How to write automated unit tests for java annotation processor?

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I'm experimenting with java annotation processors. I'm able to write integration tests using the "JavaCompiler" (in fact I'm using "hickory" at the moment). I can run the compile process and analyse the output. The Problem: a single test runs for about half a second even without any code in my annotation processor. This is way too long to using it in TDD style.

Mocking away the dependencies seems very hard for me (I would have to mock out the entire "javax.lang.model.element" package). Have someone succeed to write unit tests for an annotation processor (Java 6)? If not ... what would be your approach?

annotation-processing annotations unit-testing java
7 Answers

jOOR is a small Java reflection library that also provides simplified access to the in-memory Java compilation API in javax.tool.JavaCompiler. We added support for this to unit test jOOQ's annotation processors. You can easily write unit tests like this:

@Test
public void testCompileWithAnnotationProcessors() {
    AProcessor p = new AProcessor();

    try {
        Reflect.compile(
            "org.joor.test.FailAnnotationProcessing",
            "package org.joor.test; " +
            "@A " +
            "public class FailAnnotationProcessing { " +
            "}",
            new CompileOptions().processors(p)
        ).create().get();
        Assert.fail();
    }
    catch (ReflectException expected) {
        assertFalse(p.processed);
    }
}

The above example has been taken from this blog post

user_5217990

I ran into the same problem awhile ago and found this question. Although the other answers provided are decent, I felt that that there was still room for improvement. Based on the other answers for this question, I created Elementary, a suite of JUnit 5 extensions that provide a real annotation processing environment for unit tests.

Most libraries test annotation processors by running them. However, most annotation processors are pretty complex and broken into more fine-grained components. It is not feasible to test individual components by running the annotation processor. Instead, we make the annotation processing environment available to these tests.

The following code snippet illustrates how to test a Lint component:

import com.karuslabs.elementary.junit.Cases;
import com.karuslabs.elementary.junit.Tools;
import com.karuslabs.elementary.junit.ToolsExtension;
import com.karuslabs.elementary.junit.annotations.Case;
import com.karuslabs.elementary.junit.annotations.Introspect;
import com.karuslabs.utilitary.type.TypeMirrors;

@ExtendWith(ToolsExtension.class)
@Introspect
class ToolsExtensionExampleTest {

    Lint lint = new Lint(Tools.typeMirrors());
    
    @Test
    void lint_string_variable(Cases cases) {
        var first = cases.one("first");
        assertTrue(lint.lint(first));
    }
    
    @Test
    void lint_method_that_returns_string(Cases cases) {
        var second = cases.get(1);
        assertFalse(lint.lint(second));
    }
    
    @Case("first") String first;
    @Case String second() { return "";}
    
}

class Lint {
    
    final TypeMirrors types;
    final TypeMirror expectedType;
    
    Lint(TypeMirrors types) {
        this.types = types;
        this.expectedType = types.type(String.class);
    }
    
    public boolean lint(Element element) {
        if (!(element instanceof VariableElement)) {
            return false;
        }
        
        var variable = (VariableElement) element;
        return types.isSameType(expectedType, variable.asType());
    }
    
}

By annotating the test class with @Introspect and test cases with @Case, we can declare test cases in the same file as the tests. The corresponding Element representation of the test cases can be retrieved by a test using Cases.

If anyone is interested, I wrote an article, The Problem with Annotation Processors that details the problems with unit testing annotation processors.

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