一文搞懂SpringMVC中@InitBinder注解的使用

@RestController
@RequestMapping("/demos")
public class DemoController {
  @InitBinder // 1
  public void bind(WebDataBinder binder) { // 2
    binder.registerCustomEditor(Long.class, new PropertyEditorSupport() { // 3
      @Override
      public void setAsText(String text) throws IllegalArgumentException {
        setValue(Long.valueOf(text) + 666L) ;
      }
    }) ;
  }
  @GetMapping("/index")
  public Object index(Long id) {
    return "index - " + id ;
  }
}

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter implements BeanFactoryAware, InitializingBean {
  protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
    // ...
    // 这里会查找当前执行的Controller中定义的所有@InitBinder注解的方法
    WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
    ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
    invocableMethod.invokeAndHandle(webRequest, mavContainer);
    // ...
  }
}

public class ServletInvocableHandlerMethod extends InvocableHandlerMethod {
  public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
    // 调用父类方法
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    // ...
  }
}
// 执行父类方法调用
public class InvocableHandlerMethod extends HandlerMethod {
  public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
    Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    return doInvoke(args);
  }
  protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
    Object[] args = new Object[parameters.length];
    for (int i = 0; i < parameters.length; i++) {
      // 解析参数
      args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
    }
  }
}

public abstract class AbstractNamedValueMethodArgumentResolver {
  public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
    // 封装方法参数的名称这里为：id
    NamedValueInfo namedValueInfo = getNamedValueInfo(parameter);
    // resolvedName = id
    Object resolvedName = resolveEmbeddedValuesAndExpressions(namedValueInfo.name);
    // ...
    // 获取参数名对应的请求参数值：/demos/index?id=100 , 这就返回100
    Object arg = resolveName(resolvedName.toString(), nestedParameter, webRequest);
    // ...
    if (binderFactory != null) {
      // 根据当前的Request对象及请求参数名创建WebDataBinder对象
      // 内部创建的ExtendedServletRequestDataBinder对象
      WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
      try {
        // 执行类型转换
        arg = binder.convertIfNecessary(arg, parameter.getParameterType(), parameter);
      }
    }
  }
}
// 创建WebDataBinder对象
public class DefaultDataBinderFactory implements WebDataBinderFactory {
  public final WebDataBinder createBinder(NativeWebRequest webRequest, @Nullable Object target, String objectName) throws Exception {
    WebDataBinder dataBinder = createBinderInstance(target, objectName, webRequest);
    if (this.initializer != null) {
      // 初始化WebDataBinder对象，这里最主要的就是为其设置类型转换器
      this.initializer.initBinder(dataBinder, webRequest);
    }
    // 初始化执行@InitBinder注解的方法
    initBinder(dataBinder, webRequest);
    return dataBinder;
  }
}
public class InitBinderDataBinderFactory extends DefaultDataBinderFactory {
  public void initBinder(WebDataBinder dataBinder, NativeWebRequest request) throws Exception {
    // 遍历所有@InitBinder注解的方法
    for (InvocableHandlerMethod binderMethod : this.binderMethods) {
      if (isBinderMethodApplicable(binderMethod, dataBinder)) {
        // 这里就是执行@InitBinder注解的方法
       Object returnValue = binderMethod.invokeForRequest(request, null, dataBinder);
        // 如果@InitBinder注解的方法有返回值则抛出异常
        if (returnValue != null) {
          throw new IllegalStateException("@InitBinder methods must not return a value (should be void): " + binderMethod);
        }
      }
    }
  }
}
// 解析@InitBinder注解方法的参数及方法执行
public class InvocableHandlerMethod extends HandlerMethod {
  public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
    // 解析获取@InitBinder注解方法的参数
    Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    // 执行调用
    return doInvoke(args);
  }
}

public abstract class AbstractNamedValueMethodArgumentResolver {
  public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
    if (binderFactory != null) {
      // 根据当前的Request对象及请求参数名创建WebDataBinder对象
      // 内部创建的ExtendedServletRequestDataBinder对象
      WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
      try {
        // 执行类型转换
        arg = binder.convertIfNecessary(arg, parameter.getParameterType(), parameter);
      }
    }
  }
}
// 最终通过该类调用类型转换
class TypeConverterDelegate {
  public <T> T convertIfNecessary(@Nullable String propertyName, @Nullable Object oldValue, @Nullable Object newValue, @Nullable Class<T> requiredType, @Nullable TypeDescriptor typeDescriptor) throws IllegalArgumentException {
    // Custom editor for this type?
    // 获取自定义的类型转换器（首先获取的就是我们上面自定义的）
    PropertyEditor editor = this.propertyEditorRegistry.findCustomEditor(requiredType, propertyName);
    // ...
    Object convertedValue = newValue;
    // ...
    convertedValue = doConvertValue(oldValue, convertedValue, requiredType, editor);
  }
  private Object doConvertValue(@Nullable Object oldValue, @Nullable Object newValue, @Nullable Class<?> requiredType, @Nullable PropertyEditor editor) { 
    // ...
    if (convertedValue instanceof String) {
      if (editor != null) {
        String newTextValue = (String) convertedValue;
        // 最终的调用
        return doConvertTextValue(oldValue, newTextValue, editor);
      } else if (String.class == requiredType) {
        returnValue = convertedValue;
      }
    }
    return returnValue;
  }
  // 最终得到了我们想要的值
  private Object doConvertTextValue(@Nullable Object oldValue, String newTextValue, PropertyEditor editor) {
    try {
      editor.setValue(oldValue);
    }
    // ...
    editor.setAsText(newTextValue);
    return editor.getValue();
  }
}