springmvc--8-ServletModelAttributeMethodProcessor兜底处理@ModelAttribute注解和无注解

迷南。 2022-11-03 11:57 254阅读 0赞

# springmvc–`ServletModelAttributeMethodProcessor`兜底处理`@ModelAttribute`注解和无注解 #

### 文章目录 ###

*  springmvc--\`ServletModelAttributeMethodProcessor\`兜底处理\`@ModelAttribute\`注解和无注解
 *  1 简单介绍
 *   *  1.1 创建位置
     *  1.2 构造方法
 *  2 处理方法参数
 *   *  2.1 \`supportsParameter(MethodParameter parameter)\`方法
     *  2.2 \`resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) \`方法
     *   *  2.2.1 获取参数名
         *  2.2.2 指定参数是否需要进行数据绑定
         *  2.2.3 创建参数类型的实例
         *  2.2.4 将请求参数值绑定到\`JavaBean\`对象同名字段中
         *  2.2.5 参数校验
         *  2.2.6 获取绑定结果的模型数据
 *  3 处理返回值
 *   *  3.1 \`supportsReturnType(MethodParameter returnType)\`方法
     *  3.2 \`handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest)\`方法
 *  4 \`ServletModelAttributeMethodProcessor\`拥有的一些重要方法
 *   *  4.1 \`getUriTemplateVariables(NativeWebRequest request) \`方法，获取请求路径上模板变量的映射关系
     *  4.2 \`getRequestValueForAttribute(String attributeName, NativeWebRequest request)\`方法，从模板变量或请求参数中获取方法参数名的对应值
     *  4.3 \`createAttributeFromRequestValue(String sourceValue, String attributeName,MethodParameter parameter, WebDataBinderFactory binderFactory, NativeWebRequest request)\`方法，参数名对应值进行类型转换

# 1 简单介绍 #

如其名，它就是用来兜底处理`@ModelAttribute`注解和方法参数无注解情况

我们来看一下它的类图

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzMzMDEyOTgx_size_16_color_FFFFFF_t_70_pic_center]

> 从它继承的接口我们也能发现，该`Processor`拥有处理方法参数和返回值的功能
> 
> 根据我们使用`springmvc`的经验，我们来看看`@ModelAttribute`注解和方法参数无注解这两种情况具体代表什么意思
> 
>  *  `@ModelAttribute`注解
>     
>      *  `@ModelAttribute`注解标注在方法上，这时候`springmvc`就会自动将方法返回值放入到`Model`中
>      *  `@ModelAttribute`注解标注在参数上，这时候`springmvc`就会从`Model`中找到参数名对应的值，然后绑定到参数上
>  *  方法参数无注解
>     
>      *  此种情况只适用于表单方式提交的请求，也就是说请求头的`Content-Type`字段必须为`application/x-www-form-urlencoded`
>      *  只适用于`javaBean`类型参数，可以直接根据`javaBean`中的字段名进行数据绑定
>      *  对于简单型参数，则是由`RequestParamMethodArgumentResolver`这个解析器处理的

## 1.1 创建位置 ##

`RequestMappingHandlerAdapter`实现了`InitializingBean`接口，所以`spring`在它初始化阶段自动回调`InitializingBean`接口的`afterPropertiesSet()`方法，在这个方法中创建一些默认的参数解析器，其中就包含`ServletModelAttributeMethodProcessor`

@Override
 public void afterPropertiesSet() { 
 //.....................省略一些不相关的代码
 
 //用户未进行手动配置，就会使用springmvc默认配置的解析器
 if (this.argumentResolvers == null) { 
 List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
 this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
 }
 
 //.....................省略一些不相关的代码
 }
 
 /** * Return the list of argument resolvers to use including built-in resolvers * and custom resolvers provided via {@link #setCustomArgumentResolvers}. */
 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());
 
 // Custom arguments
 if (getCustomArgumentResolvers() != null) { 
 resolvers.addAll(getCustomArgumentResolvers());
 }
 
 // Catch-all
 resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
 /** * 在这里创建了ServletModelAttributeMethodProcessor对象 * 注意，此时它的传入了true，表明可以没有@ModelAttribute注解 */
 resolvers.add(new ServletModelAttributeMethodProcessor(true));
 
 return resolvers;
 }

## 1.2 构造方法 ##

/** * Class constructor. * @param annotationNotRequired if "true", non-simple method arguments and * return values are considered model attributes with or without a * {@code @ModelAttribute} annotation */
    public ServletModelAttributeMethodProcessor(boolean annotationNotRequired) { 
        super(annotationNotRequired);
    }

看它父类`ModelAttributeMethodProcessor`的构造方法

//使用默认的参数名发现器
    private static final ParameterNameDiscoverer parameterNameDiscoverer = new DefaultParameterNameDiscoverer();
    
    //@ModelAttribute注解是不是非必须的
    private final boolean annotationNotRequired;
    
    
    /** * Class constructor. * @param annotationNotRequired if "true", non-simple method arguments and * return values are considered model attributes with or without a * {@code @ModelAttribute} annotation */
    public ModelAttributeMethodProcessor(boolean annotationNotRequired) { 
        this.annotationNotRequired = annotationNotRequired;
    }

> 可以看到，构造方法调用过程中只是简单的初始化了一个字段值
> 
> `annotationNotRequired`是一个重要的字段，它会参与判断该参数解析器能否解析指定参数（即`supportsParameter()`方法）

# 2 处理方法参数 #

这就离不开`HandlerMethodArgumentResolver`接口定义的方法了

public interface HandlerMethodArgumentResolver { 
    
        /** * 判断是否支持处理该方法参数 */
        boolean supportsParameter(MethodParameter parameter);
    
        /** * 解析该方法参数，得到参数值 */
        @Nullable
        Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
                               NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;
    
    }

那我们就按照顺序看这些方法的原理

## 2.1 `supportsParameter(MethodParameter parameter)`方法 ##

/** * Returns {@code true} if the parameter is annotated with * {@link ModelAttribute} or, if in default resolution mode, for any * method parameter that is not a simple type. */
    @Override
    public boolean supportsParameter(MethodParameter parameter) { 
        return (parameter.hasParameterAnnotation(ModelAttribute.class) ||
                (this.annotationNotRequired && !BeanUtils.isSimpleProperty(parameter.getParameterType())));
    }

> 有两种判断通过条件
> 
>  *  方法参数上有`@ModelAttribute`注解，就支持解析该参数
>  *  `annotationNotRequired`字段为`true`，且非简单属性，就支持解析该参数

那么什么是简单属性呢？我们来看一看这个`BeanUtils.isSimpleProperty()`方法

/** * Check if the given type represents a "simple" property: a simple value * type or an array of simple value types. * See {@link #isSimpleValueType(Class)} for the definition of simple * value type. * Used to determine properties to check for a "simple" dependency-check. * @param type the type to check * @return whether the given type represents a "simple" property * @see org.springframework.beans.factory.support.RootBeanDefinition#DEPENDENCY_CHECK_SIMPLE * @see org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#checkDependencies * @see #isSimpleValueType(Class) */
 public static boolean isSimpleProperty(Class<?> type) { 
 Assert.notNull(type, "'type' must not be null");
 //getComponentType()方法 获取数组内元素的类型
 return isSimpleValueType(type) || (type.isArray() && isSimpleValueType(type.getComponentType()));
 }
 
 
 
 /** * Check if the given type represents a "simple" value type: a primitive or * primitive wrapper, an enum, a String or other CharSequence, a Number, a * Date, a Temporal, a URI, a URL, a Locale, or a Class. * {@code Void} and {@code void} are not considered simple value types. * @param type the type to check * @return whether the given type represents a "simple" value type * @see #isSimpleProperty(Class) */
 public static boolean isSimpleValueType(Class<?> type) { 
 return (Void.class != type && void.class != type &&
 (ClassUtils.isPrimitiveOrWrapper(type) ||
 Enum.class.isAssignableFrom(type) ||
 CharSequence.class.isAssignableFrom(type) ||
 Number.class.isAssignableFrom(type) ||
 Date.class.isAssignableFrom(type) ||
 Temporal.class.isAssignableFrom(type) ||
 URI.class == type ||
 URL.class == type ||
 Locale.class == type ||
 Class.class == type));
 }

> 简单类型如下所示：
> 
>  *  基本类型及其包装类型
>  *  `Enum`
>  *  `CharSequence`
>  *  `Number`
>  *  `Date`
>  *  `Temporal`
>  *  `URI`
>  *  `URL`
>  *  `Locale`
>  *  `Class`
> 
> 这些类型都是这个参数解析器所不能处理的，除此之外，数组中元素类型如果是上面这些也不能处理

## 2.2 `resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory)`方法 ##

/** * Resolve the argument from the model or if not found instantiate it with * its default if it is available. The model attribute is then populated * with request values via data binding and optionally validated * if {@code @java.validation.Valid} is present on the argument. * @throws BindException if data binding and validation result in an error * and the next method parameter is not of type {@link Errors} * @throws Exception if WebDataBinder initialization fails */
 @Override
 @Nullable
 public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
 NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception { 
 
 Assert.state(mavContainer != null, "ModelAttributeMethodProcessor requires ModelAndViewContainer");
 Assert.state(binderFactory != null, "ModelAttributeMethodProcessor requires WebDataBinderFactory");
 
 //获取参数名，见2.2.1
 String name = ModelFactory.getNameForParameter(parameter);
 //获取参数上标注的@ModelAttribute注解
 ModelAttribute ann = parameter.getParameterAnnotation(ModelAttribute.class);
 if (ann != null) { 
 /** * @ModelAttribute注解的binding属性指定参数是否需要进行数据绑定 * 这里通过setBinding()方法， 添加或删除不需要进行数据绑定的属性，见2.2.2 */
 mavContainer.setBinding(name, ann.binding());
 }
 
 Object attribute = null;
 BindingResult bindingResult = null;
 
 //尝试从模型中直接获取参数名的对应值
 if (mavContainer.containsAttribute(name)) { 
 attribute = mavContainer.getModel().get(name);
 }
 //模型中不包含该参数名的对应值
 else { 
 // Create attribute instance
 try { 
 //创建参数类型的实例，见2.2.3
 attribute = createAttribute(name, parameter, binderFactory, webRequest);
 }
 //捕获2.2.3中抛出的异常，再次进行处理
 catch (BindException ex) { 
 if (isBindExceptionRequired(parameter)) { 
 // No BindingResult parameter -> fail with BindException
 throw ex;
 }
 // Otherwise, expose null/empty value and associated BindingResult
 if (parameter.getParameterType() == Optional.class) { 
 attribute = Optional.empty();
 }
 bindingResult = ex.getBindingResult();
 }
 }
 
 //无异常发生
 if (bindingResult == null) { 
 // Bean property binding and validation;
 // skipped in case of binding failure on construction.
 //构造一个数据绑定器，用来将attribute对象绑定到处理器方法的name参数上
 WebDataBinder binder = binderFactory.createBinder(webRequest, attribute, name);
 //要绑定的对象不为null，即attribute不为null
 if (binder.getTarget() != null) { 
 /** * 参数name需要进行数据绑定 * isBindingDisabled()方法，实际上就是去验证2.2.2中 * 不需要数据绑定的参数是否包含name */
 if (!mavContainer.isBindingDisabled(name)) { 
 //将请求参数值绑定到JavaBean对象同名字段中，见2.2.4
 bindRequestParameters(binder, webRequest);
 }
 //校验参数，见2.2.5
 validateIfApplicable(binder, parameter);
 //绑定过程中发生了错误就在此处抛出异常
 if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) { 
 throw new BindException(binder.getBindingResult());
 }
 }
 // Value type adaptation, also covering java.util.Optional
 //参数值和参数类型不匹配就进行类型转换
 if (!parameter.getParameterType().isInstance(attribute)) { 
 attribute = binder.convertIfNecessary(binder.getTarget(), parameter.getParameterType(), parameter);
 }
 bindingResult = binder.getBindingResult();
 }
 
 // Add resolved attribute and BindingResult at the end of the model
 //获取绑定结果的模型数据，见2.2.6
 Map<String, Object> bindingResultModel = bindingResult.getModel();
 //移除掉原来的，再重新添加
 mavContainer.removeAttributes(bindingResultModel);
 mavContainer.addAllAttributes(bindingResultModel);
 
 return attribute;
 }

### 2.2.1 获取参数名 ###

/** * Derive the model attribute name for the given method parameter based on * a {@code @ModelAttribute} parameter annotation (if present) or falling * back on parameter type based conventions. * @param parameter a descriptor for the method parameter * @return the derived name * @see Conventions#getVariableNameForParameter(MethodParameter) */
    public static String getNameForParameter(MethodParameter parameter) { 
        //@ModelAttribute注解的value属性值作为参数名
        ModelAttribute ann = parameter.getParameterAnnotation(ModelAttribute.class);
        String name = (ann != null ? ann.value() : null);
        //否则默认策略得到参数名
        return (StringUtils.hasText(name) ? name : Conventions.getVariableNameForParameter(parameter));
    }

> 参数名的确定有两种方式
> 
>  *  `@ModelAttribute`注解的`value`属性值作为参数名
>  *  解析参数类型，得到参数类型的完全限定名，然后截取为短类名，首字母小写

### 2.2.2 指定参数是否需要进行数据绑定 ###

//保存在该集合中属性不再参与数据绑定
 private final Set<String> noBinding = new HashSet<>(4);
 
 /** * Register whether data binding should occur for a corresponding model attribute, * corresponding to an {@code @ModelAttribute(binding=true/false)} declaration. * Note: While this flag will be taken into account by {@link #isBindingDisabled}, * a hard {@link #setBindingDisabled} declaration will always override it. * @param attributeName the name of the attribute * @since 4.3.13 */
 public void setBinding(String attributeName, boolean enabled) { 
 if (!enabled) { 
 this.noBinding.add(attributeName);
 }
 else { 
 this.noBinding.remove(attributeName);
 }
 }

### 2.2.3 创建参数类型的实例 ###

/** * Instantiate the model attribute from a URI template variable or from a * request parameter if the name matches to the model attribute name and * if there is an appropriate type conversion strategy. If none of these * are true delegate back to the base class. * @see #createAttributeFromRequestValue */
    @Override
    protected final Object createAttribute(String attributeName, MethodParameter parameter,
                                           WebDataBinderFactory binderFactory, NativeWebRequest request) throws Exception { 
    
        //尝试从模板变量或请求参数中获取方法参数名的对应值，见4.2
        String value = getRequestValueForAttribute(attributeName, request);
        if (value != null) { 
            //参数名对应值进行类型转换,见4.3
            Object attribute = createAttributeFromRequestValue(
                value, attributeName, parameter, binderFactory, request);
            if (attribute != null) { 
                return attribute;
            }
        }
    
        //父类方法完成参数类型实例的创建
        return super.createAttribute(attributeName, parameter, binderFactory, request);
    }

> 该方法被子类`ServletModelAttributeMethodProcessor`重写，添加从模板变量或请求参数中获取方法参数名的对应值的能力。

/** * Extension point to create the model attribute if not found in the model, * with subsequent parameter binding through bean properties (unless suppressed). * The default implementation typically uses the unique public no-arg constructor * if available but also handles a "primary constructor" approach for data classes: * It understands the JavaBeans {@link ConstructorProperties} annotation as well as * runtime-retained parameter names in the bytecode, associating request parameters * with constructor arguments by name. If no such constructor is found, the default * constructor will be used (even if not public), assuming subsequent bean property * bindings through setter methods. * @param attributeName the name of the attribute (never {@code null}) * @param parameter the method parameter declaration * @param binderFactory for creating WebDataBinder instance * @param webRequest the current request * @return the created model attribute (never {@code null}) * @throws BindException in case of constructor argument binding failure * @throws Exception in case of constructor invocation failure * @see #constructAttribute(Constructor, String, MethodParameter, WebDataBinderFactory, NativeWebRequest) * @see BeanUtils#findPrimaryConstructor(Class) */
 protected Object createAttribute(String attributeName, MethodParameter parameter,
 WebDataBinderFactory binderFactory, NativeWebRequest webRequest) throws Exception { 
 
 //获取到Optional容器中真实的方法参数
 MethodParameter nestedParameter = parameter.nestedIfOptional();
 //获取方法参数的嵌套类型
 Class<?> clazz = nestedParameter.getNestedParameterType();
 
 //获取主构造器，一般都是null，我们并没有指定
 Constructor<?> ctor = BeanUtils.findPrimaryConstructor(clazz);
 if (ctor == null) { 
 //得到所有的构造器
 Constructor<?>[] ctors = clazz.getConstructors();
 //只有一个构造器，就使用唯一的一个
 if (ctors.length == 1) { 
 ctor = ctors[0];
 }
 //多个构造器就获取无惨构造器
 else { 
 try { 
 ctor = clazz.getDeclaredConstructor();
 }
 catch (NoSuchMethodException ex) { 
 throw new IllegalStateException("No primary or default constructor found for " + clazz, ex);
 }
 }
 }
 
 //使用构造器完成参数类型实例的创建
 Object attribute = constructAttribute(ctor, attributeName, parameter, binderFactory, webRequest);
 //很明显是Optional容器，包装一下
 if (parameter != nestedParameter) { 
 attribute = Optional.of(attribute);
 }
 return attribute;
 }

使用构造器完成参数类型实例的创建

/** * Construct a new attribute instance with the given constructor. * Called from * {@link #createAttribute(String, MethodParameter, WebDataBinderFactory, NativeWebRequest)} * after constructor resolution. * @param ctor the constructor to use * @param attributeName the name of the attribute (never {@code null}) * @param binderFactory for creating WebDataBinder instance * @param webRequest the current request * @return the created model attribute (never {@code null}) * @throws BindException in case of constructor argument binding failure * @throws Exception in case of constructor invocation failure * @since 5.1 */
 @SuppressWarnings("deprecation")
 protected Object constructAttribute(Constructor<?> ctor, String attributeName, MethodParameter parameter,
 WebDataBinderFactory binderFactory, NativeWebRequest webRequest) throws Exception { 
 
 //该方法已经被废弃掉了，内部实现为空
 Object constructed = constructAttribute(ctor, attributeName, binderFactory, webRequest);
 if (constructed != null) { 
 return constructed;
 }
 
 //无参构造器，直接实例化
 if (ctor.getParameterCount() == 0) { 
 // A single default constructor -> clearly a standard JavaBeans arrangement.
 return BeanUtils.instantiateClass(ctor);
 }
 
 //有参构造器，解析构造器参数
 // A single data class constructor -> resolve constructor arguments from request parameters.
 //@ConstructorProperties注解，显式为构造器中的参数指定参数名
 ConstructorProperties cp = ctor.getAnnotation(ConstructorProperties.class);
 //获取构造器中所有的参数名
 String[] paramNames = (cp != null ? cp.value() : parameterNameDiscoverer.getParameterNames(ctor));
 Assert.state(paramNames != null, () -> "Cannot resolve parameter names for constructor " + ctor);
 //获取构造器中所有参数的类型
 Class<?>[] paramTypes = ctor.getParameterTypes();
 Assert.state(paramNames.length == paramTypes.length,
 () -> "Invalid number of parameter names: " + paramNames.length + " for constructor " + ctor);
 
 Object[] args = new Object[paramTypes.length];
 //为处理器方法参数创建一个绑定器
 WebDataBinder binder = binderFactory.createBinder(webRequest, null, attributeName);
 //fieldDefaultPrefix = "!"
 String fieldDefaultPrefix = binder.getFieldDefaultPrefix();
 //fieldMarkerPrefix = "_"
 String fieldMarkerPrefix = binder.getFieldMarkerPrefix();
 boolean bindingFailure = false;
 Set<String> failedParams = new HashSet<>(4);
 
 for (int i = 0; i < paramNames.length; i++) { 
 String paramName = paramNames[i];
 Class<?> paramType = paramTypes[i];
 //首先尝试直接通过构造器参数名获取
 Object value = webRequest.getParameterValues(paramName);
 if (value == null) { 
 if (fieldDefaultPrefix != null) { 
 //其次尝试通过"!"+构造器参数名获取
 value = webRequest.getParameter(fieldDefaultPrefix + paramName);
 }
 if (value == null && fieldMarkerPrefix != null) { 
 //最后尝试通过"_"+构造器参数名获取
 if (webRequest.getParameter(fieldMarkerPrefix + paramName) != null) { 
 value = binder.getEmptyValue(paramType);
 }
 }
 }
 
 try { 
 /** * 构造器参数对象 * 这个类很简单，提供了两个字段 * String parameterName 参数名 * Annotation[] combinedAnnotations 合并的注解 * ，并重写了getParameterAnnotations()方法 * 用来合并父子类构造器参数上的注解信息 */
 MethodParameter methodParam = new FieldAwareConstructorParameter(ctor, i, paramName);
 /** * 从请求参数中获取不到构造器参数名的对应值，且参数类型是Optional容器 * 那么就直接创建一个空的Optional容器 */
 if (value == null && methodParam.isOptional()) { 
 args[i] = (methodParam.getParameterType() == Optional.class ? Optional.empty() : null);
 }
 //有值，类型转换
 else { 
 args[i] = binder.convertIfNecessary(value, paramType, methodParam);
 }
 }
 catch (TypeMismatchException ex) { 
 ex.initPropertyName(paramName);
 args[i] = value;
 failedParams.add(paramName);
 binder.getBindingResult().recordFieldValue(paramName, paramType, value);
 binder.getBindingErrorProcessor().processPropertyAccessException(ex, binder.getBindingResult());
 //只要出现异常就置为true，表明绑定失败
 bindingFailure = true;
 }
 }
 
 /** * 绑定失败的异常处理 * 此处抛出的异常在外层方法中，又会被捕获 */
 if (bindingFailure) { 
 BindingResult result = binder.getBindingResult();
 for (int i = 0; i < paramNames.length; i++) { 
 String paramName = paramNames[i];
 if (!failedParams.contains(paramName)) { 
 Object value = args[i];
 result.recordFieldValue(paramName, paramTypes[i], value);
 validateValueIfApplicable(binder, parameter, ctor.getDeclaringClass(), paramName, value);
 }
 }
 throw new BindException(result);
 }
 
 //实例化
 return BeanUtils.instantiateClass(ctor, args);
 }

> 总结一下创建参数实例的流程：
> 
>  *  先尝试从模板变量或请求参数中获取方法参数名的对应值，有值就直接进行类型转换
>  *  否则尝试获取处理器方法参数的真实类型，使用反射得到它得构造器
>  *  若是无参构造器，直接实例化
>  *  若是有参构造器
>     
>      *  先尝试根据构造器参数名从请求参数中去获取对应值，有值就进行类型转换
>      *  无值，则构造器参数类型是不是`Optional`容器，如果是，好办，直接创建一个空的`Optional`容器，否则构造器参数值为`null`
>      *  上述过程中发送任何异常，被捕获之后，先进性初步处理，再抛出

### 2.2.4 将请求参数值绑定到`JavaBean`对象同名字段中 ###

/** * This implementation downcasts {@link WebDataBinder} to * {@link ServletRequestDataBinder} before binding. * @see ServletRequestDataBinderFactory */
    @Override
    protected void bindRequestParameters(WebDataBinder binder, NativeWebRequest request) { 
        //获取原生的请求对象
        ServletRequest servletRequest = request.getNativeRequest(ServletRequest.class);
        Assert.state(servletRequest != null, "No ServletRequest");
        ServletRequestDataBinder servletBinder = (ServletRequestDataBinder) binder;
        //请求参数值绑定到JavaBean对象同名字段中
        servletBinder.bind(servletRequest);
    }

> 核心是数据绑定器的`bind()`方法，它内部完成请求参数值到`JavaBean`对象同名字段的绑定

### 2.2.5 参数校验 ###

/** * Validate the model attribute if applicable. * The default implementation checks for {@code @javax.validation.Valid}, * Spring's {@link org.springframework.validation.annotation.Validated}, * and custom annotations whose name starts with "Valid". * @param binder the DataBinder to be used * @param parameter the method parameter declaration * @see WebDataBinder#validate(Object...) * @see SmartValidator#validate(Object, Errors, Object...) */
 protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) { 
 //遍历参数上的所有注解，若标注了@Validated注解就进行校验
 for (Annotation ann : parameter.getParameterAnnotations()) { 
 //获取@Validated注解的value属性值
 Object[] validationHints = determineValidationHints(ann);
 if (validationHints != null) { 
 binder.validate(validationHints);
 break;
 }
 }
 }
 
 
 /** * Determine any validation triggered by the given annotation. * @param ann the annotation (potentially a validation annotation) * @return the validation hints to apply (possibly an empty array), * or {@code null} if this annotation does not trigger any validation * @since 5.1 */
 @Nullable
 private Object[] determineValidationHints(Annotation ann) { 
 //获取参数上标注的@Validated注解信息
 Validated validatedAnn = AnnotationUtils.getAnnotation(ann, Validated.class);
 if (validatedAnn != null || ann.annotationType().getSimpleName().startsWith("Valid")) { 
 Object hints = (validatedAnn != null ? validatedAnn.value() : AnnotationUtils.getValue(ann));
 if (hints == null) { 
 return new Object[0];
 }
 return (hints instanceof Object[] ? (Object[]) hints : new Object[] { hints});
 }
 return null;
 }

### 2.2.6 获取绑定结果的模型数据 ###

/** * Return a model Map for the obtained state, exposing an Errors * instance as '{@link #MODEL_KEY_PREFIX MODEL_KEY_PREFIX} + objectName' * and the object itself. * Note that the Map is constructed every time you're calling this method. * Adding things to the map and then re-calling this method will not work. * The attributes in the model Map returned by this method are usually * included in the ModelAndView for a form view that uses Spring's bind tag, * which needs access to the Errors instance. * @see #getObjectName * @see #MODEL_KEY_PREFIX */
 @Override
 public Map<String, Object> getModel() { 
 Map<String, Object> model = new LinkedHashMap<>(2);
 // Mapping from name to target object.
 //绑定的参数名->要绑定的对象
 model.put(getObjectName(), getTarget());
 // Errors instance, even if no errors.
 /** * MODEL_KEY_PREFIX=org.springframework.validation.BindingResult. * 前缀+绑定的参数名->绑定结果 */
 model.put(MODEL_KEY_PREFIX + getObjectName(), this);
 return model;
 }

> 保存两个映射关系
> 
>  *  绑定的参数名->要绑定的对象
>  *  前缀+绑定的参数名->绑定结果

# 3 处理返回值 #

这就离不开`HandlerMethodReturnValueHandler`接口定义的方法了

public interface HandlerMethodReturnValueHandler { 
    
       //判断是否支持处理该返回值
       boolean supportsReturnType(MethodParameter returnType);
    
       //处理返回值，写入响应体中
       void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
             ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
    
    }

那我们就按照顺序看这些方法的原理

## 3.1 `supportsReturnType(MethodParameter returnType)`方法 ##

/** * Return {@code true} if there is a method-level {@code @ModelAttribute} * or, in default resolution mode, for any return value type that is not * a simple type. */
    @Override
    public boolean supportsReturnType(MethodParameter returnType) { 
        return (returnType.hasMethodAnnotation(ModelAttribute.class) ||
                (this.annotationNotRequired && !BeanUtils.isSimpleProperty(returnType.getParameterType())));
    }

> 有两种判断通过条件
> 
>  *  方法上有`@ModelAttribute`注解，就支持解析该返回值
>  *  `annotationNotRequired`字段为`true`，且方法声明的返回值类型是非简单属性，就支持解析该返回值
> 
> 什么是非简单属性，见`2.1`

## 3.2 `handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest)`方法 ##

/** * Add non-null return values to the {@link ModelAndViewContainer}. */
    @Override
    public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
                                  ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception { 
    
        if (returnValue != null) { 
            //生成返回值保存在模型中的名字
            String name = ModelFactory.getNameForReturnValue(returnValue, returnType);
            //返回值保存到模型中
            mavContainer.addAttribute(name, returnValue);
        }
    }

生成返回值保存在模型中的名字

/** * Derive the model attribute name for the given return value. Results will be * based on: * <ol> * <li>the method {@code ModelAttribute} annotation value * <li>the declared return type if it is more specific than {@code Object} * <li>the actual return value type * </ol> * @param returnValue the value returned from a method invocation * @param returnType a descriptor for the return type of the method * @return the derived name (never {@code null} or empty String) */
 public static String getNameForReturnValue(@Nullable Object returnValue, MethodParameter returnType) { 
 //@ModelAttribute注解的value属性作为返回值的名字
 ModelAttribute ann = returnType.getMethodAnnotation(ModelAttribute.class);
 if (ann != null && StringUtils.hasText(ann.value())) { 
 return ann.value();
 }
 //根据返回值的完全限定名，生成一个首字母小写的短类名作为名字
 else { 
 Method method = returnType.getMethod();
 Assert.state(method != null, "No handler method");
 //包含返回值的类的clazz对象
 Class<?> containingClass = returnType.getContainingClass();
 //方法声明的返回值的泛型类型
 Class<?> resolvedType = GenericTypeResolver.resolveReturnType(method, containingClass);
 //一般都是首字母小写的短类名
 return Conventions.getVariableNameForReturnType(method, resolvedType, returnValue);
 }
 }

> 返回值保存在模型中的名字，有两种选择
> 
>  *  用户可以在方法标注的`@ModelAttribute`注解上指定名字
>  *  系统自动根据返回值类型生成一个首字母小写的短类名作为名字

# 4 `ServletModelAttributeMethodProcessor`拥有的一些重要方法 #

> 我将方法的实现细节抽取到该章节，有兴趣的就看一下

## 4.1 `getUriTemplateVariables(NativeWebRequest request)`方法，获取请求路径上模板变量的映射关系 ##

protected final Map<String, String> getUriTemplateVariables(NativeWebRequest request) { 
 /** * HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE= * org.springframework.web.servlet.HandlerMapping.uriTemplateVariables * * RequestAttributes.SCOPE_REQUEST=0,表示request作用域 */
 Map<String, String> variables = (Map<String, String>) request.getAttribute(
 HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
 return (variables != null ? variables : Collections.emptyMap());
 }

> 在`RequestMappingHandlerMapping`获取请求路径对应的处理器方法时，就将请求路径完全解析了
> 
>  *  模板变量保存在请求域`org.springframework.web.servlet.HandlerMapping.uriTemplateVariables`属性中
>  *  矩阵变量保存在请求域`org.springframework.web.servlet.HandlerMapping.matrixVariables`属性中
> 
> 不熟悉的建议去了解一下`RequestMappingHandlerMapping`的处理过程

## 4.2 `getRequestValueForAttribute(String attributeName, NativeWebRequest request)`方法，从模板变量或请求参数中获取方法参数名的对应值 ##

/** * Obtain a value from the request that may be used to instantiate the * model attribute through type conversion from String to the target type. * The default implementation looks for the attribute name to match * a URI variable first and then a request parameter. * @param attributeName the model attribute name * @param request the current request * @return the request value to try to convert, or {@code null} if none */
 @Nullable
 protected String getRequestValueForAttribute(String attributeName, NativeWebRequest request) { 
 //获取路径中模板变量的映射关系，见4.1
 Map<String, String> variables = getUriTemplateVariables(request);
 //从映射关系中获取方法参数名的对应值
 String variableValue = variables.get(attributeName);
 if (StringUtils.hasText(variableValue)) { 
 return variableValue;
 }
 //从请求参数中获取方法参数名的对应值
 String parameterValue = request.getParameter(attributeName);
 if (StringUtils.hasText(parameterValue)) { 
 return parameterValue;
 }
 return null;
 }

> 该方法从两个途径获取参数名的对应值
> 
>  *  模板变量
>  *  请求参数（请求参数包括表单提交的`POST`请求的请求体）

## 4.3 `createAttributeFromRequestValue(String sourceValue, String attributeName,MethodParameter parameter, WebDataBinderFactory binderFactory, NativeWebRequest request)`方法，参数名对应值进行类型转换 ##

/** * Create a model attribute from a String request value (e.g. URI template * variable, request parameter) using type conversion. * The default implementation converts only if there a registered * {@link Converter} that can perform the conversion. * @param sourceValue the source value to create the model attribute from * @param attributeName the name of the attribute (never {@code null}) * @param parameter the method parameter * @param binderFactory for creating WebDataBinder instance * @param request the current request * @return the created model attribute, or {@code null} if no suitable * conversion found */
 @Nullable
 protected Object createAttributeFromRequestValue(String sourceValue, String attributeName,
 MethodParameter parameter, WebDataBinderFactory binderFactory, NativeWebRequest request)
 throws Exception { 
 
 //创建一个数据绑定器，主要是为了得到转换服务
 DataBinder binder = binderFactory.createBinder(request, null, attributeName);
 //使用统一转换服务将String->参数类型
 ConversionService conversionService = binder.getConversionService();
 if (conversionService != null) { 
 TypeDescriptor source = TypeDescriptor.valueOf(String.class);
 TypeDescriptor target = new TypeDescriptor(parameter);
 if (conversionService.canConvert(source, target)) { 
 return binder.convertIfNecessary(sourceValue, parameter.getParameterType(), parameter);
 }
 }
 return null;
 }

> 该方法会将在`4.2`中得到的方法名对应值进行类型转换，转换为方法参数的实际类型。

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