Class Reflection and Introspection

JVM Tutorials - Herong's Tutorial Examples

∟Class Reflection and Introspection

This section provides a tutorial example on how to reflect a class as Class object and introspect its members: fields, methods, etc.

java.lang.Class also offers methods to support class reflection and introspection:

String getName() - Returns the name of the entity (class, interface, array class, primitive type, or void) represented by this Class object, as a String.
Constructor<?>[] getDeclaredConstructors() - Returns an array of Constructor objects reflecting all the constructors declared by the class represented by this Class object.
Field[] getDeclaredFields() - Returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object.
Method[] getDeclaredMethods() - Returns an array of Method objects reflecting all the methods declared by the class or interface represented by this Class object.
Class<?>[] getDeclaredClasses() - Returns an array of Class objects reflecting all the classes and interfaces declared as members of the class represented by this Class object.
Class<? super T> getSuperclass() - Returns the Class representing the superclass of the entity (class, interface, primitive type or void) represented by this Class.

Here is tutorial example program showing you how to introspect a Java class:

/* ClassReflection.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.lang.reflect.*;
class ClassReflection {
   public static void main(String[] a) {
      java.io.PrintStream out = System.out;
      Class cls = java.io.FilterOutputStream.class;

      out.println("Class name:");
      out.println("   "+cls.getName());

      Class p = cls.getSuperclass();
      out.println("Parent class:");
      out.println("   "+p.getName());

      Constructor[] c  = cls.getDeclaredConstructors();
      out.println("Constructors:");
      for (int i = 0; i<c.length; i++)
         out.println("   "+c[i].toString());

      Field[] f  = cls.getDeclaredFields();
      out.println("Fields:");
      for (int i = 0; i<f.length; i++)
         out.println("   "+f[i].toString());

      Method[] m  = cls.getDeclaredMethods();
      out.println("Methods:");
      for (int i = 0; i<m.length; i++)
         out.println("   "+m[i].toString());

      Class[] s  = cls.getDeclaredClasses();
      out.println("Inner classes:");
      for (int i = 0; i<s.length; i++)
         out.println("   "+s[i].toString());
   }
}

When executed, I got this result:

herong> java ClassReflection

Class name:
   java.io.FilterOutputStream
Parent class:
   java.io.OutputStream
Constructors:
   public java.io.FilterOutputStream(java.io.OutputStream)
Fields:
   protected java.io.OutputStream java.io.FilterOutputStream.out
   private volatile boolean java.io.FilterOutputStream.closed
   private final java.lang.Object java.io.FilterOutputStream.closeLock
Methods:
   public void java.io.FilterOutputStream.write(int) throws java.i...
   public void java.io.FilterOutputStream.write(byte[]) throws jav...
   public void java.io.FilterOutputStream.write(byte[],int,int) th...
   public void java.io.FilterOutputStream.close() throws java.io.I...
   public void java.io.FilterOutputStream.flush() throws java.io.I...
Inner classes:

The class reflection feature in Java can be useful to introspect any class by retrieving fields, methods and other members out of the class as reflection objects.