Converter - JiyangM/spring GitHub Wiki
spring 提供了3中类型转换器。
public interface GenericConverter {
@Nullable
Set<ConvertiblePair> getConvertibleTypes();
@Nullable
Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
final class ConvertiblePair {
private final Class<?> sourceType;
private final Class<?> targetType;
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(@Nullable Object other) {
if (this == other) {
return true;
}
if (other == null || other.getClass() != ConvertiblePair.class) {
return false;
}
ConvertiblePair otherPair = (ConvertiblePair) other;
return (this.sourceType == otherPair.sourceType && this.targetType == 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());
}
}
}GenericConverter 中提供了两个方法:
- Set getConvertibleTypes(); 用于获取转换的源类型和目标类型；
- Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType); 用于转换类型
还定义了一个内部类，里面的结构很简单，指定源和目标的类型。
GenericConverter 比较常用的一个实现类是ConditionalGenericConverter。
ConditionalGenericConverter的作用只是将GenericConverter 和ConditionalConverter接口的功能聚合。
ConditionalConverter的作用是根据源和目标的类型判断该转换器是否支持。
public interface ConditionalConverter {
boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);
}
以ArrayToCollectionConverter 为类说明ConditionalGenericConverter的作用：
final class ArrayToCollectionConverter implements ConditionalGenericConverter {
private final ConversionService conversionService;
public ArrayToCollectionConverter(ConversionService conversionService) {
this.conversionService = conversionService;
}
@Override
public Set<ConvertiblePair> getConvertibleTypes() {
return Collections.singleton(new ConvertiblePair(Object[].class, Collection.class));
}
@Override
public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
return ConversionUtils.canConvertElements(
sourceType.getElementTypeDescriptor(), targetType.getElementTypeDescriptor(), this.conversionService);
}
@Override
@Nullable
public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
if (source == null) {
return null;
}
int length = Array.getLength(source);
TypeDescriptor elementDesc = targetType.getElementTypeDescriptor();
Collection<Object> target = CollectionFactory.createCollection(targetType.getType(),
(elementDesc != null ? elementDesc.getType() : null), length);
if (elementDesc == null) {
for (int i = 0; i < length; i++) {
Object sourceElement = Array.get(source, i);
target.add(sourceElement);
}
}
else {
for (int i = 0; i < length; i++) {
Object sourceElement = Array.get(source, i);
Object targetElement = this.conversionService.convert(sourceElement,
sourceType.elementTypeDescriptor(sourceElement), elementDesc);
target.add(targetElement);
}
}
return target;
}
}- Set getConvertibleTypes();返回了源和目标类型。
- match方法实际上调用的是GenericConversionService#canConvert
@Override
public boolean canConvert(@Nullable TypeDescriptor sourceType, TypeDescriptor targetType) {
Assert.notNull(targetType, "Target type to convert to cannot be null");
if (sourceType == null) {
return true;
}
GenericConverter converter = getConverter(sourceType, targetType);
return (converter != null);
}@Nullable
protected GenericConverter getConverter(TypeDescriptor sourceType, TypeDescriptor targetType) {
ConverterCacheKey key = new ConverterCacheKey(sourceType, targetType);
GenericConverter converter = this.converterCache.get(key);
if (converter != null) {
return (converter != NO_MATCH ? converter : null);
}
converter = this.converters.find(sourceType, targetType);
if (converter == null) {
converter = getDefaultConverter(sourceType, targetType);
}
if (converter != null) {
this.converterCache.put(key, converter);
return converter;
}
this.converterCache.put(key, NO_MATCH);
return null;
}这段代码中 converterCache 是GenericConversionService中定义的一个软引用类型。（在jvm满的时候（不够），会回收掉不可达对象和软引用对象）
private final Map<ConverterCacheKey, GenericConverter> converterCache = new ConcurrentReferenceHashMap<>(64);