java 如何在 Jackson 生成的 JSON 中将 List 包装为顶级元素

How to wrap a List as top level element in JSON generated by Hymanson

提问by Seagull

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"

我遇到了一个问题，我试图将一个 List 作为根节点包含在内，但我似乎无法得到这个。让我解释。假设我们有一个类“TestClass”

class TestClass{
    String propertyA;       
}

Now, in some utility method this is what I do

现在，在某些实用方法中，这就是我所做的

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

The output I am trying to get in JSON is

我试图在 JSON 中获得的输出是

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

I have tried to use

我试过用

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

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

但是，我似乎仍然没有做对。

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

现在，我只是在创建一个 Map < String,TestClass > 并且我编写它来实现我想要做的事情，这可行，但显然这是一个黑客。有人可以帮我提供更优雅的解决方案吗？谢谢

回答by Perception

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

不幸的是，即使WRAP_ROOT_VALUE启用了该功能，您仍然需要额外的逻辑来控制序列化 Java 集合时生成的根名称（有关详细信息，请参阅此答案）。这让您有以下选择：

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

• 使用持有者类来定义根名称
• 使用地图。
• 使用自定义 ObjectWriter

Here is some code illustrating the three different options:

下面是一些说明三个不同选项的代码：

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()));
    }
}

Output:

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"}]}

Using an ObjectWriteris 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.

使用 anObjectWriter非常方便 - 请记住，使用它序列化的所有顶级对象都将具有相同的根名称。如果那是不可取的，那么请改用 map 或 holder 类。

回答by searlea

I'd expect the basic idea to be something like:

我希望基本的想法是这样的：

class UtilityClass {
    List listOfTestClasses;
    UtilityClass(List tests) {
        this.listOfTestClasses = tests;
    }
}

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