Springboot源碼分析之jar探秘

摘要：

• 利用IDEA等工具打包會出現springboot-0.0.1-SNAPSHOT.jar,springboot-0.0.1-SNAPSHOT.jar.original,前面說過它們之間的關系了，接下來我們就一探究竟，它們之間到底有什么聯系。

文件對比：

• 進入target目錄,unzip springboot-0.0.1-SNAPSHOT.jar -d jar命令將springboot-0.0.1-SNAPSHOT.jar解壓到jar目錄
  

• 進入target目錄,unzip springboot-0.0.1-SNAPSHOT.jar.original -d original命令將springboot-0.0.1-SNAPSHOT.jar.original解壓到original目錄
  

前面文章分析過springboot-0.0.1-SNAPSHOT.jar.original不能執行，將它進行repackage后生成springboot-0.0.1-SNAPSHOT.jar就成了我們的可執行fat jar,對比上面文件會發現可執行 fat jar和original jar目錄不一樣，最關鍵的地方是多了org.springframework.boot.loader這個包,這個就是我們平時java -jar springboot-0.0.1-SNAPSHOT.jar命令啟動的奧妙所在。MANIFEST.MF文件里面的內容包含了很多關鍵的信息

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

相信不用多說大家都能明白Main-Class: org.springframework.boot.loader.JarLauncher是我們 java -jar命令啟動的入口，后續會進行分析，Start-Class: com.github.dqqzj.springboot.SpringbootApplication才是我們程序的入口主函數。

Springboot jar啟動源碼分析

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);
    }
   /**
    * 判斷是否歸檔文件還是文件系統的目錄 可以猜想基於文件系統一樣是可以啟動的
    */
    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 {
    /**
     * 進入父類初始化構造器ExecutableArchiveLauncher
     * launch方法交給Launcher執行
     */
        (new JarLauncher()).launch(args);
    }
}

public abstract class ExecutableArchiveLauncher extends Launcher {
    private final Archive archive;

    public ExecutableArchiveLauncher() {
        try {
    /**
     * 使用父類Launcher加載資源，包括BOOT-INF的classes和lib下面的所有歸檔文件
     */
            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;
    }
    /**
     * 從歸檔文件中獲取我們的應用程序主函數
     */
    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 {
      /**
       *注冊協議處理器，由於Springboot是 jar in jar 所以要重寫jar協議才能讀取歸檔文件
       */
        JarFile.registerUrlProtocolHandler();
        ClassLoader classLoader = this.createClassLoader(this.getClassPathArchives());
      /**
       * this.getMainClass()交給子類ExecutableArchiveLauncher實現
       */
        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]));
    }
    /**
     * 該類加載器是fat jar的關鍵的一處，因為傳統的類加載器無法讀取jar in jar模型，所以springboot進行了自己實現
     */
    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();
    }
    /**
     * 創建應用程序主函數運行器
     */
    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;
   /**
		  * 得到我們的啟動jar的歸檔文件
			*/
    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;
    }
    /**
     * 最終執行的方法，可以發現是利用的反射調用的我們應用程序的主函數
     */
    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);
    }
}

小結：

內容太多了，未涉及歸檔文件，協議處理器，打包war同樣的可以用命令啟動等，感興趣的讀者請親自去調試一番，添加依賴

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

IDEA進行啟動類的配置