Looking at trying to use the GWT-Jackson-Apt library for doing certain RPC, but when looking at examples and trying to run some demos there are always interfaces with a bizarre undefined constructor call.
@JSONMapper
public interface SampleMapper extends ObjectMapper<SimpleBean> {
SampleMapper INSTANCE = new App_SampleMapperImpl();
}
I've been digging around, but there is no definition of App_SampleMapperImpl() anywhere in the source code. And it doesn't compile, saying that there is an undefined symbol
The exact same thing is done in the readme file's exmaples which can be found on this page: https://github.com/DominoKit/gwt-jackson-apt/tree/f60d0358b90bcbf78d066796f680aeae1d7156bb
can anyone explain what is going on here? How is this constructor being defined, or implied? And what do I need to do to make the example compile?
Assuming you are making a Maven project, the important thing is to include the annotation processor which generates the mappers. Then, once the project knows how to generate them, you'll be able to use them in your code.
Annotation Processors run while the compiler is running, which means you technically get to write code which doesn't appear it will compile. Then, as the compiler is running, it asks all registered annotation processors to please generate code based on the annotations and existing types (not the missing references like App_Sample_MapperImpl as you might think). The processor then runs, generates the missing class, and then the compile continues.
Usually what happens is that you build while writing code (eclipse, for example, does this every time a file is saved, intellij does it when you ask for a build, etc), and then the class exists and can be referenced going forward. Even when the project is cleaned and rebuilt, while the reference seems like it should not work, it will work as soon as the compiler runs.
In this case, we'll need to follow the example to make sure the processor is present. in https://github.com/DominoKit/gwt-jackson-apt/blob/f60d0358b90bcbf78d066796f680aeae1d7156bb/samples/shared-mappers/shared-mappers-shared/pom.xml, we see this in the dependencies:
<dependency>
<groupId>org.dominokit.jackson</groupId>
<artifactId>jackson-apt-processor</artifactId>
<version>1.0-SNAPSHOT</version>
<scope>provided</scope>
</dependency>
This is marked scope=provided since it is only needed to compile, then shouldn't be included in later dependency graphs. For each specific IDE, you may need to specify additional options to get it to re-run automatically (a checkbox in Eclipse, nothing in IntelliJ I believe, and I haven't used other IDEs in too long to say).
One final note for maven: you must use a relatively recent maven-compiler-plugin so that generated code is handled correctly: latest is 3.8.0, published July 2018, but I think anything after 3.5.1 will be sufficient if you must use an older one.