SpringBoot工程打包后执行Java -Jar就能启动的步骤原理

 	<build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.*</include>
                </includes>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.*</include>
                </includes>
            </resource>
        </resources>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-loader</artifactId>
            <version>2.2.7.RELEASE</version>
	</dependency>

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

 protected void launch(String[] args) throws Exception {
        JarFile.registerUrlProtocolHandler();
        ClassLoader classLoader = this.createClassLoader(this.getClassPathArchives());
        this.launch(args, this.getMainClass(), classLoader);
    }

protected String getMainClass() throws Exception {
        Manifest manifest = this.archive.getManifest();
        String mainClass = null;
        if (manifest != null) {
            mainClass = manifest.getMainAttributes().getValue("Start-Class");
        }
        if (mainClass == null) {
            throw new IllegalStateException("No 'Start-Class' manifest entry specified in " + this);
        } else {
            return mainClass;
        }
    }

 	private File getManifestFile(File root) {
        File metaInf = new File(root, "META-INF");
        return new File(metaInf, "MANIFEST.MF");
    }

protected void launch(String[] args, String mainClass, ClassLoader classLoader) throws Exception {
        Thread.currentThread().setContextClassLoader(classLoader);
        this.createMainMethodRunner(mainClass, args, classLoader).run();
    }

public MainMethodRunner(String mainClass, String[] args) {
        this.mainClassName = mainClass;
        this.args = args != null ? (String[])args.clone() : null;
    }
    public void run() throws Exception {
        Class<?> mainClass = Thread.currentThread().getContextClassLoader().loadClass(this.mainClassName);
        Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
        mainMethod.invoke((Object)null, this.args);
    }