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  2. How do I fix 'android.os.NetworkOnMainThreadException'?

How do I fix 'android.os.NetworkOnMainThreadException'?

由 一世执手 提交于 2019-12-02 20:41:37

问题


I got an error while running my Android project for RssReader.

Code:

URL url = new URL(urlToRssFeed);
SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser parser = factory.newSAXParser();
XMLReader xmlreader = parser.getXMLReader();
RssHandler theRSSHandler = new RssHandler();
xmlreader.setContentHandler(theRSSHandler);
InputSource is = new InputSource(url.openStream());
xmlreader.parse(is);
return theRSSHandler.getFeed();

And it shows the below error:

android.os.NetworkOnMainThreadException

How can I fix this issue?


回答1:


This exception is thrown when an application attempts to perform a networking operation on its main thread. Run your code in AsyncTask:

class RetrieveFeedTask extends AsyncTask<String, Void, RSSFeed> {

    private Exception exception;

    protected RSSFeed doInBackground(String... urls) {
        try {
            URL url = new URL(urls[0]);
            SAXParserFactory factory = SAXParserFactory.newInstance();
            SAXParser parser = factory.newSAXParser();
            XMLReader xmlreader = parser.getXMLReader();
            RssHandler theRSSHandler = new RssHandler();
            xmlreader.setContentHandler(theRSSHandler);
            InputSource is = new InputSource(url.openStream());
            xmlreader.parse(is);

            return theRSSHandler.getFeed();
        } catch (Exception e) {
            this.exception = e;

            return null;
        } finally {
            is.close();
        }
    }

    protected void onPostExecute(RSSFeed feed) {
        // TODO: check this.exception
        // TODO: do something with the feed
    }
}

How to execute the task:

In MainActivity.java file you can add this line within your oncreate() method

new RetrieveFeedTask().execute(urlToRssFeed);

Don't forget to add this to AndroidManifest.xml file:

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



回答2:


You should almost always run network operations on a thread or as an asynchronous task.

But it is possible to remove this restriction and you override the default behavior, if you are willing to accept the consequences.

Add:

StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();

StrictMode.setThreadPolicy(policy);

In your class,

and

ADD this permission in android manifest.xml file:    

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

Consequences:

Your app will (in areas of spotty internet connection) become unresponsive and lock up, the user perceives slowness and has to do a force kill, and you risk the activity manager killing your app and telling the user that the app has stopped.

Android has some good tips on good programming practices to design for responsiveness: http://developer.android.com/reference/android/os/NetworkOnMainThreadException.html




回答3:


I solved this problem using a new Thread.

Thread thread = new Thread(new Runnable() {

    @Override
    public void run() {
        try  {
            //Your code goes here
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
});

thread.start();



回答4:


The accepted answer has some significant down-sides. It is not advisable to use AsyncTask for networking unless you really know what you are doing. Some of the down-sides include:

  • AsyncTask's created as non-static inner classes have an implicit reference to the enclosing Activity object, its context, and the entire View hierarchy created by that activity. This reference prevents the Activity from being garbage collected until the AsyncTask's background work completes. If the user's connection is slow, and/or the download is large, these short-term memory leaks can become a problem - for example if the orientation changes several times (and you don't cancel the executing tasks), or the user navigates away from the Activity.
  • AsyncTask has different execution characteristics depending on the platform it executes on: prior to API level 4 AsyncTasks execute serially on a single background thread; from API level 4 through API level 10, AsyncTasks execute on a pool of up to 128 threads; from API level 11 onwards AsyncTask executes serially on a single background thread (unless you use the overloaded executeOnExecutor method and supply an alternative executor). Code that works fine when run serially on ICS may break when executed concurrently on Gingerbread, say, if you have inadvertent order-of-execution dependencies.

If you want to avoid short-term memory leaks, have well defined execution characteristics across all platforms, and have a base to build really robust network handling, you might want to consider:

  1. Using a library that does a nice job of this for you - there's a nice comparison of networking libs in this question, or
  2. Using a Service or IntentService instead, perhaps with a PendingIntent to return the result via the Activity's onActivityResult method.

IntentService approach

Down-sides:

  • More code and complexity than AsyncTask, though not as much as you might think
  • Will queue requests and run them on a single background thread. You can easily control this by replacing IntentService with an equivalent Service implementation, perhaps like this one.
  • Um, I can't think of any others right now actually

Up-sides:

  • Avoids the short-term memory leak problem
  • If your activity restarts while network operations are in-flight it can still receive the result of the download via its onActivityResult method
  • Better platform than AsyncTask to build and re-use robust networking code. Example: if you need to do an important upload, you could do it from AsyncTask in an Activity, but if the user context-switches out of the app to take a phone-call, the system may kill the app before the upload completes. It is less likely to kill an application with an active Service.
  • If you use your own concurrent version of IntentService (like the one I linked above) you can control the level of concurrency via the Executor.

Implementation summary

You can implement an IntentService to perform downloads on a single background thread quite easily.

Step 1: Create an IntentService to perform the download. You can tell it what to download via Intent extra's, and pass it a PendingIntent to use to return the result to the Activity:

import android.app.IntentService;
import android.app.PendingIntent;
import android.content.Intent;
import android.util.Log;

import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;

public class DownloadIntentService extends IntentService {

    private static final String TAG = DownloadIntentService.class.getSimpleName();

    public static final String PENDING_RESULT_EXTRA = "pending_result";
    public static final String URL_EXTRA = "url";
    public static final String RSS_RESULT_EXTRA = "url";

    public static final int RESULT_CODE = 0;
    public static final int INVALID_URL_CODE = 1;
    public static final int ERROR_CODE = 2;

    private IllustrativeRSSParser parser;

    public DownloadIntentService() {
        super(TAG);

        // make one and re-use, in the case where more than one intent is queued
        parser = new IllustrativeRSSParser();
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        PendingIntent reply = intent.getParcelableExtra(PENDING_RESULT_EXTRA);
        InputStream in = null;
        try {
            try {
                URL url = new URL(intent.getStringExtra(URL_EXTRA));
                IllustrativeRSS rss = parser.parse(in = url.openStream());

                Intent result = new Intent();
                result.putExtra(RSS_RESULT_EXTRA, rss);

                reply.send(this, RESULT_CODE, result);
            } catch (MalformedURLException exc) {
                reply.send(INVALID_URL_CODE);
            } catch (Exception exc) {
                // could do better by treating the different sax/xml exceptions individually
                reply.send(ERROR_CODE);
            }
        } catch (PendingIntent.CanceledException exc) {
            Log.i(TAG, "reply cancelled", exc);
        }
    }
}

Step 2: Register the service in the manifest:

<service
        android:name=".DownloadIntentService"
        android:exported="false"/>

Step 3: Invoke the service from the Activity, passing a PendingResult object which the Service will use to return the result:

PendingIntent pendingResult = createPendingResult(
    RSS_DOWNLOAD_REQUEST_CODE, new Intent(), 0);
Intent intent = new Intent(getApplicationContext(), DownloadIntentService.class);
intent.putExtra(DownloadIntentService.URL_EXTRA, URL);
intent.putExtra(DownloadIntentService.PENDING_RESULT_EXTRA, pendingResult);
startService(intent);

Step 4: Handle the result in onActivityResult:

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (requestCode == RSS_DOWNLOAD_REQUEST_CODE) {
        switch (resultCode) {
            case DownloadIntentService.INVALID_URL_CODE:
                handleInvalidURL();
                break;
            case DownloadIntentService.ERROR_CODE:
                handleError(data);
                break;
            case DownloadIntentService.RESULT_CODE:
                handleRSS(data);
                break;
        }
        handleRSS(data);
    }
    super.onActivityResult(requestCode, resultCode, data);
}

A github project containing a complete working Android-Studio/gradle project is available here.




回答5:


You cannot perform network I/O on the UI thread on Honeycomb. Technically, it is possible on earlier versions of Android, but it is a really bad idea as it will cause your app to stop responding, and can result in the OS killing your app for being badly behaved. You'll need to run a background process or use AsyncTask to perform your network transaction on a background thread.

There is an article about Painless Threading on the Android developer site which is a good introduction to this, and it will provide you with a much better depth of an answer than can be realistically provided here.




回答6:


  1. Do not use strictMode (only in debug mode)
  2. Do not change SDK version
  3. Do not use a separate thread

Use Service or AsyncTask

See also Stack Overflow question:

android.os.NetworkOnMainThreadException sending an email from Android




回答7:


Do the network actions on another thread

For Example:

new Thread(new Runnable(){
    @Override
    public void run() {
        // Do network action in this function
    }
}).start();

And add this to AndroidManifest.xml 

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



回答8:


You disable the strict mode using following code:

if (android.os.Build.VERSION.SDK_INT > 9) {
    StrictMode.ThreadPolicy policy = 
        new StrictMode.ThreadPolicy.Builder().permitAll().build();
    StrictMode.setThreadPolicy(policy);
}

This is not recommended: use the AsyncTask interface.

Full code for both the methods




回答9:


Network-based operations cannot be run on the main thread. You need to run all network-based tasks on a child thread or implement AsyncTask.

This is how you run a task in a child thread:

new Thread(new Runnable(){
    @Override
    public void run() {
        try {
            // Your implementation goes here
        } 
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}).start();



回答10:


Put your code inside:

new Thread(new Runnable(){
    @Override
    public void run() {
        try {
            // Your implementation
        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}).start();

Or:

class DemoTask extends AsyncTask<Void, Void, Void> {

    protected Void doInBackground(Void... arg0) {
        //Your implementation
    }

    protected void onPostExecute(Void result) {
        // TODO: do something with the feed
    }
}



回答11:


This happens in Android 3.0 and above. From Android 3.0 and above, they have restricted using network operations (functions that access the Internet) from running in the main thread/UI thread (what spawns from your on create and on resume methods in the activity).

This is to encourage using separate threads for network operations. See AsyncTask for more details on how to perform network activities the right way.




回答12:


Using Android Annotations is an option. It will allow you to simply run any method in a background thread:

// normal method
private void normal() {
    doSomething(); // do something in background
}

@Background
protected void doSomething() 
    // run your networking code here
}

Note, that although it provides benefits of simplicity and readability, it has its disadvantages.




回答13:


The error is due to executing long running operations in main thread,You can easily rectify the problem by using AsynTask or Thread. You can checkout this library AsyncHTTPClient for better handling. 

AsyncHttpClient client = new AsyncHttpClient();
client.get("http://www.google.com", new AsyncHttpResponseHandler() {

    @Override
    public void onStart() {
        // Called before a request is started
    }

    @Override
    public void onSuccess(int statusCode, Header[] headers, byte[] response) {
        // Called when response HTTP status is "200 OK"
    }

    @Override
    public void onFailure(int statusCode, Header[] headers, byte[] errorResponse, Throwable e) {
        // Called when response HTTP status is "4XX" (for example, 401, 403, 404)
    }

    @Override
    public void onRetry(int retryNo) {
        // Called when request is retried
    }
});



回答14:


You should not do any time-consuming task on the main thread (UI thread), like any network operation, file I/O, or SQLite database operations. So for this kind of operation, you should create a worker thread, but the problem is that you can not directly perform any UI related operation from your worker thread. For that, you have to use Handler and pass the Message.

To simplify all these things, Android provides various ways, like AsyncTask, AsyncTaskLoader, CursorLoader or IntentService. So you can use any of these according to your requirements.




回答15:


The top answer of spektom works perfect.

If you are writing the AsyncTask inline and not extending as a class, and on top of this, if there is a need to get a response out of the AsyncTask, one can use the get() method as below.

RSSFeed feed = new RetreiveFeedTask().execute(urlToRssFeed).get();

(From his example.)




回答16:


This is only thrown for applications targeting the Honeycomb SDK or higher. Applications targeting earlier SDK versions are allowed to do networking on their main event loop threads.

The error is the SDK warning!




回答17:


For me it was this:

<uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="10" />

The device I was testing my app on was 4.1.2 which is SDK Version 16!

Make the sure the target version is the same as your Android Target Library. If you are unsure what your target library is, right click your Project -> Build Path -> Android, and it should be the one that is ticked.

Also, as others have mentioned, include the correct permissions to access the Internet:

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



回答18:


Just to spell out something explicitly:

The main thread is basically the UI thread.

So saying that you cannot do networking operations in the main thread means you cannot do networking operations in the UI thread, which means you cannot do networking operations in a *runOnUiThread(new Runnable() { ... }* block inside some other thread, either.

(I just had a long head-scratching moment trying to figure out why I was getting that error somewhere other than my main thread. This was why; this thread helped; and hopefully this comment will help someone else.)




回答19:


Use this in Your Activity

    btnsub.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            new Thread(new Runnable() {

                @Override
                public void run() {
                    // TODO Auto-generated method stub

                    //Initialize soap request + add parameters
                    SoapObject request = new SoapObject(NAMESPACE, METHOD_NAME1);

                    //Use this to add parameters
                    request.addProperty("pincode", txtpincode.getText().toString());
                    request.addProperty("bg", bloodgroup.getSelectedItem().toString());

                    //Declare the version of the SOAP request
                    SoapSerializationEnvelope envelope = new SoapSerializationEnvelope(SoapEnvelope.VER11);

                    envelope.setOutputSoapObject(request);
                    envelope.dotNet = true;

                    try {
                        HttpTransportSE androidHttpTransport = new HttpTransportSE(URL);

                        //this is the actual part that will call the webservice
                        androidHttpTransport.call(SOAP_ACTION1, envelope);

                        // Get the SoapResult from the envelope body.
                        SoapObject result = (SoapObject) envelope.getResponse();
                        Log.e("result data", "data" + result);
                        SoapObject root = (SoapObject) result.getProperty(0);
                        // SoapObject s_deals = (SoapObject) root.getProperty(0);
                        // SoapObject s_deals_1 = (SoapObject) s_deals.getProperty(0);
                        //

                        System.out.println("********Count : " + root.getPropertyCount());

                        value = new ArrayList<Detailinfo>();

                        for (int i = 0; i < root.getPropertyCount(); i++) {
                            SoapObject s_deals = (SoapObject) root.getProperty(i);
                            Detailinfo info = new Detailinfo();

                            info.setFirstName(s_deals.getProperty("Firstname").toString());
                            info.setLastName(s_deals.getProperty("Lastname").toString());
                            info.setDOB(s_deals.getProperty("DOB").toString());
                            info.setGender(s_deals.getProperty("Gender").toString());
                            info.setAddress(s_deals.getProperty("Address").toString());
                            info.setCity(s_deals.getProperty("City").toString());
                            info.setState(s_deals.getProperty("State").toString());
                            info.setPinecode(s_deals.getProperty("Pinecode").toString());
                            info.setMobile(s_deals.getProperty("Mobile").toString());
                            info.setEmail(s_deals.getProperty("Email").toString());
                            info.setBloodgroup(s_deals.getProperty("Bloodgroup").toString());
                            info.setAdddate(s_deals.getProperty("Adddate").toString());
                            info.setWaight(s_deals.getProperty("waight").toString());
                            value.add(info);
                        }

                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    Intent intent = new Intent(getApplicationContext(), ComposeMail.class);
                    //intent.putParcelableArrayListExtra("valuesList", value);

                    startActivity(intent);
                }
            }).start();
        }
    });



回答20:


This exception occurs due to any heavy task performed on the main thread if that performing task takes too much time.

To avoid this, we can handle it using threads or executers

Executors.newSingleThreadExecutor().submit(new Runnable() {
    @Override
    public void run() {
        // You can perform your task here.
    }
});



回答21:


There are many great answers already on this question, but a lot of great libraries have come out since those answers were posted. This is intended as a kind of newbie-guide.

I will cover several use cases for performing network operations and a solution or two for each.

ReST over HTTP

Typically Json, can be XML or something else

Full API Access

Let's say you are writing an app that lets users track stock prices, interest rates and currecy exchange rates. You find an Json API that looks something like this:

http://api.example.com/stocks                       //ResponseWrapper<String> object containing a list of Srings with ticker symbols
http://api.example.com/stocks/$symbol               //Stock object
http://api.example.com/stocks/$symbol/prices        //PriceHistory<Stock> object
http://api.example.com/currencies                   //ResponseWrapper<String> object containing a list of currency abbreviation
http://api.example.com/currencies/$currency         //Currency object
http://api.example.com/currencies/$id1/values/$id2  //PriceHistory<Currency> object comparing the prices of the first currency (id1) to the second (id2)

Retrofit from Square

This is an excellent choice for an API with multiple endpoints and allows you to declare the ReST endpoints instead of having to code them individually as with other libraries like ion or Volley. (website: http://square.github.io/retrofit/)

How do you use it with the finances API?

build.gradle

Add these lines to your Module level buid.gradle:

implementation 'com.squareup.retrofit2:retrofit:2.3.0' //retrofit library, current as of September 21, 2017
implementation 'com.squareup.retrofit2:converter-gson:2.3.0' //gson serialization and deserialization support for retrofit, version must match retrofit version

FinancesApi.java

public interface FinancesApi {
    @GET("stocks")
    Call<ResponseWrapper<String>> listStocks();
    @GET("stocks/{symbol}")
    Call<Stock> getStock(@Path("symbol")String tickerSymbol);
    @GET("stocks/{symbol}/prices")
    Call<PriceHistory<Stock>> getPriceHistory(@Path("symbol")String tickerSymbol);

    @GET("currencies")
    Call<ResponseWrapper<String>> listCurrencies();
    @GET("currencies/{symbol}")
    Call<Currency> getCurrency(@Path("symbol")String currencySymbol);
    @GET("currencies/{symbol}/values/{compare_symbol}")
    Call<PriceHistory<Currency>> getComparativeHistory(@Path("symbol")String currency, @Path("compare_symbol")String currencyToPriceAgainst);
}

FinancesApiBuilder

public class FinancesApiBuilder {
    public static FinancesApi build(String baseUrl){
        return new Retrofit.Builder()
                    .baseUrl(baseUrl)
                    .addConverterFactory(GsonConverterFactory.create())
                    .build()
                    .create(FinancesApi.class);
    }
}

FinancesFragment snippet

FinancesApi api = FinancesApiBuilder.build("http://api.example.com/"); //trailing '/' required for predictable behavior
api.getStock("INTC").enqueue(new Callback<Stock>(){
    @Override
    public void onResponse(Call<Stock> stockCall, Response<Stock> stockResponse){
        Stock stock = stockCall.body();
        //do something with the stock
    }
    @Override
    public void onResponse(Call<Stock> stockCall, Throwable t){
        //something bad happened
    }
}

If your API requires an API Key or other header like a user token, etc. to be sent, Retrofit makes this easy (see this awesome answer for details: https://stackoverflow.com/a/42899766/1024412).

One off ReST API access

Let's say you're building a "mood weather" app that looks up the users GPS location and checks the current temperature in that area and tells them the mood. This type of app doesn't need to declare API endpoints; it just needs to be able to access one API endpoint.

Ion

This is a great library for this type of access.

Please read msysmilu's great answer (https://stackoverflow.com/a/28559884/1024412)

Load images via HTTP

Volley

Volley can also be used for ReST APIs, but due to the more complicated setup required I prefer to use Retrofit from Square as above (http://square.github.io/retrofit/)

Let's say you are building a social networking app and want to load profile pictures of friends.

build.gradle

Add this line to your Module level buid.gradle:

implementation 'com.android.volley:volley:1.0.0'

ImageFetch.java

Volley requires more setup than Retrofit. You will need to create a class like this to setup a RequestQueue, an ImageLoader and an ImageCache, but it's not too bad:

public class ImageFetch {
    private static ImageLoader imageLoader = null;
    private static RequestQueue imageQueue = null;

    public static ImageLoader getImageLoader(Context ctx){
        if(imageLoader == null){
            if(imageQueue == null){
                imageQueue = Volley.newRequestQueue(ctx.getApplicationContext());
            }
            imageLoader = new ImageLoader(imageQueue, new ImageLoader.ImageCache() {
                Map<String, Bitmap> cache = new HashMap<String, Bitmap>();
                @Override
                public Bitmap getBitmap(String url) {
                    return cache.get(url);
                }
                @Override
                public void putBitmap(String url, Bitmap bitmap) {
                    cache.put(url, bitmap);
                }
            });
        }
        return imageLoader;
    }
}

user_view_dialog.xml

Add the following to your layout xml file to add an image:

<com.android.volley.toolbox.NetworkImageView
    android:id="@+id/profile_picture"
    android:layout_width="32dp"
    android:layout_height="32dp"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    app:srcCompat="@android:drawable/spinner_background"/>

UserViewDialog.java

Add the following code to the onCreate method (Fragment, Activity) or the constructor (Dialog):

NetworkImageView profilePicture = view.findViewById(R.id.profile_picture);
profilePicture.setImageUrl("http://example.com/users/images/profile.jpg", ImageFetch.getImageLoader(getContext());

Picasso

Another excellent library from Square. Please see the site for some great examples: http://square.github.io/picasso/




回答22:


In simple words,

DO NOT DO NETWORK WORK IN THE UI THREAD

For example, if you do an HTTP request, that is a network action.

Solution:

  1. You have to create a new Thread
  2. Or use AsyncTask class

Way:

Put all your works inside

  1. run() method of new thread
  2. Or doInBackground() method of AsyncTask class.

But:

When you get something from Network response and want to show it on your view (like display response message in TextView), you need to return back to the UI thread.

If you don't do it, you will get ViewRootImpl$CalledFromWrongThreadException.

How to?

  1. While using AsyncTask, update view from onPostExecute() method
  2. Or call runOnUiThread() method and update view inside the run() method.



回答23:


Although above there is a huge solution pool, no one mentioned com.koushikdutta.ion: https://github.com/koush/ion

It's also asynchronous and very simple to use:

Ion.with(context)
.load("http://example.com/thing.json")
.asJsonObject()
.setCallback(new FutureCallback<JsonObject>() {
   @Override
    public void onCompleted(Exception e, JsonObject result) {
        // do stuff with the result or error
    }
});



回答24:


New Thread and AsyncTask solutions have been explained already.

AsyncTask should ideally be used for short operations. Normal Thread is not preferable for Android.

Have a look at alternate solution using HandlerThread and Handler

HandlerThread

Handy class for starting a new thread that has a looper. The looper can then be used to create handler classes. Note that start() must still be called.

Handler:

A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue.

Solution:

  1. Create HandlerThread

  2. Call start() on HandlerThread

  3. Create Handler by getting Looper from HanlerThread

  4. Embed your Network operation related code in Runnable object

  5. Submit Runnable task to Handler

Sample code snippet, which address NetworkOnMainThreadException

HandlerThread handlerThread = new HandlerThread("URLConnection");
handlerThread.start();
handler mainHandler = new Handler(handlerThread.getLooper());

Runnable myRunnable = new Runnable() {
    @Override
    public void run() {
        try {
            Log.d("Ravi", "Before IO call");
            URL page = new URL("http://www.google.com");
            StringBuffer text = new StringBuffer();
            HttpURLConnection conn = (HttpURLConnection) page.openConnection();
            conn.connect();
            InputStreamReader in = new InputStreamReader((InputStream) conn.getContent());
            BufferedReader buff = new BufferedReader(in);
            String line;
            while ( (line =  buff.readLine()) != null) {
                text.append(line + "\n");
            }
            Log.d("Ravi", "After IO call");
            Log.d("Ravi",text.toString());

        }catch( Exception err){
            err.printStackTrace();
        }
    }
};
mainHandler.post(myRunnable);

Pros of using this approach:

  1. Creating new Thread/AsyncTask for each network operation is expensive. The Thread/AsyncTask will be destroyed and re-created for next Network operations. But with Handler and HandlerThread approach, you can submit many network operations (as Runnable tasks) to single HandlerThread by using Handler.



回答25:


You are able to move a part of your code into another thread to offload the main thread and avoid getting ANR, NetworkOnMainThreadException, IllegalStateException(e.g. Cannot access database on the main thread since it may potentially lock the UI for a long period of time).

There are some approaches that you should choose depends on the situation

Java Thread or Android HandlerThread

Java threads are one-time use only and die after executing its run method.

HandlerThread is a handy class for starting a new thread that has a looper.

AsyncTask

AsyncTask is designed to be a helper class around Thread and Handler and does not constitute a generic threading framework. AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent package such as Executor, ThreadPoolExecutor and FutureTask.

Thread pool implementation ThreadPoolExecutor, ScheduledThreadPoolExecutor...

ThreadPoolExecutor class that implements ExecutorService which gives fine control on the thread pool (Eg, core pool size, max pool size, keep alive time, etc.)

ScheduledThreadPoolExecutor - a class that extends ThreadPoolExecutor. It can schedule tasks after a given delay or periodically.

FutureTask

FutureTask performs asynchronous processing, however, if the result is not ready yet or processing has not complete, calling get() will be block the thread

AsyncTaskLoaders

AsyncTaskLoaders as they solve a lot of problems that are inherent to AsyncTask

IntentService

This is the defacto choice for long running processing on Android, a good example would be to upload or download large files. The upload and download may continue even if the user exits the app and you certainly do not want to block the user from being able to use the app while these tasks are going on.

JobScheduler

Effectively, you have to create a Service and create a job using JobInfo.Builder that specifies your criteria for when to run the service.

RxJava

Library for composing asynchronous and event-based programs by using observable sequences.

Coroutines (Kotlin)

The main gist of it is, it makes asynchronous code looks so much like synchronous

Read more here, here, here, here




回答26:


RxAndroid is another better alternative to this problem and it saves us from hassles of creating threads and then posting results on Android UI thread. We just need to specify threads on which tasks need to be executed and everything is handled internally.

Observable<List<String>> musicShowsObservable = Observable.fromCallable(new Callable<List<String>>() { 

  @Override 
  public List<String> call() { 
    return mRestClient.getFavoriteMusicShows(); 
  }
});

mMusicShowSubscription = musicShowsObservable
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer<List<String>>() {

    @Override 
    public void onCompleted() { }

    @Override 
    public void onError(Throwable e) { }

    @Override 
    public void onNext(List<String> musicShows){
        listMusicShows(musicShows);
    }
});

  1. By specifiying (Schedulers.io()),RxAndroid will run getFavoriteMusicShows() on a different thread.

  2. By using AndroidSchedulers.mainThread() we want to observe this Observable on the UI thread, i.e. we want our onNext() callback to be called on the UI thread




回答27:


This works. Just made Dr.Luiji's answer a little simpler.

new Thread() {
    @Override
    public void run() {
        try {
            //Your code goes here
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}.start();



回答28:


On Android, network operations cannot be run on the main thread. You can use Thread, AsyncTask (short-running tasks), Service (long-running tasks) to do network operations.




回答29:


Accessing network resources from the main (UI) thread cause this exception. Use a separate thread or AsyncTask for accessing a network resource to avoid this problem.




回答30:


There is another very convenient way for tackling this issue - use rxJava's concurrency capabilities. You can execute any task in background and post results to main thread in a very convenient way, so these results will be handed to processing chain.

The first verified answer advice is to use AsynTask. Yes, this is a solution, but it is obsolete nowadays, because there are new tools around.

String getUrl() {
    return "SomeUrl";
}

private Object makeCallParseResponse(String url) {
    return null;
    //
}

private void processResponse(Object o) {

}

The getUrl method provides the URL address, and it will be executed on the main thread.

makeCallParseResponse(..) - does actual work

processResponse(..) - will handle result on main thread.

The code for asynchronous execution will look like:

rx.Observable.defer(new Func0<rx.Observable<String>>() {
    @Override
    public rx.Observable<String> call() {
        return rx.Observable.just(getUrl());
    }
})
    .subscribeOn(Schedulers.io())
    .observeOn(Schedulers.io())
    .map(new Func1<String, Object>() {
        @Override
        public Object call(final String s) {
            return makeCallParseResponse(s);
        }
    })
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(new Action1<Object>() {
        @Override
        public void call(Object o) {
             processResponse(o);
        }
    },
    new Action1<Throwable>() {
        @Override
        public void call(Throwable throwable) {
            // Process error here, it will be posted on
            // the main thread
        }
    });

Compared to AsyncTask, this method allow to switch schedulers an arbitrary number of times (say, fetch data on one scheduler and process those data on another (say, Scheduler.computation()). You can also define you own schedulers.

In order to use this library, include following lines into you build.gradle file:

   compile 'io.reactivex:rxjava:1.1.5'
   compile 'io.reactivex:rxandroid:1.2.0'

The last dependency includes support for the .mainThread() scheduler.

There is an excellent ebook for rx-java.



来源：https://stackoverflow.com/questions/52072664/how-to-connect-android-device-with-3d-printer

标签
java
android
android-networking
networkonmainthread
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