java - 启动服务时出错

Question

我正在尝试附加一个简单的服务来下载文件，但在运行该服务方法时出现错误。看起来观察者没有得到 watcher = binder.getService();，但不确定为什么，因为我正在按照给定的示例进行操作。此应用程序的目的是使用前台服务从网络下载文件，检查更新，并在更新时保存新文件。我只是想首先下载该服务。

04-12 13:19:07.023    2801-2801/com.peterchappy.filewatcher E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.peterchappy.filewatcher, PID: 2801
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.peterchappy.filewatcher/com.peterchappy.filewatcher.activities.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.peterchappy.filewatcher.services.WatcherService.lookForUpdates(java.lang.String)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.peterchappy.filewatcher.services.WatcherService.lookForUpdates(java.lang.String)' on a null object reference
            at com.peterchappy.filewatcher.activities.MainActivity.onCreate(MainActivity.java:33)
            at android.app.Activity.performCreate(Activity.java:5937)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

MainActivity.java

package com.peterchappy.filewatcher.activities;

import android.app.Activity;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.IBinder;
import android.os.Bundle;


import com.peterchappy.filewatcher.R;
import com.peterchappy.filewatcher.services.WatcherService;


public class MainActivity extends Activity {

    String dataURL;
    boolean connected;
    WatcherService watcher;

    @Override
    public void onStart(){
        super.onStart();
        Intent serviceIntent = new Intent(this, WatcherService.class);
        bindService(serviceIntent, myConnection, Context.BIND_AUTO_CREATE);
        watcher.lookForUpdates("http://www.reddit.com");
    }

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

    }

    @Override
    public void onStop(){
        super.onStop();
        unbindService(myConnection);

    }

    ServiceConnection myConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            WatcherService.TestBinder binder = (WatcherService.TestBinder) service;
            watcher = binder.getService();

            connected = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            connected = false;
        }
    };
}

WatcherService.java

package com.peterchappy.filewatcher.services;

import android.content.Intent;
import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.net.URLConnection;
import android.app.Notification;
import android.app.PendingIntent;
import android.app.Service;
import android.os.Binder;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.util.Log;

import com.peterchappy.filewatcher.activities.MainActivity;

public class WatcherService extends Service {


    IBinder myBinder = new TestBinder();

    public WatcherService() {
    }

    @Override
    public IBinder onBind(Intent intent) {


        Notification.Builder n;

        Intent i = new Intent(this, MainActivity.class);
        i.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        i.setAction("SOME_ACTION");
        PendingIntent pIntent = PendingIntent.getActivity(this, 0, i, 0);
        n  = new Notification.Builder(this)
                .setContentTitle("FileWatcher")
                .setContentText("Your FileWatcher is running")
                .setContentIntent(pIntent)
                .setAutoCancel(false);

        startForeground(1, n.build());
        return myBinder;
    }

    public class TestBinder extends Binder {
        public WatcherService getService(){
            return WatcherService.this;
        }
    }

    public void lookForUpdates(final String urlToDownload){

        Thread t = new Thread () {
            @Override
            public void run (){
                try {
                    URL url = new URL(urlToDownload);
                    URLConnection connection = url.openConnection();
                    connection.connect();

                    InputStream input = new BufferedInputStream(connection.getInputStream());
                    OutputStream output = new FileOutputStream("/" + urlToDownload + "_temp");

                    byte data[] = new byte[1024];
                    int count;
                    while ((count = input.read(data)) != -1) {
                        output.write(data, 0, count);
                    }

                    output.flush();
                    output.close();
                    input.close();

                    Log.d("Download", "Complete");
                } catch (IOException e) {
                    Log.d("Download", e.getMessage().toString());
                }
            }
        };
        t.start();
    }
}

list

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".activities.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
    <service android:name=".WatcherService"/>
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

</manifest>

Answer 1

bindService() 是异步的。当 bindService() 返回时，watcher 仍将为 null，因为该服务甚至还不存在，更不用说为您提供 Binder 了。相反，请将 lookForUpdates() 调用移至 onServiceConnected()。