[Java] java中ThreadLocal的应用场景实例分析

//Spring TransactionSynchronizationManager类
@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {
  DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
  Connection con = null;
  try {
  //此处省略N行代码
  if (txObject.isNewConnectionHolder()) {
    //绑定数据库连接到线程中
TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
  }
  }
  catch (Throwable ex) {
  if (txObject.isNewConnectionHolder()) {
    //当发生异常时，移除线程中的连接
    DataSourceUtils.releaseConnection(con, obtainDataSource());
    txObject.setConnectionHolder(null, false);
  }
  throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
  }
}

private static final ThreadLocal<LoginUser> loginUserLocal = new ThreadLocal<LoginUser>();
public static LoginUser getLoginUser() {
  return loginUserLocal.get();
}
public static void setLoginUser(LoginUser loginUser) {
  loginUserLocal.set(loginUser);
}
public static void clear() {
  loginUserLocal.remove();
}
//在使用完后一定要清理防止内存泄露
try{
  loginUserLocal.set(loginUser);
  //执行其他业务逻辑
}finally{
  loginUserLocal.remove();
}

/**
 * 日期工具类(使用了ThreadLocal获取SimpleDateFormat,其他方法可以直接拷贝common-lang)
 * @author Niu Li
 * @date 2016/11/19
 */
public class DateUtil {
 
 private static Map<String,ThreadLocal<SimpleDateFormat>> sdfMap = new HashMap<String, ThreadLocal<SimpleDateFormat>>();
 
 private static Logger logger = LoggerFactory.getLogger(DateUtil.class);
 
 public final static String MDHMSS = "MMddHHmmssSSS";
 public final static String YMDHMS = "yyyyMMddHHmmss";
 public final static String YMDHMS_ = "yyyy-MM-dd HH:mm:ss";
 public final static String YMD = "yyyyMMdd";
 public final static String YMD_ = "yyyy-MM-dd";
 public final static String HMS = "HHmmss";
 
 /**
  * 根据map中的key得到对应线程的sdf实例
  * @param pattern map中的key
  * @return 该实例
  */
 private static SimpleDateFormat getSdf(final String pattern){
  ThreadLocal<SimpleDateFormat> sdfThread = sdfMap.get(pattern);
  if (sdfThread == null){
   //双重检验,防止sdfMap被多次put进去值,和双重锁单例原因是一样的
   synchronized (DateUtil.class){
  sdfThread = sdfMap.get(pattern);
  if (sdfThread == null){
   logger.debug("put new sdf of pattern " + pattern + " to map");
   sdfThread = new ThreadLocal<SimpleDateFormat>(){
    @Override
    protected SimpleDateFormat initialValue() {
     logger.debug("thread: " + Thread.currentThread() + " init pattern: " + pattern);
     return new SimpleDateFormat(pattern);
    }
   };
   sdfMap.put(pattern,sdfThread);
  }
   }
  }
  return sdfThread.get();
 }
 
 /**
  * 按照指定pattern解析日期
  * @param date 要解析的date
  * @param pattern 指定格式
  * @return 解析后date实例
  */
 public static Date parseDate(String date,String pattern){
  if(date == null) {
   throw new IllegalArgumentException("The date must not be null");
  }
  try {
   return getSdf(pattern).parse(date);
  } catch (ParseException e) {
   e.printStackTrace();
   logger.error("解析的格式不支持:"+pattern);
  }
  return null;
 }
 /**
  * 按照指定pattern格式化日期
  * @param date 要格式化的date
  * @param pattern 指定格式
  * @return 解析后格式
  */
 public static String formatDate(Date date,String pattern){
  if (date == null){
   throw new IllegalArgumentException("The date must not be null");
  }else {
   return getSdf(pattern).format(date);
  }
 }
}