jackson快速入门

1. 背景

目前维护的产品使用jackson处理json，现整理一下jackson相关资料，希望对初次接触jackson的新手有所帮助。

jackson主页: http://jackson.codehaus.org/
jackson document: http://wiki.fasterxml.com/JacksonJavaDocs
JacksonInFiveMinutes: http://wiki.fasterxml.com/JacksonInFiveMinutes
本文主要内容译自JacksonInFiveMinutes，增加了部分示例，转载请注明出处。

受java平台上各种处理xml的类库(StAX, JAXB等)启发，Jackson提供一种处理json的java类库。Jackson的目标是为开发者提供快速、准确、轻量级和用起来最爽的json处理类库。本文将概括介绍Jackson的主要功能和相关功能的使用示例。

2. 使用方式

Jackson提供三种可选的json处理方式：

1） Streaming API

又称Incremental parsing/generation， 受StAX API启发，以非关联递增方式读写json内容。 通过
org.codehaus.jackson.JsonParser读取，通过org.codehaus.jackson.JsonGenerator写入。

2） Tree Model

通过基于内存的树形结构来描述json数据，和 XML
DOM类似。通过org.codehaus.jackson.map.ObjectMapper构建树，树由JsonNode节点组成。

3） Data Binding

基于属性访问或注解的方式将json和POJO对象互相转换， 受JAXB基于注解的处理方式启发。通过org.codehaus.jackson.map.ObjectMapper读写json数据。它包含两种类型:

3.1 Simple Data Binding

用于json和Java Maps, Lists, Strings, Numbers, Booleans and nulls的相互转换。

3.2 Full Data Binding

用于json和Java Bean的相互转换。

下面从使用的角度上比较三种处理方式：

Streaming API 执行效率最高，读写速度最快，另外两种方式都基于它实现；

Tree Model 是最灵活的；

Data Binding 通常是最方便使用的；

3. 示例

快速上手最好的方式就是看示例，下面的例子演示了上面三种方式的用法，建议通过附件下载完整的示例工程文件。

package com.learnworld.jackson.main;

import java.io.File;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import org.codehaus.jackson.JsonEncoding;
import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.JsonToken;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.node.ObjectNode;
import org.codehaus.jackson.type.TypeReference;

import com.learnworld.jackson.annotation.Account;
import com.learnworld.jackson.pojo.Name;
import com.learnworld.jackson.pojo.User;
import com.learnworld.jackson.pojo.User.Gender;

public class JacksonMain {

/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
ObjectMapper mapper = new ObjectMapper();

// 1.1 Simple Data Binding
simpleDataBinding(mapper);

// 1.2 Full Data Binding
fullDataBinding(mapper);

// 1.3 Data Binding with Generics
dataBindingWithGenerics(mapper);

// 1.4 Data Binding with Annotation
dataBindingWithAnnotation(mapper);

// 2.1 Tree Model
treeModel(mapper);

// 2.2 Construct a Tree
constructTreeModel(mapper);

// 3.1 Streaming API (write Json)
streamingAPIWrite();

// 3.2 Streaming API (read Json)
streamingAPIRead();
}

@SuppressWarnings("unchecked")
public static void simpleDataBinding(ObjectMapper mapper) throws Exception {
// json -> Map
Map<String, Object> userDataRead = mapper.readValue(new File("user.json"),
Map.class);
System.out.println("simpleDataBinding(): " + userDataRead);

// Map -> json
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);
}

public static void fullDataBinding(ObjectMapper mapper) throws Exception {
// json -> Object
User user = mapper.readValue(new File("user.json"), User.class);
System.out.println("fullDataBinding(): " + user);

// Object -> json
mapper.writeValue(new File("user-modified.json"), user);
}

public static void dataBindingWithGenerics(ObjectMapper mapper) throws Exception {
// json -> Map
Map<String, Name> genericData = mapper.readValue(new File("generic.json"),
new TypeReference<Map<String,Name>>() { });
System.out.println("dataBindingWithGenerics():" + genericData);
}

public static void dataBindingWithAnnotation(ObjectMapper mapper) throws Exception {
// json -> Object
Account account = mapper.readValue(new File("account.json"), Account.class);
System.out.println("dataBindingWithAnnotation(): " + account);

account.setGmtCreate(new Date());
// Object -> json
mapper.writeValue(new File("account.json"), account);
}

public static void treeModel(ObjectMapper mapper) throws Exception {
// can either use mapper.readTree(JsonParser), or bind to JsonNode
JsonNode rootNode = mapper.readValue(new File("user.json"), JsonNode.class);

// 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");
}
// write it out
mapper.writeValue(new File("user-modified.json"), rootNode);
}

public static void constructTreeModel(ObjectMapper mapper) throws Exception {
ObjectMapper objectMapper = new ObjectMapper();
ObjectNode userOb = objectMapper.createObjectNode();
ObjectNode nameOb = userOb.putObject("name");
nameOb.put("first", "Thomas");
nameOb.put("last", "Zhou");
userOb.put("gender", User.Gender.MALE.toString());
userOb.put("verified", false);
userOb.put("userImage", "Foobar!".getBytes());
// write it out
mapper.writeValue(new File("user-modified.json"), userOb);
}

public static void streamingAPIRead() throws Exception {
JsonFactory f = new JsonFactory();
JsonGenerator g = f.createJsonGenerator(new File("user.json"), JsonEncoding.UTF8);

g.writeStartObject();
g.writeObjectFieldStart("name");
g.writeStringField("first", "Thomas");
g.writeStringField("last", "Zhou");
g.writeEndObject(); // for field 'name'
g.writeStringField("gender", Gender.MALE.name());
g.writeBooleanField("verified", false);
g.writeFieldName("userImage"); // no 'writeBinaryField' (yet?)
byte[] binaryData = "Foobar!".getBytes();
g.writeBinary(binaryData);
g.writeEndObject();
g.close(); // important: will force flushing of output, close underlying output stream
}

public static void streamingAPIWrite() throws Exception {
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_ARRAY
if ("name".equals(fieldname)) { // contains an object
Name name = new Name();
while (jp.nextToken() != JsonToken.END_OBJECT) {
String namefield = jp.getCurrentName();
jp.nextToken(); // move to value
if ("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(); // ensure resources get cleaned up timely and properly
System.out.println("streamingAPIWrite(): " + user);
}
}

4. 其他

除了使用上文中演示的ObjectMapper方式使用data binding和tree model，以下方法也可以满足你的需求：

JsonParser.readValueAs()

JsonParser.readValueAsTree()

JsonGenerator.writeObject()

JsonGenerator.writeTree()