[jsii-pacmak] Java code generation - extend a base class instead of generating the properties/methods #2014
Description
Given a TypeScript implementation:
export interface IResource {
readonly stack: Stack;
}The Java generated code will include a Jsii$Proxy class that Implements all (including inherited) members obtained from .jsii:
final static class Jsii$Proxy extends software.amazon.jsii.JsiiObject implements software.amazon.awscdk.core.IResource {
public @org.jetbrains.annotations.NotNull software.amazon.awscdk.core.Stack getStack() {
return this.jsiiGet("stack", software.amazon.awscdk.core.Stack.class);
}
}In the above example, the IResource interface includes a single property Stack which translates to an implementation of a get method - getStack(), satisfying the interface requirement.
A common pattern in the AWS CDK is to create an interface and an abstract class which implement the interface, in the IResource example we will have the abstract class Resource which implements IResource:
export abstract class Resource implements IResource {
...
}Every class that wishes to implement IResource will extends Resource.
We can use the same logic when generating Jsii$Proxy class, instead of implementing all of the memebers required by the interface, Jsii$Proxy can extend the base class:
final static class Jsii$Proxy extends software.amazon.jsii.JsiiObject implements software.amazon.awscdk.core.IResource extends software.amazon.awscdk.core.Resource {
public @org.jetbrains.annotations.NotNull software.amazon.awscdk.core.Stack getStack() {
return this.jsiiGet("stack", software.amazon.awscdk.core.Stack.class);
}
}Why should we do it?
First it will reduce the amount of generated code.
It will also prevent a specific situation in which Jsii$Proxy class that was generated from a .jsii file of an old AWS CDK version might break when compiled against java class files that were generated with a newer version of the AWS CDK.
To make it clearer lets look at an example, assume the above Jsii$Proxy was created using a .jsii file generated for AWS CDK version 1.59.0. In version 1.60.0 of the AWS CDK we add a property to IResource - env and we also add it to Resource. Since the Jsii$Proxy class was created with no knowledge of the newly added env property, if we try to compile it with Java class files generated by version 1.60.0 we will get a compilation error as it no longer implements IResource. We can avoid this by having Jsii$Proxy extending Resource.