java - 在 Java 中获取 URL 响应抛出异常

Question

我对 Java 还很陌生，正在做一个让我抓狂的类(class)项目。我正在尝试从 iTunes API 中提取 JSON 信息。提取的信息来自某个专辑。我在 WebClass.java 类的 getURLStringResponse() 方法中使用了 try/catch，它抛出了内置异常，我不知道为什么，也根本无法从老师那里得到帮助。所以我正在接触大脑!

如果有帮助，这是我的 github 链接

https://github.com/agvinsant/Java1/tree/master/week3

这是我的主要 Activity

package com.agvinsant.java1application;

import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import com.agvinsant.lib.BasicLayout;
import com.agvinsant.lib.WebClass;

import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.content.Context;
import android.content.res.Resources;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup.LayoutParams;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity {

Context context;
String[] songName;
Resources res;
TextView results;
Spinner viewSpinner;
TextView jsonView;
TextView connectedView;
String trackName;
String artistName;
String albumName;
String trackSite;

ArrayList<String> trackNameList = new ArrayList<String>();
ArrayList<String> artistNameList = new ArrayList<String>();
ArrayList<String> albumNameList = new ArrayList<String>();
ArrayList<String> trackSiteList = new ArrayList<String>();

Boolean connected = false;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    context = this;
    res = getResources();

    // setting the linear layout
    LinearLayout ll = new LinearLayout(this);
    LinearLayout ml = BasicLayout.layoutWithButton(this, "Show Info");
    ll.setOrientation(LinearLayout.VERTICAL);
    LayoutParams lp = new LayoutParams(LayoutParams.MATCH_PARENT,LayoutParams.MATCH_PARENT);
    ll.setLayoutParams(lp);

    // Creating button from BasicLayout class
    Button mb = (Button) ml.findViewById(1);
    mb.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // Get selected song info
            int pos = viewSpinner.getSelectedItemPosition();
            String tName = trackNameList.get(pos).toString();
            String arName = artistNameList.get(pos).toString();
            String alName = albumNameList.get(pos).toString();  
            String tSite = trackSiteList.get(pos).toString();

            jsonView.setText("Song Name: " +tName+ "\r\n" + "Artist Name: " +arName+ "\r\n" +"Album Name: "+alName+ "\r\n" + "Song Website: " +tSite);
        }

    });



    connectedView = new TextView(context);

    //Detecting network settings
            connected = WebClass.getConnectionStatus(context);
            if(connected){
                Log.i("Network Connection", WebClass.getConnectionType(context));
                connectedView.setText("Network Connection: " + WebClass.getConnectionType(context)+"\n");
            }
            else{
                    connectedView.setText(""+WebClass.getConnectionType(context)+"\n");
            }


    // calling the getSongInfo function 
    getSongInfo();

    // song length display
    int songNum = res.getStringArray(R.array.songArray).length;
    TextView tv = new TextView(context);
    tv.setText("Check out one of the "+songNum+" songs on the album");

    //spinner adapter
    ArrayAdapter<String> spinnerAdapter = new ArrayAdapter<String>(context, android.R.layout.simple_spinner_item, songName);
    spinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

    //creating the spinner
    viewSpinner = new Spinner(context);
    viewSpinner.setAdapter(spinnerAdapter);
    lp = new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.WRAP_CONTENT);
    viewSpinner.setLayoutParams(lp);

    //spinner onClick function
    viewSpinner.setOnItemSelectedListener(new OnItemSelectedListener(){

        @Override
        public void onItemSelected(AdapterView<?> parent, View view,
                int position, long id) {
            Toast.makeText(context, "You selected " + songName[position], Toast.LENGTH_LONG).show();
        }

        @Override
        public void onNothingSelected(AdapterView<?> arg0) {
            // TODO Auto-generated method stub

        }

    });


    // setting different layout parts to the main layout
    ll.addView(ml);
    ll.addView(tv);
    ll.addView(viewSpinner);
    //ll.addView(connectedView);
    ll.addView(jsonView);


    // setting the content view
    setContentView(ll);




}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

//get URL
private void getSongInfo(){
    String baseURL = "https://itunes.apple.com/search?term=groove+logic+logical+thinking";
    URL finalURL;
    try{
        finalURL = new URL(baseURL);
        songRequest sr = new songRequest();
        sr.execute(finalURL);
    } catch (MalformedURLException e){
        Log.e("BAD URL", "MALFORMED URL");
        finalURL = null;
    }
}

//get data from URL
private class songRequest extends AsyncTask<URL, Void, String>{
    @Override
    protected String doInBackground(URL... urls){
        String response = "";
        for(URL url: urls){
            response = WebClass.getURLStringResponse(url);
        }
        return response;
    }

    //get data and add to arrays.
    @Override
    protected void onPostExecute(String result){
        try {

            Log.i("URL Response", result);
            JSONArray jsonArray = new JSONArray(result);

            int n = jsonArray.length();
            for(int i = 0;i<n; i++){
                JSONObject jsonObject = jsonArray.getJSONObject(i);

                trackName = jsonObject.getString("trackName");
                artistName= jsonObject.getString("artistName");
                albumName = jsonObject.getString("collectionName");
                trackSite= jsonObject.getString("trackViewUrl");
                trackNameList.add(trackName);
                artistNameList.add(artistName);
                albumNameList.add(albumName);  
                trackSiteList.add(trackSite);
            }

        } catch (JSONException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }
    }   
}

}

这是 WebClass

package com.agvinsant.lib;

import java.io.BufferedInputStream;
import java.net.URL;
import java.net.URLConnection;

import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.util.Log;

public class WebClass {

    static Boolean connection = false;
    static String connectionType = "Unavailable";

    public static String getConnectionType(Context context){

        webInfo(context);
        return connectionType;
    }

    public static Boolean getConnectionStatus(Context context){

        webInfo(context);
        return connection;
    }

    private static void webInfo(Context context){


        ConnectivityManager cm = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo ni = cm.getActiveNetworkInfo();
        if(ni != null){

            if(ni.isConnected()){
                connectionType = ni.getTypeName();
                connection = true;

            }
        }
    }

    public static String getURLStringResponse(URL url){
        String response = "";

        try{

            URLConnection conn = url.openConnection();
            BufferedInputStream bin = new BufferedInputStream(conn.getInputStream());

            byte[] contentBytes = new byte[1024];
            int bytesRead = 0;
            StringBuffer responseBuffer = new StringBuffer();

            while((bytesRead = bin.read(contentBytes)) != -1){

                response = new String(contentBytes,0,bytesRead);
                responseBuffer.append(response);
            }
            return responseBuffer.toString();
        }
        catch(Exception e){
            // This is the exception that is being thrown
            Log.e("URL RESPONSE ERROR", "getURLStringResponse");
        }

        return response;
    }
}

这也是一个处理一些文件响应的 FileClass

package com.agvinsant.lib;

import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

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

// TODO: Auto-generated Javadoc
/**
 * The Class FileClass.
 */
public class FileClass {

    /**
     * Store string file.
     *
     * @param context the context
     * @param filename the filename
     * @param content the content
     * @param external the external
     * @return the boolean
     */
    @SuppressWarnings("resource")
    public static Boolean storeStringFile(Context context, String filename, String content, Boolean external){
        try{
            File file;
            FileOutputStream fos;
            if(external){
                file = new File(context.getExternalFilesDir(null), filename);
                fos = new FileOutputStream(file);
            }else{
                fos = context.openFileOutput(filename, Context.MODE_PRIVATE);
            }
            fos.write(content.getBytes());
            fos.close();
        } catch (IOException e){
            Log.e("WRITE ERROR", filename);
        }
        return true;
    }


    /**
     * Store object file.
     *
     * @param context the context
     * @param filename the filename
     * @param external the external
     * @return the boolean
     */
    public static Boolean storeObjectFile(Context context, String filename, Boolean external){

        return true;
    }


    /**
     * Store object file.
     *
     * @param context the context
     * @param filename the filename
     * @param content the content
     * @param external the external
     * @return the boolean
     */
    @SuppressWarnings("resource")
    public static Boolean storeObjectFile(Context context, String filename, Object content, Boolean external){
        try{
            File file;
            FileOutputStream fos;
            ObjectOutputStream oos;
            if(external){
                file = new File(context.getExternalFilesDir(null), filename);
                fos = new FileOutputStream(file);
            } else {
                fos = context.openFileOutput(filename, Context.MODE_PRIVATE);
            }
            oos = new ObjectOutputStream(fos);
            oos.writeObject(content);
            oos.close();
            fos.close();
        } catch (IOException e){
            Log.e("WRITE ERROR", filename);
        }
        return true;
    }


    /**
     * Read string file.
     *
     * @param context the context
     * @param filename the filename
     * @param external the external
     * @return the string
     */
    @SuppressWarnings("resource")
    public static String readStringFile(Context context, String filename, Boolean external){
        String content = "";
        try{
            File file;
            FileInputStream fin;
            if(external){
                file = new File(context.getExternalFilesDir(null), filename);
                fin = new FileInputStream(file);
            } else {
                file = new File(filename);
                fin = context.openFileInput(filename);
            }
            BufferedInputStream bin = new BufferedInputStream(fin);
            byte[] contentBytes = new byte[1024];
            int bytesRead = 0;
            StringBuffer contentBuffer = new StringBuffer();

            while((bytesRead = bin.read(contentBytes)) != -1){
                content = new String(contentBytes,0,bytesRead);
                contentBuffer.append(content);
            }
            content = contentBuffer.toString();
            fin.close();
        } catch (FileNotFoundException e) {
            Log.e("READ ERROR", "FILE NOT FOUND " + filename);
        } catch (IOException e){
            Log.e("READ ERROR", "I/O ERROR");
        }
        return content;
    }


    /**
     * Read object file.
     *
     * @param context the context
     * @param filename the filename
     * @param external the external
     * @return the object
     */
    @SuppressWarnings("resource")
    public static Object readObjectFile(Context context, String filename, Boolean external){
        Object content = new Object();
        try{
            File file;
            FileInputStream fin;
            if(external){
                file = new File(context.getExternalFilesDir(null), filename);
                fin = new FileInputStream(file);
            } else {
                file = new File(filename);
                fin = context.openFileInput(filename);
            }

            ObjectInputStream ois = new ObjectInputStream(fin);
            try{
                content = (Object) ois.readObject();
            } catch (ClassNotFoundException e){
                Log.e("READ ERROR", "INVALID JAVA OBJECT FILE");
            }
            ois.close();
            fin.close();
        } catch (FileNotFoundException e) {
            Log.e("READ ERROR", "FILE NOT FOUND " + filename);
            return null;
        } catch (IOException e){
            Log.e("READ ERROR", "I/O ERROR");
        }
        return content;
    }

}

这是 LogCat 运行时显示的内容

09-11 21:11:25.496: E/Trace(2500): error opening trace file: No such file or directory (2)
09-11 21:11:25.676: I/Network Connection(2500): mobile
09-11 21:11:25.707: D/AndroidRuntime(2500): Shutting down VM
09-11 21:11:25.707: W/dalvikvm(2500): threadid=1: thread exiting with uncaught exception (group=0xb2f55288)
09-11 21:11:25.756: E/AndroidRuntime(2500): FATAL EXCEPTION: main
09-11 21:11:25.756: E/AndroidRuntime(2500): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.agvinsant.java1application/com.agvinsant.java1application.MainActivity}: java.lang.NullPointerException
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.ActivityThread.access$600(ActivityThread.java:130)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.os.Looper.loop(Looper.java:137)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.ActivityThread.main(ActivityThread.java:4745)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at java.lang.reflect.Method.invokeNative(Native Method)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at java.lang.reflect.Method.invoke(Method.java:511)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at dalvik.system.NativeStart.main(Native Method)
09-11 21:11:25.756: E/AndroidRuntime(2500): Caused by: java.lang.NullPointerException
09-11 21:11:25.756: E/AndroidRuntime(2500):     at java.util.Arrays$ArrayList.<init>(Arrays.java:38)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at java.util.Arrays.asList(Arrays.java:154)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:128)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at com.agvinsant.java1application.MainActivity.onCreate(MainActivity.java:121)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.Activity.performCreate(Activity.java:5008)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
09-11 21:11:25.756: E/AndroidRuntime(2500):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
09-11 21:11:25.756: E/AndroidRuntime(2500):     ... 11 more
09-11 21:11:25.766: E/URL RESPONSE ERROR(2500): getURLStringResponse

Answer 1

在您的 onCreate 方法中，您有

ArrayAdapter<String> spinnerAdapter = new ArrayAdapter<String>(context, android.R.layout.simple_spinner_item, songName);

但是

String[] songName;

已声明但未初始化，即。默认情况下为 null。您需要初始化它并可能填充它。

ArrayAdapter 构造函数尝试使用此代码创建一个 ArrayList(不是 java.util 类型)

   ArrayList(E[] array) {
        if (array==null)
            throw new NullPointerException();
        a = array;
    }

所以你会得到一个NullPointerException。

<小时/>

请学习使用调试器并读取堆栈跟踪。