從json-lib轉成jackson的遇到的問題
問題一：json 字串，再經過Jackson序列化之後就變成原生字串了。而json-lib經過再序列化之後，還是json格式的串。
針對這種情況，可以寫一個Serializer類，遇到json串的時候就當作原生字串寫入即可。
<<JsonStringSerializer>>
/**
* 序列化時，對Json格式的字串做特殊處理：不用引號括起來
* @author
*
*/
public class JsonStringSerializer extends JsonSerializer<Object> {
@Override
public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
if (value == null) {
jgen.writeNull();
} else {
if (value instanceof String) {
String newValue = ((String) value).trim();
// 只對Json格式的字串做處理
if (newValue.startsWith(“{“) || newValue.startsWith(“[“)) {
jgen.writeRawValue((String) value);
} else {
jgen.writeObject(value);
}
} else {
jgen.writeObject(value);
}
}
}
}
問題二、jackson和json-lib對null值的處理大不相同。對於值為null的字串型別的欄位，jackson輸出null，而json-lib輸出空字串。對於List型別，json-lib輸出空列表[]，而jackson還是輸出null。
如果是從json-lib移植到jackson，為了相容老程式碼，可以寫一個SerializerProvider，遇到null值輸出空字串等。
<<NullToEmptyStringProvider>>
/**
* Customize the DefaultSerializerProvider so that when it is looking for a
* NullSerializer it will use one that is class sensitive, writing strings as “”
* and everything else using the default value.
*
* @author
*/
public class NullToEmptyStringProvider extends DefaultSerializerProvider {
private static final long serialVersionUID = -1L;
// A couple of constructors and factory methods to keep the compiler happy
public NullToEmptyStringProvider() {
super();
}
public NullToEmptyStringProvider(NullToEmptyStringProvider provider, SerializationConfig config,
SerializerFactory jsf) {
super(provider, config, jsf);
}
@Override
public NullToEmptyStringProvider createInstance(SerializationConfig config, SerializerFactory jsf) {
return new NullToEmptyStringProvider(this, config, jsf);
}
@Override
public JsonSerializer<Object> findNullValueSerializer(BeanProperty property) throws JsonMappingException {
if (property.getType().getRawClass().equals(String.class)) {
return EmptyStringSerializer.INSTANCE;
} else if ((property.getType().isArrayType() || property.getType().isCollectionLikeType())
&& !property.getType().isMapLikeType()) {
return EmptyListSerializer.INSTANCE;
} else if (property.getType().getRawClass().equals(Long.class)
|| property.getType().getRawClass().equals(Short.class)
|| property.getType().getRawClass().equals(Integer.class)
|| property.getType().getRawClass().equals(Double.class)
|| property.getType().getRawClass().equals(Float.class)
|| property.getType().getRawClass().equals(BigDecimal.class)) {
return EmptyNumberSerializer.INSTANCE;
} else {
return super.findNullValueSerializer(property);
}
}
}
/**
* Output null of String to empty string.
*
* @author
*
*/
class EmptyStringSerializer extends JsonSerializer<Object> {
public static final JsonSerializer<Object> INSTANCE = new EmptyStringSerializer();
private EmptyStringSerializer() {
}
// Since we know we only get to this seralizer in the case where the value
// is null and the type is String, we can
// do our handling without any additional logic and write that empty string
// we are so desperately wanting.
@Override
public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
throws IOException {
jsonGenerator.writeString(“”);
}
}
/**
* For null list
* @author
*
*/
class EmptyListSerializer extends JsonSerializer<Object> {
public static final JsonSerializer<Object> INSTANCE = new EmptyListSerializer();
private EmptyListSerializer() {
}
// Since we know we only get to this seralizer in the case where the value
// is null and the type is String, we can
// do our handling without any additional logic and write that empty string
// we are so desperately wanting.
@Override
public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
throws IOException {
jsonGenerator.writeStartArray();
jsonGenerator.writeEndArray();
}
}
/**
* For null Number, such as Integer, Long, Short ….
* @author
*
*/
class EmptyNumberSerializer extends JsonSerializer<Object> {
public static final JsonSerializer<Object> INSTANCE = new EmptyNumberSerializer();
private EmptyNumberSerializer() {
}
// Since we know we only get to this seralizer in the case where the value
// is null and the type is String, we can
// do our handling without any additional logic and write that empty string
// we are so desperately wanting.
@Override
public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
throws IOException {
jsonGenerator.writeNumber(0);
}
}
最後，需要把這個provider配置到jackson的mapper例項中。
jsonMapper.setSerializerProvider(new NullToEmptyStringProvider());