Jackson樹模型

樹模型準備JSON檔案的記憶體樹表示。 ObjectMapper構建JsonNode節點樹。這是最靈活的方法。它類似於DOM解析器的XML。

從JSON建立樹

ObjectMapper提供一個指標樹的根節點在讀取JSON之後。根節點可用於遍歷完全樹。考慮下面的程式碼片段獲得提供JSON字串的根節點。

//Create an ObjectMapper instance
ObjectMapper mapper = new ObjectMapper();	
String jsonString = "{\"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";
//create tree from JSON
JsonNode rootNode = mapper.readTree(jsonString);

遍歷樹模型

使用相對路徑來根節點在遍歷樹，並處理該資料得到的每個節點。考慮下面的程式碼片段遍歷提供的根節點的樹。

JsonNode nameNode = rootNode.path("name");
System.out.println("Name: "+ nameNode.getTextValue());
 
JsonNode marksNode = rootNode.path("marks");
Iterator iterator = marksNode.getElements();

範例

建立一個名為JacksonTester在Java類檔案目錄 C:\>Jackson_WORKSPACE.

File: JacksonTester.java

import java.io.IOException;
import java.util.Iterator;

import org.codehaus.jackson.JsonNode;
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();
         String jsonString = "{\"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85]}";
         JsonNode rootNode = mapper.readTree(jsonString);

         JsonNode nameNode = rootNode.path("name");
         System.out.println("Name: "+ nameNode.getTextValue());

         JsonNode ageNode = rootNode.path("age");
         System.out.println("Age: " + ageNode.getIntValue());

         JsonNode verifiedNode = rootNode.path("verified");
         System.out.println("Verified: " + (verifiedNode.getBooleanValue() ? "Yes":"No"));

         JsonNode marksNode = rootNode.path("marks");
         Iterator<JsonNode> iterator = marksNode.getElements();
         System.out.print("Marks: [ ");
         while (iterator.hasNext()) {
            JsonNode marks = iterator.next();
            System.out.print(marks.getIntValue() + " "); 
         }
         System.out.println("]");
      } catch (JsonParseException e) {
         e.printStackTrace();
      } catch (JsonMappingException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

驗證輸出結果

使用javac編譯如下類：

C:\Jackson_WORKSPACE>javac JacksonTester.java

現在執行jacksonTester看到結果：

C:\Jackson_WORKSPACE>java JacksonTester

驗證輸出

Name: Mahesh Kumar
Age: 21
Verified: No
Marks: [ 100 90 85 ]

樹到JSON轉換

在這個例子中，我們已經使用JsonNode並將其寫入到一個JSON檔案，並讀回建立了一棵樹。

建立一個名為JacksonTester在Java類檔案目錄 C:\>Jackson_WORKSPACE.

File: JacksonTester.java

import java.io.File;
import java.io.IOException;
import java.util.Iterator;

import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.node.ArrayNode;
import org.codehaus.jackson.node.ObjectNode;

public class JacksonTester {
   public static void main(String args[]){
      JacksonTester tester = new JacksonTester();
      try {
         ObjectMapper mapper = new ObjectMapper();

         JsonNode rootNode = mapper.createObjectNode();
         JsonNode marksNode = mapper.createArrayNode();
         ((ArrayNode)marksNode).add(100);
         ((ArrayNode)marksNode).add(90);
         ((ArrayNode)marksNode).add(85);
         ((ObjectNode) rootNode).put("name", "Mahesh Kumar");
         ((ObjectNode) rootNode).put("age", 21);
         ((ObjectNode) rootNode).put("verified", false);
         ((ObjectNode) rootNode).put("marks",marksNode);

         mapper.writeValue(new File("student.json"), rootNode);

         rootNode = mapper.readTree(new File("student.json"));

         JsonNode nameNode = rootNode.path("name");
         System.out.println("Name: "+ nameNode.getTextValue());

         JsonNode ageNode = rootNode.path("age");
         System.out.println("Age: " + ageNode.getIntValue());

         JsonNode verifiedNode = rootNode.path("verified");
         System.out.println("Verified: " + (verifiedNode.getBooleanValue() ? "Yes":"No"));

         JsonNode marksNode1 = rootNode.path("marks");
         Iterator<JsonNode> iterator = marksNode1.getElements();
         System.out.print("Marks: [ ");
         while (iterator.hasNext()) {
            JsonNode marks = iterator.next();
            System.out.print(marks.getIntValue() + " "); 
         }
         System.out.println("]");
      } catch (JsonParseException e) {
         e.printStackTrace();
      } catch (JsonMappingException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

驗證結果

使用javac編譯如下類：

C:\Jackson_WORKSPACE>javac JacksonTester.java

現在執行jacksonTester看到的結果：

C:\Jackson_WORKSPACE>java JacksonTester

驗證輸出

Name: Mahesh Kumar
Age: 21
Verified: No
Marks: [ 100 90 85 ]

從樹到Java物件轉換

在這個例子中，我們已經使用JsonNode並將其寫入到一個JSON檔案，並回讀然後將一個Student物件其轉換為建立了一棵樹。

建立一個名為JacksonTester在Java類檔案目錄 C:\>Jackson_WORKSPACE.

File: JacksonTester.java

import java.io.File;
import java.io.IOException;
import java.util.Arrays;

import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.node.ArrayNode;
import org.codehaus.jackson.node.ObjectNode;

public class JacksonTester {
   public static void main(String args[]){
      JacksonTester tester = new JacksonTester();
      try {
         ObjectMapper mapper = new ObjectMapper();

         JsonNode rootNode = mapper.createObjectNode();
         JsonNode marksNode = mapper.createArrayNode();
         ((ArrayNode)marksNode).add(100);
         ((ArrayNode)marksNode).add(90);
         ((ArrayNode)marksNode).add(85);
         ((ObjectNode) rootNode).put("name", "Mahesh Kumar");
         ((ObjectNode) rootNode).put("age", 21);
         ((ObjectNode) rootNode).put("verified", false);
         ((ObjectNode) rootNode).put("marks",marksNode);

         mapper.writeValue(new File("student.json"), rootNode);

         rootNode = mapper.readTree(new File("student.json"));

         Student student = mapper.treeToValue(rootNode, Student.class);

         System.out.println("Name: "+ student.getName());
         System.out.println("Age: " + student.getAge());
         System.out.println("Verified: " + (student.isVerified() ? "Yes":"No"));
         System.out.println("Marks: "+Arrays.toString(student.getMarks()));
      } catch (JsonParseException e) {
         e.printStackTrace();
      } catch (JsonMappingException e) {
         e.printStackTrace();
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

class Student {
   String name;
   int age;
   boolean verified;
   int[] marks;
   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 boolean isVerified() {
      return verified;
   }
   public void setVerified(boolean verified) {
      this.verified = verified;
   }
   public int[] getMarks() {
      return marks;
   }
   public void setMarks(int[] marks) {
      this.marks = marks;
   }
}

驗證結果

使用javac編譯如下類：

C:\Jackson_WORKSPACE>javac JacksonTester.java

現在執行jacksonTester看到結果：

C:\Jackson_WORKSPACE>java JacksonTester

驗證輸出

Name: Mahesh Kumar
Age: 21
Verified: No
Marks: [ 100 90 85 ]