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I can serialize a List<Video> in my servlet on GAE, but I can't deserialize it. What am I doing wrong?

This is my class Video in GAE, which is serialized:

package legiontube; import java.util.Date; import javax.jdo.annotations.IdGeneratorStrategy; import javax.jdo.annotations.IdentityType; import javax.jdo.annotations.PersistenceCapable; import javax.jdo.annotations.Persistent; import javax.jdo.annotations.PrimaryKey; @PersistenceCapable(identityType = IdentityType.APPLICATION) public class Video { @PrimaryKey private String id; @Persistent private String titulo; @Persistent private String descricao; @Persistent private Date date; public Video(){}; public Video(String id, String titulo, String descricao, Date date) { //super(); this.id = id; this.titulo = titulo; this.descricao = descricao; this.date = date; } public String getId() { return id; } public void setId(String id) { this.id = id; } public String getTitulo() { return titulo; } public void setTitulo(String titulo) { this.titulo = titulo; } public String getDescricao() { return descricao; } public void setDescricao(String descricao) { this.descricao = descricao; } public Date getDate() { return date; } public void setDate(Date date) { this.date = date; } }

This is my class Video in my other application, where I try to deserialize:

package classes; import java.util.Date; public class Video { private String id; private String titulo; private String descricao; private Date date; public Video(String id, String titulo, String descricao, Date date) { //super(); this.id = id; this.titulo = titulo; this.descricao = descricao; this.date = date; } public String getId() { return id; } public void setId(String id) { this.id = id; } public String getTitulo() { return titulo; } public void setTitulo(String titulo) { this.titulo = titulo; } public String getDescricao() { return descricao; } public void setDescricao(String descricao) { this.descricao = descricao; } public Date getDate() { return date; } public void setDate(Date date) { this.date = date; } }
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4 Answers 4

Reset to default
374

With Gson, you'd just need to do something like:

List<Video> videos = gson.fromJson(json, new TypeToken<List<Video>>(){}.getType());

You might also need to provide a no-arg constructor on the Video class you're deserializing to.

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8 Comments

Dude, you are amazing, thank you so much, it works. My mistake was not create a constructor with no args. Thank you so much!
What is the {} in the clause new TypeToken<List<Video>>(){}.getType() ? How is it legal java syntax?
@Maxim: It's making an anonymous subclass of TypeToken<List<Video>>... it's a trick that allows the resulting type to represent List<Video>, since subclasses of fully specified generic types retain the generic type information.
The 2.1 version of Gson now has a default access constructor... hence we can't use the code above... instead now the TypeToken class has the following method: public static TypeToken<?> get(Type type) { return new TypeToken(type); } But in this case... how could we provide the correct type to it???
@Pablo: It looks to me like the no-arg constructor for TypeToken is still protected in Gson 2.1, so the above should still work.
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130

Another way is to use an array as a type, e.g.:

Video[] videoArray = gson.fromJson(json, Video[].class);

This way you avoid all the hassle with the Type object, and if you really need a list you can always convert the array to a list, e.g.:

List<Video> videoList = Arrays.asList(videoArray);

IMHO this is much more readable.

In Kotlin this looks like this:

Gson().fromJson(jsonString, Array<Video>::class.java)

To convert this array into List, just use .toList() method

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2 Comments

The accepted answer is good, but I prefer this since all I really wanted is a list of stuff and array is sufficient to serve as a "list of stuff"
ah thank you I was trying List<Video>::class.java instead of using Array<Video>::class.java
14

I recomend this one-liner

List<Video> videos = Arrays.asList(new Gson().fromJson(json, Video[].class));

Warning: the list of videos, returned by Arrays.asList is immutable - you can't insert new values. If you need to modify it, wrap in new ArrayList<>(...).

Reference:

  1. Method Arrays#asList
  2. Constructor Gson
  3. Method Gson#fromJson (source json may be of type JsonElement, Reader, or String)
  4. Interface List
  5. JLS - Arrays
  6. JLS - Generic Interfaces
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4

Be careful using the answer provide by @DevNG. Arrays.asList() returns internal implementation of ArrayList that doesn't implement some useful methods like add(), delete(), etc. If you call them an UnsupportedOperationException will be thrown. In order to get real ArrayList instance you need to write something like this:

List<Video> = new ArrayList<>(Arrays.asList(videoArray));
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3 Comments

wat? Arrays.asList returns a List interface, that provides add() and remove() as well!
@Enrichman: Arrays.asList() returns a fixed-size list backed by the specified array. This ArrayList implementation extends a class AbstractList and doesn't override the method remove(int index) from the base class. And if you look at the source code of the AbstractList.remove(int index) you would see it always thrown UnsupportedOperationException. The same is for add() method.
Wo, that's pretty ugly, I didn't know that. Thanks for pointing it out. :)

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