我正在开发Jackson序列化/反序列化.
例如,我有这样的课程：
例如,我有这样的课程：
class Base{
String baseId;
}
我想序列化列表objs;
要使用jackson,我需要指定列表的元素实际类型,因为java类型擦除.
此代码将起作用：
List<Base> data = getData(); return new ObjectMapper().writerWithType(TypeFactory.collectionType(List.class,Base.class)).writeValueAsString(data);
现在,我想序列化更复杂的类：
class Result{
List<Base> data;
}
我应该如何告诉杰克逊正确序列化这门课程？
解决方法
只是
new ObjectMapper().writeValueAsString(myResult);
由于类型擦除,列表的类型不会像第一个示例中那样丢失.
请注意,对于列表或通用列表的vanilla序列化,没有必要指定列表组件类型,如原始问题中的示例所示.以下所有三个示例序列化代表List< Bar>使用完全相同的JSON.
import java.util.ArrayList;
import java.util.List;
import org.codehaus.jackson.annotate.JsonAutoDetect.Visibility;
import org.codehaus.jackson.annotate.JsonMethod;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.ObjectWriter;
public class JacksonFoo
{
public static void main(String[] args) throws Exception
{
Baz baz = new Baz("BAZ",42);
Zab zab = new Zab("ZAB",true);
List<Bar> bars = new ArrayList<Bar>();
bars.add(baz);
bars.add(zab);
ObjectMapper mapper = new ObjectMapper().setVisibility(JsonMethod.FIELD,Visibility.ANY);
String json1 = mapper.writeValueAsString(bars);
System.out.println(json1);
// output:
// [{"name":"BAZ","size":42},{"name":"ZAB","hungry":true}]
Foo foo = new Foo(bars);
String json2 = mapper.writeValueAsString(foo);
System.out.println(json2);
// output:
// {"bars":[{"name":"BAZ","hungry":true}]}
mapper = new ObjectMapper().setVisibility(JsonMethod.FIELD,Visibility.ANY);
ObjectWriter typedWriter = mapper.writerWithType(mapper.getTypeFactory().constructCollectionType(List.class,Bar.class));
String json3 = typedWriter.writeValueAsString(bars);
System.out.println(json3);
// output:
// [{"name":"BAZ","hungry":true}]
}
}
class Foo
{
List<Bar> bars;
Foo(List<Bar> b) {bars = b;}
}
abstract class Bar
{
String name;
Bar(String n) {name = n;}
}
class Baz extends Bar
{
int size;
Baz(String n,int s) {super(n); size = s;}
}
class Zab extends Bar
{
boolean hungry;
Zab(String n,boolean h) {super(n); hungry = h;}
}
使用其他类型信息进行序列化时,类型编写器非常有用.注意下面的json1和json3输出有何不同.
import java.util.ArrayList;
import java.util.List;
import org.codehaus.jackson.annotate.JsonAutoDetect.Visibility;
import org.codehaus.jackson.annotate.JsonMethod;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.ObjectMapper.DefaultTyping;
import org.codehaus.jackson.map.ObjectWriter;
public class JacksonFoo
{
public static void main(String[] args) throws Exception
{
Baz baz = new Baz("BAZ",Visibility.ANY);
mapper.enableDefaultTypingAsProperty(DefaultTyping.OBJECT_AND_NON_CONCRETE,"type");
String json1 = mapper.writeValueAsString(bars);
System.out.println(json1);
// output:
// [
// {"type":"com.stackoverflow.q8416904.Baz","name":"BAZ",// {"type":"com.stackoverflow.q8416904.Zab","name":"ZAB","hungry":true}
// ]
Foo foo = new Foo(bars);
String json2 = mapper.writeValueAsString(foo);
System.out.println(json2);
// output:
// {
// "bars":
// [
// "java.util.ArrayList",// [
// {"type":"com.stackoverflow.q8416904.Baz",// {"type":"com.stackoverflow.q8416904.Zab","hungry":true}
// ]
// ]
// }
mapper = new ObjectMapper().setVisibility(JsonMethod.FIELD,"type");
ObjectWriter typedWriter = mapper.writerWithType(mapper.getTypeFactory().constructCollectionType(List.class,Bar.class));
String json3 = typedWriter.writeValueAsString(bars);
System.out.println(json3);
// output:
// [
// "java.util.ArrayList",// [
// {"type":"com.stackoverflow.q8416904.Baz",// {"type":"com.stackoverflow.q8416904.Zab","hungry":true}
// ]
// ]
}
}