一、简介

stopWatch是org.springframework.util包下的一个工具类，使用它可直观的输出代码执行耗时，以及执行时间百分比，瞬间感觉比之前的方式高大上了一个档次。

在未使用这个工具类之前，如果我们需要统计某段代码的耗时，我们会这样写：

public static void main(String[] args) throws InterruptedException {
        test0();
    }

    public static void test0() throws InterruptedException {
        long start = System.currentTimeMillis();
        // do something
        Thread.sleep(100);
        long end = System.currentTimeMillis();
        long start2 = System.currentTimeMillis();
        // do something
        Thread.sleep(200);
        long end2 = System.currentTimeMillis();
        System.out.println("某某1执行耗时:" + (end - start));
        System.out.println("某某2执行耗时:" + (end2 - start2));
    }

执行结果如下：

某某1执行耗时:100
某某2执行耗时:200

可以看出我们的代码比较繁琐，如果要统计更多的任务计时呢？更繁琐，甚至让代码显得有点low
下面来看一个stopWatch实现的一个示例：

public class StopWatchDemo {
    public static void main(String[] args) throws InterruptedException {
        test1();
    }

    public static void test1() throws InterruptedException {
        StopWatch sw = new StopWatch("test");
        sw.start("task1");
        // do something
        Thread.sleep(100);
        sw.stop();
        sw.start("task2");
        // do something
        Thread.sleep(200);
        sw.stop();
        System.out.println("sw.prettyPrint()~~~~~~~~~~~~~~~~~");
        System.out.println(sw.prettyPrint());
    }

}

运行结果如下：

sw.prettyPrint()~~~~~~~~~~~~~~~~~
StopWatch 'test': running time (millis) = 310
-----------------------------------------
ms     %     Task name
-----------------------------------------
00110  035%  task1
00200  065%  task2

瞬间代码简介了，也高大上了，除此之外，还有以下两个方法shortSummary,getTotalTimeMillis，查看程序执行时间。

System.out.println("sw.shortSummary()~~~~~~~~~~~~~~~~~");
System.out.println(sw.shortSummary());
System.out.println("sw.getTotalTimeMillis()~~~~~~~~~~~~~~~~~");
System.out.println(sw.getTotalTimeMillis());
运行结果
sw.shortSummary()~~~~~~~~~~~~~~~~~
StopWatch 'test': running time (millis) = 308
sw.getTotalTimeMillis()~~~~~~~~~~~~~~~~~
308

其实以上内容在该工具类中实现也极其简单，通过start与stop方法分别记录开始时间与结束时间，其中在记录结束时间时，会维护一个链表类型的tasklist属性，从而使该类可记录多个任务，最后的输出也仅仅是对之前记录的信息做了一个统一的归纳输出，从而使结果更加直观的展示出来。
我们来看下源码：

import java.text.NumberFormat;
import java.util.LinkedList;
import java.util.List;

public class StopWatch {
    private final String id;
    private boolean keepTaskList = true;
    private final List<TaskInfo> taskList = new LinkedList();
    private long startTimeMillis;
    private boolean running;
    private String currentTaskName;
    private StopWatch.TaskInfo lastTaskInfo;
    private int taskCount;
    private long totalTimeMillis;

    public StopWatch() {
        this.id = "";
    }

    public StopWatch(String id) {
        this.id = id;
    }

    public void setKeepTaskList(boolean keepTaskList) {
        this.keepTaskList = keepTaskList;
    }

    public void start() throws IllegalStateException {
        this.start("");
    }

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

    public void stop() throws IllegalStateException {
        if (!this.running) {
            throw new IllegalStateException("Can't stop StopWatch: it's not running");
        } else {
            long lastTime = System.currentTimeMillis() - this.startTimeMillis;
            this.totalTimeMillis += lastTime;
            this.lastTaskInfo = new StopWatch.TaskInfo(this.currentTaskName, lastTime);
            if (this.keepTaskList) {
                this.taskList.add(this.lastTaskInfo);
            }

            ++this.taskCount;
            this.running = false;
            this.currentTaskName = null;
        }
    }

    public boolean isRunning() {
        return this.running;
    }

    public long getLastTaskTimeMillis() throws IllegalStateException {
        if (this.lastTaskInfo == null) {
            throw new IllegalStateException("No tasks run: can't get last task interval");
        } else {
            return this.lastTaskInfo.getTimeMillis();
        }
    }

    public String getLastTaskName() throws IllegalStateException {
        if (this.lastTaskInfo == null) {
            throw new IllegalStateException("No tasks run: can't get last task name");
        } else {
            return this.lastTaskInfo.getTaskName();
        }
    }

    public StopWatch.TaskInfo getLastTaskInfo() throws IllegalStateException {
        if (this.lastTaskInfo == null) {
            throw new IllegalStateException("No tasks run: can't get last task info");
        } else {
            return this.lastTaskInfo;
        }
    }

    public long getTotalTimeMillis() {
        return this.totalTimeMillis;
    }

    public double getTotalTimeSeconds() {
        return (double) this.totalTimeMillis / 1000.0D;
    }

    public int getTaskCount() {
        return this.taskCount;
    }

    public StopWatch.TaskInfo[] getTaskInfo() {
        if (!this.keepTaskList) {
            throw new UnsupportedOperationException("Task info is not being kept!");
        } else {
            return (StopWatch.TaskInfo[]) this.taskList.toArray(new StopWatch.TaskInfo[this.taskList.size()]);
        }
    }

    public String shortSummary() {
        return "StopWatch '" + this.id + "': running time (millis) = " + this.getTotalTimeMillis();
    }

    public String prettyPrint() {
        StringBuilder sb = new StringBuilder(this.shortSummary());
        sb.append('\n');
        if (!this.keepTaskList) {
            sb.append("No task info kept");
        } else {
            sb.append("-----------------------------------------\n");
            sb.append("ms     %     Task name\n");
            sb.append("-----------------------------------------\n");
            NumberFormat nf = NumberFormat.getNumberInstance();
            nf.setMinimumIntegerDigits(5);
            nf.setGroupingUsed(false);
            NumberFormat pf = NumberFormat.getPercentInstance();
            pf.setMinimumIntegerDigits(3);
            pf.setGroupingUsed(false);
            StopWatch.TaskInfo[] var7;
            int var6 = (var7 = this.getTaskInfo()).length;

            for (int var5 = 0; var5 < var6; ++var5) {
                StopWatch.TaskInfo task = var7[var5];
                sb.append(nf.format(task.getTimeMillis())).append("  ");
                sb.append(pf.format(task.getTimeSeconds() / this.getTotalTimeSeconds())).append("  ");
                sb.append(task.getTaskName()).append("\n");
            }
        }

        return sb.toString();
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder(this.shortSummary());
        if (this.keepTaskList) {
            StopWatch.TaskInfo[] var5;
            int var4 = (var5 = this.getTaskInfo()).length;

            for (int var3 = 0; var3 < var4; ++var3) {
                StopWatch.TaskInfo task = var5[var3];
                sb.append("; [").append(task.getTaskName()).append("] took ").append(task.getTimeMillis());
                long percent = Math.round(100.0D * task.getTimeSeconds() / this.getTotalTimeSeconds());
                sb.append(" = ").append(percent).append("%");
            }
        } else {
            sb.append("; no task info kept");
        }

        return sb.toString();
    }

    public static final class TaskInfo {
        private final String taskName;
        private final long timeMillis;

        TaskInfo(String taskName, long timeMillis) {
            this.taskName = taskName;
            this.timeMillis = timeMillis;
        }

        public String getTaskName() {
            return this.taskName;
        }

        public long getTimeMillis() {
            return this.timeMillis;
        }

        public double getTimeSeconds() {
            return (double) this.timeMillis / 1000.0D;
        }
    }

}

看源码就知道为何会打印如上那么高大上了。

最后来说下StopWatch优缺点：
优点：
1.spring自带工具类，可直接使用；
2.代码实现简单，使用更简单；
3.统一归纳，展示每项任务耗时与占用总时间的百分比，展示结果直观，性能消耗相对较小，并且最大程度的保证了start与stop之间的时间记录的准确性；
4.可在start时直接指定任务名字，从而更加直观的显示记录结果。

缺点：
1.一个StopWatch实例一次只能开启一个task，不能同时start多个task，并且在该task未stop之前不能,start一个新的task，必须在该task stop之后才能开启新的task，若要一次开启多个，需要new不同的StopWatch实例；
3.代码侵入式使用，需要改动多处代码。