Give an example to explain the usage of synchronized keyword in Java

The synchronized keyword, as its name suggests, is used for synchronization and mutual exclusion.

Here is a brief description of its usage and meaning:

1. When synchronized modifies this or a non-static method or an instance, the synchronized lock is added to this or the instance object reference. For example, a and b are both instantiated objects of the Main class. If a calls a synchronized method, and b calls a synchronized method, there is no mutual exclusion. However, the synchronized methods called by objects a from different threads are mutually exclusive.

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

2. Different from 1, when synchronized modifies the class name.class or static method, even if they are different objects, mutual exclusion will occur

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;
  
/**
 * 附带token的控制器
 * @version 1.0 Copyright 2013年7月29日 下午3:38:30
 */
public abstract class TokenBaseController extends BaseController {
  
  private static Map<String, String> springmvc_token = new HashMap<String, String>();
  
  /** 生成一个唯一值的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;
  }
  
  /** 验证表单token值和session中的token值是否一致 */
  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=&#39;" + inputToken + "&#39;,sessionToken = &#39;" + sessionToken + "&#39;");
      return false;
    }
    tokenMap.remove(Constants.DEFAULT_TOKEN_NAME + "." + inputToken);
    session.setAttribute(Constants.SPRING_MVC_TOKENNAME, tokenMap);
  
    return true;
  }
  
  /** 获取表单中token值 */
  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];
  }
}

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