【原创】遨游springmvc之Converter

时间 2019-11-08

标签原创遨游 springmvc converter 栏目 Spring 繁體版

原文原文链接

1.前言

在前一篇WebDataBinder中讲述了了一个PropertyEditor,它经过setAsText知足了字符串到指定类型的转换，可是它实现不了从任意类型转换到目标类型，因此在spring3.x以后引入了Converter，它实现了上述需求的转换。java

2.原理

2.1 工做原理图

2.2 原理介绍

①：类型转换：内部的ConversionService会根据S源类型/T目标类型自动选择相应的Converter SPI进行类型转换，并且是强类型的，能在任意类型数据之间进行转换；
②：数据验证：支持JSR-303验证框架，如将@Valid放在须要验证的目标类型上便可；
③：格式化显示：其实就是任意目标类型---->String的转换，彻底可使用Converter SPI完成。

Spring为了更好的诠释格式化/解析功能提供了Formatter SPI，支持根据Locale信息进行格式化/解析，并且该套SPI能够支持字段/参数级别的细粒度格式化/解析，流程以下：
①：类型解析（转换）：String---->T类型目标对象的解析，和PropertyEditor相似；
②：数据验证：支持JSR-303验证框架，如将@Valid放在须要验证的目标类型上便可；
③：格式化显示：任意目标类型---->String的转换，和PropertyEditor相似。web

3.Converter

3.1 接口介绍

public interface Converter<S, T> {

	/**
	 * Convert the source object of type {@code S} to target type {@code T}.
	 * @param source the source object to convert, which must be an instance of {@code S} (never {@code null})
	 * @return the converted object, which must be an instance of {@code T} (potentially {@code null})
	 * @throws IllegalArgumentException if the source cannot be converted to the desired target type
	 */
	T convert(S source);

}

Converter只提供了一个convert方法，其中S表明源类型，T表明目标类型spring

3.2 实例

3.2.1 TelephoneConverter

public class TelephoneConverter implements Converter<String, Telephone> {
    @Override
    public Telephone convert(String source) {
        if (source.matches("\\d{3,4}-\\d{7,8}")) {
            String[] telephoneArray = source.split("-");
            return new Telephone(telephoneArray[0], telephoneArray[1]);
        }
        return null;
    }
}

3.2.2 控制器

@RequestMapping (value="/converter/1",method= RequestMethod.GET)
    @ResponseBody
    public Person demo1(Person p) {
        return p;
    }

3.2.3 配置

若是咱们同时配置了PropertyEditor和Converter，spring默认先做用PropertyEditor，再做用Converter，可是最好不要这样子2个都上。数组

<mvc:annotation-driven conversion-service="myConverterService"></mvc:annotation-driven>
    
    <bean id="myConverterService" class="org.springframework.context.support.ConversionServiceFactoryBean">
        <property name="converters">
            <set>
                <bean class="com.kings.template.mvc.TelephoneConverter"></bean>
            </set>
        </property>
    </bean>

3.3 spring提供的转换器

在spring-core下的org.springframework.core.convert包下提供了许多内置的convertermvc

标量转换器app

转换器类	功能
StringToBooleanConverter	String----->Boolean true: true/on/yes/1；false: false/off/no/0
ObjectToStringConverter	Object----->String 调用toString方法转换
StringToNumberConverterFactory	String----->Number（如Integer、Long等）
NumberToNumberConverterFactory	Number子类型(Integer、Long、Double等)-----> Number子类型(Integer、Long、Double等)
StringToCharacterConverter	String----->java.lang.Character 取字符串第一个字符
NumberToCharacterConverter	Number子类型(Integer、Long、Double等)-----> java.lang.Character
CharacterToNumberFactory	java.lang.Character ----->Number子类型(Integer、Long、Double等)
StringToEnumConverterFactory	String----->enum类型
EnumToStringConverter	enum类型----->String 返回enum对象的name()值
StringToLocaleConverter	String----->java.util.Local
PropertiesToStringConverter	java.util.Properties----->String
StringToPropertiesConverter	String----->java.util.Properties

集合、数组相关转换器框架

转换器类	功能
ArrayToCollectionConverter	任意S数组---->任意T集合（List、Set）
CollectionToArrayConverter	任意T集合（List、Set）---->任意S数组
ArrayToArrayConverter	任意S数组<---->任意T数组
CollectionToCollectionConverter	任意T集合（List、Set）<---->任意T集合（List、Set）
MapToMapConverter	Map<---->Map之间的转换
ArrayToStringConverter	任意S数组---->String类型
StringToArrayConverter	String----->数组默认经过“,”分割，且去除字符串的两边空格(trim)
ArrayToObjectConverter	任意S数组---->任意Object的转换
ObjectToArrayConverter	Object----->单元素数组
CollectionToStringConverter	任意T集合（List、Set）---->String类型
StringToCollectionConverter	String----->集合（List、Set）
CollectionToObjectConverter	任意T集合---->任意Object的转换
ObjectToCollectionConverter	Object----->单元素集合

默认（fallback）转换器以前的转换器不能转换时调用ide

转换器类	功能
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)
FallbackObjectToStringConverter	Object----->String 最终转换方法，调用toString()

4.ConverterFactory

需求：须要将一个类中的String转换成Enum，并且是有多个，如：工具

@Data
public class Person {
    private String name;
    
    private Telephone telephone;
    
    private Sex sex;
    
    private Race race;

那么咱们再经过写一个SexConverter和RaceConverter是能够实现，可是咱们固然有更懒的实现方法。测试

经过工厂方法，抽象出将String转化成枚举的过程，省去每次再去定义一个方法去实现相似业务的转化器。

4.1 接口介绍

public interface ConverterFactory<S, R> {

	/**
	 * Get the converter to convert from S to target type T, where T is also an instance of R.
	 * @param <T> the target type
	 * @param targetType the target type to convert to
	 * @return A converter from S to T
	 */
	<T extends R> Converter<S, T> getConverter(Class<T> targetType);

}

4.2 实例

咱们来一个覆盖spring默认的StringConvertorFactory

4.2.1 工具类

/**
 * <p class="detail">
 * 功能:将枚举的数字转化成枚举列
 * </p>
 * @param <T> the type parameter
 *
 * @author Kings
 * @ClassName Convent tag num 2 emum list.
 * @Version V1.0.
 * @date 2016.03.24 18:46:21
 */
public class ConventNum2Emum<T extends Enum<T>> {
    
    /**
     * <p class="detail">
     * 功能:与或转化
     * </p>
     * @param status   :状态
     * @param enumType :枚举类型
     *
     * @return list
     * @author Kings
     * @date 2016.03.24 18:46:21
     */
    public List<T> convent(Long status,Class enumType) {
        List<T> tags = new ArrayList<T>();
        if(status != null){
            Field[] fields = enumType.getFields();
            for (Field f : fields) {
                Enum<T> e = Enum.valueOf(enumType,f.getName());
                Long eValue = Long.parseLong(e.toString());
                if((eValue & status) == eValue){
                    tags.add((T) e);
                }
            }
        }
        return tags;
    }
    
    /**
     * <p class="detail">
     * 功能:非与或转换
     * </p>
     * @param status   :状态
     * @param enumType :枚举类型
     *
     * @return list
     * @author Kings
     * @date 2016.06.15 10:46:35
     */
    public T conventNormal(Long status, Class enumType) {
        if (status != null) {
            Field[] fields = enumType.getFields();
            for (Field f : fields) {
                Enum<T> e = Enum.valueOf(enumType, f.getName());
                Long eVlue = Long.parseLong(e.toString());
                if (status.longValue() == eVlue.longValue()) {
                    return (T) e;
                }
            }
        }
        return null;
    }
    
}

4.2.1 自定义ConverterFactory

public final class MyStringConverterFactory implements ConverterFactory<String, Enum> {
    
    @Override
    public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
        return new LongToEnum(targetType);
    }
    
    private class LongToEnum<T extends Enum> implements Converter<String, T> {
        
        private final Class<T> enumType;
        
        public LongToEnum(Class<T> enumType) {
            this.enumType = enumType;
        }
        
        @Override
        public T convert(String source) {
            return (T) new ConventNum2Emum().conventNormal(Long.parseLong(source), enumType);
        }
    }
}

4.2.2 配置

<mvc:annotation-driven conversion-service="myConverterService"></mvc:annotation-driven>
    
    <bean id="myConverterService" class="org.springframework.context.support.ConversionServiceFactoryBean">
        <property name="converters">
            <set>
                <bean class="com.kings.template.mvc.MyStringConverterFactory"></bean>
            </set>
        </property>
    </bean>

5. ConditionalGenericConverter

ConditionalGenericConverter继承了2个重要的接口GenericConverter和ConditionConverter

5.1 接口说明

GenericConverter

/**
	 * Return the source and target types that this converter can convert between.
	 * <p>Each entry is a convertible source-to-target type pair.
	 * <p>For {@link ConditionalConverter conditional converters} this method may return
	 * {@code null} to indicate all source-to-target pairs should be considered.
	 */
	Set<ConvertiblePair> getConvertibleTypes();

	/**
	 * Convert the source object to the targetType described by the {@code TypeDescriptor}.
	 * @param source the source object to convert (may be {@code null})
	 * @param sourceType the type descriptor of the field we are converting from
	 * @param targetType the type descriptor of the field we are converting to
	 * @return the converted object
	 */
	Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

GenericConverter接口是全部的Converter接口中最灵活也是最复杂的一个类型转换接口。像咱们以前介绍的Converter接口只支持从一个原类型转换为一个目标类型；ConverterFactory接口只支持从一个原类型转换为一个目标类型对应的子类型；而GenericConverter接口支持在多个不一样的原类型和目标类型之间进行转换，这也就是GenericConverter接口灵活和复杂的地方。GenericConverter接口中一共定义了两个方法，getConvertibleTypes()和convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType)。getConvertibleTypes方法用于返回这个GenericConverter可以转换的原类型和目标类型的这么一个组合；convert方法则是用于进行类型转换的，咱们能够在这个方法里面实现咱们本身的转换逻辑。

5.2 实例

5.2.1 实体类

为了方便我就直接用User和Person

@Data
public class Person {
    private String name;
    
    private Telephone telephone;
    
    private Sex sex;
    
    private Race race;
    
    private User u;
}

5.2.1 转化器

public class MyGenericConverter implements GenericConverter {
    
    @Autowired
    private UserService userService;
    @Override
    public Set<ConvertiblePair> getConvertibleTypes() {
        Set<ConvertiblePair> pairs = new HashSet<ConvertiblePair>();
        pairs.add(new ConvertiblePair(String.class, User.class));
        //受web层的request.getParamater()的影响，在web层Long做用不了
        pairs.add(new ConvertiblePair(Long.class, User.class));
        return pairs;
    }
    
    @Override
    public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
        if (source == null) {
            return null;
        }
        if(sourceType.getType() == Long.class){
           return userService.selectByPrimaryKey(source); 
        } else if(sourceType.getType() == String.class){
            User u4q = new User();
            u4q.setName(source+"");
            return userService.selectOne(u4q);
        }
        return null;
    }
    
    public void setUserService(UserService userService) {
        this.userService = userService;
    }
}

5.2.3 控制器

@RequestMapping (value="/converter/2",method= RequestMethod.GET)
    @ResponseBody
    public Person demo2(Person p) {
        return p;
    }

直接访问:http://localhost:8080/kingstemplate/converter/2?u=ws

获得Json结果以下：

{"name":null,"telephone":null,"sex":null,"race":null,"u":{"id":1,"name":"ws","age":26}}

5.2.4 非web项目

在5.2.3里面咱们用不了Long转User，由于受web项目的影响，此次咱们来个非web项目的

首先注册一个DefaultConversionService，由于它里面有个addConverter方便我么本身加入容器中的converter

<bean id="defaultConversionService" class="org.springframework.core.convert.support.DefaultConversionService"></bean>

再上测试demo

public class ConverterDemo {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("config/spring.xml");
        
        UserService userService = (UserService) context.getBean("userServiceImpl");
        MyGenericConverter myGenericConverter = new MyGenericConverter();
        //注入接口
        myGenericConverter.setUserService(userService);
        
        DefaultConversionService defaultConversionService = (DefaultConversionService) context.getBean("defaultConversionService");
        defaultConversionService.addConverter(myGenericConverter);
        User u = defaultConversionService.convert(1L, User.class);//Long->User
        System.out.println(u.getAge());//age输出不告诉你
        User u1 = defaultConversionService.convert("ws", User.class);//String->User
        System.out.println(u1.getAge());//age输出不告诉你
    }
}

6.ConversionService

通常的ConversionService最底层都会继承ConverterRegistry和ConversionService

6.1 ConverterRegistry

6.1.1 接口说明

类型转换器注册支持，能够注册/删除相应的类型转换器

注册的时候添加了：Converter、GenericConverter、ConverterFactory以及移除某些类型的转换

void addConverter(Converter<?, ?> converter);

	/**
	 * Add a plain converter to this registry.
	 * The convertible source/target type pair is specified explicitly.
	 * <p>Allows for a Converter to be reused for multiple distinct pairs without
	 * having to create a Converter class for each pair.
	 * @since 3.1
	 */
	<S, T> void addConverter(Class<S> sourceType, Class<T> targetType, Converter<? super S, ? extends T> converter);

	/**
	 * Add a generic converter to this registry.
	 */
	void addConverter(GenericConverter converter);

	/**
	 * Add a ranged converter factory to this registry.
	 * The convertible source/target type pair is derived from the ConverterFactory's parameterized types.
	 * @throws IllegalArgumentException if the parameterized types could not be resolved.
	 */
	void addConverterFactory(ConverterFactory<?, ?> converterFactory);

	/**
	 * Remove any converters from sourceType to targetType.
	 * @param sourceType the source type
	 * @param targetType the target type
	 */
	void removeConvertible(Class<?> sourceType, Class<?> targetType);

6.2 ConversionService

6.2.1 接口说明

提供运行期类型转换的支持

/**
	 * Return {@code true} if objects of {@code sourceType} can be converted to the {@code targetType}.
	 * <p>If this method returns {@code true}, it means {@link #convert(Object, Class)} is capable
	 * of converting an instance of {@code sourceType} to {@code targetType}.
	 * <p>Special note on collections, arrays, and maps types:
	 * For conversion between collection, array, and map types, this method will return {@code true}
	 * even though a convert invocation may still generate a {@link ConversionException} if the
	 * underlying elements are not convertible. Callers are expected to handle this exceptional case
	 * when working with collections and maps.
	 * @param sourceType the source type to convert from (may be {@code null} if source is {@code null})
	 * @param targetType the target type to convert to (required)
	 * @return {@code true} if a conversion can be performed, {@code false} if not
	 * @throws IllegalArgumentException if {@code targetType} is {@code null}
	 */
	boolean canConvert(Class<?> sourceType, Class<?> targetType);

	/**
	 * Return {@code true} if objects of {@code sourceType} can be converted to the {@code targetType}.
	 * The TypeDescriptors provide additional context about the source and target locations
	 * where conversion would occur, often object fields or property locations.
	 * <p>If this method returns {@code true}, it means {@link #convert(Object, TypeDescriptor, TypeDescriptor)}
	 * is capable of converting an instance of {@code sourceType} to {@code targetType}.
	 * <p>Special note on collections, arrays, and maps types:
	 * For conversion between collection, array, and map types, this method will return {@code true}
	 * even though a convert invocation may still generate a {@link ConversionException} if the
	 * underlying elements are not convertible. Callers are expected to handle this exceptional case
	 * when working with collections and maps.
	 * @param sourceType context about the source type to convert from
	 * (may be {@code null} if source is {@code null})
	 * @param targetType context about the target type to convert to (required)
	 * @return {@code true} if a conversion can be performed between the source and target types,
	 * {@code false} if not
	 * @throws IllegalArgumentException if {@code targetType} is {@code null}
	 */
	boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);

	/**
	 * Convert the given {@code source} to the specified {@code targetType}.
	 * @param source the source object to convert (may be {@code null})
	 * @param targetType the target type to convert to (required)
	 * @return the converted object, an instance of targetType
	 * @throws ConversionException if a conversion exception occurred
	 * @throws IllegalArgumentException if targetType is {@code null}
	 */
	<T> T convert(Object source, Class<T> targetType);

	/**
	 * Convert the given {@code source} to the specified {@code targetType}.
	 * The TypeDescriptors provide additional context about the source and target locations
	 * where conversion will occur, often object fields or property locations.
	 * @param source the source object to convert (may be {@code null})
	 * @param sourceType context about the source type to convert from
	 * (may be {@code null} if source is {@code null})
	 * @param targetType context about the target type to convert to (required)
	 * @return the converted object, an instance of {@link TypeDescriptor#getObjectType() targetType}
	 * @throws ConversionException if a conversion exception occurred
	 * @throws IllegalArgumentException if targetType is {@code null},
	 * or {@code sourceType} is {@code null} but source is not {@code null}
	 */
	Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);

Spring提供了两个默认实现（其都实现了ConverterRegistry、ConversionService接口）:
DefaultConversionService:默认的类型转换服务实现；

DefaultFormattingConversionService：带数据格式化支持的类型转换服务实现，通常使用该服务实现便可。