AnnotationProcessorTest with JUnit

AnnotationProcessorTest.java

import org.junit.Before;
import org.junit.BeforeClass;

import javax.annotation.processing.Processor;
import javax.tools.Diagnostic;
import javax.tools.DiagnosticCollector;
import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import java.io.ByteArrayOutputStream;
import java.io.OutputStreamWriter;
import java.nio.charset.Charset;
import java.util.List;
import java.util.Locale;

/**
 * Your unit tests of AnnotationProcessors should inherit this class
 */
public abstract class AnnotationProcessorTest {
    private static JavaCompiler compiler;
    private StandardJavaFileManager fileManager;
    private DiagnosticCollector<JavaFileObject> collector;

    /**
     * url to the files you want to compile.
     */
    public abstract String[] getClassesToCompile();

    /**
     * processors you wish to test.
     */
    public abstract Iterable<? extends Processor> getProcessors();

    @BeforeClass
    public static void initClass() throws Exception {
        compiler = ToolProvider.getSystemJavaCompiler();
    }

    @Before
    public void initTest() throws Exception {
        collector = new DiagnosticCollector<JavaFileObject>();
        fileManager = compiler.getStandardFileManager(collector, Locale.US, Charset.forName("UTF-8"));
    }

    /**
     * Call this from your unit test. It will return the results of the compilation as a CompilationResult
     */
    protected CompilationResult testCompilation() throws Exception {
        OutputStreamWriter stdout = new OutputStreamWriter(new ByteArrayOutputStream());

        JavaCompiler.CompilationTask task = compiler.getTask(stdout, fileManager, collector, null,
                null, fileManager.getJavaFileObjects(getClassesToCompile()));

        task.setProcessors(getProcessors());

        Boolean result = task.call();

        String stdoutString = new String(new ByteArrayOutputStream().toByteArray());

        return new CompilationResult(collector.getDiagnostics(), stdoutString, result);
    }

    /**
     * results of the compilation.
     */
    protected static class CompilationResult {
        private List<Diagnostic<? extends JavaFileObject>> diagnostics;
        private String stdoutString;
        private Boolean result;

        public CompilationResult(List<Diagnostic<? extends JavaFileObject>> diagnostics,
                                 String stdoutString, Boolean result) {
            this.diagnostics = diagnostics;
            this.stdoutString = stdoutString;
            this.result = result;
        }

        /**
         * Any errors or diagnostics that occured during compilation
         */
        public List<Diagnostic<? extends JavaFileObject>> getDiagnostics() {
            return diagnostics;
        }

        /**
         * stdout of the compilation
         */
        public String getStdoutString() {
            return stdoutString;
        }

        /**
         * wether the compilation exited successfully
         */
        public Boolean isSuccess() {
            return result;
        }
    }
}