GWT (Google Web Toolkit) has become a popular choice for developing web applications due to its ability to write front-end code in Java and have it compiled into JavaScript. One of the key features that makes GWT stand out is its built-in serialization mechanism, which allows for easy communication between the client and server. However, in certain scenarios, developers may encounter issues with GWT's serialization policy whitelist, particularly when working with custom types. In this article, we will explore how to add a type to GWT's serialization policy whitelist and address any potential roadblocks along the way.
First, let's understand what the serialization policy whitelist is and why it is important. GWT's serialization mechanism works by sending objects between the client and server using their serialized form, which is essentially a string representation of the object. However, not all types can be safely serialized, as it may lead to security vulnerabilities. To prevent this, GWT has a whitelist of types that are allowed to be serialized. This whitelist is generated during the compilation process and is based on the types used in the code.
So, what happens when we need to serialize a custom type that is not included in the whitelist? This is where we need to add our type to GWT's serialization policy whitelist. The process involves two steps: registering the type and configuring the GWT compiler.
To register a custom type, we need to implement the `Serializable` interface in our class. This interface has no methods and is used as a marker interface to indicate that the class can be serialized. Additionally, if our custom type contains fields that are not serializable, we can annotate them with `@GwtTransient` to exclude them from serialization. Once our class is `Serializable`, it will be automatically added to the whitelist during compilation.
The next step is to configure the GWT compiler to include our custom type in the whitelist. This can be done by specifying a special command-line flag, `extra`, during compilation. For example, `-extra com.example.MyCustomClass`. This will ensure that our custom type is included in the generated whitelist.
However, there are some cases where the above steps may not work as expected. For instance, if our custom type is defined in a different module, we need to add an additional flag, `-extra`, to specify the module's name. Another scenario is when our custom type is defined in a library or jar file. In this case, we need to include the jar file in the classpath during compilation, and the `-extra` flag will be automatically added to include the type in the whitelist.
It is also worth noting that adding a type to GWT's serialization policy whitelist may result in a larger compiled output, as more types need to be included for serialization. This can affect the application's performance, especially in scenarios where many custom types are used. Therefore, it is essential to carefully consider which types need to be added to the whitelist and ensure that they are necessary for the application's functionality.
In conclusion, GWT's serialization policy whitelist plays a crucial role in ensuring the security of our web applications. By following the steps outlined in this article, we can easily add custom types to the whitelist and avoid potential serialization issues. However, it is essential to keep in mind the implications of adding more types to the whitelist and to only include those that are necessary. With this knowledge, developers can confidently use custom types in their GWT applications without worrying about serialization errors.
