StopWatch的用法

在学习spring的时候，看到关于统计时间的类，比较好奇，就记录下来，以便以后用到可以直接使用
org.springframework.util.StopWatch
StopWatch该类在统计时间的时候，必须得前一个对象关闭才能创建新的StopWatch，并且在统计完成后，只需要将其输出，就可以像报表一样，显示统计的时间
在开发中，常用于统计时间的是 使用 System.currentTimeMillis();进行统计，并且当执行完毕后，
还需要相减，才能得到最终时间值，不过，stopWatch差不多也是类似功能吧

	public static void main(String[] args) throws InterruptedException {
		StopWatch sw = new StopWatch();
		sw.start("读取文件");
		Thread.sleep(1000);
		sw.stop();
		sw.start("文件删除");
		Thread.sleep(100);
		sw.stop();
		sw.start("文件拷贝");
		Thread.sleep(10);
		sw.stop();
		System.out.println(sw.prettyPrint());
		
		long stime =System.currentTimeMillis();
		Thread.sleep(1000);
		long etime =System.currentTimeMillis();
		System.out.println("执行时间:"+(etime-stime));
	}

执行结果：
StopWatch '': running time (millis) = 1110
-----------------------------------------
ms % Task name
-----------------------------------------
01000 090% 读取文件
00100 009% 文件删除
00010 001% 文件拷贝

执行时间:1000

 查看其内部源码，我们可以看到，该类封装了System.currentTimeMillis();在同一个stopWatch对象中，将每次stop都存放在linklist中，在统计的时候，再取出输出

	private final List<TaskInfo> taskList = new LinkedList<TaskInfo>();

	public void start(String taskName) throws IllegalStateException {
		if (this.running) {
			throw new IllegalStateException("Can't start StopWatch: it's already running");
		}
		this.startTimeMillis = System.currentTimeMillis();
		this.running = true;
		this.currentTaskName = taskName;
	}

	/**
	 * Stop the current task. The results are undefined if timing
	 * methods are called without invoking at least one pair
	 * {@link #start()} / {@link #stop()} methods.
	 * @see #start()
	 */
	public void stop() throws IllegalStateException {
		if (!this.running) {
			throw new IllegalStateException("Can't stop StopWatch: it's not running");
		}
		long lastTime = System.currentTimeMillis() - this.startTimeMillis;
		this.totalTimeMillis += lastTime;
		this.lastTaskInfo = new TaskInfo(this.currentTaskName, lastTime);
		if (this.keepTaskList) {
			this.taskList.add(lastTaskInfo);
		}
		++this.taskCount;
		this.running = false;
		this.currentTaskName = null;
	}


	/**
	 * Return an array of the data for tasks performed.
	 */
	public TaskInfo[] getTaskInfo() {
		if (!this.keepTaskList) {
			throw new UnsupportedOperationException("Task info is not being kept!");
		}
		return this.taskList.toArray(new TaskInfo[this.taskList.size()]);
	}