Java Android：如何将JSON对象从AsncTask类传递到主类

我有一个

JSONObject

从

AsyncTask

类中的API调用接收数据。我想将该JSON对象传递给我的

MainActivity

类，以在GUI中显示

JSONObject

数据。我在一些搜索中找到了一些解决方案，并提出了一个接近的解决方案。但是当我从

MainActivity

类访问时，它会说

JSONObject

为空。我这里有什么不对的？这是最好的方法吗

下面是我的

AsyncTask

类

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
import org.json.JSONObject;

/**
 * Created by Nisal on 13-Sep-17.
 */

public class GetStationsAPICall extends AsyncTask<String, Void, JSONObject> {

    Context ctx;
    JSONObject responseObj;
    String result;

    public interface AsyncResponse {
        void processFinish(JSONObject output);
    }

    public AsyncResponse delegate = null;

    public GetStationsAPICall(AsyncResponse delegate){
        this.delegate = delegate;
    }

//    GetStationsAPICall(Context ctx){
//        this.ctx=ctx;
//    }

    @Override
    protected JSONObject doInBackground(String... params) {

        String method = params[0];

        if(method.equals("getStations")){

            try {
                HttpClient client = new DefaultHttpClient();
                String getURL = "http://api.gate.com/?lang=en";
                HttpGet httpGet = new HttpGet(getURL);
                httpGet .setHeader("Authorization", "Bearer 690");

                HttpResponse response = client.execute(httpGet);
                HttpEntity resEntity = response.getEntity();
                if (resEntity != null) {
                    //parse response.
                    Log.e("Response", EntityUtils.toString(resEntity));
//                    return "Successfully Connected!";
                }else{
//                    return "Connection Failed!";
                }

            } catch (Exception e) {
                e.printStackTrace();
//                return "Connection Failed!";
            }
        }

        return null;
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(JSONObject obj) {
        delegate.processFinish(obj);
    }
}

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import org.json.JSONObject;

public class MainActivity extends Activity implements GetStationsAPICall.AsyncResponse{

    Button btnSearch;
    String method = "getStations";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        GetStationsAPICall getStations = new GetStationsAPICall(this);
        new GetStationsAPICall(this).execute(method);
    }

    public void searchClicked(View view){
        Toast.makeText(MainActivity.this,"Search Clicked",Toast.LENGTH_SHORT).show();
    }

    @Override
    public void processFinish(JSONObject output) {
        Toast.makeText(MainActivity.this,"ProcessFinish",Toast.LENGTH_SHORT).show();

        if(output != null){
            Toast.makeText(MainActivity.this,"not null",Toast.LENGTH_SHORT).show();
        }else{
            Toast.makeText(MainActivity.this," null",Toast.LENGTH_SHORT).show();
        }
    }
}

---

我可以在

AsyncTask

类中获取

JSONObject

，但是当我尝试将它传递给

MainActivity

类并在那里使用它时。JSONObject变为

null

。我在这里做错了什么？

在执行之前，您正在重新创建

GetStationsAPICall

对象。因此，您的

AsyncResponse

对象将为空

按如下方式更改代码：

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        GetStationsAPICall getStations = new GetStationsAPICall(this);
        getStations.execute(method);
    }

---

在执行之前，您正在重新创建

GetStationsAPICall

对象。因此，您的

AsyncResponse

对象将为空

按如下方式更改代码：

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        GetStationsAPICall getStations = new GetStationsAPICall(this);
        getStations.execute(method);
    }

---

虽然我确信您已经找到了一个有效的解决方案，但我不建议将您的活动设置为一个可能泄漏它的侦听器（如果在返回结果之前按home按钮会发生什么情况？）。另外，如前所述，您遇到的主要问题是从

doInBackground

函数返回

null

。现在让我们来讨论这两个问题：

MainActivity.java

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import android.content.BroadcastReceiver;
import android.content.IntentFilter;
import android.support.v4.content.LocalBroadcastManager;

import org.json.JSONObject;

public class MainActivity extends Activity {

    Button btnSearch;
    String method = "getStations";
    BroadcastReceiver apiListener;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        BroadcastReceiver apiListener = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String output = intent.getStringExtra("API_Response); //Getting the string extra you input earlier in the AsyncTask.
                Toast.makeText(MainActivity.this,"ProcessFinish",Toast.LENGTH_SHORT).show();

                if(output != null){
                    Toast.makeText(MainActivity.this,"not null",Toast.LENGTH_SHORT).show();
                }else{
                    Toast.makeText(MainActivity.this," null",Toast.LENGTH_SHORT).show();
                }
            }
        };

        //Since you are starting your AsyncTask here, might want to set up the receiver prior to calling the task.
        LocalBroadcastManager.getInstance(this).registerReceiver(apiListener, new IntentFilter(""));

        GetStationsAPICall getStations = new GetStationsAPICall(this);
        new GetStationsAPICall(this).execute(method);
    }

    public void searchClicked(View view){
        Toast.makeText(MainActivity.this,"Search Clicked",Toast.LENGTH_SHORT).show();
    }

    //Registers the receiver whenever this acitivty is currently being shown or about to.
    @Override
    public void onStart() {
        super.onStart();
        LocalBroadcastManager.getInstance(this)
            .registerReceiver(apiListener, new IntentFilter("StationsAPI")); //This is the intent filter this receiver will be listening for
    }

    //This stops the receiver from listening when the activity is no longer shown.
    @Override
    public void onStop() {
        super.onStop();
        LocalBroadcastManager.getInstance(this).unregisterReceiver(apiListener);
    }
}

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;
import android.content.Intent;
import android.support.v4.content.LocalBroadcastManager;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
import org.json.JSONObject;

/**
* Created by Nisal on 13-Sep-17.
*/

public class GetStationsAPICall extends AsyncTask<String, Void, String> {

    LocalBroadcastManager localBroadcastManager; //This is the manager to send a result back to your activity

    GetStationsAPICall(Context ctx){
        localBroadcastManager = LocalBroadcastManager.getInstance(ctx); //No matter where you call this, it is the same object throughout your app.
    }

    @Override
    protected String doInBackground(String... params) {

        String method = params[0];

        if(method.equals("getStations")){

            try {
                HttpClient client = new DefaultHttpClient();
                String getURL = "http://api.gate.com/?lang=en";
                HttpGet httpGet = new HttpGet(getURL);
                httpGet .setHeader("Authorization", "Bearer 690");

                HttpResponse response = client.execute(httpGet);
                HttpEntity resEntity = response.getEntity();
                if (resEntity != null) {
                    //parse response.
                    String responseString = EntityUtils.toString(resEntity);
                    Log.e("Response", responseString;
                    return responseString; //This will be the parameter of your onPostExecute method
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        return null;
    }

    @Override
    protected void onPostExecute(String obj) {
        localBroadcastManager
            .sendBroadcast( //Sending the intent with the response!
                new Intent("StationsAPI") //Remember that IntentFilter? Here is where you declare where to send intent.
                    .putExtra("API_Response", obj)); //This is the extra data you want to send to your receivers.
    }
}

我做了不少修改，并发表了一些评论。这里需要注意的最重要的一点是，此活动不再是侦听器，因为我已经将此功能替换为。使用此功能，我可以注册任意数量的

BroadcastReceiver

对象来处理AsyncTask的响应，而无需担心中断AsyncTask的进程

如您所见，我使用了一个

IntentFilter

，让经理知道只有执行此操作的intent（“StationsAPI”）才能进入我注册的接收方（

apistener

）。另外，我没有可能泄露活动的情况，我会在活动不再可见时立即注销接收器，并在活动再次可见时重新注册

GetStationsAPICall.java

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import android.content.BroadcastReceiver;
import android.content.IntentFilter;
import android.support.v4.content.LocalBroadcastManager;

import org.json.JSONObject;

public class MainActivity extends Activity {

    Button btnSearch;
    String method = "getStations";
    BroadcastReceiver apiListener;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        BroadcastReceiver apiListener = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String output = intent.getStringExtra("API_Response); //Getting the string extra you input earlier in the AsyncTask.
                Toast.makeText(MainActivity.this,"ProcessFinish",Toast.LENGTH_SHORT).show();

                if(output != null){
                    Toast.makeText(MainActivity.this,"not null",Toast.LENGTH_SHORT).show();
                }else{
                    Toast.makeText(MainActivity.this," null",Toast.LENGTH_SHORT).show();
                }
            }
        };

        //Since you are starting your AsyncTask here, might want to set up the receiver prior to calling the task.
        LocalBroadcastManager.getInstance(this).registerReceiver(apiListener, new IntentFilter(""));

        GetStationsAPICall getStations = new GetStationsAPICall(this);
        new GetStationsAPICall(this).execute(method);
    }

    public void searchClicked(View view){
        Toast.makeText(MainActivity.this,"Search Clicked",Toast.LENGTH_SHORT).show();
    }

    //Registers the receiver whenever this acitivty is currently being shown or about to.
    @Override
    public void onStart() {
        super.onStart();
        LocalBroadcastManager.getInstance(this)
            .registerReceiver(apiListener, new IntentFilter("StationsAPI")); //This is the intent filter this receiver will be listening for
    }

    //This stops the receiver from listening when the activity is no longer shown.
    @Override
    public void onStop() {
        super.onStop();
        LocalBroadcastManager.getInstance(this).unregisterReceiver(apiListener);
    }
}

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;
import android.content.Intent;
import android.support.v4.content.LocalBroadcastManager;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
import org.json.JSONObject;

/**
* Created by Nisal on 13-Sep-17.
*/

public class GetStationsAPICall extends AsyncTask<String, Void, String> {

    LocalBroadcastManager localBroadcastManager; //This is the manager to send a result back to your activity

    GetStationsAPICall(Context ctx){
        localBroadcastManager = LocalBroadcastManager.getInstance(ctx); //No matter where you call this, it is the same object throughout your app.
    }

    @Override
    protected String doInBackground(String... params) {

        String method = params[0];

        if(method.equals("getStations")){

            try {
                HttpClient client = new DefaultHttpClient();
                String getURL = "http://api.gate.com/?lang=en";
                HttpGet httpGet = new HttpGet(getURL);
                httpGet .setHeader("Authorization", "Bearer 690");

                HttpResponse response = client.execute(httpGet);
                HttpEntity resEntity = response.getEntity();
                if (resEntity != null) {
                    //parse response.
                    String responseString = EntityUtils.toString(resEntity);
                    Log.e("Response", responseString;
                    return responseString; //This will be the parameter of your onPostExecute method
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        return null;
    }

    @Override
    protected void onPostExecute(String obj) {
        localBroadcastManager
            .sendBroadcast( //Sending the intent with the response!
                new Intent("StationsAPI") //Remember that IntentFilter? Here is where you declare where to send intent.
                    .putExtra("API_Response", obj)); //This is the extra data you want to send to your receivers.
    }
}

导入android.content.Context；
导入android.os.AsyncTask；
导入android.util.Log；
导入android.content.Intent；
导入android.support.v4.content.LocalBroadcastManager；
导入org.apache.http.HttpEntity；
导入org.apache.http.HttpResponse；
导入org.apache.http.client.HttpClient；
导入org.apache.http.client.methods.HttpGet；
导入org.apache.http.impl.client.DefaultHttpClient；
导入org.apache.http.util.EntityUtils；
导入org.json.JSONObject；
/**
*Nisal于2017年9月13日创建。
*/
公共类GetStationsAPICall扩展异步任务{
LocalBroadcastManager LocalBroadcastManager；//这是将结果发送回“活动”的管理器
GetStationsAPICall（上下文ctx）{
localBroadcastManager=localBroadcastManager.getInstance（ctx）；//无论您在哪里调用它，它在整个应用程序中都是同一个对象。
}
@凌驾
受保护的字符串doInBackground（字符串…参数）{
字符串方法=参数[0]；
if（方法等于（“getStations”））{
试一试{
HttpClient=new DefaultHttpClient（）；
字符串getURL=”http://api.gate.com/?lang=en";
HttpGet-HttpGet=新的HttpGet（getURL）；
httpGet.setHeader（“授权”、“承载690”）；
HttpResponse response=client.execute（httpGet）；
HttpEntity当前性=response.getEntity（）；
if（最近性！=null）{
//解析响应。
字符串responseString=EntityUtils.toString（最近性）；
Log.e（“响应”，响应预算；
return responseString；//这将是onPostExecute方法的参数
}
}捕获（例外e）{
e、 printStackTrace（）；
}
}
返回null；
}
@凌驾
受保护的void onPostExecute（字符串obj）{
本地广播管理器
.sendBroadcast（//发送带有响应的意图！
new Intent（“StationAPI”）//还记得IntentFilter吗？这里是声明发送Intent的位置。
.putExtra（“API_Response”，obj））；//这是要发送给接收者的额外数据。
}
}

与上面的

MainActivity

类一样，我已经用我所做的更改对部分进行了注释。

JSONObject

类是不可打包的，因此我没有创建它，然后将其重新转换为可以放在

Intent

中的内容，而是将其作为

字符串

对象保留，并在

doInBackground

方法

---

现在，无论何时调用

GetStationsAPICall

任务，您都应该在

MainActivity

（或任何其他带有

IntentFilter

用于“API\U响应”的接收器）中得到响应。尽管此解决方案本身并不完美（如果用户旋转设备会怎么样？），它应该有助于避免Android所提供的一些陷阱。

尽管我确信您已经找到了一个可行的解决方案，但我不建议将您的活动放入一个可能会泄露它的侦听器中（如果在返回结果之前按home按钮会发生什么情况？）。此外，如前所述，您遇到的主要问题是从

doInBackground

函数返回

null

。因此，现在我们来解决这两个问题：

MainActivity.java

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import android.content.BroadcastReceiver;
import android.content.IntentFilter;
import android.support.v4.content.LocalBroadcastManager;

import org.json.JSONObject;

public class MainActivity extends Activity {

    Button btnSearch;
    String method = "getStations";
    BroadcastReceiver apiListener;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        BroadcastReceiver apiListener = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String output = intent.getStringExtra("API_Response); //Getting the string extra you input earlier in the AsyncTask.
                Toast.makeText(MainActivity.this,"ProcessFinish",Toast.LENGTH_SHORT).show();

                if(output != null){
                    Toast.makeText(MainActivity.this,"not null",Toast.LENGTH_SHORT).show();
                }else{
                    Toast.makeText(MainActivity.this," null",Toast.LENGTH_SHORT).show();
                }
            }
        };

        //Since you are starting your AsyncTask here, might want to set up the receiver prior to calling the task.
        LocalBroadcastManager.getInstance(this).registerReceiver(apiListener, new IntentFilter(""));

        GetStationsAPICall getStations = new GetStationsAPICall(this);
        new GetStationsAPICall(this).execute(method);
    }

    public void searchClicked(View view){
        Toast.makeText(MainActivity.this,"Search Clicked",Toast.LENGTH_SHORT).show();
    }

    //Registers the receiver whenever this acitivty is currently being shown or about to.
    @Override
    public void onStart() {
        super.onStart();
        LocalBroadcastManager.getInstance(this)
            .registerReceiver(apiListener, new IntentFilter("StationsAPI")); //This is the intent filter this receiver will be listening for
    }

    //This stops the receiver from listening when the activity is no longer shown.
    @Override
    public void onStop() {
        super.onStop();
        LocalBroadcastManager.getInstance(this).unregisterReceiver(apiListener);
    }
}

import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;
import android.content.Intent;
import android.support.v4.content.LocalBroadcastManager;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;
import org.json.JSONObject;

/**
* Created by Nisal on 13-Sep-17.
*/

public class GetStationsAPICall extends AsyncTask<String, Void, String> {

    LocalBroadcastManager localBroadcastManager; //This is the manager to send a result back to your activity

    GetStationsAPICall(Context ctx){
        localBroadcastManager = LocalBroadcastManager.getInstance(ctx); //No matter where you call this, it is the same object throughout your app.
    }

    @Override
    protected String doInBackground(String... params) {

        String method = params[0];

        if(method.equals("getStations")){

            try {
                HttpClient client = new DefaultHttpClient();
                String getURL = "http://api.gate.com/?lang=en";
                HttpGet httpGet = new HttpGet(getURL);
                httpGet .setHeader("Authorization", "Bearer 690");

                HttpResponse response = client.execute(httpGet);
                HttpEntity resEntity = response.getEntity();
                if (resEntity != null) {
                    //parse response.
                    String responseString = EntityUtils.toString(resEntity);
                    Log.e("Response", responseString;
                    return responseString; //This will be the parameter of your onPostExecute method
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        return null;
    }

    @Override
    protected void onPostExecute(String obj) {
        localBroadcastManager
            .sendBroadcast( //Sending the intent with the response!
                new Intent("StationsAPI") //Remember that IntentFilter? Here is where you declare where to send intent.
                    .putExtra("API_Response", obj)); //This is the extra data you want to send to your receivers.
    }
}

我用一些com做了很多更改