文章目录

1. 1. 前言
2. 2. 开始编码
   1. 2.1 定义注解
   2. 2.2 定义注解处理器
   3. 2.3 注册处理器服务
   4. 2.4 测试
3. 3. 如何debug程序

[TOC]

1. 前言

在编译代码的时候，生成java的代码可以做很多事情，比如初始化各种环境（比如Butter Knife，虽然没用过），而无需手动去写各种冗余的代码。

下面准备在编译代码的时候生成如下的java文件，内容很简单：

package io.patamon.apt;

import java.util.ArrayList;
import java.util.List;

public class TestJavaSourceGenerate {
    private static final List<String> TEST = new ArrayList();

    public TestJavaSourceGenerate() {
    }

    public static List<String> getTest() {
        return TEST;
    }

    static {
        TEST.add("0");
        TEST.add("1");
    }
}

2. 开始编码

2.1 定义注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.SOURCE)
@Documented
public @interface TestJavaSource {

}

2.2 定义注解处理器

@SupportedAnnotationTypes({"io.patamon.apt.annotation.TestJavaSource"})
@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class TestJavaSourceProcessor extends AbstractProcessor {

    private Filer filer;

    @Override
    public synchronized void init(ProcessingEnvironment processingEnv) {
        super.init(processingEnv);
        this.filer = processingEnv.getFiler();
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (TypeElement element : annotations) {
            roundEnv.getElementsAnnotatedWith(element).forEach(it -> {
                generateFile("io.patamon.apt.TestJavaSourceGenerate");
            });
        }
        return false;
    }

    /**
     * 生成java文件
     */
    private void generateFile(String path) {
        BufferedWriter writer = null;
        try {
            // 1. 创建java源文件
            JavaFileObject sourceFile = filer.createSourceFile(path);
            int period = path.lastIndexOf('.');
            String myPackage = period > 0 ? path.substring(0, period) : null;
            String clazz = path.substring(period + 1);
            // 2. 写入代码
            writer = new BufferedWriter(sourceFile.openWriter());
            if (myPackage != null) {
                writer.write("package " + myPackage + ";\n\n");
            }
            writer.write("import java.util.ArrayList;\n");
            writer.write("import java.util.List;\n\n");
            writer.write("/** This class is generated by CustomProcessor, do not edit. */\n");
            writer.write("public class " + clazz + " {\n");
            writer.write("    private static final List<String> TEST;\n\n");
            writer.write("    static {\n");
            writer.write("        TEST = new ArrayList<>();\n\n");
            writer.write("        TEST.add(\"" + 1 + "\");\n");
            writer.write("        TEST.add(\"" + 2 + "\");\n");
            writer.write("    }\n\n");
            writer.write("    public static List<String> getTest() {\n");
            writer.write("        return TEST;\n");
            writer.write("    }\n\n");
            writer.write("}\n");
        } catch (IOException e) {
            throw new RuntimeException("Could not write source for " + path, e);
        } finally {
            if (writer != null) {
                try {
                    writer.close();
                } catch (IOException e) {
                    //Silent
                }
            }
        }
    }
}

2.3 注册处理器服务

略

2.4 测试

@TestJavaSource
public class TestJavaSourceAnnotationDemo {

    public static void main(String[] args) throws Exception {
        System.out.println(TestJavaSourceGenerate.getTest());
    }

}

编译代码，在工程中可以看到如下效果：

要想在代码中使用这个类的话，需要在IDEA设置将generated-sources/annotations > Make Directory As Source Root, 或者在pom中添加build配置：

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>build-helper-maven-plugin</artifactId>
    <version>1.4</version>
    <executions>
        <execution>
            <id>test</id>
            <phase>generate-sources</phase>
            <goals>
                <goal>add-source</goal>
            </goals>
            <configuration>
                <sources>
                    <source>${basedir}/target/generated-sources/*</source>
                </sources>
            </configuration>
        </execution>
    </executions>
</plugin>

参考：https://stackoverflow.com/questions/5170620/unable-to-use-intellij-with-a-generated-sources-folder

在生成java代码的时候可以使用javapoet，而不用傻瓜式一行行去写。

更详细的代码可以参考：https://github.com/gumingwei/apt-demo
基本的api可以参考：https://www.jianshu.com/p/50d95fbf635c

3. 如何debug程序

可以参考: https://stackoverflow.com/questions/8587096/how-do-you-debug-java-annotation-processors-using-intellij

public static void main(String[] args) throws Exception {
    com.sun.tools.javac.Main.main(new String[] {"-proc:only",
            "-processor", "io.patamon.apt.annotation.TestJavaSource",
            "/Users/icemimosa/Documents/github/AskMisa/apt/apt-demo/src/main/java/io/patamon/apt/TestJavaSourceAnnotationDemo.java"});
}

其实内部就是调用了Java Compiler API。