java - 从主线程调用时，对服务的 IPC 调用会导致 NullPointerException，但不会在按钮事件上调用

Question

我正在尝试使用 .aidl 文件中定义的 IPC 连接创建服务。

服务启动并且服务连接可以绑定(bind)到它，但是当从同一线程调用 IPC API 时，会导致 NullPointerException。当对 IPC API 的调用被放入按钮按下的事件中时，它就会起作用。

如何解决此问题，以便无需按钮事件即可完成调用。

我的主要 Activity

       public class MainActivity extends AppCompatActivity {

        public final String TAG = "Main Activity";
        IMyAidlInterface iMyAidlInterface;
        private ServiceConnection mConnection = new ServiceConnection() {
            public void onServiceConnected(ComponentName className, IBinder service) {
                iMyAidlInterface = IMyAidlInterface.Stub.asInterface(service);
            }

            public void onServiceDisconnected(ComponentName className) {
                iMyAidlInterface = null;
            }
        };


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

            Intent interFaceServiceIntent = new Intent(this, IMyAidlService.class);
            bindService(interFaceServiceIntent, mConnection, Context.BIND_AUTO_CREATE);

            Intent sigGenIntent = new Intent(this, signalGenerator.class);
            startService(sigGenIntent);

            Button b = findViewById(R.id.button);

/*
* The following block is what I want to be able to execute. Currently it causes * a NullPointerException 
* Caused by: java.lang.NullPointerException: Attempt to invoke interface method * 'int com.aidltest.IMyAidlInterface.getPid()' on a null object reference
*/
            try {
                iMyAidlInterface.getPid();
            } catch (RemoteException e) {
                e.printStackTrace();
            }
/*
*  The block below using the same interface from within a button event works
*   without problem.
*/

            b.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    try {
                        iMyAidlInterface.getPid();
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                }
            });
        }
    }

供引用我的 Aidl:

package com.aidltest;
interface IMyAidlInterface {
    int getPid();
    void basicTypes(int anInt, long aLong, boolean aBoolean, float aFloat,
            double aDouble, String aString);
}

还有我的 list :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.aidltest">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <service
            android:name=".IMyAidlService"
            android:enabled="true"
            android:exported="true">
        </service>
    </application>

</manifest>

Answer 1

发生这种情况是因为 onCreate 调用中的 iMyAidlInterface 为 null。直到您的连接监听器通过 onServiceConnected 回调您时才会设置它。绑定(bind)是一个异步操作。