java/jackson - 链式@JsonValue 注释和反序列化答案

【问题标题】：java/jackson - chained @JsonValue annotations and deserializationjava/jackson - 链式@JsonValue 注释和反序列化
【发布时间】：2015-02-26 02:28:00
【问题描述】：

我有以下结构。我知道这看起来很奇怪，但我正在用这个例子模拟我们的代码：

public static class StringWrapper {
    protected final String s;

    @JsonValue
    public String getS() {
        return s;
    }

    public StringWrapper(final String s) {
        this.s = s;
    }
}

public static class StringWrapperOuter {
    protected final StringWrapper s;

    @JsonValue
    public StringWrapper getS() {
        return s;
    }

    public StringWrapperOuter(final StringWrapper s) {
        this.s = s;
    }
}

public static class POJO {
    protected final List<StringWrapperOuter> data;

    public List<StringWrapperOuter> getData() {
        return data;
    }

    public POJO(final List<StringWrapperOuter> data) {
        this.data = data;
    }

    public POJO() {
        data = Collections.emptyList();
    }
}

POJO 的序列化给出了预期的 JSON 字符串数组：

<mapper>.writeValue(System.out, new POJO(Arrays.asList(new StringWrapperOuter(new StringWrapper("a")), new StringWrapperOuter(new StringWrapper("b")), new StringWrapperOuter(new StringWrapper("c")))));

{"data":["a","b","c"]}

如何将此 JSON 字符串反序列化为 POJO 类型的对象？

<mapper>.readValue("{\"data\":[\"a\",\"b\",\"c\"]}", POJO.class);

映射器难以识别输入，因为两个@JsonValue 注释被链接在一起。它给出了以下例外：

Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not instantiate value of type [simple type, class StringWrapperOuter] from String value ('a'); no single-String constructor/factory method
 at [Source: {"data":["a","b","c"]}; line: 1, column: 10] (through reference chain: POJO["data"]->java.util.ArrayList[0])

【问题讨论】：

标签： java jackson deserialization

【解决方案1】：

正如 Sotirios 在之前的回答中所说。 @JsonCreator 是这里的关键。但是，为了让所有类都能正常工作，可能需要@JsonProperty。

public static class POJO {
    protected final List<StringWrapperOuter> data;

    // In order for POJO creation to work properly the @JsonProperty
    // annotation on the arg is required
    @JsonCreator
    public POJO(@JsonProperty("data") final List<StringWrapperOuter> data) {
        this.data = data;
    }

    public List<StringWrapperOuter> getData() {
        return data;
    }
}

public static class StringWrapper {
    protected final String s;

    @JsonCreator
    public StringWrapper(final String s) {
        this.s = s;
    }

    @JsonValue
    public String getS() {
        return s;
    }
}

public static class StringWrapperOuter {
    protected final StringWrapper s;

    @JsonCreator
    public StringWrapperOuter(final StringWrapper s) {
        this.s = s;
    }

    @JsonValue
    public StringWrapper getS() {
        return s;
    }
}

【讨论】：

【解决方案2】：

@JsonValue 用于序列化。反序列化的类似注解是@JsonCreator。

注释你的构造函数

@JsonCreator
public StringWrapper(final String s) {
    this.s = s;
}

和

@JsonCreator
public StringWrapperOuter(final StringWrapper s) {
    this.s = s;
}

【讨论】：