"在onCreate()之前,活动无法使用系统服务"错误消息？

Question

当用户点击我的应用程序中的图标时,我希望应用程序首先检查设备是否已连接到互联网,然后根据收到的结果执行某些操作(因为知道它只是弹出一个对话框,通知设备是否是是否连接).所以我写了这段代码:

public class MainActivity extends Activity {

// SOME CONSTANTS WILL BE DEFINED HERE

AlertDialog.Builder builder = new AlertDialog.Builder(this);

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

    findViewById(R.id.icoMyIcon).setOnClickListener(listener);
}


private OnClickListener listener = new OnClickListener() {

    public void onClick(View v) {
        if (isNetworkConnected()) {
            builder.setMessage("Internet connected!").setCancelable(false)
            .setPositiveButton("OK", null);
            builder.create().show();
        } else {
            builder.setMessage("Internet isn\'t connected!")
            .setCancelable(false)
            .setPositiveButton("OK", null);
            builder.create().show();
        }

    }
};


// Check if the device is connected to the Internet
private boolean isNetworkConnected() {
    ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo ni = cm.getActiveNetworkInfo();
    if (ni == null) {
        // There are no active networks.
        return false;
    } else
        return true;
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}

}

当我试图在模拟器上运行此应用程序时,它一直在破碎,我在LogCat中收到此错误消息:

07-24 22:59:45.034: E/AndroidRuntime(894): FATAL EXCEPTION: main
07-24 22:59:45.034: E/AndroidRuntime(894): java.lang.RuntimeException: Unable to 
    instantiate activity ComponentInfo{com.my.app/com.my.app.MainActivity}: 
    java.lang.IllegalStateException: System services not available to Activities before onCreate()
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.os.Handler.dispatchMessage(Handler.java:99)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.os.Looper.loop(Looper.java:123)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.ActivityThread.main(ActivityThread.java:4627)
07-24 22:59:45.034: E/AndroidRuntime(894):  at java.lang.reflect.Method.invokeNative(Native Method)
07-24 22:59:45.034: E/AndroidRuntime(894):  at java.lang.reflect.Method.invoke(Method.java:521)
07-24 22:59:45.034: E/AndroidRuntime(894):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-24 22:59:45.034: E/AndroidRuntime(894):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-24 22:59:45.034: E/AndroidRuntime(894):  at dalvik.system.NativeStart.main(Native Method)
07-24 22:59:45.034: E/AndroidRuntime(894): Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate()
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.Activity.getSystemService(Activity.java:3526)
07-24 22:59:45.034: E/AndroidRuntime(894):  at com.android.internal.app.AlertController$AlertParams.<init>(AlertController.java:743)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.AlertDialog$Builder.<init>(AlertDialog.java:273)
07-24 22:59:45.034: E/AndroidRuntime(894):  at com.my.app.MainActivity.<init>(MainActivity.java:24)
07-24 22:59:45.034: E/AndroidRuntime(894):  at java.lang.Class.newInstanceImpl(Native Method)
07-24 22:59:45.034: E/AndroidRuntime(894):  at java.lang.Class.newInstance(Class.java:1429)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
07-24 22:59:45.034: E/AndroidRuntime(894):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
07-24 22:59:45.034: E/AndroidRuntime(894):  ... 11 more

为什么会这样,我该如何解决？我是新手,所以......请温柔!:)

Answer 1

我认为这是因为你在创建之前实例化一个onClick监听器被调用.尝试在onCreate()方法中实例化onClick侦听器.

这可能也可能不是AlertDialog,但我不完全确定.

从技术上讲,我认为导致问题的是以下行:

ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);

但是,因为这是isNetworkConnected()在onClick方法中调用的方法中调用的,所以移动onClick的实例化可以解决问题.

线索是在onCreate()之前的活动不可用的系统服务的例外

Answer 2

错误是由于创建此对象创建.

AlertDialog.Builder builder = new AlertDialog.Builder(this);

你应该在调用onCreate之后执行此操作.