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Java in a Nutshell

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12. Reflection

Contents:
Obtaining Class and Member Information
Invoking a Named Method

As noted in Chapter 4, What's New in Java 1.1, the Reflection API allows a Java program to inspect and manipulate itself. The Reflection API comprises the much-expanded Class class in java.lang and the java.lang.reflect package, which represents the members of a class with Method, Constructor, and Field objects.

Reflection can be used to obtain information about a class and its members. This is the technique that the JavaBeans "introspection" mechanism uses to determine the properties, events, and methods that are supported by a bean, for example. Reflection can also be used to manipulate objects in Java. You can use the Field class to query and set the values of fields, the Method class to invoke methods, and the Constructor class to create new objects. The examples in this chapter demonstrate both techniques for using the Reflection API.

12.1 Obtaining Class and Member Information

Example 12.1 shows a program that uses the Class, Constructor, Field, and Method classes to display information about a specified class. The program's output is similar to the class synopses that appear in the reference section of this book. (You might notice that the names of method arguments are not shown; argument names are not stored in class files, so they are not available through the Reflection API.)

Here is the output from using ShowClass on itself:

% java ShowClass ShowClass
public class ShowClass extends java.lang.Object {
 // Constructors
  public ShowClass();
 // Fields
 // Methods
  public static void main(java.lang.String[]);
  public static void print_class(java.lang.Class);
  public static java.lang.String typename(java.lang.Class);
  public static java.lang.String modifiers(int);
  public static void print_field(java.lang.reflect.Field);
  public static void print_method_or_constructor(java.lang.reflect.Member);
}

The code for this example is quite straightforward. It uses the Class.forName() method to dynamically load the named class, and calls various methods of Class object to look up the superclass, interfaces, and members of the class. The example uses Constructor, Field, and Method objects to obtain information about each member of the class.

Example 12.1: Obtaining Class and Member Information with the Reflection API

import java.lang.reflect.*;
/** A program that displays a class synopsis for the named class. */
public class ShowClass {
  /** The main method.  Print info about the named class. */
  public static void main(String[] args) throws ClassNotFoundException {
    Class c = Class.forName(args[0]);
    print_class(c);
  }
  /** Display the modifiers, name, superclass, and interfaces of a class
   *  or interface. Then go and list all constructors, fields, and methods. */
  public static void print_class(Class c)
  {
    // Print modifiers, type (class or interface), name, and superclass.
    if (c.isInterface()) {
      // The modifiers will include the "interface" keyword here...
      System.out.print(Modifier.toString(c.getModifiers()) +  c.getName());
    }
    else
      System.out.print(Modifier.toString(c.getModifiers()) + " class " +
                       c.getName() +
                       " extends " + c.getSuperclass().getName());
    // Print interfaces or super-interfaces of the class or interface.
    Class[] interfaces = c.getInterfaces();
    if ((interfaces != null) && (interfaces.length > 0)) {
      if (c.isInterface()) System.out.println(" extends ");
      else System.out.print(" implements ");
      for(int i = 0; i < interfaces.length; i++) {
        if (i > 0) System.out.print(", ");
        System.out.print(interfaces[i].getName());
      }
    }
    System.out.println(" {");            // Begin class member listing.
    // Now look up and display the members of the class.
    System.out.println(" // Constructors");
    Constructor[] constructors = c.getDeclaredConstructors();
    for(int i = 0; i < constructors.length; i++)      // Display constructors.
      print_method_or_constructor(constructors[i]);
    System.out.println(" // Fields");
    Field[] fields = c.getDeclaredFields();           // Look up fields.
    for(int i = 0; i < fields.length; i++)            // Display them.
      print_field(fields[i]);
    System.out.println(" // Methods");
    Method[] methods = c.getDeclaredMethods();        // Look up methods.
    for(int i = 0; i < methods.length; i++)           // Display them.
      print_method_or_constructor(methods[i]);
    System.out.println("}");             // End class member listing.
  }
  /** Return the name of an interface or primitive type, handling arrays. */
  public static String typename(Class t) {
    String brackets = "";
    while(t.isArray()) {
      brackets += "[]";
      t = t.getComponentType();
    }
    return t.getName() + brackets;
  }
  /** Return a string version of modifiers, handling spaces nicely. */
  public static String modifiers(int m) {
    if (m == 0) return "";
    else return Modifier.toString(m) + " ";
  }
  /** Print the modifiers, type, and name of a field. */
  public static void print_field(Field f) {
    System.out.println("  " +
                       modifiers(f.getModifiers()) +
                       typename(f.getType()) + " " + f.getName() + ";");
  }
  /** Print the modifiers, return type, name, parameter types, and exception
   *  type of a method or constructor.  Note the use of the Member interface
   *  to allow this method to work with both Method and Constructor objects. */
  public static void print_method_or_constructor(Member member) {
    Class returntype=null, parameters[], exceptions[];
    if (member instanceof Method) {
      Method m = (Method) member;
      returntype = m.getReturnType();
      parameters = m.getParameterTypes();
      exceptions = m.getExceptionTypes();
    } else {
      Constructor c = (Constructor) member;
      parameters = c.getParameterTypes();
      exceptions = c.getExceptionTypes();
    }
    System.out.print("  " + modifiers(member.getModifiers()) +
                     ((returntype!=null)? typename(returntype)+" " : "") +
                     member.getName() + "(");
    for(int i = 0; i < parameters.length; i++) {
      if (i > 0) System.out.print(", ");
      System.out.print(typename(parameters[i]));
    }
    System.out.print(")");
    if (exceptions.length > 0) System.out.print(" throws ");
    for(int i = 0; i < exceptions.length; i++) {
      if (i > 0) System.out.print(", ");
      System.out.print(typename(exceptions[i]));
    }
    System.out.println(";");
  }
}


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