杰克逊复杂列表序列化

Question

我正在开发Jackson序列化/反序列化.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？

Answer 1

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都表示List<Bar>具有完全相同的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","size":42},{"name":"ZAB","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","size":42},{"name":"ZAB","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.请注意下面的输出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", 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);
    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","size":42},
    //   {"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","name":"BAZ","size":42},
    //       {"type":"com.stackoverflow.q8416904.Zab","name":"ZAB","hungry":true}
    //     ]
    //   ]
    // }

    mapper = new ObjectMapper().setVisibility(JsonMethod.FIELD, Visibility.ANY);
    mapper.enableDefaultTypingAsProperty(DefaultTyping.OBJECT_AND_NON_CONCRETE, "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","name":"BAZ","size":42},
    //     {"type":"com.stackoverflow.q8416904.Zab","name":"ZAB","hungry":true}
    //   ]
    // ]
  }
}