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：

代码如下:

C:\Jackson_WORKSPACE>javac JacksonTester.java

现在运行jacksonTester看到的结果：

代码如下:

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：

代码如下:

C:\Jackson_WORKSPACE>javac JacksonTester.java

现在运行jacksonTester看到的结果：

代码如下:

C:\Jackson_WORKSPACE>java JacksonTester

验证输出

代码如下:

Student [ name: Mahesh, age: 10 ]