Spring Boot 提供了一个插件 spring-boot-maven-plugin 把程序打包成一个可执行的jar包，直接执行java -jar xxx.jar即可以启动程序

1、引用 spring-boot-maven-plugin插件

<build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins>
</build>

2、打包文件内部结构

  1）、BOOT-INF：

                class：项目代码

                lib：依赖jar包

    2）、META-INF：程序入口

                maven.xxx：程序配置文件

                MANIFEST.MF

    3）、org.springframework.boot.loader

3、MANIFEST.MF

Manifest-Version: 1.0
Implementation-Title: sk-cloud-order-sc
Implementation-Version: 1.0-SNAPSHOT
Start-Class: com.sk.order.sc.app
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Build-Jdk-Spec: 1.8
Spring-Boot-Version: 1.4.3.RELEASE
Created-By: Maven JAR Plugin 3.2.2
Main-Class: org.springframework.boot.loader.JarLauncher

    Main-Class：org.springframework.boot.loader.JarLauncher：当我们使用java -jar xxx.jar启动jar包的时候通过调用JarLauncher#Main方法，不是我们定义的Main方法。SpringBoot内部提供一个可用于执行SpringBootApplication的工具类，这就是为什么打的包包含spring-boot-loader

4、JarLauncher源码查看

      1）、添加依赖

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-loader</artifactId><version>2.7.8</version>
</dependency>

       2)、抽象Launcher类

 用于启动应用程序，分别有JarLauncher、WarLauncher、PropertiesLauncher实现类

public abstract class Launcher {private static final String JAR_MODE_LAUNCHER = "org.springframework.boot.loader.jarmode.JarModeLauncher";public Launcher() {}protected void launch(String[] args) throws Exception {if (!this.isExploded()) {JarFile.registerUrlProtocolHandler();}ClassLoader classLoader = this.createClassLoader(this.getClassPathArchivesIterator());String jarMode = System.getProperty("jarmode");String launchClass = jarMode != null && !jarMode.isEmpty() ? "org.springframework.boot.loader.jarmode.JarModeLauncher" : this.getMainClass();this.launch(args, launchClass, classLoader);}protected void launch(String[] args, String launchClass, ClassLoader classLoader) throws Exception {Thread.currentThread().setContextClassLoader(classLoader);this.createMainMethodRunner(launchClass, args, classLoader).run();}//Archive相关代码
}

    3）、Archive：归档文件获取Mainifest等URL路径（jar包中包含jar，或者jar包中class文件，那么会使用 !/ 分隔开）

public class JarFileArchive implements Archive {public URL getUrl() throws MalformedURLException {return this.url != null ? this.url : this.jarFile.getUrl();}public Manifest getManifest() throws IOException {return this.jarFile.getManifest();}
}

public class JarLauncher extends ExecutableArchiveLauncher {static final Archive.EntryFilter NESTED_ARCHIVE_ENTRY_FILTER = (entry) -> {return entry.isDirectory() ? entry.getName().equals("BOOT-INF/classes/") : entry.getName().startsWith("BOOT-INF/lib/");};public JarLauncher() {}protected JarLauncher(Archive archive) {super(archive);}protected boolean isPostProcessingClassPathArchives() {return false;}protected boolean isNestedArchive(Archive.Entry entry) {return NESTED_ARCHIVE_ENTRY_FILTER.matches(entry);}protected String getArchiveEntryPathPrefix() {return "BOOT-INF/";}public static void main(String[] args) throws Exception {(new JarLauncher()).launch(args);}
}

protected void launch(String[] args) throws Exception {if (!this.isExploded()) {JarFile.registerUrlProtocolHandler();}//自定义类加载器加载jar文件（通过active的urls加载jar文件）ClassLoader classLoader = this.createClassLoader(this.getClassPathArchivesIterator());String jarMode = System.getProperty("jarmode");//获取manifest文件的Start-Class类String launchClass = jarMode != null && !jarMode.isEmpty() ? "org.springframework.boot.loader.jarmode.JarModeLauncher" : this.getMainClass();//调用Start-Class类类main方法this.launch(args, launchClass, classLoader);
}protected void launch(String[] args, String launchClass, ClassLoader classLoader) throws Exception {Thread.currentThread().setContextClassLoader(classLoader);this.createMainMethodRunner(launchClass, args, classLoader).run();
}/*
* 调用manifest文件Start-Class类mian方法
*/
public class MainMethodRunner {private final String mainClassName;private final String[] args;public MainMethodRunner(String mainClass, String[] args) {this.mainClassName = mainClass;this.args = args != null ? (String[])args.clone() : null;}public void run() throws Exception {Class<?> mainClass = Class.forName(this.mainClassName, false, Thread.currentThread().getContextClassLoader());Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);mainMethod.setAccessible(true);mainMethod.invoke((Object)null, this.args);}
}