Changing private final fields via reflection

Question

class WithPrivateFinalField {
    private final String s = \"I’m totally safe\";
    public String toString() {
        return \"s = \" + s;
    }
}
WithPrivateFinal         


        

Answer 1

Here's a decompile of WithPrivateFinalField class file (I put it in a separate class for simplicity):

  WithPrivateFinalField();
     0  aload_0 [this]
     1  invokespecial java.lang.Object() [13]
     4  aload_0 [this]
     5  ldc  [8]
     7  putfield WithPrivateFinalField.s : java.lang.String [15]
    10  return
      Line numbers:
        [pc: 0, line: 2]
        [pc: 4, line: 3]
        [pc: 10, line: 2]
      Local variable table:
        [pc: 0, pc: 11] local: this index: 0 type: WithPrivateFinalField

  // Method descriptor #22 ()Ljava/lang/String;
  // Stack: 1, Locals: 1
  public java.lang.String toString();
    0  ldc  [23]
    2  areturn
      Line numbers:
        [pc: 0, line: 6]
      Local variable table:
        [pc: 0, pc: 3] local: this index: 0 type: WithPrivateFinalField

Note in the toString() method, the constant used at address 0 [0 ldc [23]] shows the compiler already concatenated the string literal "s = " and the private final field " I’m totally safe" together in advance and stored it. The toString() method will always return "s = I’m totally safe" regardless of how the instance variable s changes.