Jackson全局设置将数组反序列化为自定义列表实现 [英] Jackson global settings to deserialise array to custom list implementation

问题描述

默认情况下，Jackson使用java.util.ArrayList反序列化JSON数组.取而代之的是，我想使用自定义实现.例如，如果存在值，则使用Guava ImmutableList；如果JSON数组为空或null，则使用Collection.emptyList().

By default, Jackson is using java.util.ArrayList to deserialise a JSON array. Instead of this, I want to use a custom implementation. For example, Guava ImmutableList if value is present, or Collection.emptyList() if JSON array is empty or null.

我想为ObjectMapper全局配置它.有没有简单的方法可以做到这一点?

I want to configure this globally for ObjectMapper. Is there an easy way to do this?

PS:我的杰克逊版本为2.9.7

PS: My Jackson version is 2.9.7

推荐答案

常规解决方案是使用自定义模块.您可以定义要用于集合的类.对于番石榴，有一个Maven模块:

General solution is to use custom module. You can define classes you would like to use for collections. For Guava there is a Maven module:

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-guava</artifactId>
    <version>x.y.z</version>
</dependency>

现在，您可以注册新模块了:

Now, you can register your new module:

ObjectMapper mapper = new ObjectMapper();
// register module with object mapper
mapper.registerModule(new GuavaModule());

现在，您可以在POJO中定义要对列表进行不可变的实现.

Now, you can define in your POJO you want to have immutable implementation of the list.

class Pojo {

    private ImmutableList<Integer> ints;

    public ImmutableList<Integer> getInts() {
        return ints;
    }

    public void setInts(ImmutableList<Integer> ints) {
        this.ints = ints;
    }

    @Override
    public String toString() {
        return "Pojo{" +
                "ints=" + ints + " " + ints.getClass() + '}';
    }
}

及以下示例:

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new GuavaModule());

String json = "{\"ints\":[1,2,3,4]}";

System.out.println(mapper.readValue(json, Pojo.class));

打印:

Pojo{ints=[1, 2, 3, 4] class com.google.common.collect.RegularImmutableList}

如果您不想将POJO类与List实现联系在一起，则需要使用SimpleModule类添加一些额外的配置.因此，您的POJO如下所示:

If you do not want to tie your POJO classes with List implementation you need to add some extra configuration using SimpleModule class. So, your POJO looks like below:

class Pojo {

    private List<Integer> ints;

    public List<Integer> getInts() {
        return ints;
    }

    public void setInts(List<Integer> ints) {
        this.ints = ints;
    }

    @Override
    public String toString() {
        return "Pojo{" +
                "ints=" + ints + " " + ints.getClass() + '}';
    }
}

您的示例如下:

SimpleModule useImmutableList = new SimpleModule("UseImmutableList");
useImmutableList.addAbstractTypeMapping(List.class, ImmutableList.class);

GuavaModule module = new GuavaModule();

ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(module);
mapper.registerModule(useImmutableList);

String json = "{\"ints\":[1,2,3,4]}";

System.out.println(mapper.readValue(json, Pojo.class));

上面的代码显示:

Pojo{ints=[1, 2, 3, 4] class com.google.common.collect.RegularImmutableList}

在上面的代码打印中删除多余的SimpleModule时:

When you remove extra SimpleModule above code prints:

Pojo{ints=[1, 2, 3, 4] class java.util.ArrayList}

在Collections.emptyList()为空的情况下，我看不出有什么要使用的地方. Guava的模块将RegularImmutableList用于非空数组和空数组.

I do not see any point to use Collections.emptyList() in case it is empty. Guava's module uses RegularImmutableList for not empty and empty arrays.

要转换null -> empty，请参见以下问题:

For converting null -> empty see this question:

1. Jackson反序列化器-将null集合更改为空一个

，但我建议将其设置为POJO中的empty，如下所示:

but I recommend to set it to empty in POJO like below:

private List<Integer> ints = Collections.emptyList();

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