Jackson的简单使用

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<User> users;    public void addUser(User u) {        if(users==null) {            users = new ArrayList<User>();        }        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

<span style="white-space:pre"></span>@Testpublic void test01() {StringWriter out = new StringWriter();JsonGenerator jg = null;try {//1、创建JsonFactoryJsonFactory jf = new JsonFactory();//2、创建JsonGeneratorjg = 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 blocke.printStackTrace();} finally {try {if(jg!=null) jg.close();} catch (IOException e) {e.printStackTrace();}}}@Testpublic void test02() {StringWriter out = new StringWriter();JsonGenerator jg = null;try {//1、创建JsonFactoryJsonFactory jf = new JsonFactory();//2、创建JsonGeneratorjg = 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<User> us = new ArrayList<User>();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 blocke.printStackTrace();} finally {try {if(jg!=null) jg.close();} catch (IOException e) {e.printStackTrace();}}}@Testpublic void test02_1() {List<User> us = new ArrayList<User>();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));}@Testpublic 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();}}@Testpublic 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<User> 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<Map<String,Object>> us = mapper.readValue(json, List.class);for(Map<String,Object> 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();}}@Testpublic 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;}}