Programmatically invoke Annotation Processors

Question

This is the first time I am writing a Annotation Processor and I want to invoke it programmatically. Is it possible?

I have written small code for processor:

@SupportedAnnotationTypes({"app.dev.ems.support.annotation.HBMModel"})
public class HBMModelProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(HBMModel.class);
        System.out.println(elements);
        return true;
    }

}

Now If I want to invoke the process method, then how can I do this? Can I do it in following way:

HBMModelProcessor modelProcessor = new HBMModelProcessor();
modelProcessor.process(annotations, roundEnv)

Any information will be very helpful to me.

Thanks.

Answer 1

You can call the Java compiler with annotation processors programmatically, inside the same process, like this:

import com.sun.tools.javac.processing.PrintingProcessor;
import fi.jumi.actors.generator.JavaSourceFromString;
import org.junit.*;
import org.junit.rules.TemporaryFolder;

import javax.annotation.processing.Processor;
import javax.tools.*;
import javax.tools.JavaCompiler.CompilationTask;
import java.io.IOException;
import java.util.Arrays;

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.is;

public class ExampleTest {

    @Rule
    public final TemporaryFolder tempDir = new TemporaryFolder();

    @Test
    public void example() throws IOException {
        JavaFileObject src = new JavaSourceFromString(
                "com.example.GuineaPig",
                "package com.example;\n" +
                "public interface GuineaPig {\n" +
                "    void foo();\n" +
                "}"
        );
        compile(new PrintingProcessor(), src);
    }

    private void compile(Processor processor, JavaFileObject... compilationUnits) throws IOException {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<JavaFileObject>();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, null);
        fileManager.setLocation(StandardLocation.CLASS_OUTPUT, Arrays.asList(tempDir.getRoot()));

        CompilationTask task = compiler.getTask(null, fileManager, diagnostics, null, null, Arrays.asList(compilationUnits));
        task.setProcessors(Arrays.asList(
                processor
        ));
        boolean success = task.call();
        for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
            System.err.println(diagnostic);
        }
        assertThat("compile succeeded", success, is(true));
    }
}

If you remove the call to setProcessors then it will detect annotation processors automatically based on the META-INF/services/javax.annotation.processing.Processor files on classpath.

Answer 2

jOOR has an API to simplify access to javax.tools.JavaCompiler as shown in this answer. You can trigger it easily as follows:

Reflect.compile(
    "com.example.MyClass",
    "package com.example; "
  + "@app.dev.ems.support.annotation.HBMModel "
  + "class MyClass {}",
    new CompileOptions().processors(new HBMModelProcessor())
);

This is specifically useful for unit testing annotation processors. See also this blog post here: https://blog.jooq.org/2018/12/07/how-to-unit-test-your-annotation-processor-using-joor

Disclaimer, I work for the company behind jOOR.