java-apt编程实践（Annotatino Processing Tool+maven）

今天药忘吃喽~ 2023-07-23 06:58 4阅读 0赞

# 背景 #

最近在研究mapStruct，而mapStruct核心技巧就是apt，通过编译期注解+freemarker进行java文件生成，从而省去了很多编码。  
本wiki将阐述一个apt的编程实践。

# 实践过程 #

## 创建一个hello工程 ##

工程由两个模块组成：

*  application模块，将使用自定义的编译器注解
 *  apt模块，自定义注解，并且完成AbstractProcessor的继承和实现

根pom如下：

application的pom如下

<?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <parent>
            <artifactId>apt-test</artifactId>
            <groupId>com.mine</groupId>
            <version>1.0-SNAPSHOT</version>
        </parent>
        <modelVersion>4.0.0</modelVersion>
    
        <artifactId>application</artifactId>
        <dependencies>
            <dependency>
                <groupId>com.mine</groupId>
                <artifactId>apt</artifactId>
                <version>1.0-SNAPSHOT</version>
            </dependency>
            
            <dependency>
                <groupId>org.projectlombok</groupId>
                <artifactId>lombok</artifactId>
                <version>1.18.12</version>
                <scope>provided</scope>
            </dependency>
        </dependencies>
    </project>

apt的pom如下

<?xml version="1.0" encoding="UTF-8"?>
    <project xmlns="http://maven.apache.org/POM/4.0.0"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
        <parent>
            <artifactId>apt-test</artifactId>
            <groupId>com.mine</groupId>
            <version>1.0-SNAPSHOT</version>
        </parent>
        <modelVersion>4.0.0</modelVersion>
        <properties>
            <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
            <java.version>1.8</java.version>
        </properties>
    
        <artifactId>apt</artifactId>
        <build>
            <plugins>
                <plugin>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>2.5.1</version>
                    <configuration>
                        <source>1.7</source>
                        <target>1.7</target>
                        <compilerArgument>-proc:none</compilerArgument>
                    </configuration>
                </plugin>
            </plugins>
        </build>
    </project>

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70]  
创建一个Main类，并且运行一下看看结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 1]  
![在这里插入图片描述][20200407153502643.png]

## 编写apt代码 ##

自定义一个注解  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 2]  
实现AbstractProcessor，实现process过程

package com.mine.processor;
    
    import com.mine.anno.CustomAnnotation;
    
    import javax.annotation.processing.AbstractProcessor;
    import javax.annotation.processing.RoundEnvironment;
    import javax.annotation.processing.SupportedAnnotationTypes;
    import javax.lang.model.element.Element;
    import javax.lang.model.element.TypeElement;
    import javax.tools.JavaFileObject;
    import java.io.IOException;
    import java.io.Writer;
    import java.util.Set;
    
    @SupportedAnnotationTypes({
        
            "com.mine.anno.CustomAnnotation"
    })
    public class MyProcessor extends AbstractProcessor {
        
        @Override
        public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        
            //然后文件会被解释为.class文件
    
            StringBuilder builder = new StringBuilder()
                    .append("package com.mine.annotationprocessor.generated;\n\n")
                    .append("public class GeneratedClass {\n\n")
                    .append("\tpublic String getMessage() {\n")
                    .append("\t\treturn \"");
    
            //获取所有被CustomAnnotation修饰的代码元素
            for (Element element : roundEnv.getElementsAnnotatedWith(CustomAnnotation.class)) {
        
                String objectType = element.getSimpleName().toString();
                builder.append(objectType).append(" exists!\\n");
            }
    
            builder.append("\";\n")
                    .append("\t}\n")
                    .append("}\n");
    
            try {
        
                JavaFileObject source = processingEnv.getFiler().createSourceFile(
                        "com.mine.annotationprocessor.generated.GeneratedClass");
    
                Writer writer = source.openWriter();
                writer.write(builder.toString());
                writer.flush();
                writer.close();
            } catch (IOException e) {
        
                //
            }
            return false;
        }
    }

过程很简单，通过StringBuilder来写一个java文件

配置resources  
在resources/META-INF/services/创建javax.annotation.processing.Processor  
内容为：  
com.mine.processor.MyProcessor

这里表示，java compile编译器会通过配置文件去找到对应的processor，进行编译期的调用

## 编写application代码 ##

创建一个DemoClass类

package com.mine;
    
    import com.mine.anno.CustomAnnotation;
    
    // 使用我们自定义的注解
    @CustomAnnotation
    public class DemoClass {
        
    
    }

创建一个MyModel类  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 3]

package com.mine;
    
    import lombok.Data;
    
    // 使用lombok的Data，这里也可以自动生成代码
    @Data
    public class MyModel {
        
        private String name;
        private Integer age;
    }

## 编译 ##

在根目录执行  
`mvn compile`  
或者直接使用idea执行  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 4]  
查看class字节码文件的生成情况  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 5]  
可以看到已经有我们生成的代码了  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 6]  
MyModel使用了lombok，可以自动生成get set和toString等方法  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dhbmdqaWU1NTQw_size_16_color_FFFFFF_t_70 7]

# 评价 #

通过apt可以生成很多方便的代码生成模板，比如mapStruct，lombok等。可以参考博主的其他博客查看mapStruct的使用。

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[20200407153502643.png]: /images/20230528/69b7257dff9e454eb4050c1fbbdf48bc.png
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