Parceler

In Android, Parcelables are a great way to serialize Java Objects between Contexts. Compared with traditional Serialization, Parcelables take on the order of 10x less time to both serialize and deserialize. There is a major flaw with Parcelables, however. Parcelables contain a ton of boilerplate code. To implement a Parcelable, you must mirror the writeToParcel() and createFromParcel() methods such that they read and write to the Parcel in the same order. Also, a Parcelable must define a public final static Parcelable.Creator CREATOR in order for the Android infrastructure to be able to leverage the serialization code.

Parceler is a code generation library that generates the Android Parcelable boilerplate source code. No longer do you have to implement the Parcelable interface, the writeToParcel() or createFromParcel() or the public static final CREATOR. You simply annotate a POJO with @Parcel and Parceler does the rest. Because Parceler uses the Java JSR-269 Annotation Processor, there is no need to run a tool manually to generate the Parcelable code. Just annotate your Java Bean, compile and you are finished. By default, Parceler will serialize the fields of your instance directly:

@Parcel
public class Example {
    String name;
    int age;

    public Example(){ /*Required empty bean constructor*/ }

    public Example(int age, String name) {
        this.age = age;
        this.name = name;
    }

    public String getName() { return name; }

    public int getAge() { return age; }
}

Be careful not to use private fields when using the default field serialization strategy as it will incur a performance penalty due to reflection.

To use the generated code, you may reference the generated class directly, or via the Parcels utility class:

Parcelable wrapped = Parcels.wrap(new Example("Andy", 42));

To dereference the @Parcel, just call the Parcels.unwrap() method:

Example example = Parcels.unwrap(wrapped);
example.getName(); // Andy
example.getAge(); // 42

Of course, the wrapped Parcelable can be added to an Android Bundle to transfer from Activity to Activity:

Bundle bundle = new Bundle();
bundle.putParcelable("example", Parcels.wrap(example));

And dereferenced in the onCreate() method:

Example example = Parcels.unwrap(this.getIntent().getExtras().get("example"));

Parcel attribute types

Only a select number of types may be used as attributes of a @Parcel class. The following list includes the mapped types:

• byte

• double

• float

• int

• long

• char

• boolean

• String

• IBinder

• Bundle

• SparseArray of any of the mapped types*

• SparseBooleanArray

• List, ArrayList and LinkedList of any of the mapped types*

• Map, HashMap, LinkedHashMap, SortedMap, and TreeMap of any of the mapped types*

• Set, HashSet, SortedSet, TreeSet, LinkedHashSet of any of the mapped types*

• Parcelable

• Serializable

• Array of any of the mapped types

• Any other class annotated with @Parcel

*Parcel will error if the generic parameter is not mapped.

Parceler also supports any of the above types directly. This is especially useful when dealing with collections of classes annotated with @Parcel:

Parcelable listParcelable = Parcels.wrap(new ArrayList<Example>());
Parcelable mapParcelable = Parcels.wrap(new HashMap<String, Example>());

Polymorphism

Note that Parceler does not unwrap inheritance hierarchies, so any polymorphic fields will be unwrapped as instances of the base class. This is because Parceler opts for performance rather than checking .getClass() for every piece of data.

@Parcel
public class Example {
    public Parent p;
    Example(Parent p) { this.p = p; }
}

@Parcel public class Parent {}
@Parcel public class Child extends Parent {}

Example example = new Example(new Child());
System.out.println("%b", example.p instanceof Child); // true
example = Parcels.unwrap(Parcels.wrap(example));
System.out.println("%b", example.p instanceof Child); // false

Refer to the Custom Serialization section for an example of working with polymorphic fields.

Serialization techniques

Parceler offers several choices for how to serialize and deserialize an object in addition to the field-based serialization seen above.

Getter/setter serialization

Parceler may be configured to serialize using getter and setter methods and a non-empty constructor. In addition, fields, methods and constructor parameters may be associated using the @ParcelParameter annotation. This supports a number of bean strategies including immutability and traditional getter/setter beans.

To configure default method serialization, simply configure the @Parcel annotation with Serialization.BEAN:

@Parcel(Serialization.BEAN)
public class Example {
    private String name;
    private int age;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
}

To use a constructor with serialization, annotate the desired constructor with the @ParcelConstructor annotation:

@Parcel(Serialization.BEAN)
public class Example {
    private final String name;
    private final int age;

    @ParcelConstructor
    public Example(int age, String name) {
        this.age = age;
        this.name = name;
    }

    public String getName() { return name; }

    public int getAge() { return age; }
}

If an empty constructor is present, Parceler will use that constructor unless another constructor is annotated.

Mixing getters/setters and fields

You may also mix and match serialization techniques using the @ParcelParameter annotation. In the following example, firstName and lastName are written to the bean using the constructor while firstName is read from the bean using the field and lastName is read using the getLastName() method. The parameters firstName and lastName are coordinated by the parameter names "first" and "last" respectfully.

@Parcel
public class Example {
    @ParcelParameter("first")
    String firstName;
    String lastName;

    @ParcelConstructor
    public Example(@ParcelParam("first") String firstName, @ParcelParam("last") String lastName){
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public String getFirstName() { return firstName; }

    @ParcelParameter("last");
    public String getLastName() { return lastName; }
}

For attributes that should not be serialized with Parceler, the attribute field, getter or setter may be annotated by @Transient.

Parceler supports many different styles centering around the POJO. This allows @Parcel annotated classes to be used with other POJO based libraries, including GSON, Cupboard, and Simple XML to name a few.

Static Factory support

As an alternative to using a constructor directly, Pareler supports using an annotated Static Factory to build an instance of the given class. This style supports Google’s AutoValue annoation processor / code generation library for generating immutable beans. Parceler interfaces with AutoValue via the @ParcelFactory annotation, which maps a static factory method into the annotated @Parcel serialization:

@AutoValue
@Parcel
public abstract class AutoValueParcel {

    @ParcelProperty("value") public abstract String value();

    @ParcelFactory
    public static AutoValueParcel create(String value) {
        return new AutoValue_AutoValueParcel(value);
    }
}

AutoValue generates a different class than the annotated @Parcel, therefore, you need to specify which class Parceler should build in the Parcels utility class:

Parcelable wrappedAutoValue = Parcels.wrap(AutoValueParcel.class, AutoValueParcel.create("example"));

And to deserialize:

AutoValueParcel autoValueParcel = Parcels.unwrap(wrappedAutoValue);

Custom serialization

@Parcel includes an optional parameter to include a manual serializer ParcelConverter for the case where special serialization is necessary. This provides a still cleaner option for using Parcelable classes than implementing them by hand.

The following code demonstrates using a ParcelConverter to unwrap the inheritance hierarchy during deserialization.

@Parcel
public class Item {
    @ParcelPropertyConverter(ItemListParcelConverter.class)
    public List<Item> itemList;
}
@Parcel public class SubItem1 extends Item {}
@Parcel public class SubItem2 extends Item {}

public class ItemListParcelConverter implements ParcelConverter<List<Item>> {
    @Override
    public void toParcel(List<Item> input, Parcel parcel) {
        if (input == null) {
            parcel.writeInt(-1);
        }
        else {
            parcel.writeInt(input.size());
            for (Item item : input) {
                parcel.writeParcelable(Parcels.wrap(item), 0);
            }
        }
    }

    @Override
    public List<Item> fromParcel(Parcel parcel) {
        int size = parcel.readInt();
        if (size < 0) return null;
        List<Item> items = new ArrayList<Item>();
        for (int i = 0; i < size; ++i) {
            items.add((Item) Parcels.unwrap(parcel.readParcelable(Item.class.getClassLoader())));
        }
        return items;
    }
}

Classes without Java source

For classes whose corresponding Java source is not available, one may include the class as a Parcel by using the @ParcelClass annotation. This annotation may be declared anywhere in the compiled source that is convenient. For instance, one could include the @ParcelClass along with the Android Application:

@ParcelClass(LibraryParcel.class)
public class AndroidApplication extends Application{
    //...
}

Multiple @ParcelClass annotations may be declared using the @ParcelClasses annotation.

Advanced configuration

Skipping analysis

It is a common practice for some libraries to require a bean to extend a base class. Although it is not the most optimal case, Parceler supports this practice by allowing the configuration of what classes in the inheritance hierarchy to analyze via the analyze parameter:

@Parcel(analyze = {One.class, Three.class})
class One extends Two {}
class Two extends Three {}
class Three extends BaseClass {}

In this example, only fields of the One and Three classes will be serialized, avoiding both the BaseClass and Two class parameters.

Specific wrapping

The Parcels utility class looks up the given class for wrapping by class. For performance reasons this ignores inheritance, both super and base classes. There are two solutions to this problem. First, one may specify additional types to associate to the given type via the implementations parameter:

class ExampleProxy etends Example {}
@Parcel(implementations = {ExampleProxy.class})
class Example {}

ExampleProxy proxy = new ExampleProxy();
Parcels.wrap(proxy);  // ExampleProxy will be serialized as a Example

Second, one may also specify the class type when using the Parcels.wrap() method:

ExampleProxy proxy = new ExampleProxy();
Parcels.wrap(Example.class, proxy);

Avoiding Parcels indexing

Using Parceler in libraries can be challenging because Parceler writes a single mapping class Parceler$$Parcels to associate a given type with a Parcelable. This mapping class can collide giving the following error during compilation:

Error Code:
    2
Output:
    UNEXPECTED TOP-LEVEL EXCEPTION:
    com.android.dex.DexException: Multiple dex files define Lorg/parceler/Parceler$$Parcels$1;
        at com.android.dx.merge.DexMerger.readSortableTypes(DexMerger.java:594)
        at com.android.dx.merge.DexMerger.getSortedTypes(DexMerger.java:552)
        at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:533)
        ....

To avoid writing this common mapping class, set parcelsIndex = false to each of the library classes. Parceler will not write a Parceler$$Parcels mapping class if no indexable classes exist and the Parcels utiltiy class will fallback to looking up the generated class by name.

Alternatively, using @ParcelClass in the root project, instead of annotating classes directly in the library can avoid this issue.

Getting Parceler

You may download Parceler as a Maven dependency:

<dependency>
    <groupId>org.parceler</groupId>
    <artifactId>parceler</artifactId>
    <version>${parceler.version}</version>
    <scope>provided</scope>
</dependency>
<dependency>
    <groupId>org.parceler</groupId>
    <artifactId>parceler-api</artifactId>
    <version>${parceler.version}</version>
</dependency>

or Gradle:

compile "org.parceler:parceler-api:${parcelerVersion}"
provided "org.parceler:parceler:${parcelerVersion}"

Or from Maven Central.

License

Copyright 2011-2015 John Ericksen

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.