JScrollBar does not show up even if there are enough lebgth of textstring in textField

When text filed is re-freshed, JScrollBar does not show up even if there are enough lebgth of textstring in textField

CONFIGURATION:
JDK: 7/6u26/5u30

REPRODUCE:
1. Compile and invoke the attached test cae(Test.java)
2. Click the button "change text"

EXPECTED:
attached "good.jpg"

ACTUAL:
attahched "bad.jpg"(ScrollBar does not show up)

FREQUENCT :
This always occurs.