1.说明

本文介绍Protobuf生成Java代码的方法，
配置对应的Maven插件，
把.proto文件生成Java代码。

2.插件配置

创建Maven工程grpc-compile，
修改pom.xml，
引入生成代码需要的jar包依赖，
以及构建配置：

<dependencies><dependency><groupId>io.grpc</groupId><artifactId>grpc-protobuf</artifactId><version>1.30.2</version></dependency><dependency><groupId>io.grpc</groupId><artifactId>grpc-stub</artifactId><version>1.30.2</version></dependency>
</dependencies>
<build><extensions><extension><groupId>kr.motd.maven</groupId><artifactId>os-maven-plugin</artifactId><version>1.6.2</version></extension></extensions><plugins><plugin><groupId>org.xolstice.maven.plugins</groupId><artifactId>protobuf-maven-plugin</artifactId><version>0.6.1</version><configuration><protocArtifact>com.google.protobuf:protoc:3.12.0:exe:${os.detected.classifier}</protocArtifact><pluginId>grpc-java</pluginId><pluginArtifact>io.grpc:protoc-gen-grpc-java:1.32.1:exe:${os.detected.classifier}</pluginArtifact><protoSourceRoot>src/main/proto</protoSourceRoot><outputDirectory>src/main/java</outputDirectory><clearOutputDirectory>false</clearOutputDirectory></configuration><executions><execution><goals><goal>compile</goal><goal>compile-custom</goal></goals></execution></executions></plugin></plugins>
</build>

3.创建.proto文件

新建src/main/proto目录，
在里面创建helloworld.proto文件：

// 显示声明使用proto3, 否则使用默认的proto2
syntax = "proto3";// 生成类的包名
option java_package = "com.asiainfo.yuwen.grpc.helloworld";
// 生成类的文件名，否则默认生成的类名为proto文件名的驼峰命名
option java_outer_classname = "HelloWorldProto";
// 定义的所有消息、枚举和服务生成对应的多个类文件，而不是以内部类的形式出现
option java_multiple_files = false;// greeting服务定义
service Greeter {// sayHello方法，格式为"方法名 请求参数 返回参数"rpc SayHello (HelloRequest) returns (HelloReply) {}// 另一个sayHello方法rpc SayHelloAgain (HelloRequest) returns (HelloReply) {}
}// 方法请求,包含用户名
message HelloRequest {string name = 1;
}// 方法响应,包含响应的消息
message HelloReply {string message = 1;
}

4.生成Java代码

在grpc-compile下执行Maven编译命令：

mvn clean compile

编译成功日志：

[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Detecting the operating system and CPU architecture
[INFO] ------------------------------------------------------------------------
[INFO] os.detected.name: windows
[INFO] os.detected.arch: x86_64
[INFO] os.detected.version: 10.0
[INFO] os.detected.version.major: 10
[INFO] os.detected.version.minor: 0
[INFO] os.detected.classifier: windows-x86_64
[INFO]
[INFO] ------------------< com.asiainfo.yuwen:grpc-compile >-------------------
[INFO] Building grpc-compile 0.0.1-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ grpc-compile ---
[INFO] Deleting D:\Code\Work\Telemetry\grpc-demo\grpc-compile\target
[INFO]
[INFO] --- protobuf-maven-plugin:0.6.1:compile (default) @ grpc-compile ---
[INFO] Compiling 1 proto file(s) to D:\Code\Work\Telemetry\grpc-demo\grpc-compile\src\main\java
[INFO]
[INFO] --- protobuf-maven-plugin:0.6.1:compile-custom (default) @ grpc-compile ---
[INFO] Compiling 1 proto file(s) to D:\Code\Work\Telemetry\grpc-demo\grpc-compile\src\main\java
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ grpc-compile ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory D:\Code\Work\Telemetry\grpc-demo\grpc-compile\src\main\resources
[INFO] Copying 1 resource
[INFO] Copying 1 resource
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ grpc-compile ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 2 source files to D:\Code\Work\Telemetry\grpc-demo\grpc-compile\target\classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  3.998 s
[INFO] Finished at: 2021-09-29T17:33:22+08:00
[INFO] ------------------------------------------------------------------------

会在src/main/java目录下生成代码:

5.插件参数说明

下面详细说明protobuf-maven-plugin插件配

configuration下的配置项：

1. protocArtifact：指定Protobuf编译器protoc具体版本，用于生成Java消息对象
2. pluginId：指定protoc的插件Id
3. pluginArtifact：指定生成Java代码的具体插件版本,用于生成Java接口服务
4. protoSourceRoot：proto文件所在目录
5. outputDirectory：Java类输出目录
6. clearOutputDirectory：是否需要清理输出目录

goals目标说明：

1. compile：编译消息对象
2. compile-custom :依赖上一步生成的消息对象,生成接口服务

说明一下，
上面clearOutputDirectory设置为false,
不清理输出目录，
是因为compile-custom时需要依赖上一步生成的消息对象，
如果清理了输出目录，
则compile-custom执行时会报错，
但是关闭了自动清理，
如果重新生成代码，
需要手动删除输出目录下的所有代码。

6.查看插件帮助手册

查看插件帮助手册(简略)
mvn help:describe -Dplugin=protobuf
查看插件帮助手册(详细，包含配置参数说明)
mvn help:describe -Dplugin=protobuf -Ddetail
查看插件帮助手册(详细，指定goal的说明)
mvn help:describe -Dplugin=protobuf -Dgoal=compile-custom -Ddetail

命令输出内容如下，
有部分省略：

Name: Maven Protocol Buffers Plugin
Description: Maven Plugin that executes the Protocol Buffers (protoc)compiler
Group Id: org.xolstice.maven.plugins
Artifact Id: protobuf-maven-plugin
Version: 0.6.1
Goal Prefix: protobufThis plugin has 15 goals:protobuf:compileDescription: This mojo executes the protoc compiler for generating mainJava sources from protocol buffer definitions. It also searches dependencyartifacts for .proto files and includes them in the proto_path so that theycan be referenced. Finally, it adds the .proto files to the project asresources so that they are included in the final artifact.
protobuf:compile-cppDescription: This mojo executes the protoc compiler for generating main C++sources from protocol buffer definitions. It also searches dependencyartifacts for .proto files and includes them in the proto_path so that theycan be referenced. Finally, it adds the .proto files to the project asresources so that they are included in the final artifact.
protobuf:compile-csharpDescription: This mojo executes the protoc compiler for generating main C#sources from protocol buffer definitions. It also searches dependencyartifacts for .proto files and includes them in the proto_path so that theycan be referenced. Finally, it adds the .proto files to the project asresources so that they are included in the final artifact.
protobuf:compile-customDescription: This mojo executes the protoc compiler with the specifiedplugin executable to generate main sources from protocol bufferdefinitions. It also searches dependency artifacts for .proto files andincludes them in the proto_path so that they can be referenced. Finally, itadds the .proto files to the project as resources so that they are includedin the final artifact.
protobuf:helpDescription: Display help information on protobuf-maven-plugin.Call mvn protobuf:help -Ddetail=true -Dgoal=<goal-name> to displayparameter details.
protobuf:test-compileDescription: This mojo executes the protoc compiler for generating testJava sources from protocol buffer definitions. It also searches dependencyartifacts in the test scope for .proto files and includes them in theproto_path so that they can be referenced. Finally, it adds the .protofiles to the project as test resources so that they can be included in thetest-jar artifact.
......
For more information, run 'mvn help:describe [...] -Ddetail'

7.参考文章

Maven Protocol Buffers Plugin Usage