JSON in Java and Jackson Part 1 Return JSON from the server

at first

Let's look back on the handling of JSON using Jackson in Java. In the case of my project, Java is almost a Web system, and the cooperation between Servlet and JavaScript is important. First of all, I tried to return JSON from the server this time. Except for the implementation in Servlet, most of the references are diverted.

environment

• Java 1.8
• Tomcat 8.0.53
• Jackson 2.10.1

Source

Java class creation for JSON

First of all, in order to convert to JSON, it is necessary to create a Java class to store the object for it. The properties of id (number type), name (string type), and datas (array) are prepared as follows. The key is the annotation by @JsonProperty. Without it, an exception will be thrown.

JsonBean.java

package servletTest;
import java.util.List;
import com.fasterxml.jackson.annotation.JsonProperty;

public class JsonBean {

	@JsonProperty("id")
	private int id;

	@JsonProperty("name")
	private String name;

	@JsonProperty("datas")
	private List<String> datas;

	public void setId(int id) {
		this.id = id;
	}

	public void setName(String name) {
		this.name = name;
	}

	public void setDatas(List<String> datas) {
		this.datas = datas;
	}
}

Creating a Servlet

Create an object using the Java class created in the previous section, convert it to a Json string with the writeValueAsString method of the ObjectMapper class, and return it to the client.

SetvletTest.java

package servletTest;

import java.io.IOException;
import java.util.List;
import java.util.ArrayList;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;


//Reference https://itsakura.com/java-jackson

@WebServlet("/helloworld")
public class ServletTest extends HttpServlet {
    private static final long serialVersionUID = 1L;
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

    	//Set a value in a Java object
    	JsonBean jsonBean = new JsonBean();
    	jsonBean.setId(1);
    	jsonBean.setName("kimisyo");
    	List<String> datas = new ArrayList<>();
    	datas.add("Programmer");
    	datas.add("Data Scientist");
    	jsonBean.setDatas(datas);

    	ObjectMapper mapper = new ObjectMapper();
    	try {
    		//Convert from Java object to JSON
    		String testJson = mapper.writeValueAsString(jsonBean);
    		//JSON output
    		response.getWriter().write(testJson);
    	} catch (JsonProcessingException e) {
    		e.printStackTrace();
    	}
    }
}

Execution result

When the above is executed in Eclipse, the following character string is displayed in the browser.

{"id":1,"name":"kimisyo","datas":["Programmer","Data Scientist"]}

in conclusion

Originally, it would be better to set Content-Type as well, but I think I will have to do it from the next time onwards, so I postponed it. From the next time onward (if any), we plan to proceed with the following subjects. -~~ Get the JSON received on the client side with JavaScript ~~ --Get JSON sent from client on server side --~~ Escape of JSON string, ~~ XSS countermeasures --Embed JSON in HTML and use it from JavaScript (Java version)

reference

-Convert JSON and Java objects with Java Jackson