手写springboot的核心流程

spring容器以及Tomcat启动流程

1. 在CasSpringApplication的run方法中启动spring容器

public class CasSpringApplication { 
    public static void run(Class clazz) {
        // 启动sprinng容器
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        context.register(clazz);
        context.refresh();
        // 启动服务
        start(context);
   }
}

2. 定义服务
   2.1 定义接口

public interface WebServer {
    void start();
}

2.2 Tomcat实现类

public class TomcatWebServer implements WebServer {
    @Override
    public void start() {
        System.out.println("tomcat 启动了");
    }
}

2.3 Jetty实现类

public class JettyWebServer implements WebServer {
    @Override
    public void start() {
        System.out.println("jetty 启动了");
    }
}

2.3 获取实现类

 public static void start(AnnotationConfigWebApplicationContext applicationContext) {
    Map<String, WebServer> webserver = applicationContext.getBeansOfType(WebServer.class);
    if (webserver.size() > 1) {
        throw new RuntimeException("获取到多个服务");
    }
    webserver.values().stream().findFirst().get().start();
}

3. 动态选择服务

• 如果项目中有Tomcat就启动Tomcat
• 如果项目中有Jetty则启动Jetty
  3.1 条件注解

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Conditional(CondationOnExitClass.class)
public @interface CondationOnExit {
    String value() default "";
}

3.2 CondationOnExitClass实现类

public class CondationOnExitClass implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        MultiValueMap<String, Object> map = metadata.getAllAnnotationAttributes(CondationOnExit.class.getName());
        String className = (String) map.getFirst("value");
        try {
            context.getClassLoader().loadClass(className);
            return true;
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            return false;
        }
    }
}

3.3 向spring容器中注入bean

@Configuration
public class WebServerBean {
    @Bean
    @CondationOnExit("org.apache.catalina.startup.Tomcat")
    public TomcatWebServer tomcatWebServer() {
        return new TomcatWebServer();
    }

    @Bean
    @CondationOnExit("org.eclipse.jetty.server.Server")
    public JettyWebServer jettyWebServer() {
        return new JettyWebServer();
    }
}

springboot的包扫描

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Configuration
@ComponentScan
public @interface CasSpringBoot {
}

springboot 自动装配

• spring有一套自己实现的spi机制 在这里使用java自带的spi 暂时不做太复杂

public class AutoImportSelect implements DeferredImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        // java spi
        ServiceLoader<AutoConfiguation> configuations = ServiceLoader.load(AutoConfiguation.class);
        List<String> list = new ArrayList<>();
        for (AutoConfiguation configuation : configuations) {
           list.add(configuation.getClass().getName());
        }
        return list.toArray(new String[0]);
    }
}

这个类无法被spring包扫描到 我们需要手动导入

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Configuration
@ComponentScan
@Import({AutoImportSelect.class}) // 导入自动装配类
public @interface CasSpringBoot {
}

测试

至此我们一个极简版springboot就实现完毕了

@CasSpringBoot
public class CasApplication {
    public static void main(String[] args) {
        CasSpringApplication.run(CasApplication.class);
    }
}