如何将List包装为Jackson生成的JSON中的顶级元素 [英] How to wrap a List as top level element in JSON generated by Jackson

问题描述

我遇到了一个问题，我试图将List包含为根节点，但我似乎无法得到它。让我解释。假设我们有一个类TestClass

I am running into a problem where I am trying to include a List as the root node, but I can't seem to be able to get this. Let me explain. Let's say we have a class "TestClass"

class TestClass{
    String propertyA;       
}

现在，在某种实用方法中，这就是我的工作

Now, in some utility method this is what I do

String utilityMethod(){
   List<TestClass> list = someService.getList();
   new ObjectMapper().writeValueAsString(list); 
}

我想用JSON获得的输出是

The output I am trying to get in JSON is

{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}

我试图使用

objMapper.getSerializationConfig().set(Feature.WRAP_ROOT_VALUE, true);

但是，我似乎仍然没有做对。

But, I still don't seem to get it right.

现在，我正在创建一个Map< String，TestClass>我写的是为了实现我想要做的事情，但这很有效但很明显这是一个黑客。有人可以帮我一个更优雅的解决方案吗？谢谢

Right now, I am just creating a Map < String,TestClass > and I write that to achieve what I am trying to do, which works but clearly this is a hack. Could someone please help me with a more elegant solution? Thanks

推荐答案

不幸的是，即使启用了 WRAP_ROOT_VALUE 功能，您仍然需要用于控制序列化Java集合时生成的根名称的额外逻辑（请参阅这个答案详情为什么）。这使您可以选择：

Unfortunately, even with the WRAP_ROOT_VALUE feature enabled you still need extra logic to control the root name generated when serializing a Java collection (see this answer for details why). Which leaves you with the options of:

• 使用持有人类来定义根名称


• 使用地图。


• 使用自定义 ObjectWriter

• using a holder class to define the root name
• using a map.
• using a custom ObjectWriter

以下是一些说明三种不同选项的代码：

Here is some code illustrating the three different options:

public class TestClass {
    private String propertyA;

    // constructor/getters/setters
}

public class TestClassListHolder {

    @JsonProperty("ListOfTestClasses")
    private List<TestClass> data;

    // constructor/getters/setters
}

public class TestHarness {

    protected List<TestClass> getTestList() {
        return Arrays.asList(new TestClass("propertyAValue"), new TestClass(
                "someOtherPropertyValue"));
    }

    @Test
    public void testSerializeTestClassListDirectly() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
        System.out.println(mapper.writeValueAsString(getTestList()));
    }

    @Test
    public void testSerializeTestClassListViaMap() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        final Map<String, List<TestClass>> dataMap = new HashMap<String, List<TestClass>>(
                4);
        dataMap.put("ListOfTestClasses", getTestList());
        System.out.println(mapper.writeValueAsString(dataMap));
    }

    @Test
    public void testSerializeTestClassListViaHolder() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        final TestClassListHolder holder = new TestClassListHolder();
        holder.setData(getTestList());
        System.out.println(mapper.writeValueAsString(holder));
    }

    @Test
    public void testSerializeTestClassListViaWriter() throws Exception {
        final ObjectMapper mapper = new ObjectMapper();
        final ObjectWriter writer = mapper.writer().withRootName(
                "ListOfTestClasses");
        System.out.println(writer.writeValueAsString(getTestList()));
    }
}

输出：

{ArrayList：[{propertyA：propertyAValue}，{propertyA：someOtherPropertyValue}]}

{ListOfTestClasses ：[{propertyA：propertyAValue}，{propertyA：someOtherPropertyValue}]}

{ListOfTestClasses：[{propertyA：propertyAValue}，{propertyA ：someOtherPropertyValue}]}

{ListOfTestClasses：[{propertyA：propertyAValue}，{propertyA：someOtherPropertyValue}]}

{"ArrayList":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}
{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}
{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}
{"ListOfTestClasses":[{"propertyA":"propertyAValue"},{"propertyA":"someOtherPropertyValue"}]}

使用 ObjectWriter 非常方便 - 请记住，用它序列化的所有顶级对象都将具有相同的根名称。如果那不可取，那么请改用地图或持有人类。

Using an ObjectWriter is very convenient - just bare in mind that all top level objects serialized with it will have the same root name. If thats not desirable then use a map or holder class instead.

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