@TrisceliVersion
public static final String version = "";

@TrisceliVersion
public static final String version = "1.0.31-SNAPSHOT";

@Target({ElementType.FIELD, ElementType.TYPE})
@Retention(RetentionPolicy.SOURCE)
public @interface Setter {
   // 略...
}

@Documented
@Retention(RetentionPolicy.SOURCE) //只在編譯期有效，最終不會(huì)打進(jìn)class文件中
@Target({ElementType.FIELD}) //僅允許作用于類(lèi)屬性之上
public @interface TrisceliVersion {
}

/**
 * {@link AbstractProcessor} 就屬于 Pluggable Annotation Processing API
 */
public class TrisceliVersionProcessor extends AbstractProcessor {

    private JavacTrees javacTrees;
    private TreeMaker treeMaker;
    private ProcessingEnvironment processingEnv;

    /**
     * 初始化處理器
     *
     * @param processingEnv 提供了一系列的實(shí)用工具
     */
    @SneakyThrows
    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        this.processingEnv = processingEnv;
        this.javacTrees = JavacTrees.instance(processingEnv);
        Context context = ((JavacProcessingEnvironment) processingEnv).getContext();
        this.treeMaker = TreeMaker.instance(context);
    }


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

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        HashSet<String> set = new HashSet<>();
        set.add(TrisceliVersion.class.getName()); // 支持解析的注解
        return set;
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (TypeElement t : annotations) {
            for (Element e : roundEnv.getElementsAnnotatedWith(t)) { // 獲取到給定注解的element（element可以是一個(gè)類(lèi)、方法、包等）
                // JCVariableDecl為字段/變量定義語(yǔ)法樹(shù)節(jié)點(diǎn)
                JCTree.JCVariableDecl jcv = (JCTree.JCVariableDecl) javacTrees.getTree(e);
                String varType = jcv.vartype.type.toString();
                if (!"java.lang.String".equals(varType)) { // 限定變量類(lèi)型必須是String類(lèi)型，否則拋異常
                    printErrorMessage(e, "Type '" + varType + "'" + " is not support.");
                }
                jcv.init = treeMaker.Literal(getVersion()); // 給這個(gè)字段賦值，也就是getVersion的返回值
            }
        }
        return true;
    }

    /**
     * 利用processingEnv內(nèi)的Messager對(duì)象輸出一些日志
     *
     * @param e element
     * @param m error message
     */
    private void printErrorMessage(Element e, String m) {
        processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, m, e);
    }

    private String getVersion() {
        /**
         * 獲取version，這里省略掉復(fù)雜的代碼，直接返回固定值
         */
        return "v1.0.1";
    }