Instantiate the class Set s = null; try { s = (Set) clnewInstance(); } catch(IllegalAccessException e) Systemerrprintln("Class not Systemexit(1); } catch(InstantiationException e) Systemerrprintln("Class not Systemexit(1); }
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// Exercise the set saddAll(ArraysasList(args)subList(1, argslength-1)); Systemoutprintln(s); }
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While this program is just a toy, the technique that it demonstrates is very powerful The toy program could easily be turned into a generic set tester that validates the specified Set implementation by aggressively manipulating one or more instances and checking that they obey the Set contract Similarly, it could be turned into a generic set performance analysis tool In fact, the technique that it demonstrates is sufficient to implement a full-blown service provider framework (Item 1) Most of the time, this technique is all that you need in the way of reflection You can see two disadvantages of reflection in the example First, the example is capable of generating three run-time errors, all of which would have been compile-time errors if reflective instantiation were not used Second, it takes twenty lines of tedious code to generate an instance of the class from its name, whereas a constructor invocation would fit neatly on a single line These disadvantages are, however, restricted to the part of the program that instantiates the object Once instantiated, it is indistinguishable from any other Set instance In a real program, the great bulk of the code is thus unaffected by this limited use of reflection A legitimate, if rare, use of reflection is to break a class's dependencies on other classes, methods, or fields that may be absent at run time This can be useful if you are writing a package that must run against multiple versions of some other package The technique is to compile your package against the minimal environment required to support it, typically the oldest version, and to access any newer classes or methods reflectively To make this work, you have to take appropriate action if a newer class or method that you are attempting to access does not exist at run time Appropriate action might consist of using some alternate means to accomplish the same goal or operating with reduced functionality
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