1. Getting started
There is an ObjectMapper class in Jackson that is very practical, used to interchange Java objects and JSON.
1.JAVA object to JSON[JSON serialization]
import java.io.IOException; import java.text.ParseException; import java.text.SimpleDateFormat; import com.fasterxml.jackson.databind.ObjectMapper; public class JacksonDemo { public static void main(String[] args) throws ParseException, IOException { User user = new User(); user.setName("Xiaomin"); user.setEmail("[email protected]"); user.setAge(20); SimpleDateFormat dateformat = new SimpleDateFormat("yyyy-MM-dd"); user.setBirthday(dateformat.parse("1996-10-01")); /** * ObjectMapper is the core of JSON operations, and all JSON operations of Jackson are implemented in ObjectMapper. * ObjectMapper has multiple JSON serialization methods, which can save JSON strings in different media such as File, OutputStream, etc. * writeValue(File arg0, Object arg1) converts arg1 into a json sequence and saves it to arg0 file. * writeValue(OutputStream arg0, Object arg1) converts arg1 into a json sequence and saves it to the arg0 output stream. * writeValueAsBytes(Object arg0) converts arg0 into a json sequence and outputs the result into a byte array. * writeValueAsString(Object arg0) converts arg0 into a json sequence and outputs the result into a string. */ ObjectMapper mapper = new ObjectMapper(); //User class to JSON //Output result: {"name":"Xiaomin","age":20,"birthday":844099200000,"email":"[email protected]"} String json = mapper.writeValueAsString(user); System.out.println(json); //Java collection to JSON //Output result: [{"name":"Xiaomin","age":20,"birthday":844099200000,"email":"[email protected]"}] List<User> users = new ArrayList<User>(); users.add(user); String jsonlist = mapper.writeValueAsString(users); System.out.println(jsonlist); } }2.JSON to Java class [JSON deserialization]
import java.io.IOException; import java.text.ParseException; import com.fasterxml.jackson.databind.ObjectMapper; public class JacksonDemo { public static void main(String[] args) throws ParseException, IOException { String json = "{/"name/":/"Xiaomin/",/"age/":20,/"birthday/":844099200000,/"email/":/"[email protected]/"}"; /** * ObjectMapper supports JSON deserialization from byte[], File, InputStream, strings, etc. */ ObjectMapper mapper = new ObjectMapper(); User user = mapper.readValue(json, User.class); System.out.println(user); } }
2. Jackson supports 3 usage methods:
1. Data Binding: The most convenient to use.
(1)Full Data Binding:
private static final String MODEL_BINDING = "{/"name/":/"name1/",/"type/":1}"; public void fullDataBinding() throws Exception{ ObjectMapper mapper = new ObjectMapper(); Model user = mapper.readValue(MODEL_BINDING, Model.class);//readValue to an entity class. System.out.println(user.getName()); System.out.println(user.getType()); }Model class:
private static class Model{ private String name; private int type; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getType() { return type; } public void setType(int type) { this.type = type; } }(2)Raw Data Binding:
/** Concrete Java types that Jackson will use for simple data binding are: JSON Type Java Type object LinkedHashMap<String,Object> array ArrayList<Object> string String number(no fraction) Integer, Long or BigInteger (smallest applicable) number(fraction) Double(configurable to use BigDecimal) true|false Boolean null null */ public void rawDataBinding() throws Exception{ ObjectMapper mapper = new ObjectMapper(); HashMap map = mapper.readValue(MODEL_BINDING,HashMap.class);//readValue to a primitive data type. System.out.println(map.get("name")); System.out.println(map.get("type")); }(3)generic Data Binding:
private static final String GENERIC_BINDING = "{/"key1/":{/"name/":/"name2/",/"type/":2},/"key2/":{/"name/":/"name3/",/"type/":3}}"; public void genericDataBinding() throws Exception{ ObjectMapper mapper = new ObjectMapper(); HashMap<String,Model> modelMap = mapper.readValue(GENERIC_BINDING,new TypeReference<HashMap<String,Model>>(){});//readValue into a paradigm data. Model model = modelMap.get("key2"); System.out.println(model.getName()); System.out.println(model.getType()); }2. Tree Model: The most flexible.
private static final String TREE_MODEL_BINDING = "{/"treekey1/":/"treevalue1/",/"treekey2/":/"treevalue2/",/"children/":[{/"childkey1/":/"childkey1/"}]}"; public void treeModelBinding() throws Exception{ ObjectMapper mapper = new ObjectMapper(); JsonNode rootNode = mapper.readTree(TREE_MODEL_BINDING); //path and get function the same as get, but when the node cannot be found, it returns missing node instead of Null. String treekey2value = rootNode.path("treekey2").getTextValue();// System.out.println("treekey2value:" + treekey2value); JsonNode childrenNode = rootNode.path("children"); String childkey1Value = childrenNode.get(0).path("childkey1").getTextValue(); System.out.println("childkey1Value:"+childkey1Value); //Create the root node ObjectNode root = mapper.createObjectNode(); //Create child node1 ObjectNode node1 = mapper.createObjectNode(); node1.put("nodekey1",1); node1.put("nodekey2",2); //Bind child node 1 root.put("child",node1); //ArrayNode arrayNode = mapper.createArrayNode(); arrayNode.add(node1); arrayNode.add(1); //Bind the array node root.put("arraynode", arrayNode); //JSON reads the tree node JsonNode valueToTreeNode = mappper.valueToTree(TREE_MODEL_BINDING); //Bind JSON node root.put("valuetotreenode",valueToTreeNode); //Bind the JSON node object JsonNode bindJsonNode = mappper.readValue(GENERIC_BINDING, JsonNode.class); //Bind JSON to the JSON node object. //Bind JSON node root.put("bindJsonNode",bindJsonNode); System.out.println(mapper.writeValueAsString(root)); } 3. Streaming API: Best performance.
For programs with high performance requirements, it is recommended to use the streaming API. Otherwise, using other methods, whether it is to create a JsonGenerator or a JsonParser, JsonFactory is used.
package com.jingshou.jackson; import java.io.File; import java.io.IOException; import com.fasterxml.jackson.core.JsonEncoding; import com.fasterxml.jackson.core.JsonFactory; import com.fasterxml.jackson.core.JsonGenerator; import com.fasterxml.jackson.core.JsonParser; import com.fasterxml.jackson.core.JsonToken; public class JacksonTest6 { public static void main(String[] args) throws IOException { JsonFactory jfactory = new JsonFactory(); /*** write to file ***/ JsonGenerator jGenerator = jfactory.createGenerator(new File( "c://user.json"), JsonEncoding.UTF8); jGenerator.writeStartObject(); // { jGenerator.writeStringField("name", "mkyong"); // "name" : "mkyong" jGenerator.writeNumberField("age", 29); // "age" : 29 jGenerator.writeFieldName("messages"); // "messages" : jGenerator.writeStartArray(); // [ jGenerator.writeString("msg 1"); // "msg 1" jGenerator.writeString("msg 2"); // "msg 2" jGenerator.writeString("msg 3"); // "msg 3" jGenerator.writeEndArray(); // ] jGenerator.writeEndObject(); // } jGenerator.close(); /*** read from file ***/ JsonParser jParser = jfactory.createParser(new File("c://user.json")); // loop until token equal to "}" while (jParser.nextToken() != JsonToken.END_OBJECT) { String fieldname = jParser.getCurrentName(); if ("name".equals(fieldname)) { // current token is "name", // move to next, which is "name"'s value jParser.nextToken(); System.out.println(jParser.getText()); // display mkyong } if ("age".equals(fieldname)) { // current token is "age", // move to next, which is "name"'s value jParser.nextToken(); System.out.println(jParser.getIntValue()); // display 29 } if ("messages".equals(fieldname)) { jParser.nextToken(); // current token is "[", move next // messages is array, loop until token equal to "]" while (jParser.nextToken() != JsonToken.END_ARRAY) { // display msg1, msg2, msg3 System.out.println(jParser.getText()); } } } jParser.close(); } }