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);
    }
}