使用mvn編譯一個java project: mvn clean compile exec:java 結果出現如下錯誤：

[INFO] [INFO] — exec-maven-plugin:1.5.0:java (default-cli) @ snmpclient — [INFO] ------------------------------------------------------------------------ [INFO] BUILD FAILURE [INFO] ------------------------------------------------------------------------ [INFO] Total time: 11.477 s [INFO] Finished at: 2018-11-11T10:31:08+08:00 [INFO] Final Memory: 23M/125M [INFO] ------------------------------------------------------------------------ [ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.5.0:java (default-cli) on project snmpclient: The parameters ‘mainClass’ for goal org.codehaus.mojo:exec-maven-plugin:1.5.0:java are missing or invalid -> [Help 1]

<build>
            <plugins>
               ......
                    <plugin>
                            <groupId>org.codehaus.mojo</groupId>
                            <artifactId>exec-maven-plugin</artifactId>
                            <version>1.2.1</version>
                            <executions>
                                    <execution>
                                            <goals>
                                                    <goal>java</goal>
                                            </goals>
                                    </execution>
                            </executions>
                            <configuration>
                                    <mainClass>pl.stasiak.pytel.SnmpClientApplication</mainClass>
                            </configuration>
                    </plugin>
            </plugins>
    </build>