概念

注解处理器（Annotation Processeor）是java内置的注解处理工具。它可以在代码编译阶段通过扫描代码源码读取代码中的注解。换句话说我们可以利用它在代码编译阶段扫描源码中的注解（Annotation）动态生成一些java文件，让机器帮助我们生成大量重复代码，提高工作效率。

使用方式

实例

以下代码帮助我们根据源码中的Scene注解里声明的SceneId自动生成SceneIds.java文件，达到给场景起别名的目的。下边的例子包括了注解处理器（Annotation Processeor）的使用、注解配置、代码生等功能。

代码

SceneProcessor.java

package com.hyena.framework.processor;

import com.google.auto.service.AutoService;
import com.hyena.framework.annotation.Scene;
import com.squareup.javapoet.FieldSpec;
import com.squareup.javapoet.JavaFile;
import com.squareup.javapoet.MethodSpec;
import com.squareup.javapoet.TypeSpec;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Filer;
import javax.annotation.processing.Messager;
import javax.annotation.processing.ProcessingEnvironment;
import javax.annotation.processing.Processor;
import javax.annotation.processing.RoundEnvironment;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.Modifier;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.Elements;
import javax.tools.Diagnostic;

/**
 * Created by yangzc on 17/11/23.
 */
@AutoService(Processor.class)
public class SceneProcessor extends AbstractProcessor {

    private static final String APPLICATION_ID = "applicationId";

    private Filer filer;
    private Elements elementUtils;
    private Messager messager;

    private String applicationId = "com.hyena.ids";
    private HashMap<String, String> mSceneMap = new HashMap<>();

    @Override
    public synchronized void init(ProcessingEnvironment processingEnvironment) {
        super.init(processingEnvironment);
        this.filer = processingEnvironment.getFiler();
        this.elementUtils = processingEnvironment.getElementUtils();
        this.messager = processingEnvironment.getMessager();

        if (processingEnvironment.getOptions() != null) {
            Map<String, String> options = processingEnvironment.getOptions();
            String applicationId = options.get(APPLICATION_ID);
            if (applicationId != null && !"".equals(applicationId)) {
                this.applicationId = applicationId;
                messager.printMessage(Diagnostic.Kind.NOTE, "applicationId: " + applicationId);
            }
        }
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        if (roundEnv.getRootElements().isEmpty() || isIgnore(roundEnv)) {
            return true;
        }
        messager.printMessage(Diagnostic.Kind.NOTE, "process!!!");
        mSceneMap.clear();
        for (Element e : roundEnv.getElementsAnnotatedWith(Scene.class)) {
            Scene scene = e.getAnnotation(Scene.class);
            mSceneMap.put(scene.value(), e.toString());
        }
        TypeSpec.Builder javaClassBuilder = TypeSpec.classBuilder("SceneIds").addModifiers(Modifier.PUBLIC);
        List<String> methodLine = new ArrayList();
        if (!mSceneMap.isEmpty()) {
            //添加静态变量
            Iterator<String> iterator = mSceneMap.keySet().iterator();
            while (iterator.hasNext()) {
                String sceneId = iterator.next();
                String className = mSceneMap.get(sceneId);
                FieldSpec fieldSpec = FieldSpec.builder(String.class, sceneId)
                        .addModifiers(Modifier.PUBLIC, Modifier.FINAL, Modifier.STATIC)
                        .initializer("$S", sceneId).build();
                javaClassBuilder.addField(fieldSpec);
                methodLine.add("sceneMap.put(" + sceneId + ", \"" + className + "\")");
            }
        }

        //添加getSceneMaps方法
        MethodSpec.Builder methodGetSceneMapSpecBuild = MethodSpec.methodBuilder("getSceneMaps")
                .returns(HashMap.class)
                .addModifiers(Modifier.STATIC, Modifier.PUBLIC)
                .addStatement("$T sceneMap = new $T()", HashMap.class, HashMap.class);
        for (int i = 0; i < methodLine.size(); i++) {
            methodGetSceneMapSpecBuild.addStatement(methodLine.get(i));
        }
        methodGetSceneMapSpecBuild.addStatement("return sceneMap");
        javaClassBuilder.addMethod(methodGetSceneMapSpecBuild.build());
        TypeSpec javaClass = javaClassBuilder.build();
        JavaFile javaFile = JavaFile.builder(applicationId, javaClass).build();

        try {
            javaFile.writeTo(filer);
        } catch (IOException e) {
            messager.printMessage(Diagnostic.Kind.NOTE, e.getMessage());
        }
        return true;
    }

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

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        Set<String> types = new HashSet<String>();
        types.add(Scene.class.getCanonicalName());
        return types;
    }

    @Override
    public Set<String> getSupportedOptions() {
        return Collections.singleton(APPLICATION_ID);
    }

    private boolean isIgnore(RoundEnvironment roundEnv) {
        Set<? extends Element> rootElements = roundEnv.getRootElements();
        if (rootElements.isEmpty())
            return true;
        for (Element element: rootElements) {
            String name = element.getSimpleName().toString();
            if ("SceneIds".equals(name))
                return true;
        }
        return false;
    }
}

Scene.java

package com.hyena.framework.annotation;

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

/**
 * Created by yangzc on 17/11/23.
 */

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface Scene {

    /**
     * 场景ID
     * @return 场景ID
     */
    String value();
}

build.gradle

apply plugin: 'java'

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    compile 'com.google.auto.service:auto-service:1.0-rc2'
    //谷歌的帮助我们快速实现注解处理器
    compile 'com.squareup:javapoet:1.7.0'
    //用来生成java文件的，避免字符串拼接
}
sourceCompatibility = "1.7"
targetCompatibility = "1.7"

dependencies中添加

annotationProcessor project(":processor")

defaultConfig中添加

javaCompileOptions {
    //配置注解处理器参数
    annotationProcessorOptions {
        arguments = [applicationId: defaultConfig.applicationId]
    }
}