问题

Maven可以使用mvn package指令对项目进行打包，如果使用java -jar xxx.jar执行运行jar文件，

会出现"no main manifest attribute, in xxx.jar"（没有设置Main-Class）、ClassNotFoundException（找不到依赖包）等错误。

解决方法

要想jar包能直接通过java -jar xxx.jar运行，需要满足：

1、在jar包中的META-INF/MANIFEST.MF中指定Main-Class，这样才能确定程序的入口在哪里；

2、要能加载到依赖包。

方法一

使用maven-jar-plugin和maven-dependency-plugin插件打包

打包效果

生成一个jar文件和lib目录

相关插件

maven-jar-plugin

https://mvnrepository.com/artifact/org.apache.maven.plugins/maven-jar-plugin/

maven-jar-plugin用于生成jar包中META-INF/MANIFEST.MF文件的部分内容

<mainClass>com.xxg.Main</mainClass>指定MANIFEST.MF中的Main-Class

<addClasspath>true</addClasspath>会在MANIFEST.MF加上Class-Path项并配置依赖包，<classpathPrefix>lib/</classpathPrefix>指定依赖包所在目录。

maven-dependency-plugin

https://mvnrepository.com/artifact/org.apache.maven.plugins/maven-dependency-plugin

maven-dependency-plugin插件用于将依赖包拷贝到指定路径下

<outputDirectory>${project.build.directory}/lib</outputDirectory>

示例POM

<plugins>
	<plugin>
		<groupId>org.apache.maven.plugins</groupId>
		<artifactId>maven-jar-plugin</artifactId>
		<version>2.6</version>
		<configuration>
			<archive>
				<manifest>
					<addClasspath>true</addClasspath>
					<classpathPrefix>lib/</classpathPrefix>
					<mainClass>com.xxg.Main</mainClass>
				</manifest>
			</archive>
		</configuration>
	</plugin>
	
	<plugin>
		<groupId>org.apache.maven.plugins</groupId>
		<artifactId>maven-dependency-plugin</artifactId>
		<version>2.10</version>
		<executions>
			<execution>
				<id>copy-dependencies</id>
				<phase>package</phase>
				<goals>
					<goal>copy-dependencies</goal>
				</goals>
				<configuration>
					<outputDirectory>${project.build.directory}/lib</outputDirectory>
				</configuration>
			</execution>
		</executions>
	</plugin>
</plugins>

方法二

使用maven-assembly-plugin插件打包

打包效果

相关插件

maven-assembly-plugin

https://mvnrepository.com/artifact/org.apache.maven.plugins/maven-assembly-plugin

示例POM

<plugins>
	<plugin>
		<groupId>org.apache.maven.plugins</groupId>
		<artifactId>maven-assembly-plugin</artifactId>
		<version>2.5.5</version>
		<configuration>
			<archive>
				<manifest>
					<mainClass>com.xxg.Main</mainClass>
				</manifest>
			</archive>
			<descriptorRefs>
				<descriptorRef>jar-with-dependencies</descriptorRef>
			</descriptorRefs>
		</configuration>
	</plugin>
</plugins>

方法三

使用maven-shade-plugin插件打包

打包效果

生成一个jar包，jar包里面是将class类拷贝

相关插件

maven-shade-plugin

https://mvnrepository.com/artifact/org.apache.maven.plugins/maven-shade-plugin

示例POM

<plugins>
	<plugin>
		<groupId>org.apache.maven.plugins</groupId>
		<artifactId>maven-shade-plugin</artifactId>
		<version>2.4.1</version>
		<executions>
			<execution>
				<phase>package</phase>
				<goals>
					<goal>shade</goal>
				</goals>
				<configuration>
					<transformers>
						<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
							<mainClass>com.xxg.Main</mainClass>
						</transformer>
					</transformers>
				</configuration>
			</execution>
		</executions>
	</plugin>
</plugins>

遇到过的问题

如果项目中用到了Spring Framework，将依赖打到一个jar包中，运行时会出现读取XML schema文件出错。原因是Spring Framework的多个jar包中包含相同的文件spring.handlers和spring.schemas，如果生成一个jar包会互相覆盖。为了避免互相影响，可以使用AppendingTransformer来对文件内容追加合并：

<plugins>

	<plugin>
		<groupId>org.apache.maven.plugins</groupId>
		<artifactId>maven-shade-plugin</artifactId>
		<version>2.4.1</version>
		<executions>
			<execution>
				<phase>package</phase>
				<goals>
					<goal>shade</goal>
				</goals>
				<configuration>
					<transformers>
						<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
							<mainClass>com.xxg.Main</mainClass>
						</transformer>
						<transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
							<resource>META-INF/spring.handlers</resource>
						</transformer>
						<transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
							<resource>META-INF/spring.schemas</resource>
						</transformer>
					</transformers>
				</configuration>
			</execution>
		</executions>
	</plugin>

</plugins>

方法四

使用spring-boot-maven-plugin插件打包，多用于spring项目。

打包效果

生成一个jar包

相关插件

spring-boot-maven-plugin

https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-maven-plugin

示例POM

<plugins>
    <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
        <version>2.7.9</version>
        <executions>
            <execution>
                <goals>
                    <goal>repackage</goal>
                </goals>
            </execution>
        </executions>
        <configuration>
            <mainClass>com.example.service.DubboProvider</mainClass>
        </configuration>
    </plugin>
</plugins>

参考

https://blog.csdn.net/xiao__gui/article/details/47341385#

maven多模块项目springboot打包：

https://blog.csdn.net/Blackjoker0/article/details/126954209