springboot的jar为何能独立运行

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.bolingcavalry</groupId>
    <artifactId>springbootstarterdemo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>springbootstarterdemo</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

package com.bolingcavalry.springbootstarterdemo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Date;

@SpringBootApplication
@RestController
public class SpringbootstarterdemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringbootstarterdemoApplication.class, args);
    }
    @RequestMapping(value = "/hello")
    public String hello(){
        return "hello " + new Date();
    }
}

mvn clean package -U -DskipTests

Spring-Boot-Classpath-Index: BOOT-INF/classpath.idx
Implementation-Title: springbootstarterdemo
Implementation-Version: 0.0.1-SNAPSHOT
Start-Class: com.bolingcavalry.springbootstarterdemo.Springbootstarter
 demoApplication
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Build-Jdk-Spec: 1.8
Spring-Boot-Version: 2.3.1.RELEASE
Created-By: Maven Jar Plugin 3.2.0
Implementation-Vendor: Pivotal Software, Inc.
Main-Class: org.springframework.boot.loader.JarLauncher

Start-Class: com.bolingcavalry.springbootstarterdemo.Springbootstarter
 demoApplication

public static void main(String[] args) throws Exception {
	new JarLauncher().launch(args);
}

protected void launch(String[] args) throws Exception {
    // 将jar解压后运行的方式叫做exploded mode
    // 如果是exploded mode，就不能支持通过URL加载jar
    // 如果不是exploded mode,就可以通过URL加载jar
	if (!isExploded()) {
	    // 如果允许通过URL加载jar，就在此注册对应的处理类
		JarFile.registerUrlProtocolHandler();
	}
	// 创建classLoader
	ClassLoader classLoader = createClassLoader(getClassPathArchivesIterator());
	// jarmode是创建docker镜像时用到的参数，使用该参数是为了生成带有多个layer信息的镜像
	// 这里暂时不关注jarmode
	String jarMode = System.getProperty("jarmode");
	//如果没有jarmode参数，launchClass的值就来自getMainClass()返回
	String launchClass = (jarMode != null && !jarMode.isEmpty()) ? JAR_MODE_LAUNCHER : getMainClass();
	launch(args, launchClass, classLoader);
}

public ExecutableArchiveLauncher() {
		try {
			this.archive = createArchive();
			this.classPathIndex = getClassPathIndex(this.archive);
		}
		catch (Exception ex) {
			throw new IllegalStateException(ex);
		}
	}

protected final Archive createArchive() throws Exception {
		ProtectionDomain protectionDomain = getClass().getProtectionDomain();
		CodeSource codeSource = protectionDomain.getCodeSource();
		URI location = (codeSource != null) ? codeSource.getLocation().toURI() : null;
		String path = (location != null) ? location.getSchemeSpecificPart() : null;
		if (path == null) {
			throw new IllegalStateException("Unable to determine code source archive");
		}
		File root = new File(path);
		if (!root.exists()) {
			throw new IllegalStateException("Unable to determine code source archive from " + root);
		}
		return (root.isDirectory() ? new ExplodedArchive(root) : new JarFileArchive(root));
	}

@Override
	protected String getMainClass() throws Exception {
	    // 对应的是JarFileArchive.getManifest方法，
	    // 进去后发现对应的就是JarFile.getManifest方法，
	    // JarFile.getManifest对应的就是META-INF/MANIFEST.MF文件的内容
		Manifest manifest = this.archive.getManifest();
		String mainClass = null;
		if (manifest != null) {
		    // 对应的是META-INF/MANIFEST.MF文件中的Start-Class的属性
			mainClass = manifest.getMainAttributes().getValue(START_CLASS_ATTRIBUTE);
		}
		if (mainClass == null) {
			throw new IllegalStateException("No 'Start-Class' manifest entry specified in " + this);
		}
		return mainClass;
	}

protected void launch(String[] args) throws Exception {
		if (!isExploded()) {
			JarFile.registerUrlProtocolHandler();
		}
		ClassLoader classLoader = createClassLoader(getClassPathArchivesIterator());
		String jarMode = System.getProperty("jarmode");
		// 这里的launchClass等于"com.bolingcavalry.springbootstarterdemo.SpringbootstarterdemoApplication"
		String launchClass = (jarMode != null && !jarMode.isEmpty()) ? JAR_MODE_LAUNCHER : getMainClass();
		// 这里就是启动SpringbootstarterdemoApplication的地方
		launch(args, launchClass, classLoader);
	}

public class MainMethodRunner {

	private final String mainClassName;

	private final String[] args;

	/**
	 * Create a new {@link MainMethodRunner} instance.
	 * @param mainClass the main class
	 * @param args incoming arguments
	 */
	public MainMethodRunner(String mainClass, String[] args) {
	    // mainClassName被赋值为"com.bolingcavalry.springbootstarterdemo.SpringbootstarterdemoApplication"
		this.mainClassName = mainClass;
		this.args = (args != null) ? args.clone() : null;
	}

	public void run() throws Exception {
	    // 得到SpringbootstarterdemoApplication的Class对象
		Class<?> mainClass = Class.forName(this.mainClassName, false, Thread.currentThread().getContextClassLoader());
		// 得到SpringbootstarterdemoApplication的main方法对象
		Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
		mainMethod.setAccessible(true);
		// 通过反射执行main方法
		mainMethod.invoke(null, new Object[] { this.args });
	}
}