How to use Parcel in Android?

Question

I'm trying to use Parcel to write and then read back a Parcelable. For some reason, when I read the object back from the file, it's coming back as null.

public void testFoo() {     final Foo orig = new Foo("blah blah");      // Wrote orig to a parcel and then byte array     final Parcel p1 = Parcel.obtain();     p1.writeValue(orig);     final byte[] bytes = p1.marshall();       // Check to make sure that the byte array seems to contain a Parcelable     assertEquals(4, bytes[0]); // Parcel.VAL_PARCELABLE       // Unmarshall a Foo from that byte array     final Parcel p2 = Parcel.obtain();     p2.unmarshall(bytes, 0, bytes.length);     final Foo result = (Foo) p2.readValue(Foo.class.getClassLoader());       assertNotNull(result); // FAIL     assertEquals( orig.str, result.str ); }   protected static class Foo implements Parcelable {     protected static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {         public Foo createFromParcel(Parcel source) {             final Foo f = new Foo();             f.str = (String) source.readValue(Foo.class.getClassLoader());             return f;         }          public Foo[] newArray(int size) {             throw new UnsupportedOperationException();         }      };       public String str;      public Foo() {     }      public Foo( String s ) {         str = s;     }      public int describeContents() {         return 0;     }      public void writeToParcel(Parcel dest, int ignored) {         dest.writeValue(str);     }   } 

What am I missing?

UPDATE: To simplify the test I've removed the reading and writing of files in my original example.

Answer 1

Ah, I finally found the problem. There were two in fact.

1. CREATOR must be public, not protected. But more importantly,
2. You must call setDataPosition(0) after unmarshalling your data.

Here is the revised, working code:

public void testFoo() {     final Foo orig = new Foo("blah blah");     final Parcel p1 = Parcel.obtain();     final Parcel p2 = Parcel.obtain();     final byte[] bytes;     final Foo result;      try {         p1.writeValue(orig);         bytes = p1.marshall();          // Check to make sure that the byte stream seems to contain a Parcelable         assertEquals(4, bytes[0]); // Parcel.VAL_PARCELABLE          p2.unmarshall(bytes, 0, bytes.length);         p2.setDataPosition(0);         result = (Foo) p2.readValue(Foo.class.getClassLoader());      } finally {         p1.recycle();         p2.recycle();     }       assertNotNull(result);     assertEquals( orig.str, result.str );  }  protected static class Foo implements Parcelable {     public static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {         public Foo createFromParcel(Parcel source) {             final Foo f = new Foo();             f.str = (String) source.readValue(Foo.class.getClassLoader());             return f;         }          public Foo[] newArray(int size) {             throw new UnsupportedOperationException();         }      };       public String str;      public Foo() {     }      public Foo( String s ) {         str = s;     }      public int describeContents() {         return 0;     }      public void writeToParcel(Parcel dest, int ignored) {         dest.writeValue(str);     }   } 

Answer 2

Beware! Dont use Parcel for serialization to a file

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.

from http://developer.android.com/reference/android/os/Parcel.html