When creating a REST API servlet,\u00a0Jackson provides a huge list of possibilities to manipulate the JSON data, mostly using annotations.<\/p>\n
Let’s demonstrate some of them with this\u00a0little class, which has only two properties:<\/p>\n
public class Demo {\r\n\r\n private String foo;\r\n<\/code> private String bar;\r\n\r\n<\/code> public String getFoo() { \r\n return foo;\r\n }\r\n public void setFoo(String foo) {\r\n this.foo = foo;\r\n }\r\n\r\n<\/code> public String getBar() { \r\n return bar;\r\n }\r\n\r\n public void setBar(String bar) {\r\n this.bar = bar;\r\n }<\/code>\r\n}<\/code><\/pre>\nThe playground converts the content<\/em>\u00a0string to a POJO and back to a string:<\/p>\nString content = \"{ \\\"foo\\\": \\\"bar\\\" }\";\r\n \r\n \/\/ init the ObjectMapper\r\n ObjectMapper mapper = new ObjectMapper();\r\n \r\n \/\/ build the Object\r\n Demo test = null;\r\n try {\r\n test = mapper.readValue(content, Demo.class);\r\n } catch (Exception e) {\r\n e.printStackTrace();\r\n }\r\n\r\n \/\/ and now convert it back to a String\r\n String data = null;\r\n try {\r\n data = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(test);\r\n } catch (Exception e) {\r\n e.printStackTrace();\r\n }\r\n\r\n System.out.println( data );<\/code><\/pre>\nIf we run this code, the result is not really spectacular:<\/p>\n
{\r\n \"foo\" : \"bar\",\r\n \"bar\" : null\r\n}<\/code><\/pre>\nSo let’s ignore the property foo by adding the annotation @JsonIgnoreProperties<\/em> to the Demo<\/em> class:<\/p>\n@JsonIgnoreProperties({\"foo\"})\r\npublic class Demo { ... }<\/code><\/pre>\nNow, foo<\/em> is no longer in our resulting JSON:<\/p>\n{\r\n \"bar\" : null\r\n}<\/code><\/pre>\nThe property bar<\/em> is null, and we don’t like nulled properties in our JSON. That’s why we add another annotation, @JsonInclude<\/em>:<\/p>\n@JsonInclude(JsonInclude.Include.NON_EMPTY)\r\npublic class Demo { ... }<\/code><\/pre>\nAfter removing the previously added @JsonIgnoreProperties<\/em> annotation, our result looks like this (the empty property bar<\/em> was skipped):<\/p>\n{\r\n \"foo\" : \"bar\"\r\n}<\/code><\/pre>\nWhat happens if we change our content<\/em> string, and add an unknown property?<\/p>\nString content = \"{ \\\"foo\\\": \\\"bar\\\", \\\"undefined\\\": \\\"property\\\" }\";<\/code><\/pre>\nAn error occurs because Jackson does not know how to handle the new property:<\/p>\n
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field \"undefined\" (class ch.hasselba.JacksonPlayground.Demo), not marked as ignorable (2 known properties: \"foo\", \"bar\"])\r\n at [Source: { \"foo\": \"bar\", \"undefined\": \"property\" }; line: 1, column: 31] (through reference chain: ch.hasselba.JacksonPlayground.Demo[\"undefined\"])\r\n at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:51)\r\n at com.fasterxml.jackson.databind.DeserializationContext.reportUnknownProperty(DeserializationContext.java:817)\r\n at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:954)\r\n at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1315)\r\n at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1293)\r\n at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:249)\r\n at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:136)\r\n at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3560)\r\n at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2576)\r\n at ch.hasselba.JacksonPlayground.App.main(App.java:24)\r\nnull<\/code><\/pre>\nBut there are two annotations to the rescue, @JsonAnyGetter<\/em> &\u00a0@JsonAnySetter<\/em>. By changing our Demo<\/em> class and adding the following lines of code…<\/p>\nprivate Map<String, Object> others = new ConcurrentHashMap<String, Object>();\r\n\r\n@JsonAnyGetter\r\npublic Map<String, Object> getOthers() {\r\n return this.others;\r\n}\r\n\r\n@JsonAnySetter\r\npublic void addOther(final String name, final Object value) {\r\n this.others.put(name, value);\r\n}<\/code><\/pre>\n… Jackson now puts all the unknown\/undefined properties in the others<\/em> map (uses the method defined by @JsonSetter<\/em>). And then it uses the method with the @JsonGetter<\/em> annotation when producing the JSON from the Demo instance.<\/p>\n{\r\n \"foo\" : \"bar\",\r\n \"bar\" : null,\r\n \"undefined\" : \"property\"\r\n}\r\n<\/code><\/pre>\nWhat if we want to handle multiple „Demo“ objects in a JSON Array?<\/p>\n
String content = \"[ { \\\"foo\\\": \\\"bar\\\" }, {\\\"foo\\\": \\\"bar2\\\" } ]\";<\/code><\/pre>\nIn this case we change our reading routine to work with lists:<\/p>\n
\/\/ build the Object\r\nList<Demo> test = null;\r\ntry {\r\n test = mapper.readValue(content, mapper.getTypeFactory()\r\n .constructCollectionType(List.class, Demo.class));\r\n} catch (Exception e) {\r\n e.printStackTrace();\r\n}<\/code><\/pre>\nIn the result all entries are now contained in the list of Demo<\/em>\u00a0objects:<\/p>\n[ {\r\n \"foo\" : \"bar\",\r\n \"bar\" : null\r\n}, {\r\n \"foo\" : \"bar2\",\r\n \"bar\" : null\r\n} ]<\/code><\/pre>\nBack to our\u00a0RestApiApplication<\/a>, have a look at this line:<\/p>\nobjMapper.setSerializationInclusion(Include.NON_EMPTY);<\/code><\/pre>\nThis removes all empty properties globally from the generated output of our the servlet. So there is no need to add the @JsonIgnore<\/em> annotation to any class. You can modifiy the globally used ObjectMapper in your servlet with multiple option, more will follow in another blog post.<\/p>\n","protected":false},"excerpt":{"rendered":"When creating a REST API servlet,\u00a0Jackson provides a huge list of possibilities to manipulate the JSON data, mostly using annotations. Let’s demonstrate some of them with this\u00a0little class, which has only two properties: public class Demo { private String foo; … Weiterlesen