[System.currentTimeMillis]/[Calendar.getInstance().getTimeInMillis()]/[new Date().getTime()]

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在Java中,生成当前的时间戳大致上有这么几种方法,分别是:

System.currentTimeMillis(),它属于java.lang.System
Calendar.getInstance().getTimeInMillis(),它属于java.util.Calendar
new Date().getTime(),它属于java.util.Date;

他们都是返回从1970/1/1返回到现在所经过的毫秒数，从实现上来看new Date().getTime()也是依据System.currentTimeMillis()

    public Date() {
        this(System.currentTimeMillis());
    }

单从性能方面考虑，优先使用System.currentTimeMillis()，采用如下方式比较性能，输入结果为：

System.currentTimeMillis(): 477 Calendar.getInstance().getTimeInMillis(): 16415 new Date().getTime(): 433

		startTime = System.currentTimeMillis();
		for (int i = 0; i < times; i++) {
			System.currentTimeMillis();
		}
		endTime = System.currentTimeMillis();
		System.out.println("System.currentTimeMillis(): " + (endTime - startTime));
		
		startTime = System.currentTimeMillis();
		for (int i = 0; i < times; i++) {
			Calendar.getInstance().getTimeInMillis();
		}
		endTime = System.currentTimeMillis();
		System.out.println("Calendar.getInstance().getTimeInMillis(): " + (endTime - startTime));
		
		startTime = System.currentTimeMillis();
		for (int i = 0; i < times; i++) {
			new Date().getTime();
		}
		endTime = System.currentTimeMillis();
		System.out.println("new Date().getTime(): " + (endTime - startTime));