计时任务之StopWatch
StopWatch对应的中文名称为秒表，经常我们对一段代码耗时检测的代码如下：
long startTime = System.currentTimeMillis(); // 业务处理代码 doSomeThing(); long endTime = System.currentTimeMillis(); long costTime = endTime -startTime; System.err.println("该段代码耗时：" + costTime + " ms");
改进的代码写法：
我们可以利用已有的工具类中的秒表，常见的秒表工具类有org.springframework.util.StopWatch、org.apache.commons.lang.time.StopWatch以及谷歌提供的guava中的秒表。这里重点讲下，org.springframework.util.StopWatch的用法。
org.springframework.util.StopWatch的用法：
源码：
package org.springframework.util; 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<TaskInfo>(); /** Start time of the current task */ private long startTimeMillis; /** Is the stop watch currently running? */ private boolean running; /** Name of the current task */ private String currentTaskName; private TaskInfo lastTaskInfo; private int taskCount; /** Total running time */ private long totalTimeMillis; /** * Construct a new stop watch. Does not start any task. */ public StopWatch() { this.id = ""; } /** * Construct a new stop watch with the given id. * Does not start any task. * @param id identifier for this stop watch. * Handy when we have output from multiple stop watches * and need to distinguish between them. */ public StopWatch(String id) { this.id = id; } /** * Determine whether the TaskInfo array is built over time. Set this to * "false" when using a StopWatch for millions of intervals, or the task * info structure will consume excessive memory. Default is "true". */ public void setKeepTaskList(boolean keepTaskList) { this.keepTaskList = keepTaskList; } /** * Start an unnamed task. The results are undefined if {@link #stop()} * or timing methods are called without invoking this method. * @see #stop() */ public void start() throws IllegalStateException { start(""); } /** * Start a named task. The results are undefined if {@link #stop()} * or timing methods are called without invoking this method. * @param taskName the name of the task to start * @see #stop() */ public void start(String taskName) throws IllegalStateException { if (this.running) { throw new IllegalStateException("Can't start StopWatch: it's already running"); } this.running = true; this.currentTaskName = taskName; this.startTimeMillis = System.currentTimeMillis(); } /** * Stop the current task. The results are undefined if timing * methods are called without invoking at least one pair * {@code start()} / {@code 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 whether the stop watch is currently running. */ public boolean isRunning() { return this.running; } /** * Return the time taken by the last task. */ public long getLastTaskTimeMillis() throws IllegalStateException { if (this.lastTaskInfo == null) { throw new IllegalStateException("No tasks run: can't get last task interval"); } return this.lastTaskInfo.getTimeMillis(); } /** * Return the name of the last task. */ public String getLastTaskName() throws IllegalStateException { if (this.lastTaskInfo == null) { throw new IllegalStateException("No tasks run: can't get last task name"); } return this.lastTaskInfo.getTaskName(); } /** * Return the last task as a TaskInfo object. */ public TaskInfo getLastTaskInfo() throws IllegalStateException { if (this.lastTaskInfo == null) { throw new IllegalStateException("No tasks run: can't get last task info"); } return this.lastTaskInfo; } /** * Return the total time in milliseconds for all tasks. */ public long getTotalTimeMillis() { return this.totalTimeMillis; } /** * Return the total time in seconds for all tasks. */ public double getTotalTimeSeconds() { return this.totalTimeMillis / 1000.0; } /** * Return the number of tasks timed. */ public int getTaskCount() { return this.taskCount; } /** * 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()]); } /** * Return a short description of the total running time. */ public String shortSummary() { return "StopWatch '" + this.id + "': running time (millis) = " + getTotalTimeMillis(); } /** * Return a string with a table describing all tasks performed. * For custom reporting, call getTaskInfo() and use the task info directly. */ public String prettyPrint() { StringBuilder sb = new StringBuilder(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); for (TaskInfo task : getTaskInfo()) { sb.append(nf.format(task.getTimeMillis())).append(" "); sb.append(pf.format(task.getTimeSeconds() / getTotalTimeSeconds())).append(" "); sb.append(task.getTaskName()).append("\n"); } } return sb.toString(); } /** * Return an informative string describing all tasks performed * For custom reporting, call {@code getTaskInfo()} and use the task info directly. */ @Override public String toString() { StringBuilder sb = new StringBuilder(shortSummary()); if (this.keepTaskList) { for (TaskInfo task : getTaskInfo()) { sb.append("; [").append(task.getTaskName()).append("] took ").append(task.getTimeMillis()); long percent = Math.round((100.0 * task.getTimeSeconds()) / getTotalTimeSeconds()); sb.append(" = ").append(percent).append("%"); } } else { sb.append("; no task info kept"); } return sb.toString(); } /** * Inner class to hold data about one task executed within the stop watch. */ public static final class TaskInfo { private final String taskName; private final long timeMillis; TaskInfo(String taskName, long timeMillis) { this.taskName = taskName; this.timeMillis = timeMillis; } /** * Return the name of this task. */ public String getTaskName() { return this.taskName; } /** * Return the time in milliseconds this task took. */ public long getTimeMillis() { return this.timeMillis; } /** * Return the time in seconds this task took. */ public double getTimeSeconds() { return (this.timeMillis / 1000.0); } } }
可以看到有一个List<StopWatch.TaskInfo> taskList，用于存储一组任务的耗时时间。这个很有用。比如：我们可以记录多段代码耗时时间，然后一次性打印（这里：org.springframework.util.StopWatch提供了一个prettyString()函数用于按照指定格式打印出耗时）
举个例子：
package com.coco.controller; import org.springframework.util.StopWatch; public class TestStopWatch { public static void main(String[] args) throws InterruptedException { // 定义一个计数器 StopWatch stopWatch = new StopWatch("统计一组任务耗时"); // 统计任务一耗时 stopWatch.start("任务一"); Thread.sleep(1000); stopWatch.stop(); // 统计任务二耗时 stopWatch.start("任务二"); Thread.sleep(2000); stopWatch.stop(); // 打印出耗时 String result = stopWatch.prettyPrint(); System.out.println(result); } }
结果为：
分析：
可以看到可以统一定义一个计数器为“统计一组任务耗时”，然后看到整段代码的耗时。以及各个部分程序代码的执行时间，并且输出的格式帮我们整理好了。
