Constant won't change after class replaced in Tomcat

Question

I have deployed a application on Tomcat 6 and after I deployed I wanted to do some changes on my constant class and I uploaded only the constant class (.class file) into exploded war file.

And even after I restart the server several times the changes I made wouldn't show.

All I changed was some strings in constants. What would you suggest me to do other than uploading war file again?

Answer 1

You will have to recompile all classes that reference those String constants.

Note that a static final field of a primitive type or of type String that is initialized with a compile time constant value (a so called constant variable) will be inlined when they are used in other classes.

In other words if you have these classes:

public class Constants {
  public static final int FOO = 42;
}

public class Bar {
  public void frobnicate() {
    System.out.println(Constants.FOO);
  }
}

Then at compile time the value of FOO will be compiled into the .class file of Bar, meaning that Bar no longer references Constants at runtime!

This also means that any change of FOO will have no effect on Bar until you re-compile Bar with the new Constants.class.

This effect is discussed at length in JLS §13.4.9 final Fields and Constants.

One way to avoid this problem in the future is to ensure that your "constants" are not interpreted as constant variables by the compiler. One way to do this is to move the assignment of a value from an initializer to a simple assignment via a static initializer block:

public class Constants {
  public static final int FOO;

  static {
    FOO = 42;
  }
}