Spring扫描逻辑原码解析（带图好理解）

    public static void main(String[] args) {
        XsmApplicationContext applicationContext=new XsmApplicationContext(AppConfig.class);
        System.out.println(applicationContext.getBean("userService"));
        System.out.println(applicationContext.getBean("userService"));
        System.out.println(applicationContext.getBean("userService"));
    }

@ComponentScan("com.xiashiman.service")
public class AppConfig {}
@Component("userService")
//@Scope("prototype")
public class UserService {}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface ComponentScan {
    String value();
}
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Component {
    String value();
}
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Scope {
    String value();
}

public class BeanDefinition {
    private Class clazz;
    private String scope;
    public BeanDefinition(Class clazz, String scope) {
        this.clazz = clazz;
        this.scope = scope;
    }
    public BeanDefinition() {
    }
    public Class getClazz() {
        return clazz;
    }
    public void setClazz(Class clazz) {
        this.clazz = clazz;
    }
    public String getScope() {
        return scope;
    }
    public void setScope(String scope) {
        this.scope = scope;
    }
}

public class XsmApplicationContext {
    private  Class configClass;
    private ConcurrentHashMap<String,Object> singletonObjects=new ConcurrentHashMap<>();//单例池
    private ConcurrentHashMap<String,BeanDefinition> beanDefinitionMap=new ConcurrentHashMap<>();
    public XsmApplicationContext(Class configClass) {
        this.configClass = configClass;
        //ComponentScan注解--->扫描路径---》扫描---》Beandefinition--->BeanDefinitionMap
        scan(configClass);
        //在启动时把所有单例bean创建好
        for(Map.Entry<String,BeanDefinition> entry:beanDefinitionMap.entrySet()){
            String beanName=entry.getKey();
            BeanDefinition beanDefinition=entry.getValue();
            if(beanDefinition.getScope().equals("singleton")){
                Object bean= createBean(beanDefinition); //单例bean
                singletonObjects.put(beanName,bean);
            }
        }
    }
    //根据bean的生命周期创建对象
    public Object createBean(BeanDefinition beanDefinition){
        Class clazz = beanDefinition.getClazz();
        try {
            Object instance = clazz.getDeclaredConstructor().newInstance();
            return instance;//返回实例对象
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
        return null;
    }
    private void scan(Class configClass) {
        if(configClass.isAnnotationPresent(ComponentScan.class)){
            //解析配置类
            //ComponentScan注解--->扫描路径---》扫描
            ComponentScan declaredAnnotation = (ComponentScan) configClass.getDeclaredAnnotation(ComponentScan.class);
            String path = declaredAnnotation.value(); //扫描路径 com.xiashiman.service
            path=path.replace(".","/"); //把·替换成/---》 com/xiashiman/service
            //扫描--->找出包下面的所有类
            //类加载器
            //Bootstrap----jre/lib
            //Ext----->jre/ext/lib
            //App----->classpath-----:E:\project\common\shousiCode\target\classes
            ClassLoader classLoader=XsmApplicationContext.class.getClassLoader();//app类加载器
            URL resource = classLoader.getResource(path);//通过路径获取资源
            File file=new File(resource.getFile());
            //判断是否是目录
            if(file.isDirectory()){
                File[] files = file.listFiles();//拿到目录下的所以文件
                for (File f:files){
                    String fileName = f.getAbsolutePath(); //E:\project\common\shousiCode\target\classes\com\xiashiman\service\UserService.class
                    //判断是否是class文件
                    if(fileName.endsWith(".class")){
                        //截取com到.class前的部分
                        String className= fileName.substring(fileName.indexOf("com\\"), fileName.indexOf(".class"));
                        className=className.replace("\\",".");  //替换成：com.xiashiman.service.UserService
                        //准备通过反射实例化对象
                        Class clazz=null;
                        try{
                            clazz=classLoader.loadClass(className); //反射获取类模板对象
                            if(clazz.isAnnotationPresent(Component.class)){
                                //表示当前类是一个bean
                                //解析类：判断是单例 还是原型
                                // BeanDefinition
                                Component componentAnntotaion= (Component) clazz.getDeclaredAnnotation(Component.class);
                                String beanName = componentAnntotaion.value();
                                BeanDefinition beanDefinition=new BeanDefinition();
                                beanDefinition.setClazz(clazz);
                                if(clazz.isAnnotationPresent(Scope.class)){
                                    Scope scopeAnntation= (Scope) clazz.getDeclaredAnnotation(Scope.class);
                                    beanDefinition.setScope(scopeAnntation.value());
                                }else{
                                    beanDefinition.setScope("singleton");
                                }
                                beanDefinitionMap.put(beanName,beanDefinition);
                            }
                        }catch (ClassNotFoundException e){
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }
    public Object getBean(String beanName){
        if(beanDefinitionMap.containsKey(beanName)){
            BeanDefinition beanDefinition=beanDefinitionMap.get(beanName);
            if(beanDefinition.getScope().equals("singleton")){
                Object o=singletonObjects.get(beanName);
                return o;
            }else{
                //创建bean对象  怎么创建呢
                Object bean=createBean(beanDefinition);
                return bean;
            }
        }else{
            throw new NullPointerException("不存在对应的bean");
        }
    }
}