杰克逊的readValue和readTree：何时使用哪个？ [英] readValue and readTree in Jackson: when to use which?

问题描述

我刚刚开始使用Jackson JSON库。杰克逊是一个非常强大的库，但它有一个非常广泛的API。很多事情可以通过多种方式完成。这使得很难在杰克逊找到自己的方式 - 如何知道什么是正确/最好的做事方式？

I'm just starting using the Jackson JSON library. Jackson is a very powerful library, but it has a terribly extensive API. A lot of things can be done in multiple ways. This makes it hard to find your way in Jackson - how to know what is the correct/best way of doing things?

为什么我会使用这个解决方案：

Why would I use this solution:

String json = "{\"a\":2, \"b\":\"a string\", \"c\": [6.7, 6, 5.6, 8.0]}";
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readValue(json, JsonNode.class);
if (node.isObject()) {
    ObjectNode obj = mapper.convertValue(node, ObjectNode.class);
    if (obj.has("a")) {
        System.out.println("a=" + obj.get("a").asDouble());
    }
}

通过这样的解决方案：

String json = "{\"a\":2, \"b\":\"a string\", \"c\": [6.7, 6, 5.6, 8.0]}";
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readTree(json);
if (node.isObject()) {
    ObjectNode obj = (ObjectNode) node;
    if (obj.has("a")) {
        System.out.println("a=" + obj.get("a").asDouble());
    }
}     

或者我使用JsonFactory和JsonParser遇到的解决方案可能还有更多的选择...

Or over solutions that I came across using JsonFactory and JsonParser and maybe even more options...

看来mapper.readValue是最通用的，可以在很多情况下使用：读取JsonNode，ObjectNode， ArrayNode，PoJo等等。那么为什么我要使用mapper.readTree？

It seems to mee that mapper.readValue is most generic and can be used in a lot of cases: read to JsonNode, ObjectNode, ArrayNode, PoJo, etc. So why would I want to use mapper.readTree?

将JsonNode转换为ObjectNode的最佳方法是什么？刚刚转换为ObjectNode？或者使用像mapper.convertValue这样的东西？

And what is the best way to convert a JsonNode to an ObjectNode? Just cast to ObjectNode? Or use something like mapper.convertValue?

推荐答案

readValue（）可以用于任何和所有类型，包括 JsonNode 。 readTree（）仅适用于 JsonNode （树模型）;并且是为了方便而添加的。

readValue() can be used for any and all types, including JsonNode. readTree() only works for JsonNode (tree model); and is added for convenience.

请注意，您永远不想使用您的第一个示例：它相当于将您的节点写为JSON，然后将其读回 - 只是施展它。

Note that you NEVER want to use your first example: it is equivalent to writing out your node as JSON, then reading it back -- just cast it.

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