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real usage of SPI explanation(JDBC for example)?


real usage of SPI explanation(JDBC for example)?

By : Wilson Daniel Ospina
Date : November 17 2020, 03:01 PM
To fix this issue A services definition allows for dynamic discovery of implementations of an interface (or class). As an example, in JDBC, the services mechanism is used by java.sql.DriverManager to find and load implementations of java.sql.Driver.
Specifically it does (code from Java 9):
code :
/**
 * Load the initial JDBC drivers by checking the System property
 * jdbc.properties and then use the {@code ServiceLoader} mechanism
 */
static {
    loadInitialDrivers();
    println("JDBC DriverManager initialized");
}

// ...

private static void loadInitialDrivers() {
    String drivers;
    try {
        drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
            public String run() {
                return System.getProperty("jdbc.drivers");
            }
        });
    } catch (Exception ex) {
        drivers = null;
    }
    // If the driver is packaged as a Service Provider, load it.
    // Get all the drivers through the classloader
    // exposed as a java.sql.Driver.class service.
    // ServiceLoader.load() replaces the sun.misc.Providers()

    AccessController.doPrivileged(new PrivilegedAction<Void>() {
        public Void run() {

            ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
            Iterator<Driver> driversIterator = loadedDrivers.iterator();

            /* Load these drivers, so that they can be instantiated.
             * It may be the case that the driver class may not be there
             * i.e. there may be a packaged driver with the service class
             * as implementation of java.sql.Driver but the actual class
             * may be missing. In that case a java.util.ServiceConfigurationError
             * will be thrown at runtime by the VM trying to locate
             * and load the service.
             *
             * Adding a try catch block to catch those runtime errors
             * if driver not available in classpath but it's
             * packaged as service and that service is there in classpath.
             */
            try{
                while(driversIterator.hasNext()) {
                    driversIterator.next();
                }
            } catch(Throwable t) {
            // Do nothing
            }
            return null;
        }
    });

    println("DriverManager.initialize: jdbc.drivers = " + drivers);

    if (drivers == null || drivers.equals("")) {
        return;
    }
    String[] driversList = drivers.split(":");
    println("number of Drivers:" + driversList.length);
    for (String aDriver : driversList) {
        try {
            println("DriverManager.Initialize: loading " + aDriver);
            Class.forName(aDriver, true,
                    ClassLoader.getSystemClassLoader());
        } catch (Exception ex) {
            println("DriverManager.Initialize: load failed: " + ex);
        }
    }
}


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How to extend a sealed class - need simple program explanation and usage in real time

How to extend a sealed class - need simple program explanation and usage in real time


By : Joshua Kyle Smith
Date : March 29 2020, 07:55 AM
I hope this helps . As @ReedCopsey has already pointed out, the way to extend the functionality of a sealed class is with an Extension Method. Here is one that will do what you are asking:
code :
public sealed class MyClass       
{ 
    int a; int b;    
    int Add (int x, int y)    
    {
        return x + y;
    }
}

public static class MyClassExtensions
{
    public static decimal Average(this MyClass value,  int x, int y)
    {
        return (x + y)/2M;
    }
}
    var myClass = new MyClass();

    // returns 15
    var avg = myClass.Average(10, 20);
using System;

namespace ConsoleApplication1
{
    public sealed class MyClass
    {
        public int X { get; private set; }
        public int Y { get; private set; }

        public MyClass(int x, int y)
        {
            this.X = x;
            this.Y = y;
        }

        int Add()
        {
            return this.X + this.Y;
        }
    }

    public static class MyClassExtensions
    {
        public static decimal Average(this MyClass value)
        {
            return (value.X + value.Y) / 2M;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var myClass = new MyClass(10, 20);
            var avg = myClass.Average();

            Console.WriteLine(avg);
            Console.ReadLine();
        }
    }
}
A brief explanation : how JDBC works?

A brief explanation : how JDBC works?


By : Asad
Date : March 29 2020, 07:55 AM
Does that help can anyone in simple words explain what exactly the "steps" in implementing a JDBC mean? What is the importance of each step? Why DriverManager class is required? what is it? What is a Driver in first place? what does it do? Basically I want the internal working of JDBC (with respect to MySqL) , and how is it carried out? And also while connecting java with MySql, what is the importance of port? and why is username and password required? ,
Why DriverManager class is required? what is it?
Glvalue real examples and explanation?

Glvalue real examples and explanation?


By : user2858012
Date : March 29 2020, 07:55 AM
hope this fix your issue A glvalue is anything that isn't a prvalue. Examples are names of entities, or expressions that have reference type (regardless of the kind of the reference).
code :
int i;
int* p = &i;
int& f();
int&& g();

int h();

h() // prvalue
g() // glvalue (xvalue)
f() // glvalue (lvalue)
i   // glvalue (lvalue)
*p  // glvalue (lvalue)

std::move(i)  // glvalue (xvalue)
explanation for real-time behavior

explanation for real-time behavior


By : Hao Jiang
Date : March 29 2020, 07:55 AM
With these it helps Computers can't do anything simultaneously. So if you watch a movie, sound and video do not come at the same time. In reality, your CPU does it in short sequences, like show a movie-frame, play a few notes of music, show another frame etc. If you have n cores (or n processors), you computer can do at most n things at the same time. Now multithreading (running a few threads at the same time) allows for better real-time feeling for the end user, as more things happen at the same time.
Explanation to why connection = null in JDBC

Explanation to why connection = null in JDBC


By : swetab singh
Date : March 29 2020, 07:55 AM
it should still fix some issue Every time I check out a tutorial for JDBC programmers seem to set the connection as null , Usually, it's so the finally block can include a check like
code :
} finally {
    // ...
    if (conn != null) {
        try {
            conn.close();
        } catch (Exception e) {
        }
    }
}
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