Singleton and invariant modes of concurrent mode

The singleton pattern

Singleton mode is to ensure that there is only one instance of a class in the system .
In many ways , Static inner class method is recommended , Use the class load once feature to ensure that there is only one instance （ Thread safety ）,
At the same time, it has the advantages of lock free and lazy creation .
Example ：

public class StaticInnerClassSingleton {
    //  Privatized construction method 
    private StaticInnerClassSingleton(){}
    //  Inner class 
    static class Holder{
        static StaticInnerClassSingleton INSTANCE = new StaticInnerClassSingleton();
    }
    //  Get instance object 
    public static StaticInnerClassSingleton getInstance(){
        return Holder.INSTANCE;
    }

    public static void main(String[] args) {
        for(int i=0;i<10;i++){
            new Thread(()->{
                System.out.println(StaticInnerClassSingleton.getInstance());
            }).start();
        }
    }
}

Immutable mode

In a concurrent scenario , Invariant objects do not require synchronous operations .（ Because the object will not change , Therefore, there will be no inconsistency in the access of multiple threads ）
features ：

1. class Be modified as final
2. Properties are private and are final Embellished

JDK All metadata classes in 、 Wrapper classes are implemented using invariant patterns .

java.lang.String
java.lang.Boolean
java.lang.Byte
java.lang.Character
java.lang.Double
java.lang.Float
java.lang.Integer
java.lang.Long
java.lang.Short

Example ：

public final class FinalCat {// final  Make sure there are no subclasses 
    private final String name;// final  Ensure properties 
    private final Integer age;

    public FinalCat(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public Integer getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "FinalCat{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public static void main(String[] args) {
        FinalCat cat = new FinalCat(" Pikanzi ", 4);
        System.out.println(cat);
    }
}