尝试使用 HttpURLConnection 从 url 读取信息时出现 NullPointerException

Question

我正在尝试使用 HttpURLConnection 从 url 中读取一些信息，但是，当我尝试运行时，我得到：

java.lang.RuntimeException: Unable to start activity ComponentInfo{...}: java.lang.NullPointerException: println needs a message.

我已经看到很多这样做的例子，那么问题是什么？我已经仔细检查了 url，当我 copy/paste 它到网络浏览器时，我收到了我应该收到的消息，没问题。我使用相同的 url 将信息获取到不同的程序（用 C++ 编写）并且在那里工作得很好。是否有我错过的连接设置？

这是代码，问题出现在"InputStream stream = connection.getInputStream();":

public String download_organisations(String url){
    String jsonString = "";
    try {
        if(!url.startsWith("http://")){
            url = "http://" + url;
        }
        if(url.endsWith("/")){
            url = url.substring(0, url.lastIndexOf("/"));
        }
        url = url + SystemStatic.URL_SERVICE_WEB_ORGANISATION_MANAGER;

        Log.d("OrganisationManager-Android","url: " + url);
        URL httpUrl = new URL(url);

        HttpURLConnection connection = (HttpURLConnection) httpUrl.openConnection();
        connection.setConnectTimeout(10000);


        InputStream stream = connection.getInputStream();

        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        int nRead;
        byte[] data = new byte[1024];
        while ((nRead = stream.read(data, 0, data.length)) != -1) {
            buffer.write(data, 0, nRead);
        }

        buffer.flush();
        byte[] inputStreamByteArray = buffer.toByteArray();

        byte[] base64 = Base64.decode(inputStreamByteArray, 0);
        byte[] decrypted = CipherUtils.decrypt(base64);

        jsonString = decrypted.toString();

        if (connection.getResponseCode() == HttpURLConnection.HTTP_OK) {
            Log.d("OrganisationManager-Android", "Download succeded with response: " + connection.getResponseCode());
        } else {
            Log.d("OrganisationManager-Android", "Download failed with response: " + connection.getResponseCode());
        }
    } catch (MalformedURLException e) {
        Log.e("OrganisationManager-Android", e.getMessage());

    } catch (IOException e) {
        Log.e("OrganisationManager-Android", e.getMessage());

    } catch (Exception e) {
        Log.e("OrganisationManager-Android", e.getMessage());

    }

    return jsonString;
}

错误堆栈：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.organisationmanager.cin, PID: 2066
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.organisationmanager.cin/com.organisationmanager.ble.ScanningActivity}: java.lang.NullPointerException: println needs a message
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3302)
                  at android.app.ActivityThread.-wrap12(Unknown Source:0)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1891)
                  at android.os.Handler.dispatchMessage(Handler.java:108)
                  at android.os.Looper.loop(Looper.java:166)
                  at android.app.ActivityThread.main(ActivityThread.java:7425)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
               Caused by: java.lang.NullPointerException: println needs a message
                  at android.util.Log.println_native(Native Method)
                  at android.util.Log.e(Log.java:261)
                  at com.organisationmanager.ble.common.OrganisationManagerWebPortalCommunicationHelper.download_organisations(OrganisationManagerWebPortalCommunicationHelper.java:210)
                  at com.organisationmanager.ble.ScanningActivity.onCreate(ScanningActivity.java:184)
                  at android.app.Activity.performCreate(Activity.java:7372)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1218)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3147)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3302) 
                  at android.app.ActivityThread.-wrap12(Unknown Source:0) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1891) 
                  at android.os.Handler.dispatchMessage(Handler.java:108) 
                  at android.os.Looper.loop(Looper.java:166) 
                  at android.app.ActivityThread.main(ActivityThread.java:7425) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)

Answer 1

检查您是否在清单中的应用程序中获得了权限？

如果你不使用它试试吧：

在 AndroidManifest 中将此代码添加到 Tag Application 之外

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

Answer 2

错误堆栈显示：

Caused by: java.lang.NullPointerException: println needs a message at android.util.Log.println_native(Native Method) at android.util.Log.e(Log.java:261)

正如您从这些行中看到的，异常是由 Log.e() 在 download_organisations() 内的 catch 块之一（捕获 MalformedURLException，或捕捉 IOException 的那个，或捕捉 Exception 的那个）。

您通过记录消息来处理 catch 块中的异常。但也许他们的消息是 null，所以 println 没有要打印的有效消息并抛出 NullPointerException.

尝试记录 e，并检查是否再次抛出“java.lang.NullPointerException: println needs a message”。这样做：

} catch (IOException e) {
        Log.e("OrganisationManager-Android", e.toString());
}

否则你可以打印捕获异常的堆栈跟踪：

} catch (IOException e) {
        Log.e("OrganisationManager-Android");
        e.printStackTrace()
}

尝试使用 HttpURLConnection 从 url 读取信息时出现 NullPointerException

NullPointerException when trying to read info from an url using HttpURLConnection

java

android

inputstream

nullpointerexception

httpurlconnection