Jackson的简单使用
2023-04-12
Jackson既可以将对象转化为jason格式，又可以将jason字符串转化为java对象。他的效率要比xstream高，因为他是单例的。
首先，我们创建两个对象，以便jackson来使用：
public class User {
private int id;
private String username;
private String password;
private Group group;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Group getGroup() {
return group;
}
public void setGroup(Group group) {
this.group = group;
}
public User() {
}
public User(int id, String username, String password, Group group) {
super();
this.id = id;
this.username = username;
this.password = password;
this.group = group;
}
public User(int id, String username, String password) {
super();
this.id = id;
this.username = username;
this.password = password;
}
}
public class Group {
private int id;
private String name;
List users;
public void addUser(User u) {
if(users==null) {
users = new ArrayList();
}
users.add(u);
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Group(int id, String name) {
super();
this.id = id;
this.name = name;
}
public Group() {
}
}
下面通过代码和注释来讲解jackson的简单使用
@Test
public void test01() {
StringWriter out = new StringWriter();
JsonGenerator jg = null;
try {
//1、创建JsonFactory
JsonFactory jf = new JsonFactory();
//2、创建JsonGenerator
jg = jf.createJsonGenerator(out);
//使用一种相对漂亮的格式输出
jg.useDefaultPrettyPrinter();
//3、创建ObjectMapper,通过ObjectMapper来写对象
User u = new User(1,"劳动法","123",new Group(1,"财务处"));
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(jg, u);
System.out.println(out.toString());
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
try {
if(jg!=null) jg.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
@Test
public void test02() {
StringWriter out = new StringWriter();
JsonGenerator jg = null;
try {
//1、创建JsonFactory
JsonFactory jf = new JsonFactory();
//2、创建JsonGenerator
jg = jf.createJsonGenerator(out);
//使用一种相对漂亮的格式输出
// jg.useDefaultPrettyPrinter();
//3、创建ObjectMapper,通过ObjectMapper来写对象
List us = new ArrayList();
User u = new User(1,"劳动法","123",new Group(1,"财务处"));
us.add(u);
u = new User(2,"猪八戒","123",new Group(1,"财务处"));
us.add(u);
u = new User(3,"张学友","123",new Group(2,"教务处"));
us.add(u);
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(jg, us);
System.out.println(out.toString());
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
try {
if(jg!=null) jg.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
@Test
public void test02_1() {
List us = new ArrayList();
User u = new User(1,"劳动法","123",new Group(1,"财务处"));
us.add(u);
u = new User(2,"猪八戒","123",new Group(1,"财务处"));
us.add(u);
u = new User(3,"张学友","123",new Group(2,"教务处"));
us.add(u);
System.out.println(JsonUtil.getInstance().obj2json(us));
}
@Test
public void test03() {
try {
String json = "{\"id\":1,\"username\":\"杂货\",\"password\":\"123\",\"group\":{\"id\":1,\"name\":\"财务处\"}}";
ObjectMapper mapper = new ObjectMapper();
User u = mapper.readValue(json, User.class);
System.out.println(u.getId()+","+u.getUsername()+","+u.getGroup().getName());
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
@Test
public void test04() {
try {
String json = "[{\"id\":1,\"username\":\"劳动法\",\"password\":\"123\",\"group\":{\"id\":1,\"name\":\"财务处\"}},{\"id\":2,\"username\":\"猪八戒\",\"password\":\"123\",\"group\":{\"id\":1,\"name\":\"财务处\"}},{\"id\":3,\"username\":\"张学友\",\"password\":\"123\",\"group\":{\"id\":2,\"name\":\"教务处\"}}]";
ObjectMapper mapper = new ObjectMapper();
//在把json转换为list的时候，不能直接存储Bean对象，list中存储的是Map对象
/*List us = mapper.readValue(json,List.class);
for(User u:us) {
System.out.println(u.getUsername());
}*/
/**
* 所以的json都是通过map来存储的，不会直接存储bean，但是在开发中，把字符串转换为对象
* 一般只会对单个的对象转换，很少会用到对对象的完全转换
*/
List> us = mapper.readValue(json, List.class);
for(Map m:us) {
System.out.println(m.get("id"));
System.out.println(m.get("group"));
}
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
@Test
public void test05() {
try {
String json = "[{\"id\":1,\"username\":\"劳动法\",\"password\":\"123\",\"group\":{\"id\":1,\"name\":\"财务处\"}},{\"id\":2,\"username\":\"猪八戒\",\"password\":\"123\",\"group\":{\"id\":1,\"name\":\"财务处\"}},{\"id\":3,\"username\":\"张学友\",\"password\":\"123\",\"group\":{\"id\":2,\"name\":\"教务处\"}}]";
ObjectMapper mapper = new ObjectMapper();
/*
* 在Jaskson中提供了一种基于节点的读取方法
*/
JsonNode node = mapper.readTree(json);
//判断这个节点是否是数组
System.out.println(node.isArray());
System.out.println(node.size());
System.out.println(node.get(0).get("username"));
System.out.println(node.get(0).get("group").get("name"));
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
} 我们通过上述代码已经简单了解了jackson的使用，根据上述代码，我们可以构造一个jackson的util类，来更方便的使用他。上述代码中的test02_1也用到了此类。
public class JsonUtil {
private static JsonUtil ju;
private static JsonFactory jf;
private static ObjectMapper mapper;
private JsonUtil(){}
public static JsonUtil getInstance() {
if(ju==null) ju = new JsonUtil();
return ju;
}
public static ObjectMapper getMapper() {
if(mapper==null) {
mapper = new ObjectMapper();
}
return mapper;
}
public static JsonFactory getFactory() {
if(jf==null) jf = new JsonFactory();
return jf;
}
public String obj2json(Object obj) {
JsonGenerator jg = null;
try {
jf = getFactory();
mapper = getMapper();
StringWriter out = new StringWriter();
jg = jf.createJsonGenerator(out);
mapper.writeValue(jg, obj);
return out.toString();
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
if(jg!=null) jg.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
public Object json2obj(String json,Class clz) {
try {
mapper = getMapper();
return mapper.readValue(json,clz);
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
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