Spring 学习记录3 ConversionService

ConversionService与Environment的关系

通过之前的学习(Spring 学习记录2 Environment),我已经Environment主要是负责解析properties和profile...但是它虽然实现了相关的接口,但是具体工作并不是由它本身处理,而是委托了其他的类来帮忙..properties相关的接口方法最终主要是通过PropertySourcesPropertyResolver这个类来处理的..(它们实现了相同的接口)

在通过Environment使用properties相关的方法中,有一些方法是带泛型参数的,比如

org.springframework.core.env.PropertyResolver

    /**
     * Return the property value associated with the given key, or {@code null}
     * if the key cannot be resolved.
     * @param key the property name to resolve
     * @param targetType the expected type of the property value
     * @see #getRequiredProperty(String, Class)
     */
    <T> T getProperty(String key, Class<T> targetType);

得到properties以后肯定要通过一些类型转换,才能从String类型得到T类型.那么这个类型转换.其实用的就是ConversionService以及其相关的一套类.

properties文件中的所有值都是String类型的,而java内存里都是对象.所以需要一些工具将String(或者其他类型)转化成我们需要的java类型..(ConversionService是一套通用的转化方案,并不只是在这里用到,任何需要类型转化的地方都可以用)

ConversionService

实验1

properties文件

list=a,b,c,1,2,3

/**
 * 测试ConversionService
 */
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:test-application-context.xml")
public class PropertySourcesPropertyResolverTest  implements EnvironmentAware {

    StandardEnvironment standardEnvironment;

    @Test
    public void testPropertySourcesPropertyResolver() {
        List<String> list = standardEnvironment.getProperty("list", List.class);
        System.out.println(list); // [a, b, c, 1, 2, 3]
    }

    @Override
    public void setEnvironment(Environment environment) {
        standardEnvironment = (StandardEnvironment) environment;
    }
}

通过Environment的相关properties方法获取属性值并转化成List对象.

追踪断点发现:

PropertySourcesPropertyResolver内部得到属性值a,b,c,1,2,3以后通过conversionService去convert成List类型.

所以让我们来研究下ConversionService吧

ConversionService的结构

public abstract class AbstractPropertyResolver implements ConfigurablePropertyResolver {

    protected final Log logger = LogFactory.getLog(getClass());

    protected ConfigurableConversionService conversionService = new DefaultConversionService();

............

.............

}

conversionService是定义在AbstractPropertyResolver中的.也就是PropertySourcesPropertyResolver的父抽象类中.

/*
 * Copyright 2002-2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert;

/**
 * A service interface for type conversion. This is the entry point into the convert system.
 * Call {@link #convert(Object, Class)} to perform a thread-safe type conversion using this system.
 *
 * @author Keith Donald
 * @author Phillip Webb
 * @since 3.0
 */
public interface ConversionService {

    /**
     * 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 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 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 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);

}

查看ConversionService接口里的方法得知,这个类主要就是判断是否能够类型转化,可以的话就转化.

/*
 * Copyright 2002-2009 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.converter;

/**
 * For registering converters with a type conversion system.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @since 3.0
 */
public interface ConverterRegistry {

    /**
     * Add a plain converter to this registry.
     * The convertible sourceType/targetType pair is derived from the Converter's parameterized types.
     * @throws IllegalArgumentException if the parameterized types could not be resolved
     */
    void addConverter(Converter<?, ?> converter);

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

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

    /**
     * Add a ranged converter factory to this registry.
     * The convertible sourceType/rangeType 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);

}

查看ConverterRegistry接口里的方法得知,这个类主要就是增加Converter用的.

那么既实现了ConversionService又实现了ConverterRegistry的DefaultConversionService用处就是

1.允许添加类型转化器Converter.

2.允许调用相关方法进行类型转化.

/*
 * Copyright 2002-2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.support;

import java.util.Locale;
import java.util.UUID;

import org.springframework.core.convert.ConversionService;
import org.springframework.core.convert.converter.ConverterRegistry;
import org.springframework.util.ClassUtils;

/**
 * A specialization of {@link GenericConversionService} configured by default with
 * converters appropriate for most environments.
 *
 * <p>Designed for direct instantiation but also exposes the static
 * {@link #addDefaultConverters(ConverterRegistry)} utility method for ad hoc use against any
 * {@code ConverterRegistry} instance.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.1
 */
public class DefaultConversionService extends GenericConversionService {

    /** Java 8's java.util.Optional class available? */
    private static final boolean javaUtilOptionalClassAvailable =
            ClassUtils.isPresent("java.util.Optional", DefaultConversionService.class.getClassLoader());

    /** Java 8's java.time package available? */
    private static final boolean jsr310Available =
            ClassUtils.isPresent("java.time.ZoneId", DefaultConversionService.class.getClassLoader());


    /**
     * Create a new {@code DefaultConversionService} with the set of
     * {@linkplain DefaultConversionService#addDefaultConverters(ConverterRegistry) default converters}.
     */
    public DefaultConversionService() {
        addDefaultConverters(this);
    }


    // static utility methods

    /**
     * Add converters appropriate for most environments.
     * @param converterRegistry the registry of converters to add to (must also be castable to ConversionService,
     * e.g. being a {@link ConfigurableConversionService})
     * @throws ClassCastException if the given ConverterRegistry could not be cast to a ConversionService
     */
    public static void addDefaultConverters(ConverterRegistry converterRegistry) {
        addScalarConverters(converterRegistry);
        addCollectionConverters(converterRegistry);

        converterRegistry.addConverter(new ByteBufferConverter((ConversionService) converterRegistry));
        if (jsr310Available) {
            Jsr310ConverterRegistrar.registerZoneIdConverters(converterRegistry);
        }

        converterRegistry.addConverter(new ObjectToObjectConverter());
        converterRegistry.addConverter(new IdToEntityConverter((ConversionService) converterRegistry));
        converterRegistry.addConverter(new FallbackObjectToStringConverter());
        if (javaUtilOptionalClassAvailable) {
            converterRegistry.addConverter(new ObjectToOptionalConverter((ConversionService) converterRegistry));
        }
    }

    // internal helpers

    private static void addScalarConverters(ConverterRegistry converterRegistry) {
        converterRegistry.addConverterFactory(new NumberToNumberConverterFactory());

        converterRegistry.addConverterFactory(new StringToNumberConverterFactory());
        converterRegistry.addConverter(Number.class, String.class, new ObjectToStringConverter());

        converterRegistry.addConverter(new StringToCharacterConverter());
        converterRegistry.addConverter(Character.class, String.class, new ObjectToStringConverter());

        converterRegistry.addConverter(new NumberToCharacterConverter());
        converterRegistry.addConverterFactory(new CharacterToNumberFactory());

        converterRegistry.addConverter(new StringToBooleanConverter());
        converterRegistry.addConverter(Boolean.class, String.class, new ObjectToStringConverter());

        converterRegistry.addConverterFactory(new StringToEnumConverterFactory());
        converterRegistry.addConverter(Enum.class, String.class,
                new EnumToStringConverter((ConversionService) converterRegistry));

        converterRegistry.addConverter(new StringToLocaleConverter());
        converterRegistry.addConverter(Locale.class, String.class, new ObjectToStringConverter());

        converterRegistry.addConverter(new StringToPropertiesConverter());
        converterRegistry.addConverter(new PropertiesToStringConverter());

        converterRegistry.addConverter(new StringToUUIDConverter());
        converterRegistry.addConverter(UUID.class, String.class, new ObjectToStringConverter());
    }

    private static void addCollectionConverters(ConverterRegistry converterRegistry) {
        ConversionService conversionService = (ConversionService) converterRegistry;

        converterRegistry.addConverter(new ArrayToCollectionConverter(conversionService));
        converterRegistry.addConverter(new CollectionToArrayConverter(conversionService));

        converterRegistry.addConverter(new ArrayToArrayConverter(conversionService));
        converterRegistry.addConverter(new CollectionToCollectionConverter(conversionService));
        converterRegistry.addConverter(new MapToMapConverter(conversionService));

        converterRegistry.addConverter(new ArrayToStringConverter(conversionService));
        converterRegistry.addConverter(new StringToArrayConverter(conversionService));

        converterRegistry.addConverter(new ArrayToObjectConverter(conversionService));
        converterRegistry.addConverter(new ObjectToArrayConverter(conversionService));

        converterRegistry.addConverter(new CollectionToStringConverter(conversionService));
        converterRegistry.addConverter(new StringToCollectionConverter(conversionService));

        converterRegistry.addConverter(new CollectionToObjectConverter(conversionService));
        converterRegistry.addConverter(new ObjectToCollectionConverter(conversionService));
    }


    /**
     * Inner class to avoid a hard-coded dependency on Java 8's {@code java.time} package.
     */
    private static final class Jsr310ConverterRegistrar {

        public static void registerZoneIdConverters(ConverterRegistry converterRegistry) {
            converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
            converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());
        }
    }

}

查看DefaultConversionService的代码得知,它的构造方法里添加了一堆Converter,这些converter是Spring已经帮助我们实现的.通过这些Converter我们可以进行很多通用类型的转化.比如之前的string->list的类型转化.

Converter接口

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.converter;

/**
 * A converter converts a source object of type S to a target of type T.
 * Implementations of this interface are thread-safe and can be shared.
 *
 * <p>Implementations may additionally implement {@link ConditionalConverter}.
 *
 * @author Keith Donald
 * @since 3.0
 * @param <S> The source type
 * @param <T> The target type
 */
public interface Converter<S, T> {

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

}

Converter接口很简单,就是把S类型转化成T类型.

实验2

利用ConversionService进行类型转化

    @Test
    public void testConversionService1() {
        String s = conversionService.convert(false, String.class);
        System.out.println(s); // false
        Boolean b = conversionService.convert("true", Boolean.class);
        System.out.println(b); // true
    }

    @Before
    public void setup() {
        conversionService = standardEnvironment.getConversionService();
    }

boolean -> string 用到的是ObjectToStringConverter

string -> boolean 用到的是StringToBooleanConverter

这些都是内置的.同时我们也可以发现1个converter也可以进行N种转化.因为ObjectToStringConverter不止可以转化String.任何类型转化成String都可以用这个Converter..内部是直接调用toString()方法...

ConverterFactory和GenericConverter

Converter接口在绝大多数情况下可能都是专门进行S->T类型的转化.也就是1对1的.Spring还提供了一些其他接口来帮我们进行类型转化.比如ConverterFactory和GenericConverter

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.converter;

/**
 * A factory for "ranged" converters that can convert objects from S to subtypes of R.
 *
 * <p>Implementations may additionally implement {@link ConditionalConverter}.
 *
 * @author Keith Donald
 * @since 3.0
 * @see ConditionalConverter
 * @param <S> the source type converters created by this factory can convert from
 * @param <R> the target range (or base) type converters created by this factory can convert to;
 * for example {@link Number} for a set of number subtypes.
 */
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);

}

看源代码可以发现ConverterFactory更像是1对N的转化.

可以从S->各种R的各种子类型T..因为平时处理业务上面的各种转化基本上都是很特殊的1:1的专门的converter去转化.所以可能ConverterFactory和GenericConverter不太用得到.因此主要看看Spring是怎么用这些Converter的吧.

实验3

    /**
     * 测试ConverterFactory StringToNumberConverterFactory
     */
    @Test
    public void testConversionService2() {
        double d = conversionService.convert("1.2", double.class);
        System.out.println(d); //1.2

        int i = conversionService.convert("2", int.class);
        System.out.println(i); //2

        Byte b = conversionService.convert("0x10", Byte.class);
        System.out.println(Integer.toBinaryString(b)); //10000
    }

这里用到了StringToNumberConverterFactory把String转化成了Number的各个子类型.

        @Override
        public T convert(String source) {
            if (source.length() == 0) {
                return null;
            }
            return NumberUtils.parseNumber(source, this.targetType);
        }

StringToNumberConverterFactory通过NumberUtils的static方法进行转化

    public static <T extends Number> T parseNumber(String text, Class<T> targetClass) {
        Assert.notNull(text, "Text must not be null");
        Assert.notNull(targetClass, "Target class must not be null");
        String trimmed = StringUtils.trimAllWhitespace(text);

        if (targetClass.equals(Byte.class)) {
            return (T) (isHexNumber(trimmed) ? Byte.decode(trimmed) : Byte.valueOf(trimmed));
        }
        else if (targetClass.equals(Short.class)) {
            return (T) (isHexNumber(trimmed) ? Short.decode(trimmed) : Short.valueOf(trimmed));
        }
        else if (targetClass.equals(Integer.class)) {
            return (T) (isHexNumber(trimmed) ? Integer.decode(trimmed) : Integer.valueOf(trimmed));
        }
        else if (targetClass.equals(Long.class)) {
            return (T) (isHexNumber(trimmed) ? Long.decode(trimmed) : Long.valueOf(trimmed));
        }
        else if (targetClass.equals(BigInteger.class)) {
            return (T) (isHexNumber(trimmed) ? decodeBigInteger(trimmed) : new BigInteger(trimmed));
        }
        else if (targetClass.equals(Float.class)) {
            return (T) Float.valueOf(trimmed);
        }
        else if (targetClass.equals(Double.class)) {
            return (T) Double.valueOf(trimmed);
        }
        else if (targetClass.equals(BigDecimal.class) || targetClass.equals(Number.class)) {
            return (T) new BigDecimal(trimmed);
        }
        else {
            throw new IllegalArgumentException(
                    "Cannot convert String [" + text + "] to target class [" + targetClass.getName() + "]");
        }
    }

parseNumber方法里面各种ifelse判断需要的是哪种类型的Number然后再转化.

同理,GenericConverter应该是N:N的转化

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.converter;

import java.util.Set;

import org.springframework.core.convert.TypeDescriptor;
import org.springframework.util.Assert;

/**
 * Generic converter interface for converting between two or more types.
 *
 * <p>This is the most flexible of the Converter SPI interfaces, but also the most complex.
 * It is flexible in that a GenericConverter may support converting between multiple source/target
 * type pairs (see {@link #getConvertibleTypes()}. In addition, GenericConverter implementations
 * have access to source/target {@link TypeDescriptor field context} during the type conversion
 * process. This allows for resolving source and target field metadata such as annotations and
 * generics information, which can be used influence the conversion logic.
 *
 * <p>This interface should generally not be used when the simpler {@link Converter} or
 * {@link ConverterFactory} interfaces are sufficient.
 *
 * <p>Implementations may additionally implement {@link ConditionalConverter}.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @since 3.0
 * @see TypeDescriptor
 * @see Converter
 * @see ConverterFactory
 * @see ConditionalConverter
 */
public interface GenericConverter {

    /**
     * Return the source and target types which this converter can convert between. 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 to the targetType described by the TypeDescriptor.
     * @param source the source object to convert (may be 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);


    /**
     * Holder for a source-to-target class pair.
     */
    public static final class ConvertiblePair {

        private final Class<?> sourceType;

        private final Class<?> targetType;

        /**
         * Create a new source-to-target pair.
         * @param sourceType the source type
         * @param targetType the target type
         */
        public ConvertiblePair(Class<?> sourceType, Class<?> targetType) {
            Assert.notNull(sourceType, "Source type must not be null");
            Assert.notNull(targetType, "Target type must not be null");
            this.sourceType = sourceType;
            this.targetType = targetType;
        }

        public Class<?> getSourceType() {
            return this.sourceType;
        }

        public Class<?> getTargetType() {
            return this.targetType;
        }

        @Override
        public boolean equals(Object other) {
            if (this == other) {
                return true;
            }
            if (other == null || other.getClass() != ConvertiblePair.class) {
                return false;
            }
            ConvertiblePair otherPair = (ConvertiblePair) other;
            return (this.sourceType.equals(otherPair.sourceType) && this.targetType.equals(otherPair.targetType));
        }

        @Override
        public int hashCode() {
            return (this.sourceType.hashCode() * 31 + this.targetType.hashCode());
        }

        @Override
        public String toString() {
            return (this.sourceType.getName() + " -> " + this.targetType.getName());
        }
    }

}

1个GenericConverter支持转化的所有类型都写在了属性Set<ConvertiblePair>内.

实验四

    /**
     * 测试GenericConverter CollectionToCollectionConverter
     */
    @Test
    public void testConversionService3() {
        List<Integer> list1 = Arrays.asList(1, 2, 3, 4, 5);
        Set<String> set1 = conversionService.convert(list1, Set.class); // Set<Integer>
        System.out.println(set1); // [1, 2, 3, 4, 5]
        System.out.println(set1.toArray()[0].getClass()); // class java.lang.Integer
    }

这里用到了CollectionToCollectionConverter

/*
 * Copyright 2002-2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core.convert.support;

import java.util.Collection;
import java.util.Collections;
import java.util.Set;

import org.springframework.core.CollectionFactory;
import org.springframework.core.convert.ConversionService;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.core.convert.converter.ConditionalGenericConverter;

/**
 * Converts from a Collection to another Collection.
 *
 * <p>First, creates a new Collection of the requested targetType with a size equal to the
 * size of the source Collection. Then copies each element in the source collection to the
 * target collection. Will perform an element conversion from the source collection's
 * parameterized type to the target collection's parameterized type if necessary.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @since 3.0
 */
final class CollectionToCollectionConverter implements ConditionalGenericConverter {

    private final ConversionService conversionService;


    public CollectionToCollectionConverter(ConversionService conversionService) {
        this.conversionService = conversionService;
    }


    @Override
    public Set<ConvertiblePair> getConvertibleTypes() {
        return Collections.singleton(new ConvertiblePair(Collection.class, Collection.class));
    }

    @Override
    public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
        return ConversionUtils.canConvertElements(
                sourceType.getElementTypeDescriptor(), targetType.getElementTypeDescriptor(), this.conversionService);
    }

    @Override
    public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
        if (source == null) {
            return null;
        }
        Collection<?> sourceCollection = (Collection<?>) source;

        // Shortcut if possible...
        boolean copyRequired = !targetType.getType().isInstance(source);
        if (!copyRequired && sourceCollection.isEmpty()) {
            return source;
        }
        TypeDescriptor elementDesc = targetType.getElementTypeDescriptor();
        if (elementDesc == null && !copyRequired) {
            return source;
        }

        // At this point, we need a collection copy in any case, even if just for finding out about element copies...
        Collection<Object> target = CollectionFactory.createCollection(targetType.getType(),
                (elementDesc != null ? elementDesc.getType() : null), sourceCollection.size());

        if (elementDesc == null) {
            target.addAll(sourceCollection);
        }
        else {
            for (Object sourceElement : sourceCollection) {
                Object targetElement = this.conversionService.convert(sourceElement,
                        sourceType.elementTypeDescriptor(sourceElement), elementDesc);
                target.add(targetElement);
                if (sourceElement != targetElement) {
                    copyRequired = true;
                }
            }
        }

        return (copyRequired ? target : source);
    }

}

conveter方法中如果source和target的collection是同一种类型的话是不需要转化的,直接返回source就OK了.

然后73行是我觉得很奇怪的一个地方

TypeDescriptor elementDesc = targetType.getElementTypeDescriptor();

因为泛型不同于数组,数组是协变的,泛型是编译期的功能,所以这行代码肯定返回的是null....不知道这里为什么还需要去判断是否是null....ArrayToCollection和其他一些converter都有自己的实现,似乎没走这个converter所以我这里也不是很懂什么时候elementDesc会不是null..看这个样子只有target是数组类才有可能,但是这样的话为什么会出现在CollectionToCollectionConverter中呢?很奇怪....

因为elementDesc是null,所以会进target.addAll(sourceCollection)这行,所以就是简单的把source的所有元素丢到target中了.因为没有对元素进行转化.所以Set之中仍然是Integer类型还不是String.

不过也可以理解.集合中的类型都不知道怎么能把每个元素转化成相应的其他类型呢...这是做不到的...这大概也是泛型的缺陷吧....

后面的操作

3种不同的converter在GenericConversionService类中都有对应的addConverter方法可以添加converter.通过ConverterAdapter或者ConverterFactoryAdapter最后都会转化成GenericConverter我想应该是因为这种converter是最通用的原因吧.

这些适配的GenericConverter会被添加到GenericConversionService的静态内部类Converters中,而不是List或者Map中去.可能是因为查找对应Converter方法的时候比较麻烦.

Converters中有属性converters

Map<ConvertiblePair, ConvertersForPair> converters =
                new LinkedHashMap<ConvertiblePair, ConvertersForPair>(36);

ConvertiblePair是source的class与target的Class的封装

ConvertersForPair内部含有

LinkedList<GenericConverter> converters = new LinkedList<GenericConverter>();

所以是各种genericConvrter的封装.

因为GenericConverter可以转化N种source->target的配对.所以可以对应N个ConvertiblePair,也就是说N个ConvertiblePair对应的ConvertersForPair中的GenericConverter可以是同一个.(虽然我Spring中好像没有看到这样的..基本都是对应1个ConvertiblePair)

同样,多个GenericConverter也可以转化同一个source->target的配对,所以1个ConvertiblePair对应的ConvertersForPair中可以有多个GenericConverter.(虽然Spring中也很少出现我只发现了1个)

这样情况下如果要convert source->target是会使用前面的那个converter的...每次添加converter的时候都是向linkledlist调用addFirst方法..所以后面加的应该会放到最前面.

小结

1.Spring使用ConversionService来convert各种类型.默认提供的是DefaultConversionService.同时它实现了ConverterRegistry接口,所以也可以添加你自定义的converter.

2.Spring提供了3种converter接口,分别是Converter,ConverterFactory和GenericConverter.一般用于1:1, 1:N, N:N的source->target类型转化.

3.在DefaultConversionService内部3种converter都会转化成GenericConverter放到静态内部类Converters中.

4.接口ConvertiblePair是source的class与target的Class的封装.静态内部类ConvertersForPair是多个converter对应的LinkedList的封装..静态内部类Converters中含有1个Map<ConvertiblePair, ConvertersForPair>用来储存所有converter.

1个GenericConverter可以对应N个ConvertiblePair,1个ConvertiblePair对应的ConvertersForPair中也可以有N个GenericConverter.