java.util.Calendar使用总结

问题：系统首页title显示的时间，用javascript读取本地时间，代码如下：

     function clockon(){
        var now=new Date();
        var year=now.getYear();
        var month=now.getMonth();
        var date=now.getDate();
        var day=now.getDay();
        var hour=now.getHours();
        var minute=now.getMinutes();
        var second=now.getSeconds();
        month=month+1;
        if(month<10){month="0"+month};
        if(date<10){date="0"+date};
        if(hour<10){hour="0"+hour};
        if(second<10){second="0"+second};
        var arr_week=new Array('周日','周一','周二','周三','周四','周五','周六');
        var week=arr_week[day];
        var time=year+"年"+month+"月"+date+"日"+"&nbsp;"+week;
        document.getElementById('system_time').innerHTML=time;        
        setTimeout('clockonbak()',1000);
      }

设定每1000ms执行一次，读取客户机时间。

现在要求读取服务器时间，前台改用ajax调用，代码如下：

    function clockon(){
        jQuery.ajax( {
            type : "POST",
            datatype:"text",
            url : "${pageContext.request.contextPath}/simpleCRUDAction.do?methodName=queryDBdate",
            beforeSend:function(){
            },
            error:function(){
            },
            success : function(data) {                
                document.getElementById('system_time').innerHTML=data;
            }
        });
    }

后台调取代码如下：

public ActionForward queryDBdate(ActionMapping mapping, ActionForm form,
            HttpServletRequest request, HttpServletResponse response) throws Exception {
        //今天是 2013年4月12日 周五
        Date de=WebUtils.getTimeFormDataBase();
        String currentTime="";        
        final String dayNames[] = { "星期日", "星期一", "星期二", "星期三", "星期四", "星期五","星期六" };
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(de);
        int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK) - 1;
        if (dayOfWeek < 0)
            dayOfWeek = 0;
//        System.out.println(dayNames[dayOfWeek]);
        //System.out.println(calendar.get(Calendar.MONTH)+1);
        currentTime = String.valueOf(calendar.get(Calendar.YEAR)) + "年"
                + String.valueOf(calendar.get(Calendar.MONTH)+1) + "月"
                + String.valueOf(calendar.get(Calendar.DAY_OF_MONTH)) + "日"
                + " " + dayNames[dayOfWeek];
        //System.out.println("今天是" + currentTime);
        response.getWriter().print(currentTime);
        return null;
    }

做完也没太注意看，结果今天有人反映，月份跟实际月份少一个月。仔细查了一下sdk中Calender.get(Calendar.MONTH)的用法，记录以做标记。

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The java.util.Calendar class is used to represent the date and time. The year, month, day, hour, minute, second, and milliseconds can all be set or obtained from a Calendar object. The default Calendar object has the current time in it.

Calendar t = Calendar.getInstance();

The following field names can be used as an argument to the Calendar.get(. . .) method. In all of these examples, t is a Calendar object.

Access Method	Meaning
t.get(Calendar.YEAR)	int value of the year
t.get(Calendar.MONTH)	int value of the month (0-11)
t.get(Calendar.DAY_OF_MONTH)	int value of the day of the month (1-31)
t.get(Calendar.DAY_OF_WEEK)	int value of the day of the week (0-6)
t.get(Calendar.HOUR)	int value of the hour in 12 hour notation (0-12)
t.get(Calendar.AM_PM)	returns either Calendar.AM or Calendar.PM
t.get(Calendar.HOUR_OF_DAY)	int value of the hour of the day in 24-hour notation (0-24)
t.get(Calendar.MINUTE)	int value of the minute in the hour (0-59)
t.get(Calendar.SECOND)	int value of the second within the minute (0-59).
t.get(Calendar.MILLISECOND)	int value of the milliseconds within a second (0-999).

月份的取值区间是从0到11，所以提取出来的值应该再加1。