How do I check whether a thread group has been destroyed?

By I Wayan Saryada in Core API, Lang Package 0 Comment

You can use ThreadGroup.isDestroyed() method to check whether a thread group and its subgroups has been destroyed.

package org.kodejava.lang;

public class CheckGroupDestroy {
    public static void main(String[] args) {
        ThreadGroup grandParent = new ThreadGroup("GrandParent");
        ThreadGroup uncle = new ThreadGroup(grandParent, "Uncle");
        ThreadGroup parent = new ThreadGroup(grandParent, "Parent");
        ThreadGroup son = new ThreadGroup(parent, "Son");
        ThreadGroup daughter = new ThreadGroup(parent, "Daughter");
        ThreadGroup neighbour = new ThreadGroup("Neighbour");

        ThreadGroup[] groupArray = {
                grandParent, uncle, parent, son, daughter, neighbour
        };

        // Destroy 'parent' group and all its subgroups
        parent.destroy();

        // Check whether the group is destroyed. The result is,
        // GrandParent, Uncle, and Neighbour did not destroyed
        // because they are not Parent's subgroups
        for (ThreadGroup tg : groupArray) {
            if (tg.isDestroyed()) {
                System.out.println(tg.getName() + " is destroyed");
            } else {
                System.out.println(tg.getName() + " is not destroyed");
            }
        }
    }
}

The result is:

GrandParent is not destroyed
Uncle is not destroyed
Parent is destroyed
Son is destroyed
Daughter is destroyed
Neighbour is not destroyed

How do I destroy a thread group?

By I Wayan Saryada in Core API, Lang Package 0 Comment

You can destroy a thread group by using destroy() method of ThreadGroup class. It will cleans up the thread group and removes it from the thread group hierarchy. It’s not only destroy the thread group, but also all its subgroups.

The destroy() method is of limited use: it can only be called if there are no threads presently in the thread group.

package org.kodejava.lang;

public class ThreadGroupDestroy {
    public static void main(String[] args) {
        ThreadGroup root = new ThreadGroup("Root");
        ThreadGroup server = new ThreadGroup(root, "ServerGroup");
        ThreadGroup client = new ThreadGroup(root, "ClientGroup");

        // Destroy 'root' thread groups and all its subgroup
        // ('server' & 'client')
        root.destroy();

        // Check if 'root' group and its subgroups already destroyed
        if (root.isDestroyed()) {
            System.out.println("Root group is destroyed");
        }

        if (server.isDestroyed()) {
            System.out.println("Server group is destroyed");
        }

        if (client.isDestroyed()) {
            System.out.println("Client group is destroyed");
        }
    }
}

How to automatically close resources in JDBC?

By I Wayan Saryada in JDBC 0 Comment

One thing that we need to do manually when programming using JDBC is to make sure to close all the resources that we use. All resources including the ResultSet, Statement and Connection must be closed. This will usually produce a lot of boilerplate code in our program.

Starting from JDBC 4.1, which is a part of Java 7, we can use the try-with-resources statement to automatically manage the resources that we use. This try statement closes the resources used when the block finishes its execution either normally or abruptly.

Here is an example that shows us how to use the try-with-resources statement.

package org.kodejava.jdbc;

import java.sql.*;

public class TryWithResourceJdbc {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection conn =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD);
             Statement stmt = conn.createStatement();
             ResultSet rs = stmt.executeQuery("SELECT * FROM product")) {

            while (rs.next()) {
                String code = rs.getString("code");
                String name = rs.getString("name");

                System.out.println("Code: " + code + "; Name: " + name);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

Maven Central

How do I use try-with-resources statement?

By I Wayan Saryada in Basic, Core API 0 Comment

The try-with-resources statement is introduced in the Java 7. With this new statement we can simplify resource management in our program, also known as ARM (Automatic Resource Management).

This statement is a try statement that declares one or more resources. After the program finish with the resource, it must be closed. The try-with-resources ensures that each resource is closed and the end of the statement.

Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.

package org.kodejava.basic;

import java.io.*;
import java.nio.charset.StandardCharsets;

public class TryWithResourceExample {
    public static void main(String[] args) {
        try {
            TryWithResourceExample demo = new TryWithResourceExample();
            demo.printStream("F:/tmp/data.txt");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void printStream(String fileName) throws IOException {
        char[] buffer = new char[1024];

        try (InputStream is = new FileInputStream(fileName);
             Reader reader = new BufferedReader(
                     new InputStreamReader(is, StandardCharsets.UTF_8))) {

            while (reader.read(buffer) != -1) {
                System.out.println(buffer);
            }
        }
    }
}

How do I read all lines from a file?

By I Wayan Saryada in Core API, IO 0 Comment

The java.nio.file.Files.readAllLines() method read all lines from a file. This method ensures that the file is closed when all bytes have been read or an I/O error, or other runtime exception, is thrown. Bytes from the file are decoded into characters using the specified charset.

Note that this method is intended for simple cases where it is convenient to read all lines in a single operation. It is not intended for reading in large files. This method is available since Java 7.

package org.kodejava.io;

import java.net.URI;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.List;
import java.util.Objects;

public class ReadFileAsListDemo {
    public static void main(String[] args) {
        ReadFileAsListDemo demo = new ReadFileAsListDemo();
        demo.readFileAsList();
    }

    private void readFileAsList() {
        String fileName = "/data.txt";

        try {
            URI uri = Objects.requireNonNull(this.getClass().getResource(fileName)).toURI();
            List<String> lines = Files.readAllLines(Paths.get(uri),
                    Charset.defaultCharset());

            for (String line : lines) {
                System.out.println(line);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}