Use StopWatch to print the program time-consuming 使用StopWatch打印程序耗時

### 背景

最近看到現在的項目裡，有的程序員很負責，對自己寫的代碼性能如何很上心，會有很多這樣的代碼：

```java

long start = new Date().getTime();

//業務邏輯

long end = new Date().getTime();

system.out.println("耗時：" + (end - start) + "毫秒");

```

其實可以用StopWatch，更優雅，更何況springboot啟動的時候，打印的耗時就是用StopWatch。

### 示例

##### springboot啟動源碼

```java

public ConfigurableApplicationContext run(String... args) {

StopWatch stopWatch = new StopWatch();

stopWatch.start();

ConfigurableApplicationContext context = null;

Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();

this.configureHeadlessProperty();

SpringApplicationRunListeners listeners = this.getRunListeners(args);

listeners.starting();

Collection exceptionReporters;

try {

ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);

ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);

this.configureIgnoreBeanInfo(environment);

Banner printedBanner = this.printBanner(environment);

context = this.createApplicationContext();

exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);

this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);

this.refreshContext(context);

this.afterRefresh(context, applicationArguments);

stopWatch.stop();

if (this.logStartupInfo) {

(new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);

}

listeners.started(context);

this.callRunners(context, applicationArguments);

} catch (Throwable var10) {

this.handleRunFailure(context, var10, exceptionReporters, listeners);

throw new IllegalStateException(var10);

}

try {

listeners.running(context);

return context;

} catch (Throwable var9) {

this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);

throw new IllegalStateException(var9);

}

}

```

##### 統計輸出總耗時

```java

public class SpringStopWatchExample {

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

StopWatch sw = new StopWatch();

sw.start();

//long task simulation

Thread.sleep(1000);

sw.stop();

System.out.println(sw.getTotalTimeMillis());

}

}

```

##### 輸出最後一個任務的耗時

```java

public class SpringStopWatchExample2 {

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

StopWatch sw = new StopWatch();

sw.start("A");//setting a task name

//long task simulation

Thread.sleep(1000);

sw.stop();

System.out.println(sw.getLastTaskTimeMillis());

}

}

```

##### 以優雅的格式打出所有任務的耗時以及佔比

```java

public class SpringStopWatchExample3 {

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

StopWatch sw = new StopWatch();

sw.start("A");

Thread.sleep(500);

sw.stop();

sw.start("B");

Thread.sleep(300);

sw.stop();

sw.start("C");

Thread.sleep(200);

sw.stop();

System.out.println(sw.prettyPrint());

}

}

```

輸出：

```

StopWatch '': running time (millis) = 1031

-----------------------------------------

ms % Task name

-----------------------------------------

00514 050% A

00302 029% B

00215 021% C

```

### 不同的打印結果

- getTotalTimeSeconds() 獲取總耗時秒，同時也有獲取毫秒的方法

- prettyPrint() 優雅的格式打印結果，表格形式

- shortSummary() 返回簡短的總耗時描述

- getTaskCount() 返回統計時間任務的數量

- getLastTaskInfo().getTaskName() 返回最後一個任務TaskInfo對象的名稱

- https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/util/StopWatch.html