Android: Having Difficulty passing an Object from one activity to another using Parcel

StackOverflow https://stackoverflow.com/questions/7894353

Question

I've read MANY articles on how to use Parcel and I've used it in my app but it isn't working. I've just created an activity where you can connect to a computer using Sockets. I've opened a java.io.PrintWriter on the Socket's OutputStream and I want to have another activity (which is opened on touching a button in the 1st activity) use this same PrintWriter. I think the mistake I'm making here is of not cloning PrintWriter object. I don't know how to clone it.

Here's the 1st Activity (I've removed a few unneeded functions):

public class TestActivity extends Activity implements OnClickListener
{
    String ip;
    int port;
    Socket soc;
    PrintWriter writer;   // this works fine here

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        ip = "";
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // Connect button click handler (you can skip this part)
        Button connect = (Button)findViewById(R.id.connectBTN);
        connect.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {
                EditText TB = (EditText)findViewById(R.id.ipTB);
                ip = TB.getText().toString();
                TB = (EditText)findViewById(R.id.portTB);
                port = Integer.parseInt(TB.getText().toString());
                try {
                soc = new Socket(ip,port);
                showDialog("Connected successfully!");
                writer = new PrintWriter(soc.getOutputStream(), true);
                }
                catch(Exception er)
                {
                    showDialog("Couldn't connect.\n"+er.getMessage());
                    soc = null;
                }
            }
        } );

        findViewById(R.id.controlmouseBTN).setOnClickListener(this);

        // disconnect click handler  (skip this part too)
        Button disconnectBTN = (Button)findViewById(R.id.disconnectBTN);
        disconnectBTN.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {
                try
                {
                    if(soc.isConnected()) soc.close();
                    writer.close();
                }
                catch(Exception er) { showDialog("Couldn't close connection.\n"+er.getMessage()); }
            }
        });

    }

    @Override
    public void onClick(View v)   // here is the erroneous part 
    {
        //if(soc == null || !soc.isConnected()) { showDialog("Not connected to any computer."); return; } 
        //if(writer == null) return;
        Intent i = new Intent(getApplicationContext(), PCController.class);
        Intermediate inter = new Intermediate();   // this is the Parcelable class I've created
        //inter.set(new String("hello"));  (I tried sending a string, which worked fine, just to confirm that my Parcelable class is working)
        inter.set(writer);
        i.putExtra("printer", inter);
        startActivity(i);
    }

}

Here is the code of the Parcelable class Intermediate

import android.os.*;

public class Intermediate implements Parcelable
{
    Object ob;

    public Intermediate()
    {}

    public Intermediate(Parcel parcel)
    {
        this.ob = parcel.readValue(Intermediate.class.getClassLoader());
    }

    public void set(Object o)
    { this.ob = o; }

    public Object get()
    {
        return ob;
    }

    @Override
    public int describeContents()
    { return 0; }

    @Override
    public void writeToParcel(Parcel dest, int flags)
    {
        dest.writeValue(this.ob);
    }

   // I don't understand this part though! Just copy pasted and modified the ClassName

    public static final Intermediate.Creator<Intermediate> CREATOR = new Intermediate.Creator<Intermediate>() {
        public Intermediate createFromParcel(Parcel in) {
            return new Intermediate(in); }

        public Intermediate[] newArray(int size) {
            return new Intermediate[size]; }
    };  
}

Here's the code of the Next Activity being fired. (I've posted only the essential part)

PrintWriter writer;
    String s;   // for testing purpose
    Swipe swipe;   // a SurfaceView

    public void onCreate(Bundle bundle)
    {
        super.onCreate(bundle);
        swipe = new Swipe(this);
        swipe.setOnTouchListener(this);
        setContentView(swipe);
        Intent i = getIntent();
        Intermediate ob = (Intermediate)i.getParcelableExtra("printer");
        writer = (PrintWriter)ob.get();
        //s = (String)ob.get();
        //showDialog(s);   // this shows up the string I had sent, which means the string is getting passed.
    }

Now when I don't connect (i.e., writer is null) and I start the next Activity, nothing happens (no errors). But when I connect (writer is not null) and I start the next Activity, the MAIN Activity simply crashes. I didn't bother to log the error message, since I know I'm doing something really wrong here.

Any help would be appreciated! Thanks!

Edit: Here is the Exception Stack Trace

10-26 00:37:55.445: WARN/System.err(251): java.lang.RuntimeException: Parcel: unable to marshal value java.io.PrintWriter@44eb0b40
10-26 00:37:55.474: WARN/System.err(251):     at android.os.Parcel.writeValue(Parcel.java:1087)
10-26 00:37:55.484: WARN/System.err(251):     at android.test.Intermediate.writeToParcel(Intermediate.java:33)
10-26 00:37:55.484: WARN/System.err(251):     at android.os.Parcel.writeParcelable(Parcel.java:1106)
10-26 00:37:55.494: WARN/System.err(251):     at android.os.Parcel.writeValue(Parcel.java:1029)
10-26 00:37:55.504: WARN/System.err(251):     at android.os.Parcel.writeMapInternal(Parcel.java:469)
10-26 00:37:55.514: WARN/System.err(251):     at android.os.Bundle.writeToParcel(Bundle.java:1445)
10-26 00:37:55.524: WARN/System.err(251):     at android.os.Parcel.writeBundle(Parcel.java:483)
10-26 00:37:55.534: WARN/System.err(251):     at android.content.Intent.writeToParcel(Intent.java:5237)
10-26 00:37:55.544: WARN/System.err(251):     at android.app.ActivityManagerProxy.startActivity(ActivityManagerNative.java:1204)
10-26 00:37:55.554: WARN/System.err(251):     at android.app.Instrumentation.execStartActivity(Instrumentation.java:1373)
10-26 00:37:55.564: WARN/System.err(251):     at android.app.Activity.startActivityForResult(Activity.java:2749)
10-26 00:37:55.574: WARN/System.err(251):     at android.app.Activity.startActivity(Activity.java:2855)
10-26 00:37:55.584: WARN/System.err(251):     at android.test.TestActivity.onClick(TestActivity.java:80)
10-26 00:37:55.594: WARN/System.err(251):     at android.view.View.performClick(View.java:2364)
10-26 00:37:55.606: WARN/System.err(251):     at android.view.View.onTouchEvent(View.java:4179)
10-26 00:37:55.615: WARN/System.err(251):     at android.widget.TextView.onTouchEvent(TextView.java:6541)
10-26 00:37:55.624: WARN/System.err(251):     at android.view.View.dispatchTouchEvent(View.java:3709)
10-26 00:37:55.634: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:37:55.634: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:37:55.644: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:37:55.656: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:37:55.664: WARN/System.err(251):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1659)
10-26 00:37:55.674: WARN/System.err(251):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107)
10-26 00:37:55.684: WARN/System.err(251):     at android.app.Activity.dispatchTouchEvent(Activity.java:2061)
10-26 00:37:55.694: WARN/System.err(251):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1643)
10-26 00:37:55.705: WARN/System.err(251):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1691)
10-26 00:37:55.714: WARN/System.err(251):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-26 00:37:55.724: WARN/System.err(251):     at android.os.Looper.loop(Looper.java:123)
10-26 00:37:55.734: WARN/System.err(251):     at android.app.ActivityThread.main(ActivityThread.java:4363)
10-26 00:37:55.744: WARN/System.err(251):     at java.lang.reflect.Method.invokeNative(Native Method)
10-26 00:37:55.754: WARN/System.err(251):     at java.lang.reflect.Method.invoke(Method.java:521)
10-26 00:37:55.764: WARN/System.err(251):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
10-26 00:37:55.774: WARN/System.err(251):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
10-26 00:37:55.785: WARN/System.err(251):     at dalvik.system.NativeStart.main(Native Method)
10-26 00:38:02.424: WARN/System.err(251): java.lang.RuntimeException: Parcel: unable to marshal value java.io.PrintWriter@44eb0b40
10-26 00:38:02.454: WARN/System.err(251):     at android.os.Parcel.writeValue(Parcel.java:1087)
10-26 00:38:02.454: WARN/System.err(251):     at android.test.Intermediate.writeToParcel(Intermediate.java:33)
10-26 00:38:02.464: WARN/System.err(251):     at android.os.Parcel.writeParcelable(Parcel.java:1106)
10-26 00:38:02.474: WARN/System.err(251):     at android.os.Parcel.writeValue(Parcel.java:1029)
10-26 00:38:02.486: WARN/System.err(251):     at android.os.Parcel.writeMapInternal(Parcel.java:469)
10-26 00:38:02.494: WARN/System.err(251):     at android.os.Bundle.writeToParcel(Bundle.java:1445)
10-26 00:38:02.505: WARN/System.err(251):     at android.os.Parcel.writeBundle(Parcel.java:483)
10-26 00:38:02.514: WARN/System.err(251):     at android.content.Intent.writeToParcel(Intent.java:5237)
10-26 00:38:02.524: WARN/System.err(251):     at android.app.ActivityManagerProxy.startActivity(ActivityManagerNative.java:1204)
10-26 00:38:02.534: WARN/System.err(251):     at android.app.Instrumentation.execStartActivity(Instrumentation.java:1373)
10-26 00:38:02.544: WARN/System.err(251):     at android.app.Activity.startActivityForResult(Activity.java:2749)
10-26 00:38:02.554: WARN/System.err(251):     at android.app.Activity.startActivity(Activity.java:2855)
10-26 00:38:02.554: WARN/System.err(251):     at android.test.TestActivity.onClick(TestActivity.java:80)
10-26 00:38:02.564: WARN/System.err(251):     at android.view.View.performClick(View.java:2364)
10-26 00:38:02.574: WARN/System.err(251):     at android.view.View.onTouchEvent(View.java:4179)
10-26 00:38:02.584: WARN/System.err(251):     at android.widget.TextView.onTouchEvent(TextView.java:6541)
10-26 00:38:02.594: WARN/System.err(251):     at android.view.View.dispatchTouchEvent(View.java:3709)
10-26 00:38:02.604: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:38:02.614: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:38:02.625: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:38:02.634: WARN/System.err(251):     at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:884)
10-26 00:38:02.644: WARN/System.err(251):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1659)
10-26 00:38:02.654: WARN/System.err(251):     at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107)
10-26 00:38:02.664: WARN/System.err(251):     at android.app.Activity.dispatchTouchEvent(Activity.java:2061)
10-26 00:38:02.674: WARN/System.err(251):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1643)
10-26 00:38:02.684: WARN/System.err(251):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1691)
10-26 00:38:02.694: WARN/System.err(251):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-26 00:38:02.704: WARN/System.err(251):     at android.os.Looper.loop(Looper.java:123)
10-26 00:38:02.714: WARN/System.err(251):     at android.app.ActivityThread.main(ActivityThread.java:4363)
10-26 00:38:02.714: WARN/System.err(251):     at java.lang.reflect.Method.invokeNative(Native Method)
10-26 00:38:02.724: WARN/System.err(251):     at java.lang.reflect.Method.invoke(Method.java:521)
10-26 00:38:02.734: WARN/System.err(251):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
10-26 00:38:02.744: WARN/System.err(251):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
10-26 00:38:02.754: WARN/System.err(251):     at dalvik.system.NativeStart.main(Native Method)
Was it helpful?

Solution

You've got a bigger problem here. You're not supposed use Parcels to pass things between activities. Per the android documentation:

Parcel is not a general-purpose serialization mechanism. This class (and the corresponding Parcelable API for placing arbitrary objects into a Parcel) is designed as a high-performance IPC transport. As such, it is not appropriate to place any Parcel data in to persistent storage: changes in the underlying implementation of any of the data in the Parcel can render older data unreadable.

You should use bundles instead. What I usually do is create two static methods in the class I'm trying to pass between activities called toBundle and fromBundle. The toBundle takes an instance of the class I want to transport and returns a bundle. The fromBunle takes a bundle and returns a instance of the class.

I'm not aware of any way to pass a complex object like a PrintWriter between Activities. But there's other problems with your code too. You're doing network stuff on the main UI thread. This is gonna cause your app to freeze A LOT. You need to move all of your IO off to a different thread. This is good because then you can just move all your IO stuff into a seperate class that's accessible from both activities. For instance, use an IntentService. This will make your app sooooooo much better. You might also want to read the article Painless Threading. It's a crucial read that I recommend for all android developers.

OTHER TIPS

You can't pass a PrintWriter between two Android activities using any of the normal Android data-passing methods (Bundles / Parcels). If you must share the PrintWriter between two Activities, than you can use a static variable.

That being said, you could also write a service that you access and use on each Activity. You might want to look at the LocalService example in the ApiDemos project for an example on how to do that. There are also countless tutorials online regarding this.

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