output: #fixed code
                                  @Override
      public void printStackTrace(PrintWriter writer) {
          Exception borrowedBy = this.borrowedBy;
          if (borrowedBy != null) {
              borrowedBy.printStackTrace(writer);
          }
          Exception usedBy = this.usedBy;
          if (usedBy != null) {
              usedBy.printStackTrace(writer);
          }
      }
input: #vulnerable code 
                                  @Override
      public void printStackTrace(PrintWriter writer) {
          if (borrowedBy != null) {
              borrowedBy.printStackTrace(writer);
          }
          if (usedBy != null) {
              usedBy.printStackTrace(writer);
          }
      } 
                              #location 4 
                              #vulnerability type THREAD_SAFETY_VIOLATION