android缺少权限为什么报NullPointerException

最新推荐文章于 2020-09-02 16:01:51 发布

原创最新推荐文章于 2020-09-02 16:01:51 发布 · 210 阅读

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文章标签：

#Android Http下载 #报错 #NullPointerException #缺少权限

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探讨Android应用中未声明网络权限时尝试下载文件导致的空指针异常，而非权限缺失异常。

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下面是我的一个通过Http下载文件的练习，问题是在没有为程序添加网络访问权限(ndroid.permission.INTERNET)的情况为什么报的是NullPointerException，而不是指定缺少某个权限呢？

public class FileDownload extends Activity {
	
	public URL url;
	TextView tView;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		LinearLayout linearLayout = new LinearLayout(this);
		tView = new TextView(this);
		tView.setText("将要显示下载的网络数据");
		
		Button download = new Button(this);
		linearLayout.addView(download);
		linearLayout.addView(tView);
		setContentView(linearLayout);
		download.setText("开始下载文本文件");
		
		download.setOnClickListener(new View.OnClickListener() {		
			@Override
			public void onClick(View v) {
				HttpDownloader hd = new HttpDownloader();
				//注意下面的192.168.1.106是我电脑此时的ip地址，如果使用的是动态ip，请在运行时确认修改，/myapp是我的web应用，WebData.txt是web应用根目录下的一个文本文件
				String webData = hd.download("http://192.168.1.106:8080/myapp/WebData.txt");  //此处报错，空指针异常

				tView.setText(webData);
			}
		});
	}
	class HttpDownloader {
		public String download(String urlStr){
			StringBuffer sb = new StringBuffer();
			String line = null;
			BufferedReader buffer = null;
			try {
				url = new URL(urlStr);
				HttpURLConnection urlConn = (HttpURLConnection)url.openConnection();
				buffer = new BufferedReader(new InputStreamReader(urlConn.getInputStream()));
				while((line = buffer.readLine()) != null) {
					sb.append(line);
				}
			} catch (MalformedURLException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			} finally {
				try {
					buffer.close(); //此处报错，空指针异常
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return sb.toString();
		}
	}
}

报错：

03-27 07:58:43.210: E/AndroidRuntime(1303): Uncaught handler: thread main exiting due to uncaught exception
03-27 07:58:43.220: E/AndroidRuntime(1303): java.lang.NullPointerException
03-27 07:58:43.220: E/AndroidRuntime(1303): at com.wly.summary.mechanism.FileDownload$HttpDownloader.download(FileDownload.java:64)
03-27 07:58:43.220: E/AndroidRuntime(1303): at com.wly.summary.mechanism.FileDownload$1.onClick(FileDownload.java:39)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.view.View.performClick(View.java:2364)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.view.View.onTouchEvent(View.java:4179)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.widget.TextView.onTouchEvent(TextView.java:6541)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.view.View.dispatchTouchEvent(View.java:3709)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
03-27 07:58:43.220: E/AndroidRuntime(1303): at android.view.ViewGroup.dispatchTouch.......

由错误信息可以报的是空指针异常，为什么不是缺少权限异常的描述呢？