lombok实现原理

大家在做java程序开发时会用到Lombok插件，在使用时大家都觉得其很方面，帮助我们消除Java代码的冗长，那么它的实现原理是什么呢？

在javac解析称抽象语法树之后，Lombok通过自己的注解解释器，动态修改语法树，插入新的节点到语法树中，最后通过分析生成字节码。

例如创建一个名为Getter的注解。

package annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.CLASS)
public @interface Getter {
}

为这个注解添加一个GetterProcessor解释器。JCTree、TreeMaker的相关用法见https://www.cnblogs.com/lwjblogs/p/15741065.html

import annotation.Getter;
import com.sun.source.tree.Tree;
import com.sun.tools.javac.api.JavacTrees;
import com.sun.tools.javac.code.Flags;
import com.sun.tools.javac.processing.JavacProcessingEnvironment;
import com.sun.tools.javac.tree.JCTree;
import com.sun.tools.javac.tree.TreeMaker;
import com.sun.tools.javac.tree.TreeTranslator;
import com.sun.tools.javac.util.*;

import javax.annotation.processing.*;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;
import java.util.Set;

@SupportedSourceVersion(SourceVersion.RELEASE_8)
@SupportedAnnotationTypes("annotation.Getter")
public class GetterProcessor extends AbstractProcessor {

    private Messager messager;

    private TreeMaker treeMaker;

    private JavacTrees trees;

    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.names = Names.instance(context);
        this.treeMaker = TreeMaker.instance(context);
    }

    @Override
    public 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> jcVariableDecls = List.nil();
                    for(JCTree jcTree : jcClassDecl.defs){
                        if(jcTree.getKind().equals(Tree.Kind.VARIABLE)){
                            JCTree.JCVariableDecl jcVariableDecl = (JCTree.JCVariableDecl) jcTree;
                            jcVariableDecls = jcVariableDecls.append(jcVariableDecl);
                        }
                    }
                    jcVariableDecls.forEach(jcVariableDecl -> {
                        messager.printMessage(Diagnostic.Kind.NOTE, jcVariableDecl.getName() + " has been processed");
                        jcClassDecl.defs = jcClassDecl.defs.prepend(makeGetterDecl(jcVariableDecl));
                    });
                    super.visitClassDef(jcClassDecl);
                }
            });
        });
        return true;
    }

    private JCTree makeGetterDecl(JCTree.JCVariableDecl jcVariableDecl){
        ListBuffer<JCTree.JCStatement> listBuffer = new ListBuffer<>();
        listBuffer.add(treeMaker.Return(treeMaker.Select(treeMaker.Ident(names.fromString("this")),jcVariableDecl.getName())));
        JCTree.JCBlock block = treeMaker.Block(0,listBuffer.toList());
        return treeMaker.MethodDef(treeMaker.Modifiers(Flags.PUBLIC),getNewMethodName(jcVariableDecl.getName())
                ,jcVariableDecl.vartype, List.nil(), List.nil(), List.nil(),block,null);
    }

    private Name getNewMethodName(Name name) {
        String s = name.toString();
        return names.fromString("get" + s.substring(0, 1).toUpperCase() + s.substring(1, name.length()));
    }
}

创建一个名为Demo的测试样例。

import annotation.Getter;

@Getter
public class Demo{
    private String test;

    private String value;
}

使用javac编译Getter.java和GetterProcess.java成为Getter.class和GetterProcess.class

使用javac编译Demo.java javac -processor processor.GetterProcessor Demo.java 编译成功以后会生成一个Demo.class文件。

import annotation.Getter;

@Getter
public class Demo {
    private String test;
    private String value;

    public String getValue() {
        return this.value;
    }

    public String getTest() {
        return this.test;
    }

    public Demo() {
    }
}

可以看见在Demo.class文件中，已经成功生成了test和value的get方法。

setter方法也可以通过此方式实现。