How do I create a NIO Buffer?

By I Wayan Saryada in Core API, IO 0 Comment

This post show you how to use the java.nio buffer such as ByteBuffer, CharBuffer, DoubleBuffer, etc. To create a buffer you can use the allocate(int capacity) method. Each buffer object has the allocate(int capacity) method. You need to pass the value of the capacity of the buffer to be created when calling this method.

package org.kodejava.io;

import java.nio.*;

public class BufferAllocate {
    public static void main(String[] args) {
        ByteBuffer byteBuffer = ByteBuffer.allocate(64);
        CharBuffer charBuffer = CharBuffer.allocate(64);
        ShortBuffer shortBuffer = ShortBuffer.allocate(64);
        IntBuffer intBuffer = IntBuffer.allocate(64);
        LongBuffer longBuffer = LongBuffer.allocate(64);
        FloatBuffer floatBuffer = FloatBuffer.allocate(64);
        DoubleBuffer doubleBuffer = DoubleBuffer.allocate(64);
    }
}

How do I copy file using FileChannel class?

By I Wayan Saryada in Core API, IO 2 Comments

The example below show you how to copy a file using the java.nio.channels.FileChannel class.

package org.kodejava.io;

import java.io.*;
import java.nio.channels.FileChannel;

public class FileCopy {
    public static void main(String[] args) {
        //// Define the source and target file
        File source = new File("D:/Temp/source.txt");
        File target = new File("D:/Temp/target.txt");

        // Create the source and target channel
        try (FileChannel sourceChannel = new FileInputStream(source).getChannel();
             FileChannel targetChannel = new FileOutputStream(target).getChannel()) {

            // Copy data from the source channel into the target channel
            targetChannel.transferFrom(sourceChannel, 0, sourceChannel.size());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

How do I create zip file in Servlet for download?

By I Wayan Saryada in Servlet 15 Comments

The example below is a servlet that shows you how to create a zip file and send the generated zip file for user to download. The compressing process is done by the zipFiles method of this class.

For a servlet to work you need to configure it in the web.xml file of your web application which can be found after the code snippet below.

package org.kodejava.servlet;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.util.zip.ZipEntry;
import java.util.zip.ZipOutputStream;

@WebServlet(urlPatterns = "/zipservlet")
public class ZipDownloadServlet extends HttpServlet {
    public static final String FILE_SEPARATOR = System.getProperty("file.separator");

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        try {
            // The path below is the root directory of data to be
            // compressed.
            String path = getServletContext().getRealPath("data");

            File directory = new File(path);
            String[] files = directory.list();

            // Checks to see if the directory contains some files.
            if (files != null && files.length > 0) {

                // Call the zipFiles method for creating a zip stream.
                byte[] zip = zipFiles(directory, files);

                // Sends the response back to the user / browser. The
                // content for zip file type is "application/zip". We
                // also set the content disposition as attachment for
                // the browser to show a dialog that will let user 
                // choose what action will he do to the content.
                ServletOutputStream sos = response.getOutputStream();
                response.setContentType("application/zip");
                response.setHeader("Content-Disposition", "attachment; filename=\"DATA.ZIP\"");

                sos.write(zip);
                sos.flush();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * Compress the given directory with all its files.
     */
    private byte[] zipFiles(File directory, String[] files) throws IOException {
        try (ByteArrayOutputStream baos = new ByteArrayOutputStream();
             ZipOutputStream zos = new ZipOutputStream(baos)) {
            byte[] bytes = new byte[2048];

            for (String fileName : files) {
                String path = directory.getPath() +
                        ZipDownloadServlet.FILE_SEPARATOR + fileName;
                try (FileInputStream fis = new FileInputStream(path);
                     BufferedInputStream bis = new BufferedInputStream(fis)) {

                    zos.putNextEntry(new ZipEntry(fileName));

                    int bytesRead;
                    while ((bytesRead = bis.read(bytes)) != -1) {
                        zos.write(bytes, 0, bytesRead);
                    }
                    zos.closeEntry();
                }
            }

            zos.close();
            return baos.toByteArray();
        }
    }
}

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

Maven Central

How do I convert Map into JSON?

By I Wayan Saryada in Google Gson 1 Comment

This example show you how to convert a java.util.Map into JSON string and back to Map again.

package org.kodejava.gson;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Map;

public class MapToJson {
    public static void main(String[] args) {
        Map<String, String> colours = new HashMap<>();
        colours.put("BLACK", "#000000");
        colours.put("RED", "#FF0000");
        colours.put("GREEN", "#008000");
        colours.put("BLUE", "#0000FF");
        colours.put("YELLOW", "#FFFF00");
        colours.put("WHITE", "#FFFFFF");

        // Convert a Map into JSON string.
        Gson gson = new Gson();
        String json = gson.toJson(colours);
        System.out.println("json = " + json);

        // Convert JSON string back to Map.
        Type type = new TypeToken<Map<String, String>>() {
        }.getType();
        Map<String, String> map = gson.fromJson(json, type);
        for (String key : map.keySet()) {
            System.out.println("map.get = " + map.get(key));
        }
    }
}

Here is the result of the program:

json = {"RED":"#FF0000","WHITE":"#FFFFFF","BLUE":"#0000FF","BLACK":"#000000","YELLOW":"#FFFF00","GREEN":"#008000"}
map.get = #FF0000
map.get = #FFFFFF
map.get = #0000FF
map.get = #000000
map.get = #FFFF00
map.get = #008000

Maven Dependencies

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.11.0</version>
</dependency>

Maven Central

How do I convert collections into JSON?

By I Wayan Saryada in Google Gson 0 Comment

This example show you how to convert Java collections object into JSON string. For Student class use in this example you can find it the previous example on How do I convert object into JSON?.

package org.kodejava.gson;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import org.kodejava.gson.support.Student;

import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;

public class CollectionToJson {
    public static void main(String[] args) {
        // Converts a collection of string object into JSON string.
        List<String> names = new ArrayList<>();
        names.add("Alice");
        names.add("Bob");
        names.add("Carol");
        names.add("Mallory");

        Gson gson = new Gson();
        String jsonNames = gson.toJson(names);
        System.out.println("jsonNames = " + jsonNames);

        // Converts a collection Student object into JSON string
        Student a = new Student("Alice", "Apple St", getDOB(2000, 10, 1));
        Student b = new Student("Bob", "Banana St", null);
        Student c = new Student("Carol", "Grape St", getDOB(2000, 5, 21));
        Student d = new Student("Mallory", "Mango St", null);

        List<Student> students = new ArrayList<>();
        students.add(a);
        students.add(b);
        students.add(c);
        students.add(d);

        gson = new Gson();
        String jsonStudents = gson.toJson(students);
        System.out.println("jsonStudents = " + jsonStudents);

        // Converts JSON string into a collection of Student object.
        Type type = new TypeToken<List<Student>>() {
        }.getType();
        List<Student> studentList = gson.fromJson(jsonStudents, type);

        for (Student student : studentList) {
            System.out.println("student.getName() = " + student.getName());
        }
    }

    private static Date getDOB(int year, int month, int date) {
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.YEAR, year);
        calendar.set(Calendar.MONTH, month - 1);
        calendar.set(Calendar.DATE, date);
        calendar.set(Calendar.HOUR, 0);
        calendar.set(Calendar.MINUTE, 0);
        calendar.set(Calendar.SECOND, 0);
        return calendar.getTime();
    }
}

Here is the result of our program:

jsonNames = ["Alice","Bob","Carol","Mallory"]
jsonStudents = [{"name":"Alice","address":"Apple St","dateOfBirth":"Oct 1, 2000, 12:00:00 AM"},{"name":"Bob","address":"Banana St"},{"name":"Carol","address":"Grape St","dateOfBirth":"May 21, 2000, 12:00:00 AM"},{"name":"Mallory","address":"Mango St"}]
student.getName() = Alice
student.getName() = Bob
student.getName() = Carol
student.getName() = Mallory

Maven Dependencies

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.11.0</version>
</dependency>

Maven Central