從編譯時註解解析到Lombok

時間 2019-11-07

標籤編譯註解解析 lombok 欄目 Java 简体版

原文原文鏈接

以前的文章中介紹過Java註解相關的內容（見Java Annotations），本文將介紹Lombok及其背後的編譯時註解處理技術。java

經過註解生成POJO setter／getter

首先定義一個註解@Data:shell

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
public @interface Data {
}
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實現@Data的註解處理器apache

public class DataProcessor extends AbstractProcessor {

    private Messager messager;
    private JavacTrees trees;
    // 建立AST節點工具
    private TreeMaker treeMaker;
    private Names names;

    /** 獲取處理器框架組件 */
    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        messager = processingEnv.getMessager();
        trees = JavacTrees.instance(processingEnv);
        Context context = ((JavacProcessingEnvironment) processingEnv).getContext();
        treeMaker = TreeMaker.instance(context);
        names = Names.instance(context);
    }

    /** 註解處理邏輯，在這裏修改AST */
    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        messager.printMessage(Kind.NOTE, "starting process");
        /** 獲取Data註解的元素 */
        Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(Data.class);
        for (Element element : elements) {
            JCTree jcTree = trees.getTree(element);
            // 遍歷類定義的元素
            jcTree.accept(new TreeTranslator() {
                @Override
                public void visitClassDef(JCClassDecl jcClassDecl) {
                    super.visitClassDef(jcClassDecl);
                    try {
                        java.util.List<JCVariableDecl> jcVariableDeclList = new ArrayList<>();
                        for (JCTree tree : jcClassDecl.defs) {
                            // 收集全部field
                            if (tree.getKind() == Tree.Kind.VARIABLE) {
                                jcVariableDeclList.add((JCVariableDecl) tree);
                            }
                        }

                        for (JCVariableDecl jcVariableDecl : jcVariableDeclList) {
                            // 生成相應的geeter，setter
                            generateElement(jcClassDecl, jcVariableDecl);
                        }
                    } catch (Throwable e) {
                        try {
                            e.printStackTrace(new PrintStream(new FileOutputStream("error.txt")));
                        } catch (FileNotFoundException e1) {
                            messager.printMessage(Kind.ERROR, "Error create log file");
                        }
                    }
                }
            });
        }
        return true;
    }

    /** 處理的註解類型 */
    @Override
    public Set<String> getSupportedAnnotationTypes() {
        LinkedHashSet<String> supportedAnnotations = new LinkedHashSet<>();
        supportedAnnotations.add("*");
        return supportedAnnotations;
    }

    /** 支持的JDK版本 */
    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }
}
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具體方法生成邏輯：微信

private void generateElement(JCClassDecl jcClassDecl, JCVariableDecl jcVariableDecl) throws Exception {
        // 將生成的getter方法插入類語法樹中
        JCMethodDecl getter = generateGetterMethodElement(jcVariableDecl);
        jcClassDecl.defs = jcClassDecl.defs.append(getter);

  			// 將生成的setter方法插入類語法樹中
        JCMethodDecl setter = generateSetterMethodElement(jcVariableDecl);
        jcClassDecl.defs = jcClassDecl.defs.append(setter);
    }

		/** 生成getter方法 */
    private JCMethodDecl generateGetterMethodElement(JCVariableDecl jcVariableDecl) {

        String field = jcVariableDecl.getName().toString();
        Name name = names.fromString("get" + field.substring(0, 1).toUpperCase() + field.substring(1, field.length()));

        ListBuffer<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), name, jcVariableDecl.vartype,
            List.nil(), List.nil(), List.nil(), body, null);
    }

		/** 生成setter方法 */
    private JCMethodDecl generateSetterMethodElement(JCVariableDecl jcVariableDecl) throws Exception {

        String field = jcVariableDecl.getName().toString();
        Name name = names.fromString("set" + field.substring(0, 1).toUpperCase() + field.substring(1, field.length()));

        JCTree.JCExpression returnMethodType = treeMaker
            .Type((Type) (Class.forName("com.sun.tools.javac.code.Type$JCVoidType").newInstance()));

        ListBuffer<JCStatement> statements = new ListBuffer<>();
        statements.append(treeMaker.Exec(treeMaker
            .Assign(treeMaker.Select(treeMaker.Ident(names.fromString("this")), jcVariableDecl.getName()),
                treeMaker.Ident(jcVariableDecl.getName()))));
        JCTree.JCBlock body = treeMaker.Block(0, statements.toList());

        JCTree.JCVariableDecl param = treeMaker
            .VarDef(treeMaker.Modifiers(Flags.PARAMETER, List.nil()), jcVariableDecl.name, jcVariableDecl.vartype,
                jcVariableDecl.nameexpr);
        List<JCTree.JCVariableDecl> parameters = List.of(param);

        return treeMaker.MethodDef(treeMaker.Modifiers(Flags.PUBLIC), name, returnMethodType,
            List.nil(), parameters, List.nil(), body, null);
    }
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用@Data註解去修飾一個類：app

@Data
public class Member {
    private Long id;
    private String name;
    private Date joinDate;
}
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搞定！驗證一下：框架

javac DataProcessor.java && javac -g -processor DataProcessor Member.java && javap -p Member
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經過SPI封裝成工具包

maven構建配置：eclipse

<build>
    <resources>
      <resource>
        <directory>src/main/resources</directory>
        <excludes>
          <!-- 構建時排除Service聲明，此時Processor還不可用 -->
          <exclude>META-INF/**/*</exclude>
        </excludes>
      </resource>
    </resources>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <source>8</source>
          <target>8</target>
        </configuration>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-resources-plugin</artifactId>
        <version>3.0.2</version>
        <executions>
          <execution>
            <!-- prepare-package階段再把services拷貝到生成類目標目錄 -->
            <id>process-META</id>
            <phase>prepare-package</phase>
            <goals>
              <goal>copy-resources</goal>
            </goals>
            <configuration>
              <outputDirectory>target/classes</outputDirectory>
              <resources>
                <resource>
                  <directory>${basedir}/src/main/resources/</directory>
                  <includes>
                    <include>**/*</include>
                  </includes>
                </resource>
              </resources>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
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新建項目，引入依賴：maven

<dependency>
  <groupId>org.yannis</groupId>
  <artifactId>srctool</artifactId>
  <version>1.0-SNAPSHOT</version>
</dependency>
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複製Member.java至新項目：ide

mvn clean package,觀察target生成類Member.class反編譯結果:工具

至此，已經實現了一個簡單的lombok功能！

Lombok

Lombok是我在我的項目中比較喜歡用的工具，它能使咱們僅經過簡單的註解就能夠在編譯時爲類生成一系列樣板式的代碼，源碼則能夠保持很是簡潔。下面簡單介紹幾個經常使用註解，更多用法能夠去官網查看。

Dependency:

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.16.20</version>
    <scope>provided</scope>
</dependency>
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