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
<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>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
layout=NONEcan 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;
}
}
//...
