Jackson快速指南

JSON的三种方式

    Jackson提供了三种可替代的方式来处理JSON。
    streaming API：以分离事件来读、写Json内容。
        org.codehaus.jackson.JsonParser读，org.codehaus.jackson.JsonGenerator写
    Tree Model：提供易变的JSON文档在内存中以树的方式展现。
        org.codehaus.jackson.map.ObjectMapper可以构建树，多个树构建了JsonNode节点。
    Data Binding：基于属性或annotation进行JSON和POJO之间的互转。有两种变种：simple和full 数据绑定。
        simple data binding：从Java Maps，List，String和Numbers，Booleans和nulls的互转。
        full data binding：任意java类型的互转。
        org.codehaus.jackson.map.ObjectMapper完成组装(写Json)和解组(读JSON)。
    streaming api性能最好，最小开销，最快读写，其他两种都是基于它。
    Data Binding：最便利

    Tree Model：弹性最好(扩展性好)

例子

full data binding

    jackson的ObjectMapper仅仅映射Json数据到Java对象。

{  "name" : { "first" : "Joe", "last" : "Sixpack" },  "gender" : "MALE",  "verified" : false,  "userImage" : "Rm9vYmFyIQ=="}

ObjectMapper mapper = new ObjectMapper(); // can reuse, share globallyUser user = mapper.readValue(new File("user.json"), User.class);mapper.writeValue(new File("user-modified.json"), user);public class User {public enum Gender { MALE, FEMALE };public static class Name {private String _first, _last;public String getFirst() { return _first; }public String getLast() { return _last; }public void setFirst(String s) { _first = s; }public void setLast(String s) { _last = s; }}private Gender _gender;private Name _name;private boolean _isVerified;private byte[] _userImage;public Name getName() { return _name; }public boolean isVerified() { return _isVerified; }public Gender getGender() { return _gender; }public byte[] getUserImage() { return _userImage; }public void setName(Name n) { _name = n; }public void setVerified(boolean b) { _isVerified = b; }public void setGender(Gender g) { _gender = g; }public void setUserImage(byte[] b) { _userImage = b; }}

raw数据绑定(无类型或简单数据)

    有时不想创建特定的java类来绑定Json，Untyped data binding会很好点。

Map<String,Object> userData = mapper.readValue(new File("user.json"), Map.class);Map<String,Object> userData = new HashMap<String,Object>();Map<String,String> nameStruct = new HashMap<String,String>();nameStruct.put("first", "Joe");nameStruct.put("last", "Sixpack");userData.put("name", nameStruct);userData.put("gender", "MALE");userData.put("verified", Boolean.FALSE);userData.put("userImage", "Rm9vYmFyIQ==");mapper.writeValue(new File("user-modified.json"), userData);

Json TypeJava TypeobjectLinkedHashMap<String,Object>arrayArrayList<Object>stringstringnumber (no fraction)Integer, Long or BigInteger (smallest applicable)number (fraction)Double (configurable to use BigDecimal)true|falseBooleannullnull

使用泛型绑定数据

    除了绑定POJO外和simple类型外，还有另外一个变种：绑定泛型容器。这种情况下需要特殊处理：
    Map<String,User> result = mapper.readValue(src, new TypeReference<Map<String,User>>() { });
    此处TypeReference仅fancier需要传递泛型定义：<Map<String,User>>。

Tree Model Example

    获取Json对象的另一种方式就是构建tree。类似与XML的DOM tree。Jackson使用基本JsonNode来构建树。

    可以使用streaming api或ObjectMapper读取或写tree。

ObjectMapper m = new ObjectMapper();// can either use mapper.readTree(source), or mapper.readValue(source, JsonNode.class);JsonNode rootNode = m.readTree(new File("user.json"));// ensure that "last name" isn't "Xmler"; if is, change to "Jsoner"JsonNode nameNode = rootNode.path("name");String lastName = nameNode.path("last").getTextValue().if ("xmler".equalsIgnoreCase(lastName)) {((ObjectNode)nameNode).put("last", "Jsoner");}m.writeValue(new File("user-modified.json"), rootNode);

TreeMapper treeMapper = new TreeMapper();ObjectNode userOb = treeMapper.objectNode();Object nameOb = userRoot.putObject("name");nameOb.put("first", "Joe");nameOb.put("last", "Sixpack");userOb.put("gender", User.Gender.MALE.toString());userOb.put("verified", false);byte[] imageData = getImageData(); // or wherever it comes fromuserOb.put("userImage", imageData);

Streaming API Example

生成Json:

JsonFactory f = new JsonFactory();JsonGenerator g = f.createJsonGenerator(new File("user.json"));g.writeStartObject();g.writeObjectFieldStart("name");g.writeStringField("first", "Joe");g.writeStringField("last", "Sixpack");g.writeEndObject(); // for field 'name'g.writeStringField("gender", Gender.MALE);g.writeBooleanField("verified", false);g.writeFieldName("userImage"); // no 'writeBinaryField' (yet?)byte[] binaryData = ...;g.writeBinary(binaryData);g.writeEndObject();g.close();

JsonFactory f = new JsonFactory();JsonParser jp = f.createJsonParser(new File("user.json"));User user = new User();jp.nextToken(); // will return JsonToken.START_OBJECT (verify?)while (jp.nextToken() != JsonToken.END_OBJECT) {String fieldname = jp.getCurrentName();jp.nextToken(); // move to value, or START_OBJECT/START_ARRAYif ("name".equals(fieldname)) { // contains an object    Name name = new Name();    while (jp.nextToken() != JsonToken.END_OBJECT) {String namefield = jp.getCurrentName();jp.nextToken(); // move to valueif ("first".equals(namefield)) {name.setFirst(jp.getText());} else if ("last".equals(namefield)) {name.setLast(jp.getText());} else {throw new IllegalStateException("Unrecognized field '"+fieldname+"'!");}    }    user.setName(name);} else if ("gender".equals(fieldname)) {            user.setGender(User.Gender.valueOf(jp.getText()));        } else if ("verified".equals(fieldname)) {            user.setVerified(jp.getCurrentToken() == JsonToken.VALUE_TRUE);        } else if ("userImage".equals(fieldname)) {            user.setUserImage(jp.getBinaryValue());        } else {            throw new IllegalStateException("Unrecognized field '"+fieldname+"'!");       }}jp.close();