使用背景

由于刚接手另一组开发的项目，发现其中接口耗时很长，所以决定查看到底哪里耗时长，进而思考如何进行优化

什么是StopWatch

StopWatch是org.springframework.util下的一个工具类，主要用来查看任务执行时间，查看源码可以发现其实其实底层仍然是使用System.currentTimeMillis()方法来计时，start()中给currentTaskName和startTimeMillis赋值，然后stop()方法中currentTaskName和lastTime生成TaskInfo对象，并放到taskList里，最后prettyprint()方法循环打印输出即可，源码（精简版）如下

/** Name of the current task */
private String currentTaskName;

/** Start time of the current task */
private long startTimeMillis;

private final List<TaskInfo> taskList = new LinkedList<TaskInfo>();

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();
}

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;
}

public static final class TaskInfo {
    private final String taskName;
    private final long timeMillis;
    TaskInfo(String taskName, long timeMillis) {
        this.taskName = taskName;
        this.timeMillis = timeMillis;
    }
}

如何使用

public class StopWatchTest {
      public static void main(String[] args) {
          try {
              StopWatch stopWatch = new StopWatch();
              stopWatch.start("fun1");
              fun1();
              stopWatch.stop();
              stopWatch.start("fun2");
              fun2();
              stopWatch.stop();
              System.out.println(stopWatch.prettyPrint());
          } catch (Exception e) {
              e.printStackTrace();
          }
      }

      private static void fun1() throws Exception {
          Thread.sleep(1000);
      }
    
      private static void fun2() throws Exception {
          Thread.sleep(2000);
       }
}

然后可以看到结果

StopWatch '': running time (millis) = 3011
-----------------------------------------
ms     %     Task name
-----------------------------------------
01010  034%  fun1
02001  066%  fun2