编译时注解（一）AbstractProcessor实战

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.CLASS)
public @interface MethodProcessor {
    String name() default "Method";
}

@MethodProcessor(name = "Method")
public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

//@AutoService(Processor.class) 这个有木有？这是一个注解处理器，是Google开发的，
//用来生成META-INF/services/javax.annotation.processing.Processor文件的。
//引入方式     compile 'com.google.auto.service:auto-service:1.0-rc2'
@AutoService(Processor.class)
public class MyProcessor extends AbstractProcessor {

    private Types mTypeUtils;
    private Elements mElementUtils;
    private Filer mFiler;
    private Messager mMessager;

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        Set<String> annotations = new LinkedHashSet<>();
        //把我们自己定义的注解添加进去
        annotations.add(MethodProcessor.class.getCanonicalName());
        return annotations;
    }

    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }

    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);

        mTypeUtils = processingEnv.getTypeUtils();
        mElementUtils = processingEnv.getElementUtils();
        mFiler = processingEnv.getFiler();
        mMessager = processingEnv.getMessager();
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    	// // 遍历所有被注解了的元素
        for (Element annotatedElement : roundEnv.getElementsAnnotatedWith(MethodProcessor.class)) {
            if (annotatedElement.getKind() != ElementKind.CLASS) {
                error(annotatedElement, "Only classes can be annotated with @%s", MethodProcessor.class.getSimpleName());
                return true;
            }
            // //解析，并生成代码
            analysisAnnotated(annotatedElement);
        }
        return false;
    }

    private void error(Element e, String msg, Object... args) {
        mMessager.printMessage(Diagnostic.Kind.ERROR, String.format(msg, args), e);
    }

    private static final String SUFFIX = "Test";
    private static final String packageName = "com.company.processtest";
    private static final String retStr = "";

    private void analysisAnnotated(Element classElement) {
        MethodProcessor annotation = classElement.getAnnotation(MethodProcessor.class);
        String name = annotation.name();

        String newClassName = name + SUFFIX;

        StringBuilder builder = new StringBuilder()
                .append("package " + packageName + ";\n\n")
                .append("public class ")
                .append(newClassName)
                .append(" {\n\n") // open class
                .append("\tpublic String getMessage() {\n") // open method
                .append("\t\treturn \"");

        // this is appending to the return statement
        builder.append(retStr).append(" !\\n");

        builder.append("\";\n") // end return
                .append("\t}\n") // close method
                .append("}\n"); // close class

        try { // write the file
            JavaFileObject source = mFiler.createSourceFile(packageName + "." + newClassName);
            Writer writer = source.openWriter();
            writer.write(builder.toString());
            writer.flush();
            writer.close();
        } catch (IOException e) {
        }
    }
}

package com.company.processtest;

public class MethodTest {

	public String getMessage() {
		return "this is just a test !\n";
	}
}

@MethodProcessor(name = "Method")
public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        MethodTest test = new MethodTest();
        test.getMessage();
    }
}