【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; } }
//...
