错误：无法找到或加载主类... - Eclipse 中的 Maven 项目

Question

我使用 Eclipse 创建了一个 Maven 项目，并且能够在 Eclipse 中运行 App.java。但现在我想用这个命令从 Eclipse 中运行它：

..\workspace\subscribe\target\classes> java com.mqtt.subscribe.App 

编辑： 带有可执行文件jar：

..\workspace\subscribe>java -jar target\subscribe-0.0.1-SNAPSHOT.jar

并得到这个错误：

错误：无法找到或加载主类 com.mqtt.subscribe.App

我做错了什么？没看懂。

我还尝试使用cmd 中的mvn package 和mvn clean install 重建项目。我总是得到同样的错误。

编辑：我还尝试制作一个可执行的jar 文件，仍然是同样的错误。

感谢您的帮助。

系统变量设置如下：

JAVA_HOME = C:\Program Files\Java\jdk1.8.0_121
Path = C:\Program Files\Java\jdk1.8.0_121\bin;C:\Program Files\Maven\apache-maven-3.3.9\bin

pom.xml

<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.mqtt</groupId>
  <artifactId>subscribe</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>subscribe</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <version>3.0.0</version>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <mainClass>com.mqtt.subscribe.App</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
        <plugin>
            <artifactId>maven-assembly-plugin</artifactId>
            <configuration>
                <descriptorRefs>
                    <descriptorRef>jar-with-dependencies</descriptorRef>
                </descriptorRefs>
            </configuration>
        </plugin>
    </plugins>
  </build>

  <repositories>
    <repository>
        <id>Eclipse Paho Repo</id>
        <url>https://repo.eclipse.org/content/repositories/paho-releases/</url>
    </repository>
  </repositories>

  <dependencies>
    <dependency>
        <groupId>org.eclipse.paho</groupId>
        <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
        <version>1.0.2</version>
    </dependency>
  </dependencies>

</project>

App.java

package com.mqtt.subscribe;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttAsyncClient;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class App implements MqttCallback {

    public static void main(String[] args) {
        String topic = "TEST";
        int qos = 2;
        String broker = "tcp://broker.hivemq.com:1883";
        String clientId = "Test";
        MemoryPersistence persistence = new MemoryPersistence();

        try {

            MqttAsyncClient sampleClient = new MqttAsyncClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            sampleClient.setCallback(new App());
            System.out.println("Connecting to broker: " + broker);
            sampleClient.connect(connOpts);
            System.out.println("Connected");
            Thread.sleep(500);
            sampleClient.subscribe(topic, qos);
            System.out.println("Subscribed");
        } catch (Exception me) {
            if (me instanceof MqttException) {
                System.out.println("reason " + ((MqttException) me).getReasonCode());
            }
            System.out.println("msg " + me.getMessage());
            System.out.println("loc " + me.getLocalizedMessage());
            System.out.println("cause " + me.getCause());
            System.out.println("excep " + me);
            me.printStackTrace();
        }
    }

    public void connectionLost(Throwable arg0) {
        System.err.println("connection lost");

    }

    public void deliveryComplete(IMqttDeliveryToken arg0) {
        System.err.println("delivery complete");
    }

    public void messageArrived(String topic, MqttMessage message) throws Exception {
        System.out.println("topic: " + topic);
        System.out.println("message: " + new String(message.getPayload()));
    }  
}

CMD:

C:\Users\test\workspace\subscribe>mvn clean install
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building subscribe 0.0.1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ subscribe ---
[INFO] Deleting C:\Users\test\workspace\subscribe\target
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ subscribe
---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory C:\Users\test\workspace\subscribe
\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ subscribe ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to C:\Users\test\workspace\subscribe\target\c
lasses
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ su
bscribe ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory C:\Users\test\workspace\subscribe
\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ subscri
be ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ subscribe ---
[INFO] No tests to run.
[INFO]
[INFO] --- maven-jar-plugin:3.0.0:jar (default-jar) @ subscribe ---
[INFO] Building jar: C:\Users\test\workspace\subscribe\target\subscribe-0.0.
1-SNAPSHOT.jar
[INFO]
[INFO] --- maven-install-plugin:2.4:install (default-install) @ subscribe ---
[INFO] Installing C:\Users\test\workspace\subscribe\target\subscribe-0.0.1-S
NAPSHOT.jar to C:\Users\test\.m2\repository\com\mqtt\subscribe\0.0.1-SNAPSHO
T\subscribe-0.0.1-SNAPSHOT.jar
[INFO] Installing C:\Users\test\workspace\subscribe\pom.xml to C:\Users\test\.m2\repository\com\mqtt\subscribe\0.0.1-SNAPSHOT\subscribe-0.0.1-SNAPSHOT.p
om
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.049 s
[INFO] Finished at: 2017-02-26T17:50:53+01:00
[INFO] Final Memory: 15M/212M
[INFO] ------------------------------------------------------------------------

C:\Users\test\workspace\subscribe>java -jar target\subscribe-0.0.1-SNAPSHOT.
jar
Error: Could not find or load main class com.mqtt.subscribe.App

Answer 1

您的主要问题来自 pom.xml 的构建部分，因为 程序集插件不知道您的主类是什么，并且您还忘记指出程序集插件应该运行的阶段（请参阅解决此问题的第二个解决方案）第一个解决方案是摆脱程序集插件并使用下面的内容（调整主类的名称）。第二种解决方案是在这段代码sn-p之后：

<build>
  <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <encoding>UTF-8</encoding>
                    <target>1.8</target>
                    <source>1.8</source>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-dependency-plugin</artifactId>
                <executions>
                    <execution>
                        <id>copy-dependencies</id>
                        <phase>prepare-package</phase>
                        <goals>
                            <goal>copy-dependencies</goal>
                        </goals>
                        <configuration>
                            <outputDirectory>${project.build.directory}/lib/</outputDirectory>
                            <overWriteReleases>false</overWriteReleases>
                            <overWriteSnapshots>false</overWriteSnapshots>
                            <overWriteIfNewer>true</overWriteIfNewer>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            <plugin>
<!--                Build an executable JAR -->
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.0.2</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                            <classpathPrefix>lib/</classpathPrefix>
                            <mainClass>org.test.whatever.pomtester.App</mainClass>
                        </manifest>
                    </archive>

                </configuration>
            </plugin>
    </plugins>
  </build>

另一种解决方案是只使用程序集插件，如下所示（如之前调整主类的名称）：

<build>
        <plugins>

            <plugin>
                <artifactId>maven-assembly-plugin</artifactId>
                <configuration>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies</descriptorRef>
                    </descriptorRefs>
                    <archive>
                        <manifest>
                            <mainClass>org.test.whatever.pomtester.App</mainClass>
                        </manifest>
                    </archive>
                </configuration>
                <executions>
                    <execution>

                        <phase>package</phase> <!-- bind to the packaging phase -->
                        <goals>
                            <goal>single</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>