Question

我正在写一个简单的基于AIDL的Android远程服务＆amp;客户端访问远程服务公开的API。我在互联网上查了一下，人们在按钮的onClickListener（）客户端代码方法中调用了远程服务API。但是，当我试图在onClickListener（）方法之外调用远程服务公开的API时，它会抛出NullPointerException，表明我的服务对象尚未初始化（请检查客户端代码的onCreate方法内的注释）。我已将此代码附加到此问题中。如果有人能解释我为什么这样的行为那么那将是非常好的这是客户端代码：

package com.myapp.myclient;

import android.app.Activity;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.content.ServiceConnection;
import android.os.Bundle;
import android.os.IBinder;
import android.os.RemoteException;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

import com.myapp.myservice.RemoteServiceInterface;

public class MyClient extends Activity {

RemoteServiceInterface remoteInterface;
ServiceConnection connection;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    Intent i = new Intent();
    i.setAction("com.myapp.myservice.RemoteService");

    startRemoteInterface(i);
    bindRemoteInterface(i);

    /* This code doesn't execute. Raises a Null Pointer 
      Exception, indicating that remoteInterface is not 
      initialized. */
    try {
        Toast.makeText(getBaseContext(), remoteInterface.getMessage(), Toast.LENGTH_SHORT).show();
    } catch (RemoteException e) {
        e.printStackTrace();
    }

    /* Whereas this code does work. */      
    Button getMessage = (Button)findViewById(R.id.getMessage);
    getMessage.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            String msg = null;
            try {
                msg = remoteInterface.getMessage();
            } catch (RemoteException e) {
                e.printStackTrace();
            }

            Toast.makeText(getBaseContext(), msg, Toast.LENGTH_SHORT).show();
        }
    });
}

class RemoteServiceConnection implements ServiceConnection{

    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {
        remoteInterface = RemoteServiceInterface.Stub.asInterface(service);
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {
    }
}

private void startRemoteInterface(Intent i) {
    startService(i);
}

private void bindRemoteInterface(Intent i) {
    if(connection == null){
        connection = new RemoteServiceConnection();
        bindService(i, connection, Context.BIND_AUTO_CREATE);
    } else {
        Toast.makeText(getBaseContext(), "Service cannot bind - already bound.", Toast.LENGTH_SHORT).show();
    }
}
}

这是我的远程服务代码：

package com.myapp.myservice;

import android.app.Service;
import android.content.Intent;
import android.os.IBinder;
import android.os.RemoteException;

public class RemoteService extends Service {

@Override
public void onCreate() {
    super.onCreate();
}

@Override
public void onStart(Intent intent, int startId) {
    super.onStart(intent, startId);
}

@Override
public IBinder onBind(Intent intent) {
    return mBinder;
}

@Override
public void onDestroy() {
    super.onDestroy();
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    return super.onStartCommand(intent, flags, startId);
}

@Override
public boolean onUnbind(Intent intent) {
    return super.onUnbind(intent);
}

private final RemoteServiceInterface.Stub mBinder = new RemoteServiceInterface.Stub() {

    @Override
    public String getMessage() throws RemoteException {
        return "Hello World!";
    }
};
}

这是我的aidl文件：

package com.myapp.myservice; 

interface RemoteServiceInterface {
String getMessage();
}

提前致谢，
Rupesh

Answer 1

bindRemoteInterface(i);

    /* This code doesn't execute. Raises a Null Pointer 
      Exception, indicating that remoteInterface is not 
      initialized. */
    try {
        Toast.makeText(getBaseContext(), remoteInterface.getMessage(), Toast.LENGTH_SHORT).show();
    } catch (RemoteException e) {
        e.printStackTrace();
    }

请记住，绑定是一个asychronus调用，您必须等待onServiceConnected的ServiceConnection中的回调并在此之后执行操作。

此外，您必须使用asInterface方法来获取连接的真实界面，google aidl example

    RemoteServiceInterface mIRemoteService;
private ServiceConnection mConnection = new ServiceConnection() {
    // Called when the connection with the service is established
    public void onServiceConnected(ComponentName className, IBinder service) {
        // Following the example above for an AIDL interface,
        // this gets an instance of the IRemoteInterface, which we can use to call on the service
        mIRemoteService = RemoteServiceInterface.Stub.asInterface(service);
    }

    // Called when the connection with the service disconnects unexpectedly
    public void onServiceDisconnected(ComponentName className) {
        Log.e(TAG, "Service has unexpectedly disconnected");
        mIRemoteService = null;
    }
};

然后，您可以在mIRemoteService对象上进行调用。直接在onServiceConnected回调中或通知服务。

Answer 2

在连接服务（remoteInterface调用）之前，

onServiceConnected 为NULL 。

startService是async来电，您拨打startService并不意味着该服务已启动并已连接。连接服务时，调用onServiceConnected，然后您可以使用该连接来呼叫远程服务。

实际上，您应该始终检查天气remoteInterface是否为空。

为什么我们不能直接调用android（基于AIDL）远程服务的api？

2 个答案: