如何在 Jackson 生成的 JSON 中将 List 包装为顶级元素 [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TestClass"类

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，在某些实用方法中，这就是我所做的

Now, in some utility method this is what I do

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

我试图在 JSON 中获得的输出是

The output I am trying to get in JSON is

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

我试过使用

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，我只是在创建一个 Map <String,TestClass > 并且我写了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

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，您仍然需要额外的逻辑来控制序列化 Java 集合时生成的根名称(请参阅这个答案了解详情为什么).这让您有以下选择:

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:

• 使用持有者类来定义根名称
• 使用地图.
• 使用自定义<代码>ObjectWriter

以下是说明三个不同选项的一些代码:

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，所有使用它序列化的顶级对象都将具有相同的根名称.如果那不是可取的，那么使用 map 或 holder 类代替.

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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