Jackson复杂列表序列化

Question

我正在使用 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 List objs； 要使用杰克逊，我需要指定列表元素的真实类型，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 Jackson 正确序列化这个类？

Answer 1

只是

new ObjectMapper().writeValueAsString(myResult);

列表的类型不会因为类型擦除而丢失，就像在第一个示例中一样。

---

请注意，对于列表或通用列表的普通序列化，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具有完全相同 JSON 的 List<Bar>。

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;}
}

在使用附加类型信息进行序列化时，类型化编写器很有用。请注意下面的 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", 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}
    //   ]
    // ]
  }
}