Failed to convert value of type java.lang.String to required type java.time.LocalDate

花开从不是为了别人的欣赏，努力也是。

一、提供出去的接口，请求入参带有时间类型参数

@GetMapping("/date")
public DateResponse get(@RequestParam(value = "localDate", defaultValue = "2020-12-12", required = false) LocalDate localDate, @RequestParam(value = "date", defaultValue = "2020-12-12 12:12:12", required = false) Date date) {
	System.out.println(localDate);
    System.out.println(date);
    return new DateResponse();
}

当我们没有做任何配置时，请求这个接口的时候会报这样的错误：

{
  "timestamp": 1592623507092,
  "status": 400,
  "error": "Bad Request",
  "message": "Failed to convert value of type 'java.lang.String' to required type 'java.time.LocalDate'; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [@org.springframework.web.bind.annotation.RequestParam java.time.LocalDate] for value '2020-12-12'; nested exception is java.lang.IllegalArgumentException: Parse attempt failed for value [2020-12-12]",
  "path": "/date"
}

根据Failed to convert value of type ‘java.lang.String’ to required type ‘java.time.LocalDate’ 这条信息来看，程序不能将一个String类型的值转换成LocalDate类型（前端传过来的是String类型，程序会对这些数据进行相应的convert，当匹配不到类型时，就会抛ConversionFailedException异常）。那么就按照提示，补充相应的Converter就好了。

二、几种常用的Converter
PS：我们自定义的Converter需要实现org.springframework.core.convert.converter.Converter 接口，这种方式是针对全局生效的，如果只是针对某个接口，可以使用 @DateTimeFormat(iso = DateTimeFormat.ISO.DATE) 这样的方式。
1、字符串转换成LocalDate

public class String2LocalDateConverter implements Converter<String, LocalDate> {
    @Override
    public LocalDate convert(String s) {
        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd");
        return LocalDate.parse(s, fmt);
    }
}

2、字符串转换成LocalDateTime

public class String2LocalDateTimeConverter implements Converter<String, LocalDateTime> {
    @Override
    public LocalDateTime convert(String s) {
        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        return LocalDateTime.parse(s, fmt);
    }
}

3、字符串转换成LocalTime

public class String2LocalTimeConverter implements Converter<String, LocalTime> {
    @Override
    public LocalTime convert(String s) {
        DateTimeFormatter fmt = DateTimeFormatter.ofPattern("HH:mm:ss");
        return LocalTime.parse(s, fmt);
    }
}

4、字符串转换成Date

public class String2DateConverter implements Converter<String, Date> {
    @SneakyThrows
    @Override
    public Date convert(String s) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return sdf.parse(s);
    }
}

三、注册Converters
这里我们选择通过WebMvcConfigurationSupport 的**addFormatters(FormatterRegistry registry)**方法将自定义的Converters注册到容器里面去

@Configuration
public class WebMvcConfig extends WebMvcConfigurationSupport {

    @Override
    protected void addFormatters(FormatterRegistry registry) {
        registry.addConverter(new String2LocalDateConverter());
        registry.addConverter(new String2LocalDateTimeConverter());
        registry.addConverter(new String2LocalTimeConverter());
        registry.addConverter(new String2DateConverter());
    }

我们再次请求接口，就可以正确的访问了。