上一篇《Spring MVC设计原理》中提到了Spring MVC在使用RequestMappingHandlerAdapter处理HTTP请求时会进行参数绑定，参数绑定会在两种方法上发生：
- @RequestMapping方法的参数绑定
- @ModelAttribute方法的参数绑定
1、参数解析器的初始化
这两种参数绑定分别使用一个HandlerMethodArgumentResolverComposite对象，只不过这个对象内部使用两套HandlerMethodArgumentResolver。HandlerMethodArgumentResolverComposite的初始化是在RequestMappingHandlerAdapter的afterPropertiesSet()方法中。
控制器使用的参数解析器列表，包括通过setCustomArgumentResolvers()提供的内置解析器和自定义解析器。
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>();
// 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));
resolvers.add(new ServletModelAttributeMethodProcessor(true));
return resolvers;
}
@InitBinder方法使用的参数解析器列表，包括setCustomArgumentResolvers()内置和自定义解析器。
private List<HandlerMethodArgumentResolver> getDefaultInitBinderArgumentResolvers() {
List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>();
// 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 ExpressionValueMethodArgumentResolver(getBeanFactory()));
resolvers.add(new SessionAttributeMethodArgumentResolver());
resolvers.add(new RequestAttributeMethodArgumentResolver());
// Type-based argument resolution
resolvers.add(new ServletRequestMethodArgumentResolver());
resolvers.add(new ServletResponseMethodArgumentResolver());
// Custom arguments
if (getCustomArgumentResolvers() != null) {
resolvers.addAll(getCustomArgumentResolvers());
}
// Catch-all
resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
return resolvers;
}
HandlerMethodArgumentResolver用于在给定请求的上下文中将方法参数解析为参数值的策略接口。
public interface HandlerMethodArgumentResolver {
//此解析程序是否支持给定的MethodParameter方法参数
boolean supportsParameter(MethodParameter parameter);
//将方法参数解析为来自给定请求的参数值。
//ModelAndViewContainer提供对请求模型的访问。
//WebDataBinderFactory提供了一种在需要进行数据绑定和类型转换时创建WebDataBinder实例的方法
@Nullable
Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception;
}
HandlerMethodArgumentResolverComposite也实现了HandlerMethodArgumentResolver接口，通过委托给已注册的HandlerMethodArgumentResolver列表来解析方法参数。先前解析的方法参数被缓存以便更快地查找。
@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);
}
@Nullable
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
if (result == null) {
for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
if (methodArgumentResolver.supportsParameter(parameter)) {
result = methodArgumentResolver;
this.argumentResolverCache.put(parameter, result);
break;
}
}
}
return result;
}
2、参数绑定触发时机
Spring MVC在处理请求时会流转到到RequestMappingHandlerAdapter的invokeHandlerMethod方法，这个方法中会调用modelFactory.initModel方法和invocableMethod.invokeAndHandle方法会完成参数解析。
这两个地方最终调用InvocableHandlerMethod.invokeForRequest方法使用HandlerMethodArgumentResolver进行参数解析。
下面分析一下这个方法的具体实现过程。
3、源码分析
@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Arguments: " + Arrays.toString(args));
}
return doInvoke(args);
}
Spring MVC的参数绑定是在InvocableHandlerMethod#getMethodArgumentValues()中触发的。
//获取当前请求的方法参数值，检查提供的参数值并回退到配置的参数解析器。
//生成的数组将传递到doInvoke()。
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
if (ObjectUtils.isEmpty(getMethodParameters())) {
return EMPTY_ARGS;
}
MethodParameter[] parameters = getMethodParameters();
Object[] args = new Object[parameters.length];
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
args[i] = findProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
if (!this.resolvers.supportsParameter(parameter)) {
throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
}
try {
//如果在已提供的参数值中没有与方法参数类型匹配的则使用HandlerMethodArgumentResolverComposite
args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
}
catch (Exception ex) {
// Leave stack trace for later, exception may actually be resolved and handled..
if (logger.isDebugEnabled()) {
String error = ex.getMessage();
if (error != null && !error.contains(parameter.getExecutable().toGenericString())) {
logger.debug(formatArgumentError(parameter, error));
}
}
throw ex;
}
}
return args;
}
下面分析几个HandlerMethodArgumentResolver的实现。
3.1、AbstractNamedValueMethodArgumentResolver
AbstractNamedValueMethodArgumentResolver用于从NamedValueInfo解析方法参数的抽象基类。子类定义如何执行以下操作：
- 获取方法参数的NamedValueInfo
- 将名称解析为参数值
- 在需要参数值时处理缺少的参数值
- 可选择处理已解析的值
NamedValueInfo是对参数名，参数是否必填以及默认值的封装。
protected static class NamedValueInfo {
private final String name;
private final boolean required;
@Nullable
private final String defaultValue;
public NamedValueInfo(String name, boolean required, @Nullable String defaultValue) {
this.name = name;
this.required = required;
this.defaultValue = defaultValue;
}
}
默认值字符串可以包含${...}占位符和Spring Expression Language #{...}表达式。 为了这个工作必须提供ConfigurableBeanFactory给类构造函数。创建WebDataBinder以将类型转换应用于已解析的参数值（如果它与方法参数类型不匹配）。
public abstract class AbstractNamedValueMethodArgumentResolver implements HandlerMethodArgumentResolver {
@Nullable
private final ConfigurableBeanFactory configurableBeanFactory;
@Nullable
private final BeanExpressionContext expressionContext;
private final Map<MethodParameter, NamedValueInfo> namedValueInfoCache = new ConcurrentHashMap<>(256);
public AbstractNamedValueMethodArgumentResolver() {
this.configurableBeanFactory = null;
this.expressionContext = null;
}
public AbstractNamedValueMethodArgumentResolver(@Nullable ConfigurableBeanFactory beanFactory) {
this.configurableBeanFactory = beanFactory;
this.expressionContext =
(beanFactory != null ? new BeanExpressionContext(beanFactory, new RequestScope()) : null);
}
@Override
@Nullable
public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
NamedValueInfo namedValueInfo = getNamedValueInfo(parameter);
MethodParameter nestedParameter = parameter.nestedIfOptional();
//参数名支持Spl
Object resolvedName = resolveStringValue(namedValueInfo.name);
if (resolvedName == null) {
throw new IllegalArgumentException(
"Specified name must not resolve to null: [" + namedValueInfo.name + "]");
}
//根据参数名提取参数值
Object arg = resolveName(resolvedName.toString(), nestedParameter, webRequest);
if (arg == null) {
//如不能通过webRequest取得参数值使用默认值（看子类创建namedValueInfo的实现方式）
if (namedValueInfo.defaultValue != null) {
arg = resolveStringValue(namedValueInfo.defaultValue);
}
//如果必填参数值为null通常抛出异常
else if (namedValueInfo.required && !nestedParameter.isOptional()) {
handleMissingValue(namedValueInfo.name, nestedParameter, webRequest);
}
//如果arg仍然为null,返回类型是Boolean返回false否则抛异常
arg = handleNullValue(namedValueInfo.name, arg, nestedParameter.getNestedParameterType());
}
//空字串使用默认值
else if ("".equals(arg) && namedValueInfo.defaultValue != null) {
arg = resolveStringValue(namedValueInfo.defaultValue);
}
if (binderFactory != null) {
WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
try {
//通常由字符串转换成复杂对象
arg = binder.convertIfNecessary(arg, parameter.getParameterType(), parameter);
}
catch (ConversionNotSupportedException ex) {
throw new MethodArgumentConversionNotSupportedException(arg, ex.getRequiredType(),
namedValueInfo.name, parameter, ex.getCause());
}
catch (TypeMismatchException ex) {
throw new MethodArgumentTypeMismatchException(arg, ex.getRequiredType(),
namedValueInfo.name, parameter, ex.getCause());
}
}
handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);
return arg;
}
//获取给定方法参数的命名值。
private NamedValueInfo getNamedValueInfo(MethodParameter parameter) {
NamedValueInfo namedValueInfo = this.namedValueInfoCache.get(parameter);
if (namedValueInfo == null) {
namedValueInfo = createNamedValueInfo(parameter);
namedValueInfo = updateNamedValueInfo(parameter, namedValueInfo);
this.namedValueInfoCache.put(parameter, namedValueInfo);
}
return namedValueInfo;
}
//为给定的方法参数创建{@link NamedValueInfo}对象。
//实现通常通过MethodParamete.getParameterAnnotation(Class)检索方法注释。
protected abstract NamedValueInfo createNamedValueInfo(MethodParameter parameter);
//基于具有已清理值的给定NamedValueInfo创建新的NamedValueInfo。
private NamedValueInfo updateNamedValueInfo(MethodParameter parameter, NamedValueInfo info) {
String name = info.name;
if (info.name.isEmpty()) {
name = parameter.getParameterName();
if (name == null) {
throw new IllegalArgumentException(
"Name for argument type [" + parameter.getNestedParameterType().getName() +
"] not available, and parameter name information not found in class file either.");
}
}
String defaultValue = (ValueConstants.DEFAULT_NONE.equals(info.defaultValue) ? null : info.defaultValue);
return new NamedValueInfo(name, info.required, defaultValue);
}
//解析给定的注释指定值，可能包含占位符和表达式。
@Nullable
private Object resolveStringValue(String value) {
if (this.configurableBeanFactory == null) {
return value;
}
String placeholdersResolved = this.configurableBeanFactory.resolveEmbeddedValue(value);
BeanExpressionResolver exprResolver = this.configurableBeanFactory.getBeanExpressionResolver();
if (exprResolver == null || this.expressionContext == null) {
return value;
}
return exprResolver.evaluate(placeholdersResolved, this.expressionContext);
}
//将给定的参数类型和值名称解析为参数值。
@Nullable
protected abstract Object resolveName(String name, MethodParameter parameter, NativeWebRequest request)
throws Exception;
//在需要命名值时调用，但resolveName(String,MethodParameter,NativeWebRequest)返回null并且没有默认值。
//在这种情况下，子类通常会抛出异常。
protected void handleMissingValue(String name, MethodParameter parameter, NativeWebRequest request)
throws Exception {
handleMissingValue(name, parameter);
}
//在需要命名值时调用，但resolveName(String,MethodParameter,NativeWebRequest)返回null并且没有默认值。
//在这种情况下，子类通常会抛出异常。
protected void handleMissingValue(String name, MethodParameter parameter) throws ServletException {
throw new ServletRequestBindingException("Missing argument '" + name +
"' for method parameter of type " + parameter.getNestedParameterType().getSimpleName());
}
@Nullable
private Object handleNullValue(String name, @Nullable Object value, Class<?> paramType) {
if (value == null) {
if (Boolean.TYPE.equals(paramType)) {
return Boolean.FALSE;
}
else if (paramType.isPrimitive()) {
throw new IllegalStateException("Optional " + paramType.getSimpleName() + " parameter '" + name +
"' is present but cannot be translated into a null value due to being declared as a " +
"primitive type. Consider declaring it as object wrapper for the corresponding primitive type.");
}
}
return value;
}
//在解析值后调用。
protected void handleResolvedValue(@Nullable Object arg, String name, MethodParameter parameter,
@Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest) {
}
//表示有关命名值的信息，包括名称，是否需要以及默认值。
protected static class NamedValueInfo {
private final String name;
private final boolean required;
@Nullable
private final String defaultValue;
public NamedValueInfo(String name, boolean required, @Nullable String defaultValue) {
this.name = name;
this.required = required;
this.defaultValue = defaultValue;
}
}
}
3.2、RequestParamMethodArgumentResolver
RequestParamMethodArgumentResolver解析使用@RequestParam注释的方法参数、类型为MultipartFile的参数、以及Spring的 MultipartResolver抽象、以及类型为javax.servlet.http.Part的参数、以及Servlet 3.0 multipart请求。此解析程序也可以在默认解析模式下创建，其中未使用@RequestParam注释的简单类型（int，long等）也被视为请求参数，参数名称派生自参数名称。如果方法参数类型为Map，则注释中指定的名称用于解析请求参数String值。然后，假设已经注册了合适的Converter或PropertyEditor，则通过类型转换将该值转换为Map。或者，如果未指定请求参数名称，则使用RequestParamMapMethodArgumentResolver来提供对Map形式的所有请求参数的访问。调用WebDataBinder以将类型转换应用于尚未与方法参数类型匹配的已解析请求标头值。
public class RequestParamMethodArgumentResolver extends AbstractNamedValueMethodArgumentResolver
implements UriComponentsContributor {
private static final TypeDescriptor STRING_TYPE_DESCRIPTOR = TypeDescriptor.valueOf(String.class);
private final boolean useDefaultResolution;
//useDefaultResolution在默认解析模式下，一个简单类型的方法参数（如BeanUtils.isSimpleProperty}中所定义）被视为请求参数，
//即使它未注释，请求参数名称也是从方法参数名称派生的
public RequestParamMethodArgumentResolver(boolean useDefaultResolution) {
this.useDefaultResolution = useDefaultResolution;
}
public RequestParamMethodArgumentResolver(@Nullable ConfigurableBeanFactory beanFactory,
boolean useDefaultResolution) {
super(beanFactory);
this.useDefaultResolution = useDefaultResolution;
}
//支持以下内容：
//@RequestParam注释方法参数。 这排除了Map参数，其中注释未指定名称。 有关此类参数，请参阅RequestParamMapMethodArgumentResolver
//类型为MultipartFile的参数，除非使用@RequestPart进行注释。
//Part类型的参数，除非使用@RequestPart进行注释。
//在默认解析模式下，简单类型参数即使不与@RequestParam一起使用。
@Override
public boolean supportsParameter(MethodParameter parameter) {
if (parameter.hasParameterAnnotation(RequestParam.class)) {
if (Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType())) {
RequestParam requestParam = parameter.getParameterAnnotation(RequestParam.class);
return (requestParam != null && StringUtils.hasText(requestParam.name()));
}
else {
return true;
}
}
else {
if (parameter.hasParameterAnnotation(RequestPart.class)) {
return false;
}
parameter = parameter.nestedIfOptional();
if (MultipartResolutionDelegate.isMultipartArgument(parameter)) {
return true;
}
else if (this.useDefaultResolution) {
return BeanUtils.isSimpleProperty(parameter.getNestedParameterType());
}
else {
return false;
}
}
}
@Override
protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
RequestParam ann = parameter.getParameterAnnotation(RequestParam.class);
return (ann != null ? new RequestParamNamedValueInfo(ann) : new RequestParamNamedValueInfo());
}
@Override
@Nullable
protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
HttpServletRequest servletRequest = request.getNativeRequest(HttpServletRequest.class);
if (servletRequest != null) {
Object mpArg = MultipartResolutionDelegate.resolveMultipartArgument(name, parameter, servletRequest);
if (mpArg != MultipartResolutionDelegate.UNRESOLVABLE) {
return mpArg;
}
}
Object arg = null;
MultipartRequest multipartRequest = request.getNativeRequest(MultipartRequest.class);
if (multipartRequest != null) {
List<MultipartFile> files = multipartRequest.getFiles(name);
if (!files.isEmpty()) {
arg = (files.size() == 1 ? files.get(0) : files);
}
}
if (arg == null) {
String[] paramValues = request.getParameterValues(name);
if (paramValues != null) {
arg = (paramValues.length == 1 ? paramValues[0] : paramValues);
}
}
return arg;
}
@Override
protected void handleMissingValue(String name, MethodParameter parameter, NativeWebRequest request)
throws Exception {
HttpServletRequest servletRequest = request.getNativeRequest(HttpServletRequest.class);
if (MultipartResolutionDelegate.isMultipartArgument(parameter)) {
if (servletRequest == null || !MultipartResolutionDelegate.isMultipartRequest(servletRequest)) {
throw new MultipartException("Current request is not a multipart request");
}
else {
throw new MissingServletRequestPartException(name);
}
}
else {
throw new MissingServletRequestParameterException(name,
parameter.getNestedParameterType().getSimpleName());
}
}
@Override
public void contributeMethodArgument(MethodParameter parameter, @Nullable Object value,
UriComponentsBuilder builder, Map<String, Object> uriVariables, ConversionService conversionService) {
Class<?> paramType = parameter.getNestedParameterType();
if (Map.class.isAssignableFrom(paramType) || MultipartFile.class == paramType || Part.class == paramType) {
return;
}
RequestParam requestParam = parameter.getParameterAnnotation(RequestParam.class);
String name = (requestParam == null || StringUtils.isEmpty(requestParam.name()) ?
parameter.getParameterName() : requestParam.name());
Assert.state(name != null, "Unresolvable parameter name");
if (value == null) {
if (requestParam != null &&
(!requestParam.required() || !requestParam.defaultValue().equals(ValueConstants.DEFAULT_NONE))) {
return;
}
builder.queryParam(name);
}
else if (value instanceof Collection) {
for (Object element : (Collection<?>) value) {
element = formatUriValue(conversionService, TypeDescriptor.nested(parameter, 1), element);
builder.queryParam(name, element);
}
}
else {
builder.queryParam(name, formatUriValue(conversionService, new TypeDescriptor(parameter), value));
}
}
@Nullable
protected String formatUriValue(
@Nullable ConversionService cs, @Nullable TypeDescriptor sourceType, @Nullable Object value) {
if (value == null) {
return null;
}
else if (value instanceof String) {
return (String) value;
}
else if (cs != null) {
return (String) cs.convert(value, sourceType, STRING_TYPE_DESCRIPTOR);
}
else {
return value.toString();
}
}
private static class RequestParamNamedValueInfo extends NamedValueInfo {
public RequestParamNamedValueInfo() {
super("", false, ValueConstants.DEFAULT_NONE);
}
public RequestParamNamedValueInfo(RequestParam annotation) {
super(annotation.name(), annotation.required(), annotation.defaultValue());
}
}
}
