我正在尝试编译并执行默认的Maven应用程序。我不确定我这样做是否正确，但从我可以告诉应用程序正确编译，但不会通过maven执行。有人知道我做错了什么吗？
1.创建项目
第一个月
1.在POM中调整编译器设置（我使用JDK 21）
<maven.compiler.source>1.9</maven.compiler.source><maven.compiler.target>1.9</maven.compiler.target>
1.编译
mvn compile个
成功
1.有效的测试

$ cd target/classes 
$ java com.mycompany.app.App     
Hello World!

字符串
1.现在我观察到的问题，使用mvn exec:java

$ mvn exec:java
[INFO] ---------------------< com.mycompany.app:demoapp >----------------------
[INFO] Building demoapp 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- exec:3.1.0:java (default-cli) @ demoapp ---
[INFO] ---------------
[INFO] BUILD FAILURE
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.1.0:java (default-cli) on project demoapp: The parameters 'mainClass' for goal org.codehaus.mojo:exec-maven-plugin:3.1.0:java are missing or invalid -> [Help 1]

型

1. pom.xml个

<?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">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.mycompany.app</groupId>
  <artifactId>demoapp</artifactId>
  <version>1.0-SNAPSHOT</version>

  <name>demoapp</name>
  <!-- FIXME change it to the project's website -->
  <url>http://www.example.com</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.9</maven.compiler.source>
    <maven.compiler.target>1.9</maven.compiler.target>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <build>
    <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
      <plugins>
        <!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
        <plugin>
          <artifactId>maven-clean-plugin</artifactId>
          <version>3.1.0</version>
        </plugin>
        <!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
        <plugin>
          <artifactId>maven-resources-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>3.8.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-surefire-plugin</artifactId>
          <version>2.22.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-jar-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-install-plugin</artifactId>
          <version>2.5.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-deploy-plugin</artifactId>
          <version>2.8.2</version>
        </plugin>
        <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
        <plugin>
          <artifactId>maven-site-plugin</artifactId>
          <version>3.7.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-project-info-reports-plugin</artifactId>
          <version>3.0.0</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

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1. mvn -DmainClass=com.mycompany.app.App exec:java结果

~/src/java/demoapp]$ mvn -DmainClass=com.mycompany.app.App exec:java
[INFO] Scanning for projects...
[INFO] 
[INFO] ---------------------< com.mycompany.app:demoapp >----------------------
[INFO] Building demoapp 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- exec:3.1.0:java (default-cli) @ demoapp ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.847 s
[INFO] Finished at: 2023-11-15T18:53:53-05:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.1.0:java (default-cli) on project demoapp: The parameters 'mainClass' for goal org.codehaus.mojo:exec-maven-plugin:3.1.0:java are missing or invalid -> [Help 1]

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1.目录结构

$ tree .
.
├── pom.xml
├── src
│   ├── main
│   │   └── java
│   │       └── com
│   │           └── mycompany
│   │               └── app
│   │                   └── App.java
│   └── test
│       └── java
│           └── com
│               └── mycompany
│                   └── app
│                       └── AppTest.java
└── target
    ├── classes
    │   └── com
    │       └── mycompany
    │           └── app
    │               └── App.class
    ├── generated-sources
    │   └── annotations
    └── maven-status
        └── maven-compiler-plugin
            └── compile
                ├── default-cli
                │   ├── createdFiles.lst
                │   └── inputFiles.lst
                └── default-compile
                    ├── createdFiles.lst
                    └── inputFiles.ls

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不