Jackson 入门

同事的一些测试结果看来，Jackson在处理Json方面性能相对占优，于是关注了下它，下方大部分从官方文档翻译过来。

 

原文链接：

http://jackson.codehaus.org/Tutorial

 

Jackson有三种处理方式：

Streaming API

•     使用org.codehaus.jackson.JsonParser 进行读操作 , 使用org.codehaus.jackson.JsonGenerator 写操作
•     Jackson Streaming API有点类似 Stax API

数据绑定   它允许我们把Json转换成Java对象，也可以把Java对象转换成Json对象

•    org.codehaus.jackson.map.ObjectMapper 知道如何把Json数据绑定成为（或者映射回）Java对象

树模型   通过进驻内存的树形结构来方便的访问Json数据

•   和数据绑定方式类似，树模型也使用了 org.codehaus.jackson.map.ObjectMapper进行Json数据和Java对象的绑定；只是树形结构由 org.codehaus.jackson.map.JsonNode节点组成

 

 

从使用的角度看，上述三种方式可以归纳为 ：

• Streaming API是性能最优的（开销最小，最快的读写速度）  注：在我看来，操作最随心所欲的
• 数据绑定是最方便使用的
• 树形模型又是最灵活的

 

 

Java类：

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; }
}

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; }
}

 

  JSon数据：

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

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

 

Streaming API （这部分我比较关注，就先翻译下）比数据绑定的方式会快20%到30%

写操作：

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();

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();

 读操作：

ReadJSON.java
 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(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+"'!");
  }
}

ReadJSON.java
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(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+"'!");
  }
}

 

数据绑定：

//读
 ObjectMapper mapper = new ObjectMapper(); // can reuse, share globally
 User user = mapper.readValue(new File("user.json"), User.class);

 //写
 mapper.writeValue(<SPAN class=code-keyword>new</SPAN>


  File(<SPAN class=code-quote>"user-modified.json"</SPAN>


), user);

//读
ObjectMapper mapper = new ObjectMapper(); // can reuse, share globally
User user = mapper.readValue(new File("user.json"), User.class);

//写
mapper.writeValue(new


 File("user-modified.json"


), user);

 无类型的数据绑定：

HashMap<String,Object> untyped = new HashMap<String,Object>();
 HashMap<String,String> nameStruct = new HashMap<String,String>();
 nameStruct.put("first", "Joe");
 nameStruct.put("last", "Sixpack");
 untyped.put("name", nameStruct);
 untyped.put("gender", "MALE");
 untyped.put("verified", Boolean.FALSE);
 untyped.put("userImage", "Rm9vYmFyIQ==");

HashMap<String,Object> untyped = new HashMap<String,Object>();
HashMap<String,String> nameStruct = new HashMap<String,String>();
nameStruct.put("first", "Joe");
nameStruct.put("last", "Sixpack");
untyped.put("name", nameStruct);
untyped.put("gender", "MALE");
untyped.put("verified", Boolean.FALSE);
untyped.put("userImage", "Rm9vYmFyIQ==");

 

树模型：

可以使用Stream API或者数据绑定的方式写或者ObjectMapper读取树树结构

ObjectMapper的方式：

 

ObjectMapper m = new ObjectMapper();
 // can either use mapper.readTree(JsonParser), or bind to JsonNode
 JsonNode rootNode = m.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");
}
 // and write it out:
 m.writeValue(new File("user-modified.json"), rootNode);

ObjectMapper m = new ObjectMapper();
// can either use mapper.readTree(JsonParser), or bind to JsonNode
JsonNode rootNode = m.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");
}
// and write it out:
m.writeValue(new File("user-modified.json"), rootNode);

 

绝招：可以在JsonParser和JsonGenerator中直接使用数据绑定或者树模型的方式的，看一下以下方法就知道怎么用了

• JsonParser.readValueAs()
• JsonParser.readValueAsTree()
• JsonGenerator.writeObject()
• JsonGenerator.writeTree()