jackson 入門教程

下面演示使用 Jackson 實現(xiàn) Java 對象和 JSON 的相互轉(zhuǎn)換。

1. 快速參考

1.1 Java object to JSON, writeValue(...) 

ObjectMapper mapper = new ObjectMapper();
Staff obj = new Staff();

//Object to JSON in file
mapper.writeValue(new File("c:\\file.json"), obj);

//Object to JSON in String
String jsonInString = mapper.writeValueAsString(obj);

1.2 JSON to Java object readValue(...) 

ObjectMapper mapper = new ObjectMapper();
String jsonInString = "{'name' : 'mjw'}";

//JSON from file to Object
Staff obj = mapper.readValue(new File("c:\\file.json"), Staff.class);

//JSON from URL to Object
Staff obj = mapper.readValue(new URL("http://www.lxweimin.com/u/c38e94dcec65"), Staff.class);

//JSON from String to Object
Staff obj = mapper.readValue(jsonInString, Staff.class);

2. POJO

Plain Old Java Object, 表示常規(guī)的Java對象，沒有擴展特定的類，或者實現(xiàn)特定的接口，或者說不受特定框架擴展的影響。

例如，如果想從 JMS 接收信息，則需要實現(xiàn) MessageListener 接口：

public class ExampleListener implements MessageListener {
  public void onMessage(Message message){
    ...
  }
}

這個類和JMS的 MessageListener 接口綁定，因此難以遷移到其他的信息處理框架，它不是POJO。

3. Java 和 JSON 的相互轉(zhuǎn)換

下面通過一個實例，說明如何使用 Jackson 實現(xiàn)JSON和Java對象相互轉(zhuǎn)換。
Album 類，包含一個字段：

class Album {
    private String title;
    public Album(String title) {
        this.title = title;
    }
    public String getTitle() {
        return title;
    }
}

3.1 使用 ObjectMapper 進行轉(zhuǎn)換

Jackson 默認使用 BeanSerializer 序列化POJO，要求對應(yīng)的字段為 public，或者有對應(yīng)的 getter 方法。

單字段序列化
Album album = new Album("Kind Of Blue");
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(System.out, album);

輸出：

{"title":"Kind Of Blue"}

數(shù)組序列化

現(xiàn)在繼續(xù)向 Album 添加一個數(shù)組字段及對應(yīng)的 getter 和 setter 方法：

private String[] links;
public String[] getLinks(){
    return links;
}
public void setLinks(String[] links){
    this.links = links;
}

修改 main 方法：

Album album = new Album("Kind Of Blue");
album.setLinks(new String[]{"link1", "link2"});
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(System.out, album);

輸出：

{"title":"Kind Of Blue","links":["link1","link2"]}

List 序列化

向 Album 添加 List 字段：

private List<String> songs;
public List<String> getSongs(){
    return songs;
}
public void setSongs(List<String> songs){
    this.songs = songs;
}

修改 main 方法：

Album album = new Album("Kind Of Blue");
album.setLinks(new String[]{"link1", "link2"});

List<String> songs = new ArrayList<>();
songs.add("So what");
songs.add("Flamenco Sketches");
songs.add("Freddie Freeloader");

album.setSongs(songs);

ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(System.out, album);

輸出：

{"title":"Kind Of Blue","links":["link1","link2"],"songs":["So what","Flamenco Sketches","Freddie Freeloader"]}

從輸出結(jié)構(gòu)可以看到，List和數(shù)組的輸出格式是一樣的。

Java 對象序列化

Java 對象，序列化后在JSON中被 {} 括起來。
定義Artist 類：

public class Artist{
    public String name;
    public Date birthDate;
}

在Album 中添加對應(yīng)的字段，并在 main 中設(shè)置其值：

Artist artist = new Artist();
artist.name = "Miles Davis";
SimpleDateFormat format = new SimpleDateFormat("dd-MM-yyyy");
artist.birthDate = format.parse("26-05-1926");
album.setArtist(artist);

輸出：

{"title":"Kind Of Blue","links":["link1","link2"],"songs":["So what","Flamenco Sketches","Freddie Freeloader"],"artist":{"name":"Miles Davis","birthDate":-1376035200000}}

格式化輸出

配置 ObjectMapper ，可以讓輸出更好看一些：

mapper.configure(SerializationFeature.INDENT_OUTPUT, true);

此時的JSON輸出如下：

{
  "title" : "Kind Of Blue",
  "links" : [ "link1", "link2" ],
  "songs" : [ "So what", "Flamenco Sketches", "Freddie Freeloader" ],
  "artist" : {
    "name" : "Miles Davis",
    "birthDate" : -1376035200000
  }
}
Map 序列化

向 Album 中添加如下內(nèi)容：

private Map<String, String> musicians = new HashMap<>();
public Map<String, String> getMusicians(){
    return Collections.unmodifiableMap(musicians);
}
public void addMusician(String key, String value){
    musicians.put(key, value);
}

在 main 中添加如下內(nèi)容：

album.addMusician("Miles Davis", "Trumpet, Band leader");
album.addMusician("Julian Adderley", "Alto Saxophone");
album.addMusician("Paul Chambers", "double bass");

輸出如下：

{
  "title" : "Kind Of Blue",
  "links" : [ "link1", "link2" ],
  "songs" : [ "So what", "Flamenco Sketches", "Freddie Freeloader" ],
  "artist" : {
    "name" : "Miles Davis",
    "birthDate" : -1376035200000
  },
  "musicians" : {
    "Miles Davis" : "Trumpet, Band leader",
    "Paul Chambers" : "double bass",
    "Julian Adderley" : "Alto Saxophone"
  }
}
其他

設(shè)置輸出時間格式

SimpleDateFormat outputFormat = new SimpleDateFormat("dd MMM yyyy");
mapper.setDateFormat(outputFormat);

讓Map按序輸出

mapper.configure(SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS, true);

此時輸出：

{
  "title" : "Kind Of Blue",
  "links" : [ "link1", "link2" ],
  "songs" : [ "So what", "Flamenco Sketches", "Freddie Freeloader" ],
  "artist" : {
    "name" : "Miles Davis",
    "birthDate" : "26 May 1926"
  },
  "musicians" : {
    "Julian Adderley" : "Alto Saxophone",
    "Miles Davis" : "Trumpet, Band leader",
    "Paul Chambers" : "double bass"
  }
}

3.2 使用 Tree Model 進行轉(zhuǎn)換

我們繼續(xù)使用上面的例子，來演示 Tree Model 的使用。使用 Tree 進行輸出包含如下幾個步驟：

  • 創(chuàng)建 JsonNodeFactory，用于創(chuàng)建 node。
  • 使用JsonFactory創(chuàng)建 JsonGenerator，并指定輸出方法。
  • 創(chuàng)建ObjectMapper，它使用 JsonGenerator 和樹的根節(jié)點輸出到JSON。

如下所示：

public class SerializationExampleTreeModel{
    public static void main(String[] args) throws IOException{
        JsonNodeFactory factory = new JsonNodeFactory(false);

        JsonFactory jsonFactory = new JsonFactory();
        JsonGenerator generator = jsonFactory.createGenerator(System.out);

        ObjectMapper mapper = new ObjectMapper();
        ObjectNode album = factory.objectNode();
        mapper.writeTree(generator, album);
    }
}

由于沒有添加任何實質(zhì)性內(nèi)容，所以輸出為：

{}

開始添加內(nèi)容：

album.put("Album-Title", "Kind Of Blue");

JSON:

{"Album-Title":"Kind Of Blue"}

添加數(shù)組：

ArrayNode links = factory.arrayNode();
links.add("link1").add("link2");
album.set("links", links);

JSON:

{"Album-Title":"Kind Of Blue","links":["link1","link2"]}

添加對象：

ObjectNode artist = factory.objectNode();
artist.put("Artist-Name", "Miles Davis");
artist.put("birthDate", "26 May 1926");
album.set("artist", artist);

JSON:

{"Album-Title":"Kind Of Blue","links":["link1","link2"],"artist":{"Artist-Name":"Miles Davis","birthDate":"26 May 1926"}}

添加 musicians:

ObjectNode musicians = factory.objectNode();
musicians.put("Julian Adderley", "Alto Saxophone");
musicians.put("Miles Davis", "Trumpet, Band leader");
album.set("musicians", musicians);

JSON:

{"Album-Title":"Kind Of Blue","links":["link1","link2"],"artist":{"Artist-Name":"Miles Davis","birthDate":"26 May 1926"},"musicians":{"Julian Adderley":"Alto Saxophone","Miles Davis":"Trumpet, Band leader"}}
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