一、jar包和war包的差异

1.1 war包

  1. war包是Java Web运用程序的一种打包办法符合Servlet规范,它是Web Archive的缩写,首要用于存储Web运用程序相关的文件,包括Java类文件、JSP、HTML、CSS、JavaScript、图片等资源文件。
  2. war包需要部署到web服务器中(Tomcat、Apache、IIS)

1.2 jar包

  1. jar包是类的归档文件,首要用于存储Java类文件和相关资源文件。它一般被用于封装Java运用程序或Java类库,便利程序的部署和发布
  2. jar包能够被JVM直接加载和运转。

1.3 首要差异:

  1. jar包首要用于存储Java类文件和相关资源文件,而war包首要用于存储Web运用程序相关的文件。
  2. jar包能够被JVM直接加载和运转,而war包需要被Web服务器加载和运转。
  3. jar包一般用于封装Java运用程序或Java类库,而war包用于封装Java Web运用程序。

二、SpringBoot运用war包发动

war包发动:需要先发动外部的Web服务器,完成Servlet3.0规范中引导运用发动类,然后将war包放入Web服务器下,Web服务器经过回调引导运用发动类办法发动运用。

2.1 Servlet3.0规范中引导运用发动的阐明

  • 在Servlet容器(Tomcat、Jetty等)发动运用时,会扫描运用jar包中ServletContainerInitializer的完成类。
  • 结构有必要在jar包的META-INF/services的文件夹中提供一个名为javax.servlet.ServletContainerInitializer的文件,文件内容要写明ServletContainerInitializer的完成类的全限定名。
  • 这个ServletContainerInitializer是一个接口,完成它的类有必要完成一个办法:onStartUp
  • 能够在这个ServletContainerInitializer的完成类上标注@HandlesTypes注解,在运用发动的时候自行加载一些附加的类,这些类会以字节码的集合方式传入onStartup办法的第一个参数中。
public interface ServletContainerInitializer {
    void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException;
}

2.2 SpringBootServletInitializer的作用和原理

Spirng中SpringServletContainerInitializer完成了Servlet的规范

SpringBoot项目jar、war包启动解析

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
    @Override
    public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
            throws ServletException {
        // SpringServletContainerInitializer会加载所有的WebApplicationInitializer类型的普通完成类
        List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();
        if (webAppInitializerClasses != null) {
            for (Class<?> waiClass : webAppInitializerClasses) {
                // 如果不是接口,不是抽象类
                if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
                        WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                    try {
                        // 创立该类的实例
                        initializers.add((WebApplicationInitializer) waiClass.newInstance());
                    }
                    catch (Throwable ex) {
                        throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
                    }
                }
            }
        }
        if (initializers.isEmpty()) {
            servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
            return;
        }
        servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
        AnnotationAwareOrderComparator.sort(initializers);
        // 发动Web运用onStartup办法
        for (WebApplicationInitializer initializer : initializers) {
            initializer.onStartup(servletContext);
        }
    }
}

@HandlesTypes运用BCELClassParser字节码层面读取了/WEB-INF/classes和jar中class文件的超类名和完成的接口名,判别是否与记录的注解类名相同,若相同再经过org.apache.catalina.util.Introspection类加载为Class对象保存起来,最后传入onStartup办法参数中

SpringServletContainerInitializer类上标注了@HandlesTypes(WebApplicationInitializer.class),所以会导入WebApplicationInitializer完成类

SpringBoot中SpringBootServletInitializerWebApplicationInitializer的抽象类,完成了onStartup办法

@Override
public void onStartup(ServletContext servletContext) throws ServletException {
    // Logger initialization is deferred in case an ordered
    // LogServletContextInitializer is being used
    this.logger = LogFactory.getLog(getClass());
    // 创立 父IOC容器
    WebApplicationContext rootAppContext = createRootApplicationContext(servletContext);
    if (rootAppContext != null) {
        servletContext.addListener(new ContextLoaderListener(rootAppContext) {
            @Override
            public void contextInitialized(ServletContextEvent event) {
                // no-op because the application context is already initialized
            }
        });
    }
    else {
        this.logger.debug("No ContextLoaderListener registered, as " + "createRootApplicationContext() did not "
                + "return an application context");
    }
}

创立父容器

protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
    // 运用Builder机制,前面也介绍过
    SpringApplicationBuilder builder = createSpringApplicationBuilder();
    builder.main(getClass());
    ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
    if (parent != null) {
        this.logger.info("Root context already created (using as parent).");
        servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
        builder.initializers(new ParentContextApplicationContextInitializer(parent));
    }
    // 设置Initializer
    builder.initializers(new ServletContextApplicationContextInitializer(servletContext));
    // 在这里设置了容器发动类:AnnotationConfigServletWebServerApplicationContext
    builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
    // 【引导】多态进入子类(自己定义)的办法中
    builder = configure(builder);
    builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));
    // builder.build(),创立SpringApplication
    SpringApplication application = builder.build();
    if (application.getAllSources().isEmpty()
            && AnnotationUtils.findAnnotation(getClass(), Configuration.class) != null) {
        application.addPrimarySources(Collections.singleton(getClass()));
    }
    Assert.state(!application.getAllSources().isEmpty(),
            "No SpringApplication sources have been defined. Either override the "
                    + "configure method or add an @Configuration annotation");
    // Ensure error pages are registered
    if (this.registerErrorPageFilter) {
        application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
    }
    // 发动SpringBoot运用
    return run(application);
}

所以我们只需要自定义类承继SpringBootServletInitializer并完成configure办法告诉发动类所在的方位就能够完成SpringBoot自发动了

例如:

public class MyInitializer extends SpringBootServletInitializer {
   @Override
   protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
      //MySpringBootApplication为SpingBoot发动类
      return application.sources(MySpringBootApplication.class);
   }
}

三、SpringBoot运用jar包发动

按照java官方文档规则,java -jar指令引导的具体发动类有必要配置在MANIFEST.MF中的Main-class特点中,该值代表运用程序履行入口类也便是包含main办法的类。

从MANIFEST.MF文件内容能够看到,Main-Class这个特点定义了org.springframework.boot.loader.JarLauncher,JarLauncher便是对应Jar文件的发动器。而我们项目的发动类SpringBootDemoApplication定义在Start-Class特点中,

JarLauncher会将BOOT-INF/classes下的类文件和BOOT-INF/lib下依赖的jar加入到classpath下,然后调用META-INF/MANIFEST.MF文件Start-Class特点完成运用程序的发动。

关于 jar官方规范阐明请移步

  • JAR File Specification
  • JAR (file format)

SpringBoot的jar包,会有3个文件夹:

  • BOOT-INF:寄存自己编写并编译好的 .class 文件和静态资源文件、配置文件等
  • META-INF:有一个MANIFEST.MF的文件
  • org:spring-boot-loader的一些 .class 文件

SpringBoot项目jar、war包启动解析

META-INF下面的MANIFEST.MF文件,里面的内容如下:

Manifest-Version: 1.0
Spring-Boot-Classpath-Index: BOOT-INF/classpath.idx
Implementation-Title: my-small-test
Implementation-Version: 1.0-SNAPSHOT
Spring-Boot-Layers-Index: BOOT-INF/layers.idx
Start-Class: com.small.test.SpringBootDemoApplication
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Build-Jdk-Spec: 1.8
Spring-Boot-Version: 2.4.0
Created-By: Maven Jar Plugin 3.2.0
Main-Class: org.springframework.boot.loader.JarLauncher
  • Start-Class中注明晰 SpringBoot 的主发动类
  • Main-Class中注明晰一个类:JarLauncher
package org.springframework.boot.loader;
import java.io.IOException;
import java.util.jar.Attributes;
import java.util.jar.Manifest;
import org.springframework.boot.loader.archive.Archive;
public class JarLauncher extends ExecutableArchiveLauncher {
  private static final String DEFAULT_CLASSPATH_INDEX_LOCATION = "BOOT-INF/classpath.idx";
  static final Archive.EntryFilter NESTED_ARCHIVE_ENTRY_FILTER;
  static {
    NESTED_ARCHIVE_ENTRY_FILTER = (entry -> entry.isDirectory() ? entry.getName().equals("BOOT-INF/classes/") : entry.getName().startsWith("BOOT-INF/lib/"));
  }
  public JarLauncher() {}
  protected JarLauncher(Archive archive) {
    super(archive);
  }
  protected ClassPathIndexFile getClassPathIndex(Archive archive) throws IOException {
    if (archive instanceof org.springframework.boot.loader.archive.ExplodedArchive) {
      String location = getClassPathIndexFileLocation(archive);
      return ClassPathIndexFile.loadIfPossible(archive.getUrl(), location);
    } 
    return super.getClassPathIndex(archive);
  }
  private String getClassPathIndexFileLocation(Archive archive) throws IOException {
    Manifest manifest = archive.getManifest();
    Attributes attributes = (manifest != null) ? manifest.getMainAttributes() : null;
    String location = (attributes != null) ? attributes.getValue("Spring-Boot-Classpath-Index") : null;
    return (location != null) ? location : "BOOT-INF/classpath.idx";
  }
  protected boolean isPostProcessingClassPathArchives() {
    return false;
  }
  protected boolean isSearchCandidate(Archive.Entry entry) {
    return entry.getName().startsWith("BOOT-INF/");
  }
  protected boolean isNestedArchive(Archive.Entry entry) {
    return NESTED_ARCHIVE_ENTRY_FILTER.matches(entry);
  }
  public static void main(String[] args) throws Exception {
    (new JarLauncher()).launch(args);
  }
}

父类Launcher#launch

  protected void launch(String[] args) throws Exception {
    if (!isExploded())
    //3.1 注册URL协议并铲除运用缓存
    JarFile.registerUrlProtocolHandler(); 
    //3.2 设置类加载途径
    ClassLoader classLoader = createClassLoader(getClassPathArchivesIterator());
    String jarMode = System.getProperty("jarmode");
    String launchClass = (jarMode != null && !jarMode.isEmpty()) ? "org.springframework.boot.loader.jarmode.JarModeLauncher" : getMainClass();
    //3.3 履行main办法
    launch(args, launchClass, classLoader);
  }

3.1 registerUrlProtocolHandler:注册URL协议并铲除运用缓存

先设置当前体系的一个变量java.protocol.handler.pkgs,而这个变量的作用,是设置URLStreamHandler完成类的包途径。

之后要重置缓存,目的是铲除之前发动的残留。

private static final String MANIFEST_NAME = "META-INF/MANIFEST.MF";
    private static final String PROTOCOL_HANDLER = "java.protocol.handler.pkgs";
    private static final String HANDLERS_PACKAGE = "org.springframework.boot.loader";
    public static void registerUrlProtocolHandler() {
        String handlers = System.getProperty(PROTOCOL_HANDLER, "");
        System.setProperty(PROTOCOL_HANDLER,
                ("".equals(handlers) ? HANDLERS_PACKAGE : handlers + "|" + HANDLERS_PACKAGE));
        resetCachedUrlHandlers();
    }
    // 重置任何缓存的处理程序,以防万一现已运用了jar协议。
// 我们经过测验设置null URLStreamHandlerFactory来重置处理程序,除了铲除处理程序缓存之外,它应该没有任何作用。
    private static void resetCachedUrlHandlers() {
        try {
            URL.setURLStreamHandlerFactory(null);
        }
        catch (Error ex) {
            // Ignore
        }
    }

3.2createClassLoader:设置类加载途径

 protected ClassLoader createClassLoader(Iterator<Archive> archives) throws Exception {
    List<URL> urls = new ArrayList<>(50);
    while (archives.hasNext())
      urls.add(((Archive)archives.next()).getUrl()); 
    return createClassLoader(urls.<URL>toArray(new URL[0]));
  }
  protected ClassLoader createClassLoader(URL[] urls) throws Exception {
    return new LaunchedURLClassLoader(isExploded(), getArchive(), urls, getClass().getClassLoader());
  }

3.3 履行main办法

  protected void launch(String[] args, String launchClass, ClassLoader classLoader) throws Exception {
    Thread.currentThread().setContextClassLoader(classLoader);
    createMainMethodRunner(launchClass, args, classLoader).run();
  }
  protected MainMethodRunner createMainMethodRunner(String mainClass, String[] args, ClassLoader classLoader) {
    return new MainMethodRunner(mainClass, args);
  }
package org.springframework.boot.loader;
import java.lang.reflect.Method;
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;
  }
  public void run() throws Exception {
    Class<?> mainClass = Class.forName(this.mainClassName, false, Thread.currentThread().getContextClassLoader());
    //获取主发动类的main办法
    Method mainMethod = mainClass.getDeclaredMethod("main", new Class[] { String[].class });
    mainMethod.setAccessible(true);
    //履行main办法
    mainMethod.invoke((Object)null, new Object[] { this.args });
  }
}

所以SpringBoot运用在开发期间只需要写 main 办法,引导发动即可。