【杂谈】SpringBoot为啥不用配置启动类

前言

在学习SparkJava、Vert.x等轻量级Web框架的时候，都遇到过打包问题，这两个框架打包的时候都需要添加额外的Maven配置，并指定启动类才能得到可执行的JAR包；

而springboot项目，似乎都不需要额外的配置，直接package就可以得到可执行的JAR包，这是怎么回事呢？

Vert.x要怎么配？

我们先来看看，Vert.x打包做哪些配置

1）引入maven-shade-plugin插件

2）在插件中指定在package完成时触发shade操作

3）指定启动类

<plugin>
    <artifactId>maven-shade-plugin</artifactId>
    <version>3.1.0</version>
    <executions>
        <execution>
            <!--在mvn package完成时触发-->
            <phase>package</phase>
            <!--执行shade操作-->
            <goals>
                <goal>shade</goal>
            </goals>
            <configuration>
                <transformers>
                    <transformer
                         implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                        <manifestEntries>
                            <!--指定启动类-->
                            <main-class>com.test.Starter</main-class>
                        </manifestEntries>
                    </transformer>
                </transformers>
                <artifactSet/>
            </configuration>
        </execution>
    </executions>
</plugin>

效果：

执行package操作后，将得到两个jar包

①origin-[your project].jar（Maven默认打包操作得到的jar包，该包仅包含此项目的类）

②[your project].jar（带有依赖包，且配置有启动类的可执行JAR包）

Spring Boot又是怎么做的

不用添加插件？=> 初始化时默认就有

Spring Boot 初始化得到的项目中，默认带有spring-boot-maven-plugin的Maven配置

SpringBoot打包的基本原理与前面的Vertx配置相同，都是使用maven-shade-plugin（spring-boot-maven-plugin底层使用maven-shade-plugin），在package完成之后，加入依赖的包，并指定启动类。

SpringBoot是在package时，触发repackage，将原打包结果重命名为[your project].jar.original，并得到带有依赖包和配置好启动类的[your project].jar

不用指定启动类？=> 默认扫描得到启动类

spring-boot-maven-plugin会扫描项目，并以带有@SpringBootApplication注解和main方法的类作为启动类。

默认情况下，SpringBoot项目默认启动类写死JarLauncher，该类的main方法再调用扫描得到的实际启动类（XXXApplication）的main方法

源码查看

我们从maven repository下载一个spring-boot-maven-plugin的源码进行查看，查看RepackageMojo.java。

从@Mojo注解中，我们可以知道，该Mojo绑定在PACKAGE（注解中的defaultPhase=LifecyclePhase.PACKAGE），即在package完成后触发

/**
 * Repackages existing JAR and WAR archives so that they can be executed from the command
 * line using {@literal java -jar}. With <code>layout=NONE</code> can also be used simply
 * to package a JAR with nested dependencies (and no main class, so not executable).
 *
 * @author Phillip Webb
 * @author Dave Syer
 * @author Stephane Nicoll
 * @author Björn Lindström
 * @since 1.0.0
 */
@Mojo(name = "repackage", defaultPhase = LifecyclePhase.PACKAGE, requiresProject = true, threadSafe = true,
        requiresDependencyResolution = ResolutionScope.COMPILE_PLUS_RUNTIME,
        requiresDependencyCollection = ResolutionScope.COMPILE_PLUS_RUNTIME)
public class RepackageMojo extends AbstractDependencyFilterMojo {
    //...
}

我们可以看到，该Mojo中可以指定一个mainclass作为启动类，但是如果没有指定的时候，它是如何处理的呢？

/**
* The name of the main class. If not specified the first compiled class found that
* contains a 'main' method will be used.
* @since 1.0.0
*/
@Parameter
private String mainClass;

我们跟踪这个mainClass，发现在此类中，没有对这个mainClass进行赋值的操作，只用来构造一个Repackager（也就是说在该Maven插件没有配置mainClass的时候，传给Repackager的就是一个null），我们观察到这个Repackager就是该Mojo执行

@Override
public void execute() throws MojoExecutionException, MojoFailureException {
    if (this.project.getPackaging().equals("pom")) {
        getLog().debug("repackage goal could not be applied to pom project.");
        return;
    }
    if (this.skip) {
        getLog().debug("skipping repackaging as per configuration.");
        return;
    }
    repackage();
}

private void repackage() throws MojoExecutionException {
    Artifact source = getSourceArtifact();
    File target = getTargetFile();
    Repackager repackager = getRepackager(source.getFile());
    Set<Artifact> artifacts = filterDependencies(this.project.getArtifacts(), getFilters(getAdditionalFilters()));
    Libraries libraries = new ArtifactsLibraries(artifacts, this.requiresUnpack, getLog());
    try {
        LaunchScript launchScript = getLaunchScript();
        repackager.repackage(target, libraries, launchScript); //执行repackage操作
    }
    catch (IOException ex) {
        throw new MojoExecutionException(ex.getMessage(), ex);
    }
    updateArtifact(source, target, repackager.getBackupFile());
}

private Repackager getRepackager(File source) {
    Repackager repackager = new Repackager(source, this.layoutFactory);
    repackager.addMainClassTimeoutWarningListener(new LoggingMainClassTimeoutWarningListener());
    repackager.setMainClass(this.mainClass); //将插件配置的mainClass注入，默认就是null
    if (this.layout != null) {
        getLog().info("Layout: " + this.layout);
        repackager.setLayout(this.layout.layout());
    }
    return repackager;
}

由上可知，mainClass的最终确定，应该在Repackager的中完成，我继续跟踪该代码（Repackager来自spring-boot-maven-plugin下引入的spring-boot-loader-tools），打开Repackager的代码。我们观察到Repackager的setMainClass并没有做额外的操作，只是将传入的参数set进来，但是从注释中可以得知，其在使用时如果为空，则会搜索合适的类作为MainClass

/**
 * Utility class that can be used to repackage an archive so that it can be executed using
 * '{@literal java -jar}'.
 *
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @author Stephane Nicoll
 * @since 1.0.0
 */
public class Repackager {
    //...

    /**
     * Sets the main class that should be run. If not specified the value from the
     * MANIFEST will be used, or if no manifest entry is found the archive will be
     * searched for a suitable class.
     * @param mainClass the main class name
     */
    public void setMainClass(String mainClass) {
        this.mainClass = mainClass;
    }

    //...

}

我们就从上面调用repackage方法开始看

/**
 * Repackage to the given destination so that it can be launched using '
 * {@literal java -jar}'.
 * @param destination the destination file (may be the same as the source)
 * @param libraries the libraries required to run the archive
 * @param launchScript an optional launch script prepended to the front of the jar
 * @throws IOException if the file cannot be repackaged
 * @since 1.3.0
 */
public void repackage(File destination, Libraries libraries, LaunchScript launchScript) throws IOException {
    if (destination == null || destination.isDirectory()) {
        throw new IllegalArgumentException("Invalid destination");
    }
    if (libraries == null) {
        throw new IllegalArgumentException("Libraries must not be null");
    }
    if (this.layout == null) {
        this.layout = getLayoutFactory().getLayout(this.source);
    }
    destination = destination.getAbsoluteFile();
    File workingSource = this.source;
    if (alreadyRepackaged() && this.source.equals(destination)) {
        return;
    }
    if (this.source.equals(destination)) {
        workingSource = getBackupFile();
        workingSource.delete();
        renameFile(this.source, workingSource);
    }
    destination.delete();
    try {
        try (JarFile jarFileSource = new JarFile(workingSource)) {
            repackage(jarFileSource, destination, libraries, launchScript); //这里往下查看
        }
    }
    finally {
        if (!this.backupSource && !this.source.equals(workingSource)) {
            deleteFile(workingSource);
        }
    }
}

private void repackage(JarFile sourceJar, File destination, Libraries libraries, LaunchScript launchScript)
        throws IOException {
    WritableLibraries writeableLibraries = new WritableLibraries(libraries);
    try (JarWriter writer = new JarWriter(destination, launchScript)) {
        writer.writeManifest(buildManifest(sourceJar)); //注意这里有一个buildManifest
        writeLoaderClasses(writer);
        if (this.layout instanceof RepackagingLayout) {
            writer.writeEntries(sourceJar,
                    new RenamingEntryTransformer(((RepackagingLayout) this.layout).getRepackagedClassesLocation()),
                    writeableLibraries);
        }
        else {
            writer.writeEntries(sourceJar, writeableLibraries);
        }
        writeableLibraries.write(writer);
    }
}

private Manifest buildManifest(JarFile source) throws IOException {
    Manifest manifest = source.getManifest();
    if (manifest == null) {
        manifest = new Manifest();
        manifest.getMainAttributes().putValue("Manifest-Version", "1.0"); 
    }
    manifest = new Manifest(manifest);
    String startClass = this.mainClass; //mainClass
    if (startClass == null) {
        startClass = manifest.getMainAttributes().getValue(MAIN_CLASS_ATTRIBUTE); //先尝试从mainfest中拿，这个暂时不清楚数据来源
    }
    if (startClass == null) {
        startClass = findMainMethodWithTimeoutWarning(source); //这里触发搜索mainClass
    }
    String launcherClassName = this.layout.getLauncherClassName();
    if (launcherClassName != null) {
        manifest.getMainAttributes().putValue(MAIN_CLASS_ATTRIBUTE, launcherClassName);
        if (startClass == null) {
            throw new IllegalStateException("Unable to find main class");
        }
        manifest.getMainAttributes().putValue(START_CLASS_ATTRIBUTE, startClass);
    }
    else if (startClass != null) {
        manifest.getMainAttributes().putValue(MAIN_CLASS_ATTRIBUTE, startClass);
    }
    String bootVersion = getClass().getPackage().getImplementationVersion();
    manifest.getMainAttributes().putValue(BOOT_VERSION_ATTRIBUTE, bootVersion);
    manifest.getMainAttributes().putValue(BOOT_CLASSES_ATTRIBUTE, (this.layout instanceof RepackagingLayout)
            ? ((RepackagingLayout) this.layout).getRepackagedClassesLocation() : this.layout.getClassesLocation());
    String lib = this.layout.getLibraryDestination("", LibraryScope.COMPILE);
    if (StringUtils.hasLength(lib)) {
        manifest.getMainAttributes().putValue(BOOT_LIB_ATTRIBUTE, lib);
    }
    return manifest;
}

private String findMainMethodWithTimeoutWarning(JarFile source) throws IOException {
    long startTime = System.currentTimeMillis();
    String mainMethod = findMainMethod(source); //这里往下看
    long duration = System.currentTimeMillis() - startTime;
    if (duration > FIND_WARNING_TIMEOUT) {
        for (MainClassTimeoutWarningListener listener : this.mainClassTimeoutListeners) {
            listener.handleTimeoutWarning(duration, mainMethod);
        }
    }
    return mainMethod;
}

protected String findMainMethod(JarFile source) throws IOException {
    return MainClassFinder.findSingleMainClass(source, this.layout.getClassesLocation(),
            SPRING_BOOT_APPLICATION_CLASS_NAME); //在指定Jar文件中查找MainClass
}

private static final String SPRING_BOOT_APPLICATION_CLASS_NAME = "org.springframework.boot.autoconfigure.SpringBootApplication";

/**
 * Find a single main class in a given jar file. A main class annotated with an
 * annotation with the given {@code annotationName} will be preferred over a main
 * class with no such annotation.
 * @param jarFile the jar file to search
 * @param classesLocation the location within the jar containing classes
 * @param annotationName the name of the annotation that may be present on the main
 * class
 * @return the main class or {@code null}
 * @throws IOException if the jar file cannot be read
 */
public static String findSingleMainClass(JarFile jarFile, String classesLocation, String annotationName)
        throws IOException {
    SingleMainClassCallback callback = new SingleMainClassCallback(annotationName);
    MainClassFinder.doWithMainClasses(jarFile, classesLocation, callback);
    return callback.getMainClassName();
}

从最后几步中，我们可以知道，查找的mainClass是一个带有@SpringBootApplication注解的类。不用说明，该类肯定是带有main方法，如果你想进一步确认，则可以继续查看MainClassFinder的代码（来自spring-boot-loader-tools）。

//...
private static final Type MAIN_METHOD_TYPE = Type.getMethodType(Type.VOID_TYPE, STRING_ARRAY_TYPE);

private static final String MAIN_METHOD_NAME = "main";

private static class ClassDescriptor extends ClassVisitor {

    private final Set<String> annotationNames = new LinkedHashSet<>();

    private boolean mainMethodFound;

    ClassDescriptor() {
        super(SpringAsmInfo.ASM_VERSION);
    }

    @Override
    public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
        this.annotationNames.add(Type.getType(desc).getClassName());
        return null;
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
        //如果访问方式是public static 且 方法名为 main 且 返回值为 void，则认定该类含有main方法
        if (isAccess(access, Opcodes.ACC_PUBLIC, Opcodes.ACC_STATIC) && MAIN_METHOD_NAME.equals(name) 
                && MAIN_METHOD_TYPE.getDescriptor().equals(desc)) {
            this.mainMethodFound = true;
        }
        return null;
    }

    private boolean isAccess(int access, int... requiredOpsCodes) {
        for (int requiredOpsCode : requiredOpsCodes) {
            if ((access & requiredOpsCode) == 0) {
                return false;
            }
        }
        return true;
    }

    boolean isMainMethodFound() {
        return this.mainMethodFound;
    }

    Set<String> getAnnotationNames() {
        return this.annotationNames;
    }

}
//...