Class 22: Introspection in Java

Held: Monday, March 12, 2007

Summary: Today we consider the basics of introspection in the Java Programming language and some reasons we might want to use introspection.

Related Pages:

• EBoard.
• Reading: Sun Microsystems: Trail: The Reflection API.

Notes:

• Are there questions on the exam?
• So, what did you learn from the video?

Overview:

• Introspection basics.
• A problem: Factory Methods.
• A problem: Dynamic Loading.
• A problem: Serializing.
• A problem: Profiling.

Introspection Basics

• Introspection lets you write code that looks into existing objects and classes, permitting you to use such things differently.
• In some ways, introspection is like Scheme's ability to treat the same structure as either a list or a bit of code.
• Basic introspection operations in Java:
  • Given an object, determine its class.
  • Given its class, determine its methods, fields, and constructors.
  • Given a method or constructor, call it.
• Other introspection operations in Java:
  • Build a proxy
  • Modify the class loader
  • ...
• Why have introspection?
  • Easy way to have factory methods.
  • Many many more.

Example One: Factory Methods

• Sometimes you want to be able to build a lot of objects of the same type, and don't necessarily know that type in advance.
• By grabbing an object's class, and then grabbing the constructor from that class, you can easily make new objects.

Example Two: Dynamic Loading

• You can even load classes whose names you did not know when you wrote the program.
• How? Use Class.forName(string).
• This strategy is often used for plug-in based programs.
  • That is, the program can read the name of the plug-in from the directory structure or from an initialization file, and then load the appropriate plug-ins using reflection.