Preface: This article mainly introduces the concepts and usage of data conversion in the data binding process of SpringMVC .
This article focuses on the following issues:
- SpringMVC built-in converter
-
custom converter
1. Built-in converter
There are many converters built into the Spring MVC context to do most Java type conversions .
Group 1: Scalar Converters
class name description| StringToBooleanConverter | String—–>Boolean(true:true/on/yes/1;false:false/off/no/0) |
| ObjectToStringConverter | Object—–>String (call toString method to convert) |
| StringToNumberConverterFactory | String—–>Number (such as Integer, Long, etc.) |
| NumberToNumberConverterFactory | Number subtype <——> Number subtype (Integer, Long, Double, etc.) |
| StringToCharacterConverter | String —–>java.lang.Character takes the first character of the string |
| NumberToCharacterConverter | Number subtypes (Integer, Long, Double, etc.) --> java.lang.Character |
| CharacterToNumberFactory | java.lang.Character ——>Number subtype (Integer, Long, Double, etc.) |
| StringToEnumConverterFactory | String-->enum type (convert the string to the required enum type through Enum.valueOf) |
| EnumToStringConverter | enum type—–>String (returns the name() value of the enum object) |
| StringToLocaleConverter | String—–>java.util.Local |
| PropertiesToStringConverter | java.util.Properties—–>String (defaults to ISO-8859-1 decoding) |
| StringToPropertiesConverter | String—–>java.util.Properties (default decoded by ISO-8859-1) |
The second group: collection, array related converters
| ArrayToCollectionConverter | Any S array --> any T collection (List, Set) |
| CollectionToArrayConverter | Any T collection (List, Set) --> any S array |
| ArrayToArrayConverter | Arbitrary S array <—-> any T array |
| CollectionToCollectionConverter | Any T collection (List, Set) <—-> Any T collection (List, Set), that is, type conversion between collections |
| MapToMapConverter | Conversion between Map<—->Map |
| ArrayToStringConverter | Arbitrary S array --> String type |
| StringToArrayConverter | String—->Array is split by "," by default, and the spaces on both sides of the string are removed (trim) |
| ArrayToObjectConverter | Any S array --> conversion of any Object (if the target type is compatible with the source type, the source object is returned directly; otherwise, the first element of the S array is returned and type conversion is performed) |
| ObjectToArrayConverter | Object—–> single-element array |
| CollectionToStringConverter | 任意T集合（List、Set）—->String类型 |
| StringToCollectionConverter | String—–>集合（List、Set）(默认通过“,”分割，且去除字符串的两边空格(trim)) |
| CollectionToObjectConverter | 任意T集合—->任意Object的转换 (如果目标类型和源类型兼容，直接返回源对象；否则返回S数组的第一个元素并进行类型转换) |
| ObjectToCollectionConverter | Object—–>单元素集合 |
第三组：默认（fallback）转换器：之前的转换器不能转换时调用
| ObjectToObjectConverter | Object（S）—–>Object（T）(首先尝试valueOf进行转换、没有则尝试new 构造器(S)) |
| IdToEntityConverter | Id(S)—–>Entity(T) 查找并调用public static T findEntityName获取目标对象，EntityName是T类型的简单类型 |
| FallbackObjectToStringConverter | Object—–>String ConversionService 作为恢复使用，即其他转换器不能转换时调用（执行对象的toString()方法） |
2 自定义转换器
ConversionService是Spring类型转换体系的核心接口，可以利用 ConversionServiceFactoryBean在Spring的IOC容器中定义一个ConversionService，Spring 将自动识别出IOC 容器中的ConversionService，并在 Bean属性配置及Spring MVC 处理方法入参绑定等场合使用它进行数据的转换。可通过ConversionServiceFactoryBean的converters属性注册自定义的类型转换器口。
<!-- 配置 ConversionService -->
<bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
<property name="converters">
<set>
<ref bean="addressConverter"/>
</set>
</property>
</bean>
<mvc:annotation-driven conversion-service=“conversionService”/> 会将自定义的 ConversionService 注册到Spring MVC 的上下文中。
<!-- 开启注解 --> <mvc:annotation-driven conversion-service="conversionService" />
2.1 SpringMVC支持的转化器
Spring 定义了 3 种类型的转换器接口，实现任意一个转换器接口都可以作为自定义转换器注册到ConversionServiceFactroyBean中：
- Converter<S,T>：用于转换S类型到T类型，此接口的实现必须是线程安全的且可以被共享。(后面有此转化器的实例)
- GenericConverter 和ConditionalGenericConverter：GenericConverter 接口实现能在多种类型之间进行转换，ConditionalGenericConverter 是有条件的在多种类型之间进行转换。
- ConverterFactory：工厂模式的实现，用于选择将一种S源类型转换为R类型的子类型T的转换器的工厂接口。（例如将 String 转换为 Number 及 Number 子类（Integer、Long、Double 等）对象）
2.2 自定义转换器实例
需求：前台以"江苏-南京"的形式传给后台，后台以实体类接收此信息：
1. 利用Converter<S,T>实现自定义转换器，首先明确S，T的类型，此处自定义转化器将String类型转换为实体类AddressVo，因此S为String，T为AddressVo，先看下实体类：
public class AddressVo {
private String province;
private String city;
public AddressVo(String province, String city) {
this.province = province;
this.city = city;
}
public String getProvince() {
return province;
}
public void setProvince(String province) {
this.province = province;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
@Override
public String toString() {
return "Address [province=" + province + ", city=" + city + "]";
}
}
2. 然后是转换器：
@Component
public class AddressConverter implements Converter<String, AddressVo> {
@Override
public AddressVo convert(String source) {
if(source != null){
String [] vals = source.split("-");
// 江苏-南京
if(vals != null && vals.length == 2){
String province = vals[0];
String city = vals[1];
return new AddressVo(province, city);
}
}
return null;
}
}
3. 将此转化器注册到ConversionService，正如前面所示。
自定义转化器就以上三步，比较简单。