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Private class members in reflected class

Posted on 2012-04-12
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Last Modified: 2012-04-16
Here:
http://docs.oracle.com/javase/tutorial/reflect/index.html
is a discussion of reflection. One part says:
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.

How and why does this happen? Why do class members lose their access modifier definition?
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Question by:allelopath
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3 Comments
 
LVL 19

Accepted Solution

by:
ramazanyich earned 350 total points
ID: 37841541
IT is due to setAccessible API which is available in reflection APi.
lets say you have class:
public class PrivateObject {

  private String privateString = null;

  public PrivateObject(String privateString) {
    this.privateString = privateString;
  }
}

by usin reflection API you can get privateString value

Field privateStringField = PrivateObject.class.
            getDeclaredField("privateString");

privateStringField.setAccessible(true);

String fieldValue = (String) privateStringField.get(privateObject);
System.out.println("fieldValue = " + fieldValue);
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LVL 19

Expert Comment

by:ramazanyich
ID: 37841543
same setAccesible API is applicable for Method
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Assisted Solution

by:Sharon Seth
Sharon Seth earned 150 total points
ID: 37841972
Reflective code breaks abstractions

Say the following is a property in your class and you have a setter for this property:

private List  items;

public void setItems(Map items)
{
   this.items = items;
}      

Here you have declared an abstraction over the underlying collection for items . The actual collection used can be any of the List implementations

When you go for reflection and try to access the setter on items , but items was assigned  an ArrayList , then the reflection mechanism would throw  NoSuchMethodException , clearly breaking the abstraction you had defined .
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