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如果一个JSON字符串里有一个字段,它的值又是一个完整的JSON字符串,那么这时候,它可能会含有转义字符。举个例子,我们有一个文件  codelast.json,它的内容为一行字符串:

{"aaa":"{\"ccc\":\"ccc\",\"ddd\":\"ddd\"}","bbb":{"ccc":"ccc","ddd":"ddd"}}

现在要用Jackson解析它为一个Java对象,怎么做?

我们先把这个JSON格式化,便于观察它的结构:

{
    "aaa": "{\"ccc\":\"ccc\",\"ddd\":\"ddd\"}", 
    "bbb": {
        "ccc": "ccc", 
        "ddd": "ddd"
    }
}

所以按照Jackson的套路,我们似乎很容易就可以写出对应的JSON解析代码,先定义几个POJO:
A.java 的主要代码:

public class A {
  private AAA aaa;
  private BBB bbb;

  public AAA getAaa() {
    return aaa;
  }

  public void setAaa(AAA aaa) {
    this.aaa = aaa;
  }

  public BBB getBbb() {
    return bbb;
  }

  public void setBbb(BBB bbb) {
    this.bbb = bbb;
  }
}

文章来源:https://www.codelast.com/
AAA.java 的主要代码:

public class AAA {
  private String ccc;
  private String ddd;

  public String getCcc() {
    return ccc;
  }

  public void setCcc(String ccc) {
    this.ccc = ccc;
  }

  public String getDdd() {
    return ddd;
  }

  public void setDdd(String ddd) {
    this.ddd = ddd;
  }
}

文章来源:https://www.codelast.com/
BBB.java 的主要代码:

public class BBB {
  private String ccc;
  private String ddd;

  public String getCcc() {
    return ccc;
  }

  public void setCcc(String ccc) {
    this.ccc = ccc;
  }

  public String getDdd() {
    return ddd;
  }

  public void setDdd(String ddd) {
    this.ddd = ddd;
  }
}

文章来源:https://www.codelast.com/
然后就是解析的逻辑:

String json = FileUtils.readFileToString(new File("/home/codelast/codelast.json"));
ObjectMapper mapper = new ObjectMapper();
A data = mapper.readValue(json, A.class);

我们的目的是把codelast.json这个文件里保存的JSON字符串解析成类A的一个对象data。

这不是很简单嘛?要不运行一下试试看?
马上报错:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.codelast.AAA` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{"ccc":"ccc","ddd":"ddd"}')
 at [Source: (String)"{"aaa":"{\"ccc\":\"ccc\",\"ddd\":\"ddd\"}","bbb":{"ccc":"ccc","ddd":"ddd"}}
"; line: 1, column: 8] (through reference chain: com.codelast.A["aaa"])
 
at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1342)
at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1031)
at com.fasterxml.jackson.databind.deser.ValueInstantiator._createFromStringFallbacks(ValueInstantiator.java:371)
at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createFromString(StdValueInstantiator.java:323)
at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromString(BeanDeserializerBase.java:1366)
at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:171)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:161)
at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:127)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:288)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:151)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2992)

文章来源:https://www.codelast.com/
这是什么情况?事实上,这就是由于JSON的“aaa”这个字段里面包含转义字符造成的,如果你Google一下,可能会找到这个知乎帖子,它告诉我们,要为Jackson的 ObjectMapper 设置一个属性为true:Feature.ALLOW_UNQUOTED_CONTROL_CHARS
所以我们可以试试:

String json = FileUtils.readFileToString(new File("/home/codelast/codelast.json"));
ObjectMapper mapper = new ObjectMapper();
mapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_CONTROL_CHARS, true);
A data = mapper.readValue(json, A.class);

再执行代码,会发现报一样的错。
所以上面的解决方案是无效的。
文章来源:https://www.codelast.com/
正确解决这个问题的办法,是参考 Stack Overflow的这个帖子。里面说到,应该创建一个 JsonDeserializer 来搞定这个问题。下面,我就把正确的代码依次贴上来:
A.java 的主要代码(和之前不一样,这里使用了一个自定义的 AAADeserializer):

public class A {
  @JsonDeserialize(using = AAADeserializer.class)
  private AAA aaa;

  private BBB bbb;

  public AAA getAaa() {
    return aaa;
  }

  public void setAaa(AAA aaa) {
    this.aaa = aaa;
  }

  public BBB getBbb() {
    return bbb;
  }

  public void setBbb(BBB bbb) {
    this.bbb = bbb;
  }
}

文章来源:https://www.codelast.com/
AAA.java、BBB.java 和之前完全一样,不重复贴代码上来。
下面就是关键的 AAADeserializer.java 的主要代码:

public class AAADeserializer extends JsonDeserializer<AAA> {
  private static final ObjectMapper mapper = new ObjectMapper();

  @Override
  public AAA deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
    return mapper.readValue(jsonParser.getText(), AAA.class);
  }
}

文章来源:https://www.codelast.com/
然后就可以解析JSON了:

String json = FileUtils.readFileToString(new File("/home/codelast/codelast.json"));
ObjectMapper mapper = new ObjectMapper();
A data = mapper.readValue(json, A.class);

你这时候再运行一下这段代码,就会发现不再报错,JSON已经成功解析到对象 data 中,问题解决!

[原创] 如何用Jackson解析含有转义字符的JSON到Java对象?
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