Error converting JSON string to map in Java using Jackson

问题

I have this little piece of code, and I'm trying to convert a JSON string to a map.

String json = "[{'code':':)','img':'<img src=/faccine/sorriso.gif>'}]";
ObjectMapper mapper = new ObjectMapper();
Map<String,String> userData = mapper.readValue(json,new TypeReference<HashMap<String,String>>() { });

But it returns the following error:

org.codehaus.jackson.map.JsonMappingException: Can not deserialize instance of java.util.HashMap out of START_ARRAY token
 at [Source: java.io.StringReader@1b1756a4; line: 1, column: 1]
    at org.codehaus.jackson.map.JsonMappingException.from(JsonMappingException.java:163)
    at org.codehaus.jackson.map.deser.StdDeserializationContext.mappingException(StdDeserializationContext.java:198)
    at org.codehaus.jackson.map.deser.MapDeserializer.deserialize(MapDeserializer.java:151)
    at org.codehaus.jackson.map.deser.MapDeserializer.deserialize(MapDeserializer.java:25)
    at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2131)
    at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1402)
    at CodeSnippet_19.run(CodeSnippet_19.java:13)
    at org.eclipse.jdt.internal.debug.ui.snippeteditor.ScrapbookMain1.eval(ScrapbookMain1.java:20)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.eclipse.jdt.internal.debug.ui.snippeteditor.ScrapbookMain.evalLoop(ScrapbookMain.java:54)
    at org.eclipse.jdt.internal.debug.ui.snippeteditor.ScrapbookMain.main(ScrapbookMain.java:35)

What am I doing wrong?

回答1:

From what I remember Jackson is used to convert json to java classes - it is probably expecting the first object is reads to be a map, like

String json = "{'code':':)','img':'<img src=/faccine/sorriso.gif>'}";

回答2:

Right: you're asking Jackson to map a JSON Array into an object; there is no obvious way to do that. So, tofarr's answer is correct.

But if you wanted a List or an array, you could achieved it easily by:

List<?> list = mapper.readValue(json, List.class);

Or with full type reference; optional in this case because you just want Lists, Maps and Strings.

来源：https://stackoverflow.com/questions/5018340/error-converting-json-string-to-map-in-java-using-jackson

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java

json

dictionary

hashmap

jackson