Illustrate the use of the synchronized keyword in Java

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]; 
  } 
}