CSC/ECE 517 Fall 2011/ch4 4f sl

CSC/ECE 517 Fall 2010/ch4 4f sl

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Introduction

One of the many advantages of dynamic languages such as Ruby is the ability to introspect—to examine aspects of the program from within the program itself. Java, for one, calls this feature reflection. Sometimes you need to create an instance of class depending upon the parameter passed to a function. This parameter could be the name of the class to be created.

One way to do this is to write conditional loops and create the object. But if there are too many classes then this would become messy. Here comes reflection to rescue.

The word “reflection” conjures up an image of looking at oneself in the mirror—perhaps investigating the relentless spread of that bald spot on the top of one's head. That's a pretty apt analogy: we use reflection to examine parts of our programs that aren't normally visible from where we stand.

In this deeply introspective mood, while we are contemplating our navels and burning incense (being careful not to swap the two tasks), what can we learn about our program? We might discover:

• what objects it contains,

• the current class hierarchy,

• the contents and behaviors of objects, and

• Information on methods.

Reflection in Java

Use of Reflection

Reflection<ref>http://download.oracle.com/javase/tutorial/reflect/</ref> is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine. This is a relatively advanced feature and should be used only by developers who have a strong grasp of the fundamentals of the language. With that caveat in mind, reflection is a powerful technique and can enable applications to perform operations which would otherwise be impossible.

Extensibility Features

An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names.

Class Browsers and Visual Development Environments

A class browser needs to be able to enumerate the members of classes. Visual development environments can benefit from making use of type information available in reflection to aid the developer in writing correct code.

Debuggers and Test Tools

Debuggers need to be able to examine private members on classes. Test harnesses can make use of reflection to systematically call a discoverable set APIs defined on a class, to insure a high level of code coverage in a test suite.

A Simple Example

To see how reflection works, consider this simple example:

   import java.lang.reflect.*;
   public class DumpMethods {
      public static void main(String args[])
      {
         try {
            Class c = Class.forName(args[0]);
            Method m[] = c.getDeclaredMethods();
            for (int i = 0; i < m.length; i++)
            System.out.println(m[i].toString());
         }
         catch (Throwable e) {
            System.err.println(e);
         }
      }
   }

For an invocation of:

  java DumpMethods java.util.Stack

the output is:

  public java.lang.Object java.util.Stack.push(
    java.lang.Object)
   public synchronized 
     java.lang.Object java.util.Stack.pop()
   public synchronized
      java.lang.Object java.util.Stack.peek()
   public boolean java.util.Stack.empty()
   public synchronized 
     int java.util.Stack.search(java.lang.Object) 

Drawbacks of Reflection

Reflection is powerful, but should not be used indiscriminately. If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. The following concerns should be kept in mind when accessing code via reflection.

Performance Overhead

Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications.

Security Restrictions

Reflection requires a runtime permission which may not be present when running under a security manager. This is in an important consideration for code which has to run in a restricted security context, such as in an Applet.

Exposure of Internals

Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.

References

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