获得本年开始时间，本月开始时间，本周开始时间，今日开始时间的时间戳的java代码

获取本年开始时间的时间戳：

Calendar cal = Calendar.getInstance();
cal.set(Calendar.MONTH, 0);
cal.set(Calendar.DAY_OF_MONTH, 1);
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
long timestamp = cal.getTimeInMillis();

获取本月开始时间的时间戳：

Calendar cal = Calendar.getInstance();
cal.set(Calendar.DAY_OF_MONTH, 1);
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
long timestamp = cal.getTimeInMillis();

获取本周开始时间的时间戳：

Calendar cal = Calendar.getInstance();
cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
long timestamp = cal.getTimeInMillis();

获取今日开始时间的时间戳：

Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
long timestamp = cal.getTimeInMillis();