spring MVC源码探索之AbstractHandlerMethodMapping

Love The Way You Lie 2022-12-08 15:40 108阅读 0赞

###  ###

*  为什么看源码？
 *  AbstractHandlerMethodMapping
 *   *  initHandlerMethods()
     *  isHandler()
     *  detectHandlerMethods()
     *  registerHandlerMethod()

# 为什么看源码？ #

这篇文章是第一个原因[Spring MVC controller路径是否能重复？][Spring MVC controller]  
第二个就是工作这么多年虽然看过一点框架源码，但是没有系统的看过，写上篇文章的时候看了源码，感觉还挺有意思就记录一下。  
第三个就是一直都是在用也该了解一下原理了，至于框架的设计思路、大牛的想法什么的希望能在看源码的过程中学习到。

# AbstractHandlerMethodMapping #

官方解释是这样的。

/** * Abstract base class for {@link HandlerMapping} implementations that define * a mapping between a request and a {@link HandlerMethod}. * * <p>For each registered handler method, a unique mapping is maintained with * subclasses defining the details of the mapping type {@code <T>}. * @param <T> The mapping for a {@link HandlerMethod} containing the conditions * needed to match the handler method to incoming request. */
    public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean

我的理解为AbstractHandlerMethodMapping为每个注册的handler method，对于每个子类映射类型都维护着其唯一的一个映射，就是维护handler method 和URL的关系。主要用于@Controller,@RequestMapping 等注解

AbstractHandlerMethodMapping 实现了InitializingBean接口，InitializingBean是在程序启动的时候执行其唯一的afterPropertiesSet()方法，那我们就先看一下启动时候的要做哪些操作。

## initHandlerMethods() ##

/** 在初始化的时候发现 handler methods * Detects handler methods at initialization. */
    	@Override
    	public void afterPropertiesSet() { 
    		initHandlerMethods();
    	}

在项目启动时会执行initHandlerMethods方法，它的主要功能是扫描应用上下文，发现并注册handler methods。

/** * Scan beans in the ApplicationContext, detect and register handler methods. * @see #isHandler(Class) * @see #getMappingForMethod(Method, Class) * @see #handlerMethodsInitialized(Map) */
    	protected void initHandlerMethods() { 
    		if (logger.isDebugEnabled()) { 
    			logger.debug("Looking for request mappings in application context: " + getApplicationContext());
    		}
    		// 获取IOC容器中所有bean
    		String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
    				BeanFactoryUtils.beanNamesForTypeIncludingAncestors(obtainApplicationContext(), Object.class) :
    				obtainApplicationContext().getBeanNamesForType(Object.class));
    
    		for (String beanName : beanNames) { 
    			if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) { 
    				Class<?> beanType = null;
    				try { 
    					beanType = obtainApplicationContext().getType(beanName);
    				}
    				catch (Throwable ex) { 
    					// An unresolvable bean type, probably from a lazy bean - let's ignore it.
    					if (logger.isDebugEnabled()) { 
    						logger.debug("Could not resolve target class for bean with name '" + beanName + "'", ex);
    					}
    				}
    				// 判断bean 是否有Controller 注解或者RequestMapping 注解
    				if (beanType != null && isHandler(beanType)) { 
    					detectHandlerMethods(beanName);
    				}
    			}
    		}
    		handlerMethodsInitialized(getHandlerMethods());
    	}

## isHandler() ##

**AbstractHandlerMethodMapping\#isHandler()**  
RequestMappingHandlerMapping 是目前为止AbstractHandlerMethodMapping的子类中唯一实现了isHandler()方法的子类，看下实现

protected boolean isHandler(Class<?> beanType) { 
    		return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
    				AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
    	}

isHandler()方法在这里的作用就是检查beanType是否有@Controller或@RequestMapping注解，这两个注解经常使用，应该都很熟悉了。

## detectHandlerMethods() ##

AbstractHandlerMethodMapping\#detectHandlerMethods() 这个方法的作用是从handler中获取handler method并注册

protected void detectHandlerMethods(final Object handler) { 
    		Class<?> handlerType = (handler instanceof String ?
    				obtainApplicationContext().getType((String) handler) : handler.getClass());
    
    		if (handlerType != null) { 
    			//为给定的类返回用户定义的类:通常只是给定的类，如果是cglib生成的子类，则返回原始的类。
    			final Class<?> userType = ClassUtils.getUserClass(handlerType);
    			Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
    					(MethodIntrospector.MetadataLookup<T>) method -> { 
    						try { 
    							// 为处理程序方法提供映射
    							return getMappingForMethod(method, userType);
    						}
    						catch (Throwable ex) { 
    							throw new IllegalStateException("Invalid mapping on handler class [" +
    									userType.getName() + "]: " + method, ex);
    						}
    					});
    			if (logger.isDebugEnabled()) { 
    				logger.debug(methods.size() + " request handler methods found on " + userType + ": " + methods);
    			}
    
    			// 为查找到的handler method 进行注册
    			methods.forEach((method, mapping) -> { 
    				Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
    				registerHandlerMethod(handler, invocableMethod, mapping);
    			});
    		}
    	}

## registerHandlerMethod() ##

AbstractHandlerMethodMapping\#registerHandlerMethod() 的作用是为每个handler method注册它们唯一的映射路径，源码如下：

/** * Register a handler method and its unique mapping. Invoked at startup for * each detected handler method. * @param handler the bean name of the handler or the handler instance * @param method the method to register * @param mapping the mapping conditions associated with the handler method * @throws IllegalStateException if another method was already registered * under the same mapping */
    	protected void registerHandlerMethod(Object handler, Method method, T mapping) { 
    		this.mappingRegistry.register(mapping, handler, method);
    	}

register方法是AbstractHandlerMethodMapping 内部类MappingRegistry里的方法，MappingRegistry是一个注册表，对MappingRegistry有兴趣的可以看一下源码或者看一下我之前的一篇文章 [Spring MVC controller路径是否能重复？][Spring MVC controller]

我这里只是很简单的记录一下我看的源码，也不全面最好自己能看一下源码。

水平有限，能力一般，如有错误，请多指出

[Spring MVC controller]: https://blog.csdn.net/qq_39654841/article/details/108433865