The synchronized keyword, as its name implies, is used for the purpose of synchronization and mutual exclusion.

Here is a brief note of its use and significance:

1. When synchronized modifies this or a non-static method or an instance, the synchronized lock is attached to this or an instance object reference. For example, a,b are both instantiated objects of Main class, a calls synchronized methods, and b calls synchronized methods, without mutual exclusion. But calls to the a object from different threads are mutually exclusive if they are synchronized.

public synchronized void method(){
  // ... .
}
public void method()
{
  synchronized (this){
    // ... ..
  }
}

2. Unlike 1, when synchronized modifies the class name.class or static methods, even different objects can be mutually exclusive

Class Main
{
  public synchronized static void method1(){
    // ... .
  }
  public void method2(){
    synchronized(Main.class) //
  }
}

3. eg:

package com.asiainfolinkage.ems.web.controller.base;

import java.math.BigInteger;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;

/**
 *  attached token The controller
 * @version 1.0 Copyright 2013 years 7 month 29 day   In the afternoon 3:38:30
 */
public abstract class TokenBaseController extends BaseController {

  private static Map<String, String> springmvc_token = new HashMap<String, String>();

  /**  generate 1 A wei 1 The value of the token */
  public String generateGUID(HttpSession session) {
    String token = "";
    Date date = new Date();
    synchronized (springmvc_token) {
      try {
        Object obj = session.getAttribute(Constants.SPRING_MVC_TOKENNAME);
        if (obj != null)
          springmvc_token = (Map<String, String>) session.getAttribute(Constants.SPRING_MVC_TOKENNAME);
        token = new BigInteger(165, new Random()).toString(36).toUpperCase();
        springmvc_token.put(Constants.DEFAULT_TOKEN_NAME + "." + token, token);
        session.setAttribute(Constants.SPRING_MVC_TOKENNAME, springmvc_token);
        Constants.TOKEN_VALUE = token;
      } catch (IllegalStateException e) {
        _log.error("generateGUID() mothod find bug,by token session...");
      }
    }
    return token;
  }

  /**  Validate form token Values and session In the token Values are 1 to  */
  public boolean validToken(HttpServletRequest request) {
    String inputToken = getInputToken(request);

    if (inputToken == null) {
      _log.warn("token is not valid!inputToken is NULL");
      return false;
    }
    HttpSession session = request.getSession();
    Map<String, String> tokenMap = (Map<String, String>) session.getAttribute(Constants.SPRING_MVC_TOKENNAME);
    if (tokenMap == null || tokenMap.size() < 1) {
      _log.warn("token is not valid!sessionToken is NULL");
      return false;
    }
    String sessionToken = tokenMap.get(Constants.DEFAULT_TOKEN_NAME + "." + inputToken);
    if (!inputToken.equals(sessionToken)) {
      _log.warn("token is not valid!inputToken='" + inputToken + "',sessionToken = '" + sessionToken + "'");
      return false;
    }
    tokenMap.remove(Constants.DEFAULT_TOKEN_NAME + "." + inputToken);
    session.setAttribute(Constants.SPRING_MVC_TOKENNAME, tokenMap);

    return true;
  }

  /**  Get in the form token value  */
  private String getInputToken(HttpServletRequest request) {
    Map params = request.getParameterMap();
    if (!params.containsKey(Constants.DEFAULT_TOKEN_NAME)) {
      _log.warn("Could not find token name in params.");
      return null;
    }
    String[] tokens = (String[]) (String[]) params.get(Constants.DEFAULT_TOKEN_NAME);
    if ((tokens == null) || (tokens.length < 1)) {
      _log.warn("Got a null or empty token name.");
      return null;
    }
    return tokens[0];
  }
}