Criando um Plug-in do Compilador Java

    com.sun
    tools
    1.8.0
    system
    ${java.home}/../lib/tools.jar

public class SampleJavacPlugin implements Plugin {

    @Override
    public String getName() {
        return "MyPlugin";
    }

    @Override
    public void init(JavacTask task, String... args) {
        Context context = ((BasicJavacTask) task).getContext();
        Log.instance(context)
          .printRawLines(Log.WriterKind.NOTICE, "Hello from " + getName());
    }
}

example/tutorials$ javac -cp ./core-java/target/classes -Xplugin:MyPlugin ./core-java/src/main/java/com/example/javac/TestClass.java
Hello from MyPlugin

public void init(JavacTask task, String... args) {
    task.addTaskListener(new TaskListener() {
        public void started(TaskEvent e) {
        }

        public void finished(TaskEvent e) {
            if (e.getKind() != TaskEvent.Kind.PARSE) {
                return;
            }
            // Perform instrumentation
        }
    });
}

public void finished(TaskEvent e) {
    if (e.getKind() != TaskEvent.Kind.PARSE) {
        return;
    }
    e.getCompilationUnit().accept(new TreeScanner() {
        @Override
        public Void visitClass(ClassTree node, Void aVoid) {
            return super.visitClass(node, aVoid);

        @Override
        public Void visitMethod(MethodTree node, Void aVoid) {
            return super.visitMethod(node, aVoid);
        }
    }, null);
}

@Documented
@Retention(RetentionPolicy.CLASS)
@Target({ElementType.PARAMETER})
public @interface Positive { }

public void service(@Positive int i) { }

public void service(@Positive int i) {
    if (i <= 0) {
        throw new IllegalArgumentException("A non-positive argument ("
          + i + ") is given as a @Positive parameter 'i'");
    }
}

public class SimpleSourceFile extends SimpleJavaFileObject {
    private String content;

    public SimpleSourceFile(String qualifiedClassName, String testSource) {
        super(URI.create(String.format(
          "file://%s%s", qualifiedClassName.replaceAll("\\.", "/"),
          Kind.SOURCE.extension)), Kind.SOURCE);
        content = testSource;
    }

    @Override
    public CharSequence getCharContent(boolean ignoreEncodingErrors) {
        return content;
    }
}

public class SimpleClassFile extends SimpleJavaFileObject {

    private ByteArrayOutputStream out;

    public SimpleClassFile(URI uri) {
        super(uri, Kind.CLASS);
    }

    @Override
    public OutputStream openOutputStream() throws IOException {
        return out = new ByteArrayOutputStream();
    }

    public byte[] getCompiledBinaries() {
        return out.toByteArray();
    }

    // getters
}

public class SimpleFileManager
  extends ForwardingJavaFileManager {

    private List compiled = new ArrayList<>();

    // standard constructors/getters

    @Override
    public JavaFileObject getJavaFileForOutput(Location location,
      String className, JavaFileObject.Kind kind, FileObject sibling) {
        SimpleClassFile result = new SimpleClassFile(
          URI.create("string://" + className));
        compiled.add(result);
        return result;
    }

    public List getCompiled() {
        return compiled;
    }
}

public class TestCompiler {
    public byte[] compile(String qualifiedClassName, String testSource) {
        StringWriter output = new StringWriter();

        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        SimpleFileManager fileManager = new SimpleFileManager(
          compiler.getStandardFileManager(null, null, null));
        List compilationUnits
          = singletonList(new SimpleSourceFile(qualifiedClassName, testSource));
        List arguments = new ArrayList<>();
        arguments.addAll(asList("-classpath", System.getProperty("java.class.path"),
          "-Xplugin:" + SampleJavacPlugin.NAME));
        JavaCompiler.CompilationTask task
          = compiler.getTask(output, fileManager, null, arguments, null,
          compilationUnits);

        task.call();
        return fileManager.getCompiled().iterator().next().getCompiledBinaries();
    }
}

public class TestRunner {

    public Object run(byte[] byteCode, String qualifiedClassName, String methodName,
      Class[] argumentTypes, Object... args) throws Throwable {
        ClassLoader classLoader = new ClassLoader() {
            @Override
            protected Class findClass(String name) throws ClassNotFoundException {
                return defineClass(name, byteCode, 0, byteCode.length);
            }
        };
        Class clazz;
        try {
            clazz = classLoader.loadClass(qualifiedClassName);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Can't load compiled test class", e);
        }

        Method method;
        try {
            method = clazz.getMethod(methodName, argumentTypes);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException(
              "Can't find the 'main()' method in the compiled test class", e);
        }

        try {
            return method.invoke(null, args);
        } catch (InvocationTargetException e) {
            throw e.getCause();
        }
    }
}

public class SampleJavacPluginTest {

    private static final String CLASS_TEMPLATE
      = "package com.example.javac;\n\n" +
        "public class Test {\n" +
        "    public static %1$s service(@Positive %1$s i) {\n" +
        "        return i;\n" +
        "    }\n" +
        "}\n" +
        "";

    private TestCompiler compiler = new TestCompiler();
    private TestRunner runner = new TestRunner();

    @Test(expected = IllegalArgumentException.class)
    public void givenInt_whenNegative_thenThrowsException() throws Throwable {
        compileAndRun(double.class,-1);
    }

    private Object compileAndRun(Class argumentType, Object argument)
      throws Throwable {
        String qualifiedClassName = "com.example.javac.Test";
        byte[] byteCode = compiler.compile(qualifiedClassName,
          String.format(CLASS_TEMPLATE, argumentType.getName()));
        return runner.run(byteCode, qualifiedClassName,
        "service", new Class[] {argumentType}, argument);
    }
}