Jackson无法将JSON映射到包含映射的对象

Question

public class Widget {
    private Map<String,Object> params;

    // Getter, setter, ctor, etc...
}

public Widget fromJSON(String json) {
    ObjectMapper jsonMapper = new ObjectMapper();
    return jsonMapper.readValue(json, Widget.class);
}

{
    "params": [{
        "acks": "all"
    }, {
        "retries": 0
    }, {
        "batch.size": 16384
    }, {
        "linger.ms": 1
    }, {
        "buffer.memory": 33554432
    }, {
        "key.serializer": "org.apache.kafka.common.serialization.StringSerializer"
    }, {
        "value.serializer": "org.apache.kafka.common.serialization.StringSerializer"
    }]
}

com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance
    of java.util.LinkedHashMap out of START_ARRAY token

我使用http://jsonlint.com来验证JSON是否有效。因此，我要么需要更改JSON或代码，要么可能两者都需要。有什么想法吗？

Answer 1

JSON是有效的，但对于映射不是。params的值是一个数组，它应该是一个对象，如下所示：

{
    "params": {
        "acks": "all",
        "retries": 0,
        "batch.size": 16384,
        "linger.ms": 1,
        "buffer.memory": 33554432,
        "key.serializer": "org.apache.kafka.common.serialization.StringSerializer",
        "value.serializer": "org.apache.kafka.common.serialization.StringSerializer"
    }
}

在Jackson五分钟，你可以找到以下关系：

Jackson将用于简单数据绑定的具体Java类型有：

╔══════════════════════╦══════════════════════════════╗
║      JSON Type       ║          Java Type           ║
╠══════════════════════╬══════════════════════════════╣
║ object               ║ LinkedHashMap<String,Object> ║
║ array                ║ ArrayList<Object>            ║
║ string               ║ String                       ║
║ number (no fraction) ║ Integer, Long or BigInteger  ║
║ number (fraction)    ║ Double (or BigDecimal)       ║
║ true|false           ║ Boolean                      ║
║ null                 ║ null                         ║
╚══════════════════════╩══════════════════════════════╝