Springboot源码分析之jar探秘
- 2019 年 10 月 3 日
- 筆記
摘要：
- 利用IDEA等工具打包会出现springboot-0.0.1-SNAPSHOT.jar,springboot-0.0.1-SNAPSHOT.jar.original,前面说过它们之间的关系了，接下来我们就一探究竟，它们之间到底有什么联系。
文件对比：
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进入target目录,unzip springboot-0.0.1-SNAPSHOT.jar -d jar命令将springboot-0.0.1-SNAPSHOT.jar解压到jar目录
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进入target目录,unzip springboot-0.0.1-SNAPSHOT.jar.original -d original命令将springboot-0.0.1-SNAPSHOT.jar.original解压到original目录
前面文章分析过springboot-0.0.1-SNAPSHOT.jar.original不能执行，将它进行repackage后生成springboot-0.0.1-SNAPSHOT.jar就成了我们的可执行fat jar,对比上面文件会发现可执行 fat jar和original jar目录不一样，最关键的地方是多了org.springframework.boot.loader这个包,这个就是我们平时java -jar springboot-0.0.1-SNAPSHOT.jar命令启动的奥妙所在。MANIFEST.MF文件里面的内容包含了很多关键的信息
Manifest-Version: 1.0 Start-Class: com.github.dqqzj.springboot.SpringbootApplication Spring-Boot-Classes: BOOT-INF/classes/ Spring-Boot-Lib: BOOT-INF/lib/ Build-Jdk-Spec: 1.8 Spring-Boot-Version: 2.1.6.RELEASE Created-By: Maven Archiver 3.4.0 Main-Class: org.springframework.boot.loader.JarLauncher相信不用多说大家都能明白Main-Class: org.springframework.boot.loader.JarLauncher是我们 java -jar命令启动的入口，后续会进行分析，Start-Class: com.github.dqqzj.springboot.SpringbootApplication才是我们程序的入口主函数。
Springboot jar启动源码分析
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); } /** * 判断是否归档文件还是文件系统的目录 可以猜想基于文件系统一样是可以启动的 */ 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 { /** * 进入父类初始化构造器ExecutableArchiveLauncher * launch方法交给Launcher执行 */ (new JarLauncher()).launch(args); } } public abstract class ExecutableArchiveLauncher extends Launcher { private final Archive archive; public ExecutableArchiveLauncher() { try { /** * 使用父类Launcher加载资源，包括BOOT-INF的classes和lib下面的所有归档文件 */ this.archive = this.createArchive(); } catch (Exception var2) { throw new IllegalStateException(var2); } } protected ExecutableArchiveLauncher(Archive archive) { this.archive = archive; } protected final Archive getArchive() { return this.archive; } /** * 从归档文件中获取我们的应用程序主函数 */ 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; } } protected List<Archive> getClassPathArchives() throws Exception { List<Archive> archives = new ArrayList(this.archive.getNestedArchives(this::isNestedArchive)); this.postProcessClassPathArchives(archives); return archives; } protected abstract boolean isNestedArchive(Entry entry); protected void postProcessClassPathArchives(List<Archive> archives) throws Exception { } } public abstract class Launcher { public Launcher() { } protected void launch(String[] args) throws Exception { /** *注册协议处理器，由于Springboot是 jar in jar 所以要重写jar协议才能读取归档文件 */ JarFile.registerUrlProtocolHandler(); ClassLoader classLoader = this.createClassLoader(this.getClassPathArchives()); /** * this.getMainClass()交给子类ExecutableArchiveLauncher实现 */ this.launch(args, this.getMainClass(), classLoader); } protected ClassLoader createClassLoader(List<Archive> archives) throws Exception { List<URL> urls = new ArrayList(archives.size()); Iterator var3 = archives.iterator(); while(var3.hasNext()) { Archive archive = (Archive)var3.next(); urls.add(archive.getUrl()); } return this.createClassLoader((URL[])urls.toArray(new URL[0])); } /** * 该类加载器是fat jar的关键的一处，因为传统的类加载器无法读取jar in jar模型，所以springboot进行了自己实现 */ protected ClassLoader createClassLoader(URL[] urls) throws Exception { return new LaunchedURLClassLoader(urls, this.getClass().getClassLoader()); } protected void launch(String[] args, String mainClass, ClassLoader classLoader) throws Exception { Thread.currentThread().setContextClassLoader(classLoader); this.createMainMethodRunner(mainClass, args, classLoader).run(); } /** * 创建应用程序主函数运行器 */ protected MainMethodRunner createMainMethodRunner(String mainClass, String[] args, ClassLoader classLoader) { return new MainMethodRunner(mainClass, args); } protected abstract String getMainClass() throws Exception; protected abstract List<Archive> getClassPathArchives() throws Exception; /** * 得到我们的启动jar的归档文件 */ protected final Archive createArchive() throws Exception { ProtectionDomain protectionDomain = this.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"); } else { File root = new File(path); if (!root.exists()) { throw new IllegalStateException("Unable to determine code source archive from " + root); } else { return (Archive)(root.isDirectory() ? new ExplodedArchive(root) : new JarFileArchive(root)); } } } } 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; } /** * 最终执行的方法，可以发现是利用的反射调用的我们应用程序的主函数 */ 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); } }小结：
内容太多了，未涉及归档文件，协议处理器，打包war同样的可以用命令启动等，感兴趣的读者请亲自去调试一番，添加依赖
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-loader</artifactId> </dependency>IDEA进行启动类的配置
