StopWatch
StopWatch 是一个计时器。是spring自带的一个工具类，通过它可方便的对程序部分代码进行计时操作。StopWatch类结构如下：
/* * Copyright 2002-2016 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.springframework.util; import java.text.NumberFormat; import java.util.LinkedList; import java.util.List; /** * Simple stop watch, allowing for timing of a number of tasks, * exposing total running time and running time for each named task. * * <p>Conceals use of {@code System.currentTimeMillis()}, improving the * readability of application code and reducing the likelihood of calculation errors. * * <p>Note that this object is not designed to be thread-safe and does not * use synchronization. * * <p>This class is normally used to verify performance during proof-of-concepts * and in development, rather than as part of production applications. * * @author Rod Johnson * @author Juergen Hoeller * @author Sam Brannen * @since May 2, 2001 */ public class StopWatch { /** * Identifier of this stop watch. * Handy when we have output from multiple stop watches * and need to distinguish between them in log or console output. */ 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(""); } /** * 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; } /** * Return the id of this stop watch, as specified on construction. * @return the id (empty String by default) * @since 4.2.2 * @see #StopWatch(String) */ public String getId() { return this.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. * @see #currentTaskName() */ public boolean isRunning() { return this.running; } /** * Return the name of the currently running task, if any. * @since 4.2.2 * @see #isRunning() */ public String currentTaskName() { return this.currentTaskName; } /** * 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 '" + getId() + "': 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); } } }
主要的方法有start，stop、prettyPrint、getTotalTimeSeconds、getTotalTimeMillis；
使用正常的查询某一段代码的耗时时间的写法是这样的：
public class Application { public static void main(String[] args) throws InterruptedException { long startTime1 = System.currentTimeMillis(); System.out.println("做了第一件事"); Thread.sleep(100); long endTime1 = System.currentTimeMillis(); long startTime2 = System.currentTimeMillis(); System.out.println("做了第二件事"); Thread.sleep(300); long endTime2 = System.currentTimeMillis(); System.out.println("执行结果"); System.out.println("做第一件事耗时：" + (endTime1 - startTime1)); System.out.println("做第二件事耗时：" + (endTime2 - startTime2)); } }
运行结果： 做了第一件事 做了第二件事 执行结果 做第一件事耗时：103 做第二件事耗时：300
这样的方法，简单直接，很容易书写、但是遇到一个代码块里，需要查看多个代码段的时间耗时情况，耗时比例，要是每次都这样重复写法，估计都烦了。StopWatch方式解决了这样的问题。StopWatch方式查询某一段方法耗时的写法
public static void main(String[] args) throws InterruptedException { StopWatch sw = new StopWatch("codeTime"); sw.start("s1"); System.out.println("做了第一件事"); Thread.sleep(100); sw.stop(); sw.start("s2"); System.out.println("做了第二件事"); Thread.sleep(300); sw.stop(); System.out.println("执行结果\r\n" + sw.prettyPrint()); }
运行结果：
做了第一件事 做了第二件事 执行结果 StopWatch 'codeTime': running time (millis) = 400 ----------------------------------------- ms % Task name ----------------------------------------- 00100 025% s1 00300 075% s2
StopWatch方式代码实现简单、使用简单，还可以输出各个耗时的百分比结果直观，代能消耗相对较小。spring boot源码中使用到了StopWatch类，在run方法中使用了StopWatch，在应用启动后，输出了应用启动时间：Started Application in 3.415 seconds (JVM running for 3.723)就是通过StopWatch的getTotalTimeSeconds()获取
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