springmvc第4-2章：HandlerMethodArgumentResolver

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李延 LV6 2021-05-26

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# 作用

用于将请求上下文的参数解析为方法的参数。

有2个主要方法1、supportsParameter 判断当前类是否支持解析该参数；2、resolveArgument对参数进行解析，解析为需要的值。

# 初始化与调用

## 初始化

在HandlerAdapter 的初始化方法afterPropertiesSet中，有下面一段代码：
```java
       if (this.argumentResolvers == null) {
			List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
			this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
		}

```

我们看到首先初始化了一个数组resolvers，每个元素都是HandlerMethodArgumentResolver对象。

之后创建了HandlerMethodArgumentResolverComposite对象并将数组添加到这个对象内。

我们先看一下getDefaultArgumentResolvers方法：
```java
	private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
		List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(30);

// Annotation-based argument resolution
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
		resolvers.add(new RequestParamMapMethodArgumentResolver());
		resolvers.add(new PathVariableMethodArgumentResolver());
		resolvers.add(new PathVariableMapMethodArgumentResolver());
		resolvers.add(new MatrixVariableMethodArgumentResolver());
		resolvers.add(new MatrixVariableMapMethodArgumentResolver());
		resolvers.add(new ServletModelAttributeMethodProcessor(false));
		resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new RequestHeaderMapMethodArgumentResolver());
		resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new SessionAttributeMethodArgumentResolver());
		resolvers.add(new RequestAttributeMethodArgumentResolver());

// Type-based argument resolution
		resolvers.add(new ServletRequestMethodArgumentResolver());
		resolvers.add(new ServletResponseMethodArgumentResolver());
		resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RedirectAttributesMethodArgumentResolver());
		resolvers.add(new ModelMethodProcessor());
		resolvers.add(new MapMethodProcessor());
		resolvers.add(new ErrorsMethodArgumentResolver());
		resolvers.add(new SessionStatusMethodArgumentResolver());
		resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
		if (KotlinDetector.isKotlinPresent()) {
			resolvers.add(new ContinuationHandlerMethodArgumentResolver());
		}

// Custom arguments
		if (getCustomArgumentResolvers() != null) {
			resolvers.addAll(getCustomArgumentResolvers());
		}

// Catch-all
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
		resolvers.add(new ServletModelAttributeMethodProcessor(true));

return resolvers;
	}

```

这里直接new出来了很多HandlerMethodArgumentResolver，而每一个元素，就是一种参数的解析器。

最后将所有的解析器都交给HandlerMethodArgumentResolverComposite，我们在使用的时候使用的就是HandlerMethodArgumentResolverComposite对象。

## 调用

在ServletInvocableHandlerMethod的父类InvocableHandlerMethod中调用了这个方法，用作解析参数：
![image](http://note.youdao.com/yws/res/23181/FEC1B12152084DCFB87AA68841CF2FCB)

# 方法解析
在这里我们首先解析HandlerMethodArgumentResolverComposite对象，而刚才初始化的每个HandlerMethodArgumentResolver。我们具体在demo章节中分别解析，结合测试用例一起解析源码。

# HandlerMethodArgumentResolverComposite

## supportsParameter

```java

@Override
	public boolean supportsParameter(MethodParameter parameter) {
		return getArgumentResolver(parameter) != null;
	}

@Nullable
	private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
		HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
		if (result == null) {
			for (HandlerMethodArgumentResolver resolver : this.argumentResolvers) {
				if (resolver.supportsParameter(parameter)) {
					result = resolver;
					this.argumentResolverCache.put(parameter, result);
					break;
				}
			}
		}
		return result;
	}

```

我们看到 这里是通过变量每个resolver去查看是否有支持当前参数的解析器，如果有就返回true。并记录到缓存中。

## resolveArgument

```java
	@Override
	@Nullable
	public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {

HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
		if (resolver == null) {
			throw new IllegalArgumentException("Unsupported parameter type [" +
					parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
		}
		return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
	}

```

与上面方法相同，也是交给具体的某个resolver进行处理

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