写Service Demo的时候遇到的一个问题小结一下

我写了一个  Service 的Demo，但是将启动Service的Intent声明为主Activity的成员变量时，

public class MyServiceActivity extends Activity {
	private Button startSer1;
	private Button stopSer1;
	private Button startSer2;
	private Button stopSer2;
	public static TextView log;
	private Intent intent = new Intent(MyServiceActivity.this, MyService.class);

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		startSer1 = (Button) findViewById(R.id.startSer1);
		stopSer1 = (Button) findViewById(R.id.stopSer1);

		startSer2 = (Button) findViewById(R.id.startSer2);
		stopSer2 = (Button) findViewById(R.id.stopSer2);

		log = (TextView) findViewById(R.id.log);

		startSer1.setOnClickListener(btnListener);
		stopSer1.setOnClickListener(btnListener);

		startSer2.setOnClickListener(btnListener);
		stopSer2.setOnClickListener(btnListener);
	}

	private OnClickListener btnListener = new OnClickListener() {
		@Override
		public void onClick(View v) {
			switch (v.getId()) {
				case R.id.startSer1:
					updateLog("Start Service 1 pressed");
					startService(intent);
					break;
				case R.id.startSer2:
					updateLog("Start Service 2 pressed");
					startService(intent);
					break;
				case R.id.stopSer1:
					updateLog("Stop Service 1 pressed");
					stopService(intent);
					break;
				case R.id.stopSer2:
					updateLog("Stop Service 2 pressed");
					stopService(intent);
					break;
				default:
					break;
			}
		}
	};

	public static void updateLog(String text) {
		CharSequence ch = log.getText();
		log.setText(((ch == null || ch.length() == 0) ? text : ch.toString()
				+ "\r\n" + text));
	}
}

 
系统总是在启动的报异常：

08-08 12:03:22.894: ERROR/AndroidRuntime(2662): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.archer.rainbow/com.archer.rainbow.MyServiceActivity}: java.lang.NullPointerException
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1570)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1664)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.ActivityThread.access$1600(ActivityThread.java:118)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:949)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.os.Looper.loop(Looper.java:130)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.ActivityThread.main(ActivityThread.java:3711)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at java.lang.reflect.Method.invokeNative(Native Method)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at java.lang.reflect.Method.invoke(Method.java:507)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at dalvik.system.NativeStart.main(Native Method)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662): Caused by: java.lang.NullPointerException
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.content.ContextWrapper.getPackageName(ContextWrapper.java:120)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.content.ComponentName.<init>(ComponentName.java:75)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.content.Intent.<init>(Intent.java:2936)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at com.archer.rainbow.MyServiceActivity$1.<init>(MyServiceActivity.java:40)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at com.archer.rainbow.MyServiceActivity.<init>(MyServiceActivity.java:38)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at java.lang.Class.newInstanceImpl(Native Method)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at java.lang.Class.newInstance(Class.java:1409)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1562)
08-08 12:03:22.894: ERROR/AndroidRuntime(2662):     ... 11 more

 

一开始，我还以为是我的哪个变量没有初始化呢，结果，看来看去，一共没几个变量啊，而且都应该正常赋值了，所以就很是纳闷了。但这个问题不解决不行啊，所以我就在一EOE论坛里发贴求高手解答。虽然有几个热心的网友帮忙给了些建议，但都不奏效啊。没办法，自己再想吧。后来，我换了一种写法： 

private final Intent intent = new Intent(); 
intent.setComponent(new ComponentName(MyServiceActivity.this.getPackageName(), MyService.class.toString()));

 

并在第二句打了个断点进行了DEBUG，按F5进去之后发现：

public String getPackageName() { 
        return mBase.getPackageName(); 
    } 

 

中的 mBase 竟然为 null ,无语了。这个方法可是ContextWrapper里的啊，Activity继承自它，怎么会没有传这个值给它呢？后来发现，Activity在OnCreate之前进行init的时候，调用了 

public ContextWrapper(Context base) { 
        mBase = base; 
    }

 

此时，传进去的 base 即为 null，所以这样一来，便出现上面的问题。

为了避免这样的问题，需要在Activity在执行到onCreate()方法之后(包括在onCreate()方法)，对该Intent进行赋值即可。

public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		startSer1 = (Button) findViewById(R.id.startSer1);
		stopSer1 = (Button) findViewById(R.id.stopSer1);

		startSer2 = (Button) findViewById(R.id.startSer2);
		stopSer2 = (Button) findViewById(R.id.stopSer2);

		log = (TextView) findViewById(R.id.log);

		startSer1.setOnClickListener(btnListener);
		stopSer1.setOnClickListener(btnListener);

		startSer2.setOnClickListener(btnListener);
		stopSer2.setOnClickListener(btnListener);

		intent = new Intent(MyServiceActivity.this, MyService.class);
	}

 

通过这个Demo，吸取到了一个教训：需要使用Context方面的信息，最好在onCreate()方法之后（包括之）。

 

评论

2 楼 a_lee 2012-11-29  

lz的学习态度值得学习

1 楼 Rainbow702 2011-08-09  

大家也可以参考这篇BLOG：http://hi.baidu.com/gaogaf/blog/item/7efdb01185806264ca80c4d9.html
讲得比我这个深入