CODEWITHSUNDEEP
javareflection
zaree
zaree

Reputation: 641

Reading .class file in java

Hi can anyone guide me in this problem ?

I am reading a .class file using Java reflection and that class contain some other class Reference (External References).But when i try to access/read its information using Java reflection then it will gave

Cannot Find Symbol compile time error?

Is there any way of getting the Custom data Type(means Class A got ref of Class B) of a class using Java reflection in case of reading .class file?

  1. I am reading a .class file using ClassLoader
  2. then i am reading the file and then transfer it into byte array and using defineCalss() method i will get the Class Object.
  3. After i will get information i.e. all fields of the class i use getDeclaredFields() method.

When i try to compile a .java file having external reference then it gave "Symbol not found error" & when i compile it using some IDE i.e eclipse And then try to open its .class file to read it then it gave these errors

Exception in thread "main" java.lang.NoClassDefFoundError: LTeacherClass; at java.lang.Class.getDeclaredFields0(Native Method) at java.lang.Class.privateGetDeclaredFields(Unknown Source) at java.lang.Class.getDeclaredFields(Unknown Source) at ClassExtractManager.classInfoExtracter(ClassExtractManager.java:158) at MainClass.main(MainClass.java:36) Caused by: java.lang.ClassNotFoundException: TeacherClass at java.lang.ClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) ... 5 more

If there still problem then i will post the code.

Upvotes: 1

Views: 3237

Answers (3)

andbi
andbi

Reputation: 4454

Your MyClassLoader must be aware of other referenced classes. Where are they stored? Make your loader read all these classes before you use java reflection:

public class ABC {
  public XYZ getXyz() {
  ...
  }
}

public class XYZ {
 ...
}

In example above your classloader must be aware of XYZ.class before you could use reflection on ABC.class

Upvotes: 1

Konrad Garus
Konrad Garus

Reputation: 54035

Reflection is a run-time API and I don't think it could give you a compile-time error. Most likely there is an issue with your code such as an undeclared/unimported class or variable.

I see two possibilities.

If you're writing your own classloader or whatever other solution which indeed needs dynamic class loading at run time, you cannot mix it up with references to concrete classes or interfaces unknown (unimported) at compile time. You have to rely on reflection from the beginning to the end, or cast to known types (or, more likely, interfaces, but these have to be known to compiler at compile-time).

Another possibility is that you're misusing reflection or classloading completely. I suspect you may have code that reads a .class file, then try to use that class by name later, assuming it works as if everything was interpreted. That would mean you're messing up compilation and linking with execution. Roughly speaking compiler only translates symbols such as TeacherClass to relevant code (and so it complains that it doesn't know what TeacherClass is). Reflection is used after compilation when you actually execute the code. That's why it doesn't affect the compiler.

Perhaps you don't need any of this at all? Maybe you just need to learn the basics of classpath, compilation and linking in Java?

It's all just guesswork based on the scarce information provided.

Upvotes: 2

jmg
jmg

Reputation: 7414

Try to use ClassLoader.loadClass(String) to get a class instance from a name. That's the preferred way unless you are implementing your own ClassLoader...

Upvotes: 1

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