Jackson 入门

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：

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()