lombok原理简析

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lombok原理分析和功能实现

原理

  • 定义编译期的注解
  • 利用JSR269 API（Pluggable Annotation Processing API）创建编译期的注解处理器
  • 利用tools.jar的javac api处理AST（抽象语法树）
  • 将功能注册jar包

实现

创建Getter注解的处理器

继承AbstractProcessor这个类，基本的框架大体如下：

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package com.mythsman.test;
import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;
import java.util.Set;
@SupportedAnnotationTypes("com.mythsman.test.Getter")
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class GetterProcessor extends AbstractProcessor {
    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
    }
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        return true;
    }
}

需要着重实现两个方法：

  • init : 通过ProcessingEnvironment来获取编译阶段的一些环境信息
  • process：主要实现具体逻辑的地方，也就是对AST进行处理的地方

init方法

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private Messager messager;
private JavacTrees trees;
private TreeMaker treeMaker;
private Names names;
@Override
public synchronized void init(ProcessingEnvironment processingEnv) {
    super.init(processingEnv);
    this.messager = processingEnv.getMessager();
    this.trees = JavacTrees.instance(processingEnv);
    Context context = ((JavacProcessingEnvironment) processingEnv).getContext();
    this.treeMaker = TreeMaker.instance(context);
    this.names = Names.instance(context);
}

process方法

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@Override
public synchronized boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
    Set<? extends Element> set = roundEnv.getElementsAnnotatedWith(Getter.class);
    set.forEach(element -> {
        JCTree jcTree = trees.getTree(element);
        jcTree.accept(new TreeTranslator() {
            @Override
            public void visitClassDef(JCTree.JCClassDecl jcClassDecl) {
                List<JCTree.JCVariableDecl> jcVariableDeclList = List.nil();
                for (JCTree tree : jcClassDecl.defs) {
                    if (tree.getKind().equals(Tree.Kind.VARIABLE)) {
                        JCTree.JCVariableDecl jcVariableDecl = (JCTree.JCVariableDecl) tree;
                        jcVariableDeclList = jcVariableDeclList.append(jcVariableDecl);
                    }
                }
                jcVariableDeclList.forEach(jcVariableDecl -> {
                    messager.printMessage(Diagnostic.Kind.NOTE, jcVariableDecl.getName() + " has been processed");
                    jcClassDecl.defs = jcClassDecl.defs.prepend(makeGetterMethodDecl(jcVariableDecl));
                });
                super.visitClassDef(jcClassDecl);
            }
        });
    });
    return true;
}

接下来再实现makeGetterMethodDecl方法:

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private JCTree.JCMethodDecl makeGetterMethodDecl(JCTree.JCVariableDecl jcVariableDecl) {
    ListBuffer<JCTree.JCStatement> statements = new ListBuffer<>();
    statements.append(treeMaker.Return(treeMaker.Select(treeMaker.Ident(names.fromString("this")), jcVariableDecl.getName())));
    JCTree.JCBlock body = treeMaker.Block(0, statements.toList());
    return treeMaker.MethodDef(treeMaker.Modifiers(Flags.PUBLIC), getNewMethodName(jcVariableDecl.getName()), jcVariableDecl.vartype, List.nil(), List.nil(), List.nil(), body, null);
}
private Name getNewMethodName(Name name) {
    String s = name.toString();
    return names.fromString("get" + s.substring(0, 1).toUpperCase() + s.substring(1, name.length()));
}