HttpURLConnection 在调用 connect() 时返回错误

Question

我是Android新手，正在尝试通过httpURLConnection将String json传递给API，但它总是返回错误。这是我在网上找到的代码的副本，应该可以工作。我尝试创建另一个扩展 AsyncTask 的类，但我不知道如何从 API 返回结果。有人可以告诉我我做错了什么并帮助我纠正它。

String json = "{"requests":[{"image":{"content":"" + encodedString +  ""},"features":[{"type":"LABEL_DETECTION","maxResults":"10"}]}]}";
            try{
                URL urlObj = new URL(urlVisit);
                HttpURLConnection conn = (HttpURLConnection) urlObj.openConnection();
                conn.setDoOutput(true);
                conn.setRequestMethod("POST");
                conn.setRequestProperty("Accept-Charset", "UTF-8");
                conn.setReadTimeout(10000);
                conn.setConnectTimeout(15000);
                conn.connect();
                DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
                wr.writeBytes(json);
                wr.flush();
                wr.close();
                InputStream in = new BufferedInputStream(conn.getInputStream());
                BufferedReader reader = new BufferedReader(new InputStreamReader(in));
                StringBuilder result = new StringBuilder();
                String line;
                while ((line = reader.readLine()) != null) {
                    result.append(line);
                }
                Log.d("test", "result from server: " + result.toString());
            } catch (IOException e) {
                e.printStackTrace();
            }

控制台给出的错误：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: edmt.dev.androidcamera2api, PID: 22584
android.os.NetworkOnMainThreadException
    at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1460)
    at java.net.Inet6AddressImpl.lookupHostByName(Inet6AddressImpl.java:102)
    at java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:90)
    at java.net.InetAddress.getAllByName(InetAddress.java:787)
    at com.android.okhttp.Dns$1.lookup(Dns.java:39)
    at com.android.okhttp.internal.http.RouteSelector.resetNextInetSocketAddress(RouteSelector.java:175)
    at com.android.okhttp.internal.http.RouteSelector.nextProxy(RouteSelector.java:141)
    at com.android.okhttp.internal.http.RouteSelector.next(RouteSelector.java:83)
    at com.android.okhttp.internal.http.StreamAllocation.findConnection(StreamAllocation.java:174)
    at com.android.okhttp.internal.http.StreamAllocation.findHealthyConnection(StreamAllocation.java:126)
    at com.android.okhttp.internal.http.StreamAllocation.newStream(StreamAllocation.java:95)
    at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:281)
    at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:224)
    at com.android.okhttp.internal.huc.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:461)
    at com.android.okhttp.internal.huc.HttpURLConnectionImpl.connect(HttpURLConnectionImpl.java:127)
    at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.connect(DelegatingHttpsURLConnection.java:89)
    at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.connect(Unknown Source:0)
    at edmt.dev.androidcamera2api.MainActivity$2.onClick(MainActivity.java:177)
    at android.view.View.performClick(View.java:6291)
    at android.view.View$PerformClick.run(View.java:24931)
    at android.os.Handler.handleCallback(Handler.java:808)
    at android.os.Handler.dispatchMessage(Handler.java:101)
    at android.os.Looper.loop(Looper.java:166)
    at android.app.ActivityThread.main(ActivityThread.java:7523)
    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

从堆栈跟踪中，您正在尝试从主线程执行网络调用。

E/AndroidRuntime: FATAL EXCEPTION: main
Process: edmt.dev.androidcamera2api, PID: 22584
android.os.NetworkOnMainThreadException

应将网络调用包装在 AsyncTask 或任何其他不会阻止 UI 的后台线程中。

下面是一个代码片段，可帮助您入门：

class TestAsync extends AsyncTask<String, Integer, String>
    {
        protected void onPreExecute (){
            super.onPreExecute();
        }
        protected String doInBackground(String... params) {
            // wrap the network call here
            String json = "{"requests":[{"image":{"content":"" + encodedString +  ""},"features":[{"type":"LABEL_DETECTION","maxResults":"10"}]}]}";
            StringBuilder result = new StringBuilder();
            try{
                URL urlObj = new URL(params[0]);
                HttpURLConnection conn = (HttpURLConnection) urlObj.openConnection();
                conn.setDoOutput(true);
                conn.setRequestMethod("POST");
                conn.setRequestProperty("Accept-Charset", "UTF-8");
                conn.setReadTimeout(10000);
                conn.setConnectTimeout(15000);
                conn.connect();
                DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
                wr.writeBytes(json);
                wr.flush();
                wr.close();
                InputStream in = new BufferedInputStream(conn.getInputStream());
                BufferedReader reader = new BufferedReader(new InputStreamReader(in));
                String line;
                while ((line = reader.readLine()) != null) {
                    result.append(line);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
            return result.toString();
        }
        protected void onProgressUpdate(Integer...a){
            super.onProgressUpdate(a);
        }
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            Log.d("test", "result from server: " + result);
        }
    }

要调用 AsyncTask，请执行以下操作：

 new TestAsync().execute(urlVisit);

Answer 2

您是否在AndroidManifest.xml上添加了互联网权限？

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