Singleton Pattern

Design Pattern

创建时间:2022-12-07 19:18

字数:517 阅读:

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1. why we need
2. Implementation
Reference

1. why we need

Registry Setting
Thread Pool / Connection Pool

The singleton Pattern ensures a class has only one instance, and provides a global point of access to it.

let a class to manage a single instance of itself
- also prevent other classes from creating a new instance on its own
provide a global access point to the instance

2. Implementation

2.1 1st Iteration

public class Singleton {
    private static Singleton uniqueInstance;

    private Singleton() {}

    public static Singleton getInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

Issue when multi threading

Thread A

if (uniqueInstance == null) { // return true

new Singleton()

Thread B

if (uniqueInstance == null) { // return true

new Singleton()

Under such case, we’ll create two new instances

2.2 Implementation with multi-threading support

public class Singleton {
    private static Singleton uniqueInstance;

    private Singleton() {}

    public static synchronized Singleton getInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}

synchronized
- force every thread to wait its turn before it can enter the method
- no two threads may enter the method at the same time
Issue
- we only need synchronized when there is no instance, once there are, no need to use synchronized, it’s just overhead

2.3 Implementation with multi-threading support — Eagerly created instance

public class Singleton {
    private static Singleton uniqueInstance = new Singleton();

    private Singleton() {}

    public static synchronized Singleton getInstance() {
        return uniqueInstance;
    }
}

We rely on the JVM to create the unique instance of the Singleton when the class is loaded
JVM guarantees the instance will be created before any thread accesses the static unique instance variable

2.4 Implementation with multi-threading support — Double Checked Locking

public class Singleton {
    private volatile static Singleton uniqueInstance;

    private Singleton() {}

    public static synchronized Singleton getInstance() {
        if (uniqueInstance == null) {
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;    
    }
}

volatile
- ensure multiple threads handle the unique instance variable correctly when it is initialized to the singleton instance

2.5 Implementation with multi-threading support — Using Enum

public enum Singleton {
    UNIQUE_INSTANCE;
}

public class SingletonClient {
    Singleton singleton = Singleton.UNIQUE_INSTANCE;
}

ClassLoader

Responsible for loading java classes dynamically to the JVM during runtime
Responsible for loading classes into memory

type

application class loader
- An application or system class loader loads our own files in the classpath.
extension class loader
- Extension class loaders load classes that are an extension of the standard core Java classes.
bootstrap class loader
- A bootstrap or primordial class loader is the parent of all the others.
- This is because the bootstrap class loader is written in native code, not Java, so it doesn’t show up as a Java class
how does it work
- JVM request a class
- class loader try to locate the class and load the definition into the runtime using fully qualified class name
delegation model
- delegate the search of the class/ resource to the parent class loader

Reference

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