Question about "java.lang.ThreadLocal".

Question

public class PrinterUser extends Thread {
  private Printer printer;
  private String content;
  private String user;
  public PrinterUser(Printer printer,String content,String user) {
    this.printer = printer;
    this.content = content;
    this.user = user;
  void print() {
    printer.print(content);
  public void run() {
    print();
  public static void main(String[] args) {
    Printer p = new Printer();
    ThreadLocalPrinter tlp = new ThreadLocalPrinter();
    new PrinterUser(p,"User A is printing...","A").start();
    new PrinterUser(p,"User B is printing...","B").start();
    new PrinterUser(tlp,"User C is printing..." + tlp.get(),"C").start();
    new PrinterUser(tlp,"User D is printing..." + tlp.get(),"D").start();
class Printer {
  synchronized void print(String content) {
    System.out.print("Preparing...");
    try {
      Thread.sleep(2000);
    catch(InterruptedException e) {
      System.out.println(e);
    System.out.println(content);
  synchronized void printErrors() {
    System.out.println("Errors...");
class ThreadLocalPrinter extends Printer {
  private static int nextSerialNum = 0;
  private static ThreadLocal serialNum = new ThreadLocal() {
    protected synchronized Object initialValue() {
      return new Integer(nextSerialNum++);
  public static int get() {
    return ((Integer)serialNum.get()).intValue();
}Why the result is :
Preparing...Preparing...User A is printing...
Preparing...User C is printing...0
Preparing...User B is printing...
User D is printing...0
But not
User C is printing...0
User D is printing...1
?

Answer 1

I seldom use ThreadLocal but a standard usage might be ...
public class PrinterUser extends Thread {
  private ThreadLocalPrinter printer;
  private String content;
  private String user;
  public PrinterUser(ThreadLocalPrinter printer,String content,String user) {
    this.printer = printer;
    this.content = content;
    this.user = user;
  void print() {
    printer.print(content);
  public void run() {
    print();
  public static void main(String[] args) {
    ThreadLocalPrinter tlp = new ThreadLocalPrinter();
    PrinterUser puC = new PrinterUser(tlp, "User C is printing...", "C");
    PrinterUser puD = new PrinterUser(tlp, "User D is printing...", "D");
    puC.start();
    puD.start();
abstract class Printer {
  synchronized void print(String content) {
    System.out.println("Preparing... " + getSerial());
    try {
      Thread.sleep(2000);
    catch(InterruptedException e) {
      System.out.println(e);
    System.out.println(content);
  abstract int getSerial();
  synchronized void printErrors() {
    System.out.println("Errors...");
class ThreadLocalPrinter extends Printer {
  private static int nextSerialNum = 0;
  private static ThreadLocal serialNum = new ThreadLocal() {
    protected synchronized Object initialValue() {
      return new Integer(nextSerialNum++);
  public static int get() {
    return ((Integer)serialNum.get()).intValue();
  int getSerial(){
    return get();
}