【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

【1】不使用StopWatch的案例

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 somethingThread.sleep(100);long end = System.currentTimeMillis();long start2 = System.currentTimeMillis();// do somethingThread.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

【2】使用StopWatch的案例

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 somethingThread.sleep(100);sw.stop();sw.start("task2");// do somethingThread.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

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

【二】源码分析

【1】查看源码

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();}@Overridepublic 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;}}}

【2】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。