How to solve java singleton mode and thread safety issues

Single case mode, multi-instance mode, and thread safety

Single case mode

Single case mode refers to ensuring that a class only There is a unique instance and provides a global access point.

Category: Lazy Man Style, Hungry Man Style

Why do we need singleton mode?

In some special cases, it is necessary that a class can only be used to generate a unique object. For example: there are many printers in the printer room, but its print management system only has one print task control object, which manages the print queue and assigns print tasks to each printer. The singleton pattern was created to solve such needs.

Implementation ideas:

In order to prevent the client from using the constructor to create multiple objects, declare the constructor as a private type. But this will make this class unavailable, so a static method that can obtain an instance must be provided, usually called the getInstance method, which returns an instance. This method must be static, because static methods are called based on the class name, otherwise they cannot be used.

Class diagram: Lazy Man style

##Class diagram: Hungry Man style

Test singleton class: Dog’

//懒汉式
public class Dog {
	private static Dog dog;
	private String name;
	private int age;
	
	//私有的构造器
	private Dog() {}
	
	public String getName() {
		return name;
	}

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

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	//静态工厂方法
	public static Dog getInstance() {
		if (dog == null) {
			dog = new Dog();
		}
		return dog;
	}

	@Override
	public String toString() {
		return "Dog [name=" + name + ", age=" + age + "]";
	}
}

Test singleton class ：Cat

//饿汉式
public class Cat {
	private static Cat cat = new Cat();
	private String name;
	private int age;
	
	//私有构造器
	private Cat() {}
	
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
	
	public int getAge() {
		return age;
	}
	
	public void setAge(int age) {
		this.age = age;
	}

	//静态工厂方法
	public static Cat getInstance() {
		return cat;
	}

	@Override
	public String toString() {
		return "Cat [name=" + name + ", age=" + age + "]";
	}
}

Test class

import java.util.HashSet;
import java.util.Set;

public class Client {

	public static void main(String[] args) {
		//单线程模式测试
		Dog dog1 = Dog.getInstance();
		Dog dog2 = Dog.getInstance();
		System.out.println("dog1 == dog2: "+(dog1 == dog2));
		
		Cat cat1 = Cat.getInstance();
		Cat cat2 = Cat.getInstance();
		System.out.println("cat1 == cat2: "+(cat1 == cat2));
	}
}

Run result

Creation difference

staticmethod getInstance() is called for the first time object. Hungry Chinese style is to create a singleton object when the class is loaded, that is, instantiate the singleton class when declaring a static singleton object.

Thread safety

Resource occupation

Multi-threaded class

import java.util.HashSet;
import java.util.Set;

public class DogThread extends Thread{
	private Dog dog;
	private Set set;
	
	public DogThread() {
		set = new HashSet<>();
	}
	
	//这个方法是为了测试添加的。
	public int getCount() {
		return set.size();
	}
	
	@Override
	public void run() {
		dog = Dog.getInstance();
		set.add(dog);
	}
}

Multi-threaded test class

import java.util.HashSet;
import java.util.Set;

public class Client {

	public static void main(String[] args) {
		//单线程模式测试
		Dog dog1 = Dog.getInstance();
		Dog dog2 = Dog.getInstance();
		System.out.println("dog1 == dog2: "+(dog1 == dog2));
		
		Cat cat1 = Cat.getInstance();
		Cat cat2 = Cat.getInstance();
		System.out.println("cat1 == cat2: "+(cat1 == cat2));
		
		//多线程模式测试
		DogThread dogThread = new DogThread();
		Thread thread = null;
		for (int i = 0; i < 10; i++) {
			thread = new Thread(dogThread);
			thread.start();	
		}
		
		try {
			Thread.sleep(2000); //主线程等待子线程完成！
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("dog's number: "+dogThread.getCount());
	}
}

Running results
Note: The results of multi-threading are difficult to predict. Thread competition is involved here. The result may be the same multiple times (the same multiple times does not mean it is the same). Absolutely correct), but as long as you test it multiple times, you can see different results.

Here I use a little set technique, using Set The characteristics of the collection are to store the dog object generated each time in the Set collection, and finally just call the size() method of the collection. It can be seen that two dog objects are generated, which means an error has occurred, which is a programming error. It is also important to understand that errors may not occur under multi-threading, so the generated dog objects are smaller than the number of threads.

Note: There are many methods of synchronization, and you can also use Lock for processing. Synchronization is a method, not a specific method. Those who are interested can explore more about the synchronzied keyword. And the synchronization method is usually slower, and performance must also be weighed.

	//静态同步工厂方法
	public synchronized static Dog getInstance() {
		if (dog == null) {
			dog = new Dog();
		}
		return dog;
	}

Here is a multi-instance mode, that is, the number of objects is a fixed number. You can see the promotion of the singleton pattern. Of course, there are many ways to implement it. You can try the following. Here is my way.

//固定数目实例模式
public class MultiInstance {
	//实例数量,这里为四个
	private final static int INSTANCE_COUNT = 4;
	private static int COUNT = 0;
	private static MultiInstance[] instance = new MultiInstance[4];
	
	private MultiInstance() {};
	
	public static MultiInstance getInstance() {
		//注意数组的下标只能为 COUNT - 1
		if (MultiInstance.COUNT <= MultiInstance.INSTANCE_COUNT - 1) {
			instance[MultiInstance.COUNT] = new MultiInstance();
			MultiInstance.COUNT++;
		}
		//返回实例前，执行了 COUNT++ 操作，所以 应该返回上一个实例
		return MultiInstance.instance[MultiInstance.COUNT-1];  
	}
}

import java.util.HashSet;
import java.util.Set;

public class Test {
	public static void main(String[] args) {
		
		System.out.println("------------------------");
		testMultiInstance();
	}

	//测试多实例模式（单例的扩展，固定数目实例）
	public static void testMultiInstance() {
		Set instanceSet = new HashSet<>();
		MultiInstance instance = null;
		for (int i = 0; i < 10; i++) {
			instance = MultiInstance.getInstance();
			instanceSet.add(instance);
		}
		System.out.println("8个实例中，不同的实例有："+instanceSet.size());   
	}
}

Note : If used in a multi-threaded environment, thread safety must also be considered. If you are interested, you can implement it yourself.

这里举例看一看（我只能举我知道的哈！其他的感兴趣，可以去探究一下！）
使用反射：这种办法是非常有用的，通过反射即使是私有的属性和方法也可以访问了，因此反射破坏了类的封装性，所以使用反射还是要多多小心。但是反射也有许多其他的用途，这是一项非常有趣的技术（我也只是会一点点）。

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class Client {
	public static void main(String[] args) throws 
	ClassNotFoundException, 
	NoSuchMethodException, 
	SecurityException, 
	InstantiationException, 
	IllegalAccessException, 
	IllegalArgumentException, 
	InvocationTargetException {
	
		Class clazz = Class.forName("com.Dog");
		Constructor con = clazz.getDeclaredConstructor();
		//设置可访问权限
		con.setAccessible(true);
		Dog dog1 = (Dog) con.newInstance();
		Dog dog2 = (Dog) con.newInstance();
		System.out.println(dog1 == dog2);
	}
}

import java.io.Serializable;
//懒汉式
public class Dog implements Serializable{
	private static final long serialVersionUID = 1L;
	
	private static Dog dog;
	private String name;
	private int age;
	
	//私有的构造器
	private Dog() {}
	
	public String getName() {
		return name;
	}

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

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	//静态工厂方法
	public synchronized static Dog getInstance() {
		if (dog == null) {
			dog = new Dog();
		}
		return dog;
	}

	@Override
	public String toString() {
		return "Dog [name=" + name + ", age=" + age + "]";
	}
}

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class Client {
	public static void main(String[] args) throws IOException, ClassNotFoundException {
		Dog dog1 = Dog.getInstance();
		dog1.setName("小黑");
		dog1.setAge(2);
		System.out.println(dog1.toString());
		
		ByteArrayOutputStream bos = new ByteArrayOutputStream();
		ObjectOutputStream oos = new ObjectOutputStream(bos);
		oos.writeObject(dog1);
		
		
		ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
		ObjectInputStream ois = new ObjectInputStream(bis);
		Dog dog2 = (Dog) ois.readObject();
		System.out.println(dog2.toString());
		System.out.println("dog1 == dog2: "+(dog1 == dog2));
		
	}
}

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