实例解析Java的Jackson库中的数据绑定

数据绑定api用于json转换和使用属性访问或使用注解pojo(普通java对象)。以下是它的两个类型。

简单数据绑定 - 转换json，从java maps, lists, strings, numbers, booleans 和 null 对象。

完整数据绑定 - 转换json到任何java类型。我们将在下一章分别绑定。

objectmapper读/写json两种类型的数据绑定。数据绑定是最方便的方式是类似xml的jaxb解析器。

简单的数据绑定
简单的数据绑定是指json映射到java核心数据类型。下表列出了json类型和java类型之间的关系。

让我们来看看简单的数据操作绑定。在这里，我们将映射java基本类型直接json，反之亦然。

创建一个名为jacksontester在java类文件在目录 c:\>jackson_workspace.

file: jacksontester.java

import java.io.file;
import java.io.ioexception;
import java.util.hashmap;
import java.util.map;

import org.codehaus.jackson.jsongenerationexception;
import org.codehaus.jackson.jsonparseexception;
import org.codehaus.jackson.map.jsonmappingexception;
import org.codehaus.jackson.map.objectmapper;

public class jacksontester {
  public static void main(string args[]){
   jacksontester tester = new jacksontester();
     try {
      objectmapper mapper = new objectmapper();

      map<string,object> studentdatamap = new hashmap<string,object>(); 
      int[] marks = {1,2,3};

      student student = new student();
      student.setage(10);
      student.setname("mahesh");
      // java object
      studentdatamap.put("student", student);
      // java string
      studentdatamap.put("name", "mahesh kumar");   
      // java boolean
      studentdatamap.put("verified", boolean.false);
      // array
      studentdatamap.put("marks", marks);

      mapper.writevalue(new file("student.json"), studentdatamap);
      //result student.json
  //{ 
      //  "student":{"name":"mahesh","age":10},
      //  "marks":[1,2,3],
      //  "verified":false,
      //  "name":"mahesh kumar"
      //}
      studentdatamap = mapper.readvalue(new file("student.json"), map.class);

      system.out.println(studentdatamap.get("student"));
      system.out.println(studentdatamap.get("name"));
      system.out.println(studentdatamap.get("verified"));
      system.out.println(studentdatamap.get("marks"));
   } catch (jsonparseexception e) {
     e.printstacktrace();
   } catch (jsonmappingexception e) {
     e.printstacktrace();
   } catch (ioexception e) {
      e.printstacktrace();
   }
  }
}

class student {
  private string name;
  private int age;
  public student(){}
  public string getname() {
   return name;
  }
  public void setname(string name) {
   this.name = name;
  }
  public int getage() {
   return age;
  }
  public void setage(int age) {
   this.age = age;
  }
  public string tostring(){
   return "student [ name: "+name+", age: "+ age+ " ]";
  } 
}

验证结果

使用javac编译如下类：

c:\jackson_workspace>javac jacksontester.java

现在运行jacksontester看到的结果：

c:\jackson_workspace>java jacksontester

验证输出结果

{name=mahesh, age=10}
mahesh kumar
false
[1, 2, 3]

全数据绑定
完全数据绑定是指json映射到任何java对象。

//create an objectmapper instance
objectmapper mapper = new objectmapper(); 
//map json content to student object
student student = mapper.readvalue(new file("student.json"), student.class);
//map student object to json content
mapper.writevalue(new file("student.json"), student);

让我们来看看简单的数据操作绑定。在这里，我们将直接映射java对象到json，反之亦然。

创建一个名为jacksontester在java类文件目录 c:\>jackson_workspace.

file: jacksontester.java

import java.io.file;
import java.io.ioexception;

import org.codehaus.jackson.jsongenerationexception;
import org.codehaus.jackson.jsonparseexception;
import org.codehaus.jackson.map.jsonmappingexception;
import org.codehaus.jackson.map.objectmapper;

public class jacksontester {
  public static void main(string args[]){
   jacksontester tester = new jacksontester();
   try {
     student student = new student();
     student.setage(10);
     student.setname("mahesh");
     tester.writejson(student);

     student student1 = tester.readjson();
     system.out.println(student1);

   } catch (jsonparseexception e) {
     e.printstacktrace();
   } catch (jsonmappingexception e) {
     e.printstacktrace();
   } catch (ioexception e) {
     e.printstacktrace();
   }
  }

  private void writejson(student student) throws jsongenerationexception, jsonmappingexception, ioexception{
   objectmapper mapper = new objectmapper(); 
   mapper.writevalue(new file("student.json"), student);
  }

  private student readjson() throws jsonparseexception, jsonmappingexception, ioexception{
   objectmapper mapper = new objectmapper();
   student student = mapper.readvalue(new file("student.json"), student.class);
   return student;
  }
}

class student {
  private string name;
  private int age;
  public student(){}
  public string getname() {
   return name;
  }
  public void setname(string name) {
   this.name = name;
  }
  public int getage() {
   return age;
  }
  public void setage(int age) {
   this.age = age;
  }
  public string tostring(){
   return "student [ name: "+name+", age: "+ age+ " ]";
  } 
}

验证结果

使用javac编译如下类：

c:\jackson_workspace>javac jacksontester.java

现在运行jacksontester看到的结果：

c:\jackson_workspace>java jacksontester

验证输出

student [ name: mahesh, age: 10 ]