This outline is also available in PDF.
Held: Wednesday, 27 April 2011
Summary:
We consider the basics of introspection in the Java programming
language and some reasons we might want to use introspection.
Related Pages:
Notes:
- Reading for Friday:
gc wiki
.
- Due to prereg and College committee work, I need folks to sign up for appointments. Times at http://www.cs.grinnell.edu/~rebelsky/Schedule/signup.2011S.txt. Signup via email.
- CS Picnic: Friday, May 6, 2011.
- Don't forget to vote on a t-shirt.
- EC for today's Integrating Digital Collections into the Liberal Arts College Curriculum, 4pm JRC 209. [academic]
- EC for Poetry Convo Thursday. [academic]
- EC for Thursday's Bites to Bits.
- EC for Friday's CS table.
- EC for Psychologist Candidate Talks, coming soon. [academic]
Overview:
- Introspection basics.
- Application: Factory Methods.
- Application: Dynamic Loading.
- Application: Serializing.
- Application: Profiling.
- 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.
- The class of an object is a value in class
Class.
- Given the name of a class, get its class object.
- Given its class, determine its methods, fields, and constructors.
- Each of these things is also an object.
- Given a method or constructor object, 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.
- 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.
- 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.
- You can use this strategy for some forms of live patching.
- Problem: You want to dump an object to a file for reading later.
- How do you do it?
- What if you have to do the same thing for dozens of classes?
- What if you don't have access to the
.java file?
- Problem: You want to make a count of how many times each method is
called in any object.
- How do you do it?
- What if you have to do the same thing for dozens of classes?
- What if you don't have access to the
.java file?
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