使用Jackson在Java中处理JSON

在工作中实际使用到Java处理JSON的情况，且有很大部分都使用的是开源工具Jackson实现的。

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对象转换为JSON

1. Student st=new Student(); //Java Object
2. ObjectMapper mapper = new ObjectMapper();
3. java.text.DateFormat myFormat = new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
4. mapper.getSerializationConfig().setDateFormat(myFormat);
5. try {
6. //返回字符串
7. String res = mapper.writeValueAsString(st);
8. System.out.println(res);
9. //输出格式化后的字符串(有性能损耗)
10. res = mapper.defaultPrettyPrintingWriter().writeValueAsString(st);
11. System.out.println(res);
12. mapper.writeValue(new File("D:\\st.json"), st); //指定文件写入
13. //设置序列化配置(全局),设置序列化时不输出空值.
14. mapper.getSerializationConfig().setSerializationInclusion(Inclusion.NON_NULL);
15. res = mapper.writeValueAsString(st);
16. System.out.println(res);
17. } catch (Exception e) {
18. e.printStackTrace();
19. }

Student st=new Student(); //Java Object
ObjectMapper mapper = new ObjectMapper();
java.text.DateFormat myFormat = new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
mapper.getSerializationConfig().setDateFormat(myFormat);
try {
    //返回字符串
    String res = mapper.writeValueAsString(st);
    System.out.println(res);
//输出格式化后的字符串(有性能损耗)
res = mapper.defaultPrettyPrintingWriter().writeValueAsString(st);
System.out.println(res);

mapper.writeValue(new File("D:\\st.json"), st); //指定文件写入

//设置序列化配置(全局),设置序列化时不输出空值.
mapper.getSerializationConfig().setSerializationInclusion(Inclusion.NON_NULL);

res = mapper.writeValueAsString(st);
System.out.println(res);

} catch (Exception e) {

e.printStackTrace();

}

2.JSON反序列化为Java对象

1. String json = "{\"error\":0,\"data\":{\"name\":\"ABC\",\"age\":20,\"phone\":{\"home\":\"abc\",\"mobile\":\"def\"},\"friends\":[{\"name\":\"DEF\",\"phone\":{\"home\":\"hij\",\"mobile\":\"klm\"}},{\"name\":\"GHI\",\"phone\":{\"home\":\"nop\",\"mobile\":\"qrs\"}}]},\"other\":{\"nickname\":[]}}";
2. ObjectMapper mapper = new ObjectMapper();
3. //解析器支持解析单引号
4. mapper.configure(Feature.ALLOW_SINGLE_QUOTES, true);
5. //解析器支持解析结束符
6. mapper.configure(Feature.ALLOW_UNQUOTED_CONTROL_CHARS, true);
7. try {
8. //转换为HashMap对象
9. HashMap jsonMap = mapper.readValue(json, HashMap.class);
10. Map<String, Map<String, Object>> maps = objectMapper.readValue(json, Map.class);
11. System.out.println(maps.get("error"));//0
12. System.out.println((Object) (maps.get("data").get("phone")));//{home=abc, mobile=def}
13. } catch (Exception e) {
14. e.printStackTrace();
15. }

String json = "{\"error\":0,\"data\":{\"name\":\"ABC\",\"age\":20,\"phone\":{\"home\":\"abc\",\"mobile\":\"def\"},\"friends\":[{\"name\":\"DEF\",\"phone\":{\"home\":\"hij\",\"mobile\":\"klm\"}},{\"name\":\"GHI\",\"phone\":{\"home\":\"nop\",\"mobile\":\"qrs\"}}]},\"other\":{\"nickname\":[]}}";
ObjectMapper mapper = new ObjectMapper();
//解析器支持解析单引号
mapper.configure(Feature.ALLOW_SINGLE_QUOTES, true);
//解析器支持解析结束符
mapper.configure(Feature.ALLOW_UNQUOTED_CONTROL_CHARS, true);
try {
    //转换为HashMap对象
    HashMap jsonMap = mapper.readValue(json, HashMap.class);
    Map<String, Map<String, Object>> maps = objectMapper.readValue(json, Map.class);
    System.out.println(maps.get("error"));//0
    System.out.println((Object) (maps.get("data").get("phone")));//{home=abc, mobile=def}
} catch (Exception e) {
    e.printStackTrace();
}

二.Jackson支持三种使用方式

1.Data Binding：最方便使用

(1)Full Data Binding

1. /*
2. * Full Data Binding
3. */
4. public static void fullDataBinding() {
5. ObjectMapper mapper = new ObjectMapper();
6. try {
7. Model model = mapper.readValue(MODEL_BINDING, Model.class);
8. //readValue到一个实体类中.
9. System.out.println(model.getName()); //name1
10. System.out.println(model.getType()); //1
11. } catch (Exception e) {
12. e.printStackTrace();
13. }
14. }
15. private static class Model {
16. private String name;
17. private int type;
18. public String getName() {
19. return name;
20. }
21. public void setName(String name) {
22. this.name = name;
23. }
24. public int getType() {
25. return type;
26. }
27. public void setType(int type) {
28. this.type = type;
29. }
30. }

/*
 * Full Data Binding
 */
public static void fullDataBinding() {
ObjectMapper mapper = new ObjectMapper();
try {
    Model model = mapper.readValue(MODEL_BINDING, Model.class);
    //readValue到一个实体类中.
    System.out.println(model.getName()); //name1
    System.out.println(model.getType()); //1
} catch (Exception e) {
    e.printStackTrace();
}

}
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. * Raw Data Binding
3. */
4. public static void rawDataBinding() {
5. ObjectMapper mapper = new ObjectMapper();
6. try {
7. HashMap map = mapper.readValue(MODEL_BINDING, HashMap.class);//readValue到一个原始数据类型.
8. System.out.println(map.get("name")); //name1
9. System.out.println(map.get("type")); //1
10. } catch (Exception e) {
11. e.printStackTrace();
12. }
13. }

/*
 * Raw Data Binding
 */
public static void rawDataBinding() {
    ObjectMapper mapper = new ObjectMapper();
    try {
        HashMap map = mapper.readValue(MODEL_BINDING, HashMap.class);//readValue到一个原始数据类型.
        System.out.println(map.get("name")); //name1
        System.out.println(map.get("type")); //1
    } catch (Exception e) {
        e.printStackTrace();
    }
}

(3)generic Data Binding

1. /*
2. * generic Data Binding
3. */
4. public static void genericDataBinding() {
5. ObjectMapper mapper = new ObjectMapper();
6. try {
7. HashMap<String, Model> modelMap = mapper.readValue(GENERIC_BINDING,
8. new TypeReference<HashMap<String, Model>>() {
9. });//readValue到一个范型数据中.
10. Model model = modelMap.get("key2");
11. System.out.println(model.getName()); //name3
12. System.out.println(model.getType()); //3
13. } catch (Exception e) {
14. e.printStackTrace();
15. }
16. }

/*
 * generic Data Binding
 */
public static void genericDataBinding() {
ObjectMapper mapper = new ObjectMapper();
try {
    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()); //name3
    System.out.println(model.getType()); //3
} catch (Exception e) {
    e.printStackTrace();
}

}

2.Tree Model：最灵活

1. /*
2. * Tree Model：最灵活
3. */
4. public static void treeModelBinding() {
5. ObjectMapper mapper = new ObjectMapper();
6. try {
7. JsonNode rootNode = mapper.readTree(TREE_MODEL_BINDING);
8. //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.
9. String treekey2value = rootNode.path("treekey2").getTextValue();//
10. System.out.println("treekey2value:" + treekey2value);
11. JsonNode childrenNode = rootNode.path("children");
12. String childkey1Value = childrenNode.get(0).path("childkey1").getTextValue();
13. System.out.println("childkey1Value:" + childkey1Value);
14. //创建根节点
15. ObjectNode root = mapper.createObjectNode();
16. //创建子节点1
17. ObjectNode node1 = mapper.createObjectNode();
18. node1.put("nodekey1", 1);
19. node1.put("nodekey2", 2);
20. //绑定子节点1
21. root.put("child", node1);
22. //数组节点
23. ArrayNode arrayNode = mapper.createArrayNode();
24. arrayNode.add(node1);
25. arrayNode.add(1);
26. //绑定数组节点
27. root.put("arraynode", arrayNode);
28. //JSON读到树节点
29. JsonNode valueToTreeNode = mapper.valueToTree(TREE_MODEL_BINDING);
30. //绑定JSON节点
31. root.put("valuetotreenode", valueToTreeNode);
32. //JSON绑定到JSON节点对象
33. JsonNode bindJsonNode = mapper.readValue(GENERIC_BINDING, JsonNode.class);//绑定JSON到JSON节点对象.
34. //绑定JSON节点
35. root.put("bindJsonNode", bindJsonNode);
36. System.out.println(mapper.writeValueAsString(root));
37. } catch (Exception e) {
38. e.printStackTrace();
39. }
40. }

/*
 * Tree Model：最灵活
 */
public static void treeModelBinding() {
    ObjectMapper mapper = new ObjectMapper();
    try {
        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));
} catch (Exception e) {
    e.printStackTrace();
}

}

3.Streaming API。最佳性能

见官方文档例子。

进一步学习资料:

1.http://wiki.fasterxml.com/JacksonInFiveMinutes Jackson官方教程示例

2.http://wiki.fasterxml.com/JacksonJavaDocs Jackson在线API文档

3.http://hjg1988.iteye.com/blog/561368 JSON工具性能比较:json-lib和jackson进行Java对象到json字符串序列化。

文章来源：http://shensy.iteye.com/blog/1717776