Chapter 4: The Java Platform

4.7. Types, Reflection, and Dynamic Loading

The java.lang.Class class represents data types in Java and, along with the classes in the java.lang.reflect package, gives Java programs the capability of introspection (or self-reflection); a Java class can look at itself, or any other class, and determine its superclass, what methods it defines, and so on. There are several ways you can obtain a Class object in Java:

// Obtain the Class of an arbitrary object o
Class c = o.getClass();

// Obtain a Class object for primitive types with various predefined constants
c = Void.TYPE;          // The special "no-return-value" type
c = Byte.TYPE;          // Class object that represents a byte
c = Integer.TYPE;       // Class object that represents an int
c = Double.TYPE;        // etc. See also Short, Character, Long, Float. 

// Express a class literal as a type name followed by ".class"
c = int.class;          // Same as Integer.TYPE
c = String.class;       // Same as "dummystring".getClass()
c = byte[].class;       // Type of byte arrays
c = Class[][].class;    // Type of array of arrays of Class objects

Once you have a Class object, you can perform some interesting reflective operations with it:

import java.lang.reflect.*;

Object o;                   // Some unknown object to investigate
Class c = o.getClass();     // Get its type

// If it is an array, figure out its base type
while (c.isArray()) c = c.getComponentType();

// If c is not a primitive type, print its class hierarchy
if (!c.isPrimitive()) {
  for(Class s = c; s != null; s = s.getSuperclass()) 
    System.out.println(s.getName() + " extends");
}

// Try to create a new instance of c; this requires a no-arg constructor
Object newobj = null;
try { newobj = c.newInstance(); }
catch (Exception e) { 
  // Handle InstantiationException, IllegalAccessException
}

// See if the class has a method named setText that takes a single String
// If so, call it with a string argument
try {
  Method m = c.getMethod("setText", new Class[] { String.class });
  m.invoke(newobj, new Object[] { "My Label" });
} catch(Exception e) { /* Handle exceptions here */ }

Class also provides a simple mechanism for dynamic class loading in Java. For more complete control over dynamic class loading, however, you should use a java.lang.ClassLoader object, typically a java.net.URLClassLoader. This technique is useful, for example, when you want to load a class that is named in a configuration file instead of being hardcoded into your program:

// Dynamically load a class specified by name in a config file
String classname =                     // Look up the name of the class
  config.getProperties("filterclass",  // The property name
                       "com.davidflangan.filters.Default"); // A default

try {
  Class c = Class.forName(classname);  // Dynamically load the class
  Object o = c.newInstance();          // Dynamically instantiate it
} catch (Exception e) { /* Handle exceptions */ }

// If the class to be loaded is not in the classpath, create a custom
// class loader to load it. 
// Use the config file again to specify the custom path
import java.net.URLClassLoader;
String classdir = config.getProperties("classpath");
try {
  ClassLoader loader = new URLClassLoader(new URL[] { new URL(classdir) });
  Class c = loader.loadClass(classname);
}
catch (Exception e) { /* Handle exceptions */ }

4.6. Collections		4.8. Threads

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