一、入门Jackson中有个ObjectMapper类很是实用，用于Java对象与JSON的互换。
1.JAVA对象转JSON[JSON序列化]| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 | 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( "小民" ); user.setEmail( "xiaomin@sina.com" ); user.setAge( 20 ); SimpleDateFormat dateformat = new SimpleDateFormat( "yyyy-MM-dd" ); user.setBirthday(dateformat.parse( "1996-10-01" )); /** * ObjectMapper是JSON操作的核心，Jackson的所有JSON操作都是在ObjectMapper中实现。 * ObjectMapper有多个JSON序列化的方法，可以把JSON字符串保存File、OutputStream等不同的介质中。 * writeValue(File arg0, Object arg1)把arg1转成json序列，并保存到arg0文件中。 * writeValue(OutputStream arg0, Object arg1)把arg1转成json序列，并保存到arg0输出流中。 * writeValueAsBytes(Object arg0)把arg0转成json序列，并把结果输出成字节数组。 * writeValueAsString(Object arg0)把arg0转成json序列，并把结果输出成字符串。 */ ObjectMapper mapper = new ObjectMapper(); //User类转JSON //输出结果：{"name":"小民","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"} String json = mapper.writeValueAsString(user); System.out.println(json); //Java集合转JSON //输出结果：[{"name":"小民","age":20,"birthday":844099200000,"email":"xiaomin@sina.com"}] List<User> users = new ArrayList<User>(); users.add(user); String jsonlist = mapper.writeValueAsString(users); System.out.println(jsonlist); } } |
2.JSON转Java类[JSON反序列化]
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | 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\":\"小民\",\"age\":20,\"birthday\":844099200000,\"email\":\"xiaomin@sina.com\"}" ; /** * ObjectMapper支持从byte[]、File、InputStream、字符串等数据的JSON反序列化。 */ ObjectMapper mapper = new ObjectMapper(); User user = mapper.readValue(json, User. class ); System.out.println(user); } } |
二、Jackson支持3种使用方式:1、Data Binding：最方便使用.(1)Full Data Binding：
| 1 2 3 4 5 6 7 | 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到一个实体类中. System.out.println(user.getName()); System.out.println(user.getType()); } |
Model类：
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | 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：
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | /** 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到一个原始数据类型. System.out.println(map.get( "name" )); System.out.println(map.get( "type" )); } |
(3)generic Data Binding：
| 1 2 3 4 5 6 7 8 | 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到一个范型数据中. Model model = modelMap.get( "key2" ); System.out.println(model.getName()); System.out.println(model.getType()); } |
2、Tree Model：最灵活。
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | 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与get作用相同,但是当找不到该节点的时候,返回missing node而不是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); //创建根节点 ObjectNode root = mapper.createObjectNode(); //创建子节点1 ObjectNode node1 = mapper.createObjectNode(); node1.put( "nodekey1" , 1 ); node1.put( "nodekey2" , 2 ); //绑定子节点1 root.put( "child" ,node1); //数组节点 ArrayNode arrayNode = mapper.createArrayNode(); arrayNode.add(node1); arrayNode.add( 1 ); //绑定数组节点 root.put( "arraynode" , arrayNode); //JSON读到树节点 JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING); //绑定JSON节点 root.put( "valuetotreenode" ,valueToTreeNode); //JSON绑定到JSON节点对象 JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode. class ); //绑定JSON到JSON节点对象. //绑定JSON节点 root.put( "bindJsonNode" ,bindJsonNode); System.out.println(mapper.writeValueAsString(root)); } |
3、Streaming API：最佳性能。
对于性能要求高的程序,推荐使用流API,否则使用其他方法不管是创建JsonGenerator还是JsonParser,都是使用JsonFactory。| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 | 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(); } } |