使用Jackson ObjectMapper反序列化JSON字符串时遭遇InvalidFormatException异常的求助

我来帮你分析下这个问题并给出可行的解决方案：

首先，你的JSON字符串里comments字段的值是空字符串""，但你尝试反序列化到Map<String, LinkedHashMap>类型时，Jackson默认无法将空字符串转换为LinkedHashMap对象，所以抛出了InvalidFormatException。

你的JSON数据： #

{"id" : "1234", "name" : "Name", "description" : "abcd\n\rdghsdghjdg", "risk" : "LOW", "status" : "Open", "autoGenerated" : false, "comments" : "", "category" : [ "abc", "def" ], "lastModifiedDate" : "2021-11-19T11:23:04.948074Z", "lastModifiedBy" : "amallik", "readableId" : "2553", "componentBlocks" : [ ], "threatTemplateId" : "5678", "threatTags" : [ ], "threatComponents" : [ ], "components" : { "blocks" : [ ], "lines" : [ ], "connections" : [ ] }, "mitigationList" : [ { "mitigationTemplateId" : "155", "title" : "abcd", }, { "mitigationTemplateId" : "152", "title" : "efgh", }, { "mitigationTemplateId" : "153", "title" : "ijkl", }], "rejected" : false, "edited" : true }

你使用的反序列化代码： #

ObjectMapper mapper = new ObjectMapper();
mapper.readValue(currentPayload, new TypeReference<Map<String, LinkedHashMap>>(){});

报错信息： #

com.fasterxml.jackson.databind.exc.InvalidFormatException: Cannot coerce empty String ("") to element of `java.util.LinkedHashMap<java.lang.Object,java.lang.Object>` (but could if coercion was enabled using `CoercionConfig`) at [Source: (String)"{ [truncated chars]; line: 8, column: 16] (through reference chain: java.util.LinkedHashMap["comments"])

解决方案 #

1. 改用更通用的Map<String, Object>类型 #

你的JSON包含多种类型的值（字符串、布尔、数组、嵌套对象），用Object作为值类型可以兼容所有情况，空字符串也能正常解析：

ObjectMapper mapper = new ObjectMapper();
Map<String, Object> threatMap = mapper.readValue(currentPayload, new TypeReference<Map<String, Object>>(){});

2. 配置Jackson允许空字符串转成空Map #

如果你坚持要用LinkedHashMap作为值类型，可以通过配置Jackson的强制转换规则，让空字符串被处理为空的LinkedHashMap：

ObjectMapper mapper = new ObjectMapper();
// 配置将空字符串强制转换为空的Map
mapper.coercionConfigFor(LogicalType.Map)
    .setCoercion(CoercionInputShape.EmptyString, CoercionAction.AsEmpty);

Map<String, LinkedHashMap> threatMap = mapper.readValue(currentPayload, new TypeReference<Map<String, LinkedHashMap>>(){});

3. 创建对应的实体类（推荐方案） #

如果这个JSON结构是固定的，最稳妥的方式是创建匹配的Java实体类，这样类型安全，后续操作也更方便：

import java.util.List;

public class Threat {
    private String id;
    private String name;
    private String description;
    private String risk;
    private String status;
    private boolean autoGenerated;
    private String comments; // 直接用String类型对应空字符串
    private List<String> category;
    private String lastModifiedDate;
    private String lastModifiedBy;
    private String readableId;
    private List<Object> componentBlocks;
    private String threatTemplateId;
    private List<Object> threatTags;
    private List<Object> threatComponents;
    private Components components;
    private List<Mitigation> mitigationList;
    private boolean rejected;
    private boolean edited;

    // Getters and Setters

    public static class Components {
        private List<Object> blocks;
        private List<Object> lines;
        private List<Object> connections;

        // Getters and Setters
    }

    public static class Mitigation {
        private String mitigationTemplateId;
        private String title;

        // Getters and Setters
    }
}

然后反序列化时直接使用这个实体类：

ObjectMapper mapper = new ObjectMapper();
Threat threat = mapper.readValue(currentPayload, Threat.class);

这样comments字段作为String类型，空字符串就能被正确解析，而且整个数据结构的类型更清晰，避免了Map类型带来的类型转换问题。

内容的提问来源于stack exchange，提问作者Joy