在一个测试类中，我使用了一个内置的appiumdriverlocalservice类，它跨三个方法使用，测试运行成功。

public class AppiumServer {
 AppiumDriverLocalService service;

 static String appiumMainJSPath = "/Applications/Appium.app/Contents/Resources/app/node_modules/appium/build/lib/main.js";

 public void startServer() {
 service = AppiumDriverLocalService.buildService(new AppiumServiceBuilder()
         .withAppiumJS(new File(appiumMainJSPath)));

 service.start(); //server starts successfully
 }

 public void testServer() throws FileNotFoundException {
 System.err.println(("The Server URL is: " + service.getUrl().toString()));
 System.err.println(("Is Server Running? : " + service.isRunning())) //Server is running  
}  

  public void stopServer() {
  if (service.isRunning()) {
  service.stop(); //server successfully stopped
    }
  }

现在，在一个子类中，我创建了父类的对象，并从父类调用了方法。如果我在一个方法中调用所有三个方法，那么对象是可见的。

public class LoginTest {

AppiumServer appiumServer = new AppiumServer();

public void startApp() throws MalformedURLException, InterruptedException, FileNotFoundException {
appiumServer.startServer(); //server starts successfully
appiumServer.testServer(); //server is running
Thread.sleep(2000)
appiumServer.stopServer(); //server stops
}

所以，这就是问题所在。如果在多个方法中使用该对象，则它仅在第一个方法中可见，在随后的方法中突然变为null，从而失去可见性。可能有什么问题？

public void startApp() throws MalformedURLException, InterruptedException, FileNotFoundException {
appiumServer.startServer(); //server starts successfully
appiumServer.testServer(); //server is running
}

 public void quitApp() {
 appiumServer.stopServer(); //Throws a NullPointerException. Object visibility is lost
  }