Bonus Example: A Reflection Utility in Java

Drawing Data Matrix in Java Bonus Example: A Reflection Utility

The preceding example shows how to use reflection to display the capabilities of a specific class, but it is simple to generalize the procedure Because of the streamlined design of the reflection API, information can easily be obtained about any class This example shows

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one approach It creates a short utility program called ShowClass that displays the constructors, methods, and fields declared by the class (or interface) specified on the command line (Of course, an interface won t have any constructors) Therefore, the utility lets you see the members of any class or interface This can be helpful when debugging, or for exploring the capabilities of components for which no source code or documentation is available Here is the program:

// A simple, yet effective reflection utility that shows // the constructors, methods, and fields for a class or // interface specified on the command line import javalangreflect*; class ShowClass { public static void main(String args[]) { try { // Get a Class object for the specified class or interface Class c = ClassforName(args[0]); Systemoutprintln("Reflecting " + args[0] + "\n"); // Obtain and display information about the constructors, // methods, and fields Systemoutprintln("Constructors:"); Constructor constructors[] = cgetDeclaredConstructors(); for(Constructor cons : constructors) Systemoutprintln(" " + cons); Systemoutprintln("\nMethods:"); Method methods[] = cgetDeclaredMethods(); for(Method meth : methods) Systemoutprintln(" " + meth); Systemoutprintln("\nFields:"); Field fields[] = cgetDeclaredFields(); for(Field fld : fields) Systemoutprintln(" " + fld); } catch(ArrayIndexOutOfBoundsException exc) { Systemoutprintln("Usage: ShowClass <classname>"); } catch(Exception exc2) { // Handle all other exceptions You can add more // sophisticated exception handling if you like Systemoutprintln(exc2); } } }

To use the program, specify the fully qualified name of the class or interface on the command line For example, to see the members of javalangObject, use this command line:

In addition to the use of forName( ), there are various other ways to obtain a Class object For example, given an object of the class, you can use getClass( ), which is defined by Object It returns a Class object that represents the class type of the invoking object Another way to obtain a class object is to use a class literal For example, here is another way to obtain a Class object that represents MyClass:

Of course, this approach requires that you know the name of the class at compile time The forName( ) method lets you specify the name of the class at runtime, which can be important in some situations As explained, the getDeclaredMethods( ) and getDeclaredFields( ) methods return information about the methods and fields actually declared by a class or interface If you want to obtain information about all methods and fields, including those specified by superclasses, then you will need to use the getMethods( ) and getFields( ) methods defined by Class These return information about all public methods and fields that are members of the class Class provides many additional features Here are some examples You can obtain information about a specific constructor, method, or field by calling getConstructor( ), getMethod( ), or getField( ) The getDeclaredConstructor( ), getDeclaredMethod( ), and getDeclaredField( ) methods are also provided You can obtain information about the annotations (if any) that are associated with a class by calling getAnnotations( ), getDeclaredAnnotations( ), or getAnnotation( ) Frankly, it is worth the time and effort needed to fully explore Class It contains many useful capabilities A key feature of Java s reflection API is the ability to create an instance of an object at runtime and call methods on it This aspect of reflection is demonstrated by the following recipe

Classes javalangClass<T> Methods Constructor<T> getConstructor(Class< > params) Method getMethod(String methName, Class<T> params) Class< >[ ] getParameterTypes( ) T newInstance(Object arglist) Class< >[ ] getParameterTypes( ) Class< > getReturnType( ) Object invoke(Object obj, Object arglist)