为什么引用Maven自带的插件不需要指定goal，而自定义的需要指定？

<!-- tomcat7运行插件 -->
            <plugin>
                <groupId>org.apache.tomcat.maven</groupId>
                <artifactId>tomcat7-maven-plugin</artifactId>
                <version>2.2</version>
                <configuration>
                    <uriEncoding>UTF-8</uriEncoding>
                    <url>http://localhost:${maven.tomcat.port}/</url>
                    <server>tomcat7</server>
                    <systemProperties>
                        <bootClass>${bootClass}</bootClass>
                        <!-- 微服务应用端口与上下文路径 -->
                        <server.port>${maven.tomcat.port}</server.port>
                        <server.context-path>${maven.tomcat.path}/rest</server.context-path>
                    </systemProperties>
                </configuration>
            </plugin>

<!-- 自定义插件 -->
            <plugin>
                <groupId>org.example</groupId>
                <artifactId>PluginDemo</artifactId>
                <version>1.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>checkZtbConfig</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <modelMapping>
                        <keyName>ZTBFileStoragePath</keyName>
                        <keyName>ZTBFileStorageType</keyName>
                        <keyName>Bid_Confidence_Threshold</keyName>
                    </modelMapping>
                </configuration>
            </plugin>

实际上 Maven 插件除了是绑定在生命周期的那种以外，其余所有插件都要声明 goal

请看 spring boot 的打包插件，也是要声明 goal

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <version>${spring-boot.version}</version>
    <configuration>
        <fork>true</fork>
        <mainClass>${start-class}</mainClass>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
        </execution>
    </executions>
</plugin>