如何将Java类转换为Map <String, String> 并使用jackson将非字符串成员转换为json？

Question

我在Java中有一些类要转换为Map<String, String> 。 问题是我的java类中没有明显的String表示（集合，其他类）的任何字段都应该转换为json字符串。

这是一个例子：

@Data
@AllArgsConstructor
class MyClass {
    String field1;
    Long field2;
    Set<String> field3;
    OtherClass field4;
}

@Data
@AllArgsConstructor
class OtherClass {
    String field1;
    String field2;
}

ObjectMapper mapper = new ObjectMapper();
MyClass myClass = new MyClass("value", 
                              123L, 
                              Sets.newHashSet("item1", "item2"),
                              new OtherClass("value1", "value2"));
Map<String, String> converted =
        mapper.convertValue(myClass, new TypeReference<Map<String, String>>(){});

此时， converted应如下所示：

"field1" -> "value"
"field2" -> "123"
"field3" -> "[\"item1\", \"item2\"]"
"field4" -> "{\"field1\":\"value1\",\"field2\":\"value2\"}"

相反，当尝试使用异常java.lang.IllegalArgumentException: Can not deserialize instance of java.lang.String out of START_ARRAY token去除Set的mapper.convertValue时，对mapper.convertValue的调用失败java.lang.IllegalArgumentException: Can not deserialize instance of java.lang.String out of START_ARRAY token 。

有什么特别的配置，我可以标注MyClass有或方法来配置ObjectMapper ，使这个工作，我希望它的方式？

Answer 1

这是一种方法。

private static final ObjectMapper mapper = new ObjectMapper();

public Map<String, String> toMap(Object obj) {
    // Convert the object to an intermediate form (map of strings to JSON nodes)
    Map<String, JsonNode> intermediateMap = mapper.convertValue(obj, new TypeReference<Map<String, JsonNode>>() {});

    // Convert the json nodes to strings
    Map<String, String> finalMap = new HashMap<>(intermediateMap.size() + 1); // Start out big enough to prevent resizing
    for (Map.Entry<String, JsonNode> e : intermediateMap.entrySet()) {
        String key = e.getKey();
        JsonNode val = e.getValue();

        // Get the text value of textual nodes, and convert non-textual nodes to JSON strings
        String stringVal = val.isTextual() ? val.textValue() : val.toString();

        finalMap.put(key, stringVal);
    }

    return finalMap;
}

如果你想将Map <String，String>转换回原始类......

public static <T> T fromMap(Map<String, String> map, Class<T> clazz) throws IOException {
    // Convert the data to a map of strings to JSON nodes
    Map<String, JsonNode> intermediateMap = new HashMap<>(map.size() + 1); // Start out big enough to prevent resizing
    for (Map.Entry<String, String> e : map.entrySet()) {
        String key = e.getKey();
        String val = e.getValue();

        // Convert the value to the right type of JsonNode
        JsonNode jsonVal;
        if (val.startsWith("{") || val.startsWith("[") || "null".equals(val)) {
            jsonVal = mapper.readValue(val, JsonNode.class);
        } else {
            jsonVal = mapper.convertValue(val, JsonNode.class);
        }

        intermediateMap.put(key, jsonVal);
    }

    // Convert the intermediate map to an object
    T result = mapper.convertValue(intermediateMap, clazz);

    return result;
}