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

 1 org.springframework.core.env.PropertyResolver
 2 
 3     /**
 4      * Return the property value associated with the given key, or {@code null}
 5      * if the key cannot be resolved.
 6      * @param key the property name to resolve
 7      * @param targetType the expected type of the property value
 8      * @see #getRequiredProperty(String, Class)
 9      */
10     <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
 1 /**
 2  * 测试ConversionService
 3  */
 4 @RunWith(SpringJUnit4ClassRunner.class)
 5 @ContextConfiguration("classpath:test-application-context.xml")
 6 public class PropertySourcesPropertyResolverTest  implements EnvironmentAware {
 7 
 8     StandardEnvironment standardEnvironment;
 9 
10     @Test
11     public void testPropertySourcesPropertyResolver() {
12         List<String> list = standardEnvironment.getProperty("list", List.class);
13         System.out.println(list); // [a, b, c, 1, 2, 3]
14     }
15 
16     @Override
17     public void setEnvironment(Environment environment) {
18         standardEnvironment = (StandardEnvironment) environment;
19     }
20 }

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.

 1 /*
 2  * Copyright 2002-2013 the original author or authors.
 3  *
 4  * Licensed under the Apache License, Version 2.0 (the "License");
 5  * you may not use this file except in compliance with the License.
 6  * You may obtain a copy of the License at
 7  *
 8  *      http://www.apache.org/licenses/LICENSE-2.0
 9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package org.springframework.core.convert;
18 
19 /**
20  * A service interface for type conversion. This is the entry point into the convert system.
21  * Call {@link #convert(Object, Class)} to perform a thread-safe type conversion using this system.
22  *
23  * @author Keith Donald
24  * @author Phillip Webb
25  * @since 3.0
26  */
27 public interface ConversionService {
28 
29     /**
30      * Return {@code true} if objects of {@code sourceType} can be converted to the {@code targetType}.
31      * <p>If this method returns {@code true}, it means {@link #convert(Object, Class)} is capable
32      * of converting an instance of {@code sourceType} to {@code targetType}.
33      * <p>Special note on collections, arrays, and maps types:
34      * For conversion between collection, array, and map types, this method will return {@code true}
35      * even though a convert invocation may still generate a {@link ConversionException} if the
36      * underlying elements are not convertible. Callers are expected to handle this exceptional case
37      * when working with collections and maps.
38      * @param sourceType the source type to convert from (may be {@code null} if source is {@code null})
39      * @param targetType the target type to convert to (required)
40      * @return {@code true} if a conversion can be performed, {@code false} if not
41      * @throws IllegalArgumentException if {@code targetType} is {@code null}
42      */
43     boolean canConvert(Class<?> sourceType, Class<?> targetType);
44 
45     /**
46      * Return {@code true} if objects of {@code sourceType} can be converted to the {@code targetType}.
47      * The TypeDescriptors provide additional context about the source and target locations
48      * where conversion would occur, often object fields or property locations.
49      * <p>If this method returns {@code true}, it means {@link #convert(Object, TypeDescriptor, TypeDescriptor)}
50      * is capable of converting an instance of {@code sourceType} to {@code targetType}.
51      * <p>Special note on collections, arrays, and maps types:
52      * For conversion between collection, array, and map types, this method will return {@code true}
53      * even though a convert invocation may still generate a {@link ConversionException} if the
54      * underlying elements are not convertible. Callers are expected to handle this exceptional case
55      * when working with collections and maps.
56      * @param sourceType context about the source type to convert from
57      * (may be {@code null} if source is {@code null})
58      * @param targetType context about the target type to convert to (required)
59      * @return {@code true} if a conversion can be performed between the source and target types,
60      * {@code false} if not
61      * @throws IllegalArgumentException if {@code targetType} is {@code null}
62      */
63     boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);
64 
65     /**
66      * Convert the given {@code source} to the specified {@code targetType}.
67      * @param source the source object to convert (may be null)
68      * @param targetType the target type to convert to (required)
69      * @return the converted object, an instance of targetType
70      * @throws ConversionException if a conversion exception occurred
71      * @throws IllegalArgumentException if targetType is null
72      */
73     <T> T convert(Object source, Class<T> targetType);
74 
75     /**
76      * Convert the given {@code source} to the specified {@code targetType}.
77      * The TypeDescriptors provide additional context about the source and target locations
78      * where conversion will occur, often object fields or property locations.
79      * @param source the source object to convert (may be null)
80      * @param sourceType context about the source type to convert from
81      * (may be {@code null} if source is {@code null})
82      * @param targetType context about the target type to convert to (required)
83      * @return the converted object, an instance of {@link TypeDescriptor#getObjectType() targetType}
84      * @throws ConversionException if a conversion exception occurred
85      * @throws IllegalArgumentException if targetType is {@code null},
86      * or {@code sourceType} is {@code null} but source is not {@code null}
87      */
88     Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
89 
90 }
View Code

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.

 1 /*
 2  * Copyright 2002-2009 the original author or authors.
 3  *
 4  * Licensed under the Apache License, Version 2.0 (the "License");
 5  * you may not use this file except in compliance with the License.
 6  * You may obtain a copy of the License at
 7  *
 8  *      http://www.apache.org/licenses/LICENSE-2.0
 9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package org.springframework.core.convert.converter;
18 
19 /**
20  * For registering converters with a type conversion system.
21  *
22  * @author Keith Donald
23  * @author Juergen Hoeller
24  * @since 3.0
25  */
26 public interface ConverterRegistry {
27 
28     /**
29      * Add a plain converter to this registry.
30      * The convertible sourceType/targetType pair is derived from the Converter's parameterized types.
31      * @throws IllegalArgumentException if the parameterized types could not be resolved
32      */
33     void addConverter(Converter<?, ?> converter);
34 
35     /**
36      * Add a plain converter to this registry.
37      * The convertible sourceType/targetType pair is specified explicitly.
38      * Allows for a Converter to be reused for multiple distinct pairs without having to create a Converter class for each pair.
39      * @since 3.1
40      */
41     void addConverter(Class<?> sourceType, Class<?> targetType, Converter<?, ?> converter);
42 
43     /**
44      * Add a generic converter to this registry.
45      */
46     void addConverter(GenericConverter converter);
47 
48     /**
49      * Add a ranged converter factory to this registry.
50      * The convertible sourceType/rangeType pair is derived from the ConverterFactory's parameterized types.
51      * @throws IllegalArgumentException if the parameterized types could not be resolved.
52      */
53     void addConverterFactory(ConverterFactory<?, ?> converterFactory);
54 
55     /**
56      * Remove any converters from sourceType to targetType.
57      * @param sourceType the source type
58      * @param targetType the target type
59      */
60     void removeConvertible(Class<?> sourceType, Class<?> targetType);
61 
62 }
View Code

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.

 

  1 /*
  2  * Copyright 2002-2013 the original author or authors.
  3  *
  4  * Licensed under the Apache License, Version 2.0 (the "License");
  5  * you may not use this file except in compliance with the License.
  6  * You may obtain a copy of the License at
  7  *
  8  *      http://www.apache.org/licenses/LICENSE-2.0
  9  *
 10  * Unless required by applicable law or agreed to in writing, software
 11  * distributed under the License is distributed on an "AS IS" BASIS,
 12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 13  * See the License for the specific language governing permissions and
 14  * limitations under the License.
 15  */
 16 
 17 package org.springframework.core.convert.support;
 18 
 19 import java.util.Locale;
 20 import java.util.UUID;
 21 
 22 import org.springframework.core.convert.ConversionService;
 23 import org.springframework.core.convert.converter.ConverterRegistry;
 24 import org.springframework.util.ClassUtils;
 25 
 26 /**
 27  * A specialization of {@link GenericConversionService} configured by default with
 28  * converters appropriate for most environments.
 29  *
 30  * <p>Designed for direct instantiation but also exposes the static
 31  * {@link #addDefaultConverters(ConverterRegistry)} utility method for ad hoc use against any
 32  * {@code ConverterRegistry} instance.
 33  *
 34  * @author Chris Beams
 35  * @author Juergen Hoeller
 36  * @since 3.1
 37  */
 38 public class DefaultConversionService extends GenericConversionService {
 39 
 40     /** Java 8's java.util.Optional class available? */
 41     private static final boolean javaUtilOptionalClassAvailable =
 42             ClassUtils.isPresent("java.util.Optional", DefaultConversionService.class.getClassLoader());
 43 
 44     /** Java 8's java.time package available? */
 45     private static final boolean jsr310Available =
 46             ClassUtils.isPresent("java.time.ZoneId", DefaultConversionService.class.getClassLoader());
 47 
 48 
 49     /**
 50      * Create a new {@code DefaultConversionService} with the set of
 51      * {@linkplain DefaultConversionService#addDefaultConverters(ConverterRegistry) default converters}.
 52      */
 53     public DefaultConversionService() {
 54         addDefaultConverters(this);
 55     }
 56 
 57 
 58     // static utility methods
 59 
 60     /**
 61      * Add converters appropriate for most environments.
 62      * @param converterRegistry the registry of converters to add to (must also be castable to ConversionService,
 63      * e.g. being a {@link ConfigurableConversionService})
 64      * @throws ClassCastException if the given ConverterRegistry could not be cast to a ConversionService
 65      */
 66     public static void addDefaultConverters(ConverterRegistry converterRegistry) {
 67         addScalarConverters(converterRegistry);
 68         addCollectionConverters(converterRegistry);
 69 
 70         converterRegistry.addConverter(new ByteBufferConverter((ConversionService) converterRegistry));
 71         if (jsr310Available) {
 72             Jsr310ConverterRegistrar.registerZoneIdConverters(converterRegistry);
 73         }
 74 
 75         converterRegistry.addConverter(new ObjectToObjectConverter());
 76         converterRegistry.addConverter(new IdToEntityConverter((ConversionService) converterRegistry));
 77         converterRegistry.addConverter(new FallbackObjectToStringConverter());
 78         if (javaUtilOptionalClassAvailable) {
 79             converterRegistry.addConverter(new ObjectToOptionalConverter((ConversionService) converterRegistry));
 80         }
 81     }
 82 
 83     // internal helpers
 84 
 85     private static void addScalarConverters(ConverterRegistry converterRegistry) {
 86         converterRegistry.addConverterFactory(new NumberToNumberConverterFactory());
 87 
 88         converterRegistry.addConverterFactory(new StringToNumberConverterFactory());
 89         converterRegistry.addConverter(Number.class, String.class, new ObjectToStringConverter());
 90 
 91         converterRegistry.addConverter(new StringToCharacterConverter());
 92         converterRegistry.addConverter(Character.class, String.class, new ObjectToStringConverter());
 93 
 94         converterRegistry.addConverter(new NumberToCharacterConverter());
 95         converterRegistry.addConverterFactory(new CharacterToNumberFactory());
 96 
 97         converterRegistry.addConverter(new StringToBooleanConverter());
 98         converterRegistry.addConverter(Boolean.class, String.class, new ObjectToStringConverter());
 99 
100         converterRegistry.addConverterFactory(new StringToEnumConverterFactory());
101         converterRegistry.addConverter(Enum.class, String.class,
102                 new EnumToStringConverter((ConversionService) converterRegistry));
103 
104         converterRegistry.addConverter(new StringToLocaleConverter());
105         converterRegistry.addConverter(Locale.class, String.class, new ObjectToStringConverter());
106 
107         converterRegistry.addConverter(new StringToPropertiesConverter());
108         converterRegistry.addConverter(new PropertiesToStringConverter());
109 
110         converterRegistry.addConverter(new StringToUUIDConverter());
111         converterRegistry.addConverter(UUID.class, String.class, new ObjectToStringConverter());
112     }
113 
114     private static void addCollectionConverters(ConverterRegistry converterRegistry) {
115         ConversionService conversionService = (ConversionService) converterRegistry;
116 
117         converterRegistry.addConverter(new ArrayToCollectionConverter(conversionService));
118         converterRegistry.addConverter(new CollectionToArrayConverter(conversionService));
119 
120         converterRegistry.addConverter(new ArrayToArrayConverter(conversionService));
121         converterRegistry.addConverter(new CollectionToCollectionConverter(conversionService));
122         converterRegistry.addConverter(new MapToMapConverter(conversionService));
123 
124         converterRegistry.addConverter(new ArrayToStringConverter(conversionService));
125         converterRegistry.addConverter(new StringToArrayConverter(conversionService));
126 
127         converterRegistry.addConverter(new ArrayToObjectConverter(conversionService));
128         converterRegistry.addConverter(new ObjectToArrayConverter(conversionService));
129 
130         converterRegistry.addConverter(new CollectionToStringConverter(conversionService));
131         converterRegistry.addConverter(new StringToCollectionConverter(conversionService));
132 
133         converterRegistry.addConverter(new CollectionToObjectConverter(conversionService));
134         converterRegistry.addConverter(new ObjectToCollectionConverter(conversionService));
135     }
136 
137 
138     /**
139      * Inner class to avoid a hard-coded dependency on Java 8's {@code java.time} package.
140      */
141     private static final class Jsr310ConverterRegistrar {
142 
143         public static void registerZoneIdConverters(ConverterRegistry converterRegistry) {
144             converterRegistry.addConverter(new ZoneIdToTimeZoneConverter());
145             converterRegistry.addConverter(new ZonedDateTimeToCalendarConverter());
146         }
147     }
148 
149 }
View Code

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);

}
View Code

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

 1 /*
 2  * Copyright 2002-2015 the original author or authors.
 3  *
 4  * Licensed under the Apache License, Version 2.0 (the "License");
 5  * you may not use this file except in compliance with the License.
 6  * You may obtain a copy of the License at
 7  *
 8  *      http://www.apache.org/licenses/LICENSE-2.0
 9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package org.springframework.core.convert.converter;
18 
19 /**
20  * A factory for "ranged" converters that can convert objects from S to subtypes of R.
21  *
22  * <p>Implementations may additionally implement {@link ConditionalConverter}.
23  *
24  * @author Keith Donald
25  * @since 3.0
26  * @see ConditionalConverter
27  * @param <S> the source type converters created by this factory can convert from
28  * @param <R> the target range (or base) type converters created by this factory can convert to;
29  * for example {@link Number} for a set of number subtypes.
30  */
31 public interface ConverterFactory<S, R> {
32 
33     /**
34      * Get the converter to convert from S to target type T, where T is also an instance of R.
35      * @param <T> the target type
36      * @param targetType the target type to convert to
37      * @return A converter from S to T
38      */
39     <T extends R> Converter<S, T> getConverter(Class<T> targetType);
40 
41 }
View Code

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

 1     /**
 2      * 测试ConverterFactory StringToNumberConverterFactory
 3      */
 4     @Test
 5     public void testConversionService2() {
 6         double d = conversionService.convert("1.2", double.class);
 7         System.out.println(d); //1.2
 8 
 9         int i = conversionService.convert("2", int.class);
10         System.out.println(i); //2
11 
12         Byte b = conversionService.convert("0x10", Byte.class);
13         System.out.println(Integer.toBinaryString(b)); //10000
14     }

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

1         @Override
2         public T convert(String source) {
3             if (source.length() == 0) {
4                 return null;
5             }
6             return NumberUtils.parseNumber(source, this.targetType);
7         }

StringToNumberConverterFactory通过NumberUtils的static方法进行转化

 1     public static <T extends Number> T parseNumber(String text, Class<T> targetClass) {
 2         Assert.notNull(text, "Text must not be null");
 3         Assert.notNull(targetClass, "Target class must not be null");
 4         String trimmed = StringUtils.trimAllWhitespace(text);
 5 
 6         if (targetClass.equals(Byte.class)) {
 7             return (T) (isHexNumber(trimmed) ? Byte.decode(trimmed) : Byte.valueOf(trimmed));
 8         }
 9         else if (targetClass.equals(Short.class)) {
10             return (T) (isHexNumber(trimmed) ? Short.decode(trimmed) : Short.valueOf(trimmed));
11         }
12         else if (targetClass.equals(Integer.class)) {
13             return (T) (isHexNumber(trimmed) ? Integer.decode(trimmed) : Integer.valueOf(trimmed));
14         }
15         else if (targetClass.equals(Long.class)) {
16             return (T) (isHexNumber(trimmed) ? Long.decode(trimmed) : Long.valueOf(trimmed));
17         }
18         else if (targetClass.equals(BigInteger.class)) {
19             return (T) (isHexNumber(trimmed) ? decodeBigInteger(trimmed) : new BigInteger(trimmed));
20         }
21         else if (targetClass.equals(Float.class)) {
22             return (T) Float.valueOf(trimmed);
23         }
24         else if (targetClass.equals(Double.class)) {
25             return (T) Double.valueOf(trimmed);
26         }
27         else if (targetClass.equals(BigDecimal.class) || targetClass.equals(Number.class)) {
28             return (T) new BigDecimal(trimmed);
29         }
30         else {
31             throw new IllegalArgumentException(
32                     "Cannot convert String [" + text + "] to target class [" + targetClass.getName() + "]");
33         }
34     }

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

  1 /*
  2  * Copyright 2002-2015 the original author or authors.
  3  *
  4  * Licensed under the Apache License, Version 2.0 (the "License");
  5  * you may not use this file except in compliance with the License.
  6  * You may obtain a copy of the License at
  7  *
  8  *      http://www.apache.org/licenses/LICENSE-2.0
  9  *
 10  * Unless required by applicable law or agreed to in writing, software
 11  * distributed under the License is distributed on an "AS IS" BASIS,
 12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 13  * See the License for the specific language governing permissions and
 14  * limitations under the License.
 15  */
 16 
 17 package org.springframework.core.convert.converter;
 18 
 19 import java.util.Set;
 20 
 21 import org.springframework.core.convert.TypeDescriptor;
 22 import org.springframework.util.Assert;
 23 
 24 /**
 25  * Generic converter interface for converting between two or more types.
 26  *
 27  * <p>This is the most flexible of the Converter SPI interfaces, but also the most complex.
 28  * It is flexible in that a GenericConverter may support converting between multiple source/target
 29  * type pairs (see {@link #getConvertibleTypes()}. In addition, GenericConverter implementations
 30  * have access to source/target {@link TypeDescriptor field context} during the type conversion
 31  * process. This allows for resolving source and target field metadata such as annotations and
 32  * generics information, which can be used influence the conversion logic.
 33  *
 34  * <p>This interface should generally not be used when the simpler {@link Converter} or
 35  * {@link ConverterFactory} interfaces are sufficient.
 36  *
 37  * <p>Implementations may additionally implement {@link ConditionalConverter}.
 38  *
 39  * @author Keith Donald
 40  * @author Juergen Hoeller
 41  * @since 3.0
 42  * @see TypeDescriptor
 43  * @see Converter
 44  * @see ConverterFactory
 45  * @see ConditionalConverter
 46  */
 47 public interface GenericConverter {
 48 
 49     /**
 50      * Return the source and target types which this converter can convert between. Each
 51      * entry is a convertible source-to-target type pair.
 52      * <p>For {@link ConditionalConverter conditional} converters this method may return
 53      * {@code null} to indicate all source-to-target pairs should be considered.
 54      */
 55     Set<ConvertiblePair> getConvertibleTypes();
 56 
 57     /**
 58      * Convert the source to the targetType described by the TypeDescriptor.
 59      * @param source the source object to convert (may be null)
 60      * @param sourceType the type descriptor of the field we are converting from
 61      * @param targetType the type descriptor of the field we are converting to
 62      * @return the converted object
 63      */
 64     Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
 65 
 66 
 67     /**
 68      * Holder for a source-to-target class pair.
 69      */
 70     public static final class ConvertiblePair {
 71 
 72         private final Class<?> sourceType;
 73 
 74         private final Class<?> targetType;
 75 
 76         /**
 77          * Create a new source-to-target pair.
 78          * @param sourceType the source type
 79          * @param targetType the target type
 80          */
 81         public ConvertiblePair(Class<?> sourceType, Class<?> targetType) {
 82             Assert.notNull(sourceType, "Source type must not be null");
 83             Assert.notNull(targetType, "Target type must not be null");
 84             this.sourceType = sourceType;
 85             this.targetType = targetType;
 86         }
 87 
 88         public Class<?> getSourceType() {
 89             return this.sourceType;
 90         }
 91 
 92         public Class<?> getTargetType() {
 93             return this.targetType;
 94         }
 95 
 96         @Override
 97         public boolean equals(Object other) {
 98             if (this == other) {
 99                 return true;
100             }
101             if (other == null || other.getClass() != ConvertiblePair.class) {
102                 return false;
103             }
104             ConvertiblePair otherPair = (ConvertiblePair) other;
105             return (this.sourceType.equals(otherPair.sourceType) && this.targetType.equals(otherPair.targetType));
106         }
107 
108         @Override
109         public int hashCode() {
110             return (this.sourceType.hashCode() * 31 + this.targetType.hashCode());
111         }
112 
113         @Override
114         public String toString() {
115             return (this.sourceType.getName() + " -> " + this.targetType.getName());
116         }
117     }
118 
119 }
View Code

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

 1 /*
 2  * Copyright 2002-2014 the original author or authors.
 3  *
 4  * Licensed under the Apache License, Version 2.0 (the "License");
 5  * you may not use this file except in compliance with the License.
 6  * You may obtain a copy of the License at
 7  *
 8  *      http://www.apache.org/licenses/LICENSE-2.0
 9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package org.springframework.core.convert.support;
18 
19 import java.util.Collection;
20 import java.util.Collections;
21 import java.util.Set;
22 
23 import org.springframework.core.CollectionFactory;
24 import org.springframework.core.convert.ConversionService;
25 import org.springframework.core.convert.TypeDescriptor;
26 import org.springframework.core.convert.converter.ConditionalGenericConverter;
27 
28 /**
29  * Converts from a Collection to another Collection.
30  *
31  * <p>First, creates a new Collection of the requested targetType with a size equal to the
32  * size of the source Collection. Then copies each element in the source collection to the
33  * target collection. Will perform an element conversion from the source collection's
34  * parameterized type to the target collection's parameterized type if necessary.
35  *
36  * @author Keith Donald
37  * @author Juergen Hoeller
38  * @since 3.0
39  */
40 final class CollectionToCollectionConverter implements ConditionalGenericConverter {
41 
42     private final ConversionService conversionService;
43 
44 
45     public CollectionToCollectionConverter(ConversionService conversionService) {
46         this.conversionService = conversionService;
47     }
48 
49 
50     @Override
51     public Set<ConvertiblePair> getConvertibleTypes() {
52         return Collections.singleton(new ConvertiblePair(Collection.class, Collection.class));
53     }
54 
55     @Override
56     public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
57         return ConversionUtils.canConvertElements(
58                 sourceType.getElementTypeDescriptor(), targetType.getElementTypeDescriptor(), this.conversionService);
59     }
60 
61     @Override
62     public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
63         if (source == null) {
64             return null;
65         }
66         Collection<?> sourceCollection = (Collection<?>) source;
67 
68         // Shortcut if possible...
69         boolean copyRequired = !targetType.getType().isInstance(source);
70         if (!copyRequired && sourceCollection.isEmpty()) {
71             return source;
72         }
73         TypeDescriptor elementDesc = targetType.getElementTypeDescriptor();
74         if (elementDesc == null && !copyRequired) {
75             return source;
76         }
77 
78         // At this point, we need a collection copy in any case, even if just for finding out about element copies...
79         Collection<Object> target = CollectionFactory.createCollection(targetType.getType(),
80                 (elementDesc != null ? elementDesc.getType() : null), sourceCollection.size());
81 
82         if (elementDesc == null) {
83             target.addAll(sourceCollection);
84         }
85         else {
86             for (Object sourceElement : sourceCollection) {
87                 Object targetElement = this.conversionService.convert(sourceElement,
88                         sourceType.elementTypeDescriptor(sourceElement), elementDesc);
89                 target.add(targetElement);
90                 if (sourceElement != targetElement) {
91                     copyRequired = true;
92                 }
93             }
94         }
95 
96         return (copyRequired ? target : source);
97     }
98 
99 }
View Code

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

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

ConvertiblePair是source的class与target的Class的封装

ConvertersForPair内部含有

1 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.

 

posted @ 2017-08-29 11:28  abcwt112  阅读(10074)  评论(1)  编辑  收藏  举报