Question

长话短说，我有一个处理我的应用程序共享首选项的类。我从各种其他类中调用它没有问题，但当我尝试从我的服务（从相同的APK）调用它时，我得到一个null异常。我猜它是从错误的上下文或类似的东西中调用的。这是相关的代码。

MainActivity.java

package com.deskwizard.audiomanager;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;

import com.deskwizard.audiomanager.DataSaveRestore;

public class MainActivity extends FragmentActivity {

    public static Context contextOfApplication;

    final FragmentManager fm = getFragmentManager();

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

        contextOfApplication = getApplicationContext();

        FragmentTransaction ft = fm.beginTransaction();
        ft.replace(R.id.fragment_settings, new FadeBalanceFragment());
        ft.commit();

        // TODO: Load previously saved settings for all values
        DataSaveRestore.restore_all();
        // TODO: init I2C
    }

    public static Context getContextOfApplication() {
        return contextOfApplication;
    }
}

DataSaveRestore.java（defaultpreferences类）

package com.deskwizard.audiomanager;

import android.app.Application;
import android.content.Context;
import android.content.SharedPreferences;
import android.preference.PreferenceManager;
import android.util.Log;

public class DataSaveRestore extends Application {

    // Data variables
    public static int Bass_level, Bass_CFreq, Bass_Qfact, Sub_level,
            Sub_Lowpass, Treble_level, Treble_CFreq, Mid_level, Mid_CFreq,
            Mid_Qfact, Fade, Balance, Loudness_level, Loudness_freq,
            Loudness_boost;

    static boolean Bass_DCMode, Loudness_state;

    static Context applicationContext = MainActivity.getContextOfApplication();

    public static void restore_all() {

        SharedPreferences prefs = PreferenceManager
                .getDefaultSharedPreferences(applicationContext);

        if (prefs.getInt("data_saved", 0) == 0) {
            set_defaults();
            load_defaults();
        } else {
            load_defaults();
        }
        //TODO: send settings to TDA7418
        DS3903.set_lowpass(DataSaveRestore.Sub_Lowpass);
    };

服务代码段：

public class AudioManagerService extends Service {

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // TODO do something useful
        Log.d("com.deskwizard.audiomanager", "starting service...");

        DataSaveRestore.restore_all(); // restore settings to TDA7418/DS3903
        start();
        return Service.START_STICKY;
    }

Null Exception错误引用此行，只有从服务调用时，它才能从主应用程序和其他类中正常工作：

    SharedPreferences prefs = PreferenceManager
            .getDefaultSharedPreferences(applicationContext);

如果有任何进一步的代码可以帮助缩小范围，请告诉我。谢谢， DW

Answer 1

因为，在您的服务中，当您致电时，DataSaveRestore.restore_all();它会参考，（因为Servce没有可用的MainActivity上下文）

static Context applicationContext = MainActivity.getContextOfApplication();

在这一行，applicationContext将 null ，因为它无法找到MainActivity初始化

简单地说，只需从Application类更改restore_all()方法即可。

首先删除静态并使用Android应用程序类方法的getApplicationContext()来获取应用程序上下文，如服务中所示，

public void restore_all() {

    SharedPreferences prefs = PreferenceManager
            .getDefaultSharedPreferences(getApplicationContext());

    if (prefs.getInt("data_saved", 0) == 0) {
        set_defaults();
        load_defaults();
    } else {
        load_defaults();
    }
    //TODO: send settings to TDA7418
    DS3903.set_lowpass(DataSaveRestore.Sub_Lowpass);
};

现在通过初始化Application类的对象而不是静态方式来调用restore_all();。

像，

DataSaveRestore  dataSaveRestore  = (DataSaveRestore) getApplicationContext();

dataSaveRestore.restore_all();

从服务访问类中的defaultSharedPreferences

1 个答案: