环境：Spring5.3.23

概述

通过Spring定义请求接口非常容器，通过几个注解就可以完成，如下：

@RestController
@RequestMApping("/demos")
public class DemoController {
  @GetMapping("/index")
  public Object index() {
    return "index" ;
  }
}

通过上面的@RestController, @RequestMapping就完成了一个简单的接口定义。

实际Spring Web底层是做了很多的工作，其核心组件有HandlerMapping, HandlerAdapter, ViewResolver等组件。

1. HandlerMapping
   根据当前请求的URI，查找对应的Handler，如:HandlerExecutionChAIn，包装的HandlerMethod
2. HandlerAdapter
   根据上面的确定的HandlerMethod, 找到能够处理该Handler的Adapter，进行调用
3. ViewResolver
   如果返回的ModelAndView对象那么会通过相应的ViewResolver进行渲染输出

了解了上面的几个核心组件之后，接下来就是自定义实现上面的核心类，来完成接口的请求处理。

自定义Endpoint

自定义注解，标记Controller类及请求参数

 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 public @interface PackEndpoint {
 }

参数标记，用来对接口参数进行注解

 @Target(ElementType.PARAMETER)
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 public @interface PackParam {
 }

Endpoint接口参数封装对象

该对象用来保存记录，方法参数由@PackParam注解的参数

public class PackMethodParameter {

  // 用来解析接口参数的名称
  private ParameterNameDiscoverer parameterNameDiscoverer = new LocalVariableTableParameterNameDiscoverer() ;
  private String name ;
  private Executable executable ;
  private int parameterIndex ;
  private Class<?> type ;
  
  public PackMethodParameter(String name, int parameterIndex, Executable executable) {
    this.name = name;
    this.parameterIndex = parameterIndex ;
    this.executable = executable ;
  }
  
  public PackMethodParameter(int parameterIndex, Executable executable, Class<?> type) {
    this.parameterIndex = parameterIndex ;
    this.executable = executable ;
    this.type = type ;
  }
  
  public boolean hasParameterAnnotation(Class<? extends Annotation> clazz) {
    Method method = (Method) this.executable ;
    Parameter[] parameters = method.getParameters() ;
    return parameters[this.parameterIndex].isAnnotationPresent(clazz) ;
  }
  
  public String getParameterName() {
    String[] parameterNames = parameterNameDiscoverer.getParameterNames((Method) this.executable) ;
    return parameterNames[this.parameterIndex] ;
  }
  
}

自定义HandlerMapping

自定义实现了SpringMVC标准的HandlerMapping，这样在DispatcherServlet中才能够识别。

public class PackHandlerMapping implements HandlerMapping, InitializingBean, ApplicationContextAware {

  private ApplicationContext context;
  private Map<String, PackMethodHandler> mapping = new HashMap<>();

  @Override
  public HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    String requestPath = request.getRequestURI();
    Optional<PackMethodHandler> opt = mapping.entrySet().stream().filter(entry -> entry.getKey().equals(requestPath)).findFirst()
        .map(Map.Entry::getValue);
    if (opt.isPresent()) {
      HandlerExecutionChain executionChain = new HandlerExecutionChain(opt.get()) ;
      return executionChain ;
    }
    return null;
  }

  // Bean初始化时，从容器中查找所有符合条件的Bean对象，即Bean对象上有@PackEndpoint注解
  @Override
  public void afterPropertiesSet() throws Exception {
    String[] beanNames = context.getBeanNamesForType(Object.class) ;
    for (String beanName : beanNames) {
      Object bean = this.context.getBean(beanName) ;
      Class<?> clazz = bean.getClass() ;
      // 判断当前的Bean上是否有PackEndpoint注解，只对有该注解的类进行处理
      if (clazz.getAnnotation(PackEndpoint.class) != null) {
        RequestMapping clazzMapping = clazz.getAnnotation(RequestMapping.class) ;
        String rootPath = clazzMapping.value()[0] ;
        if (clazzMapping != null) {
          ReflectionUtils.doWithMethods(clazz, method -> {
            RequestMapping nestMapping = AnnotatedElementUtils.findMergedAnnotation(method, RequestMapping.class) ;
            if (nestMapping != null) {
              String nestPath = nestMapping.value()[0] ;
              String path = rootPath + nestPath ;
              PackMethodHandler handler = new PackMethodHandler(method, bean) ;
              mapping.put(path, handler) ;
            }
          }) ;
        }
      }
    }
  }

  @Override
  public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
    this.context = applicationContext;
  }

  // 该类的作用：用来记录接口对应的信息，方法，对应的实例，参数信息
  public static class PackMethodHandler {
    private Method method;
    private Object instance;
    private PackMethodParameter[] parameters ;
    public Method getMethod() {
      return method;
    }
    public void setMethod(Method method) {
      this.method = method;
    }
    public Object getInstance() {
      return instance;
    }
    public void setInstance(Object instance) {
      this.instance = instance;
    }
    public PackMethodHandler(Method method, Object instance) {
      super();
      this.method = method;
      this.instance = instance;
      Parameter[] params = method.getParameters() ;
      this.parameters = new PackMethodParameter[params.length] ;
      for (int i = 0; i < params.length; i++) {
        this.parameters[i] = new PackMethodParameter(i, method, params[i].getType()) ;
      }
    }
    public PackMethodParameter[] getParameter() {
      return this.parameters ;
    }
  }
}

自定义参数解析器

专门用来解析处理接口方法中的参数信息然后从请求中读取。

public interface PackHandlerMethodArgumentResolver {

  boolean supportsParameter(PackMethodParameter methodParameter) ;
  
  Object resolveArgument(PackMethodParameter methodParameter, HttpServletRequest request);
  
}
public class PackParamHandlerMethodArgumentResolver implements PackHandlerMethodArgumentResolver {

  @Override
  public boolean supportsParameter(PackMethodParameter methodParameter) {
    return methodParameter.hasParameterAnnotation(PackParam.class) ;
  }

  @Override
  public Object resolveArgument(PackMethodParameter methodParameter, HttpServletRequest request) {
    String name = methodParameter.getParameterName() ;
    Object arg = null;
    String[] parameterValues = request.getParameterValues(name) ;
    if (parameterValues != null) {
      arg = parameterValues.length == 1 ? parameterValues[0] : parameterValues ;
    }
    return arg ;
  }

}

自定义HandlerAdapter

自定义实现了SpringMVC标准的HandlerAdatper，这样在DispatcherServlet中才能够识别。

public class PackHandlerAdapter implements HandlerAdapter{
  
  @Resource
  private ConversionService conversionService ;

  private PackParamHandlerMethodArgumentResolver argumentResolver = new PackParamHandlerMethodArgumentResolver() ;
  
  @Override
  public boolean supports(Object handler) {
    return handler instanceof PackMethodHandler;
  }

  @Override
  public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
      throws Exception {
    PackMethodHandler methodHandler = (PackMethodHandler) handler ;
    
    PackMethodParameter[] parameters = methodHandler.getParameter() ;
    Object[] args = new Object[parameters.length] ;
    for (int i = 0; i < args.length; i++) {
      if (this.argumentResolver.supportsParameter(parameters[i])) {
        // 解析对应的方法参数
        args[i] = this.argumentResolver.resolveArgument(parameters[i], request) ;
        // 类型转换
        args[i] = this.conversionService.convert(args[i], parameters[i].getType()) ;
      }
    }
    // 调用目标方法
    Object result = methodHandler.getMethod().invoke(methodHandler.getInstance(), args) ;
    // 设置响应header，输出内容
    response.setHeader("Content-Type", "text/plain;charset=utf8") ;
    PrintWriter out = response.getWriter() ;
    out.write((String) result) ;
    out.flush() ;
    out.close() ; 
    return null ;
  }

  @Override
  public long getLastModified(HttpServletRequest request, Object handler) {
    return -1 ;
  }

}

通过以上的步骤就完成了一个完全自定义SpringMVC核心组件的实现。

在下一篇中我们将介绍如何将上面的代码跑到一个嵌入式的Tomcat中，敬请期待。