单例模式

饿汉式静态变量

public class Singleton01 {
    public static void main(String[] args) {
        Singleton instance1 = Singleton.getinstance();
        Singleton instance2 = Singleton.getinstance();
        System.out.println(instance1 == instance2);
    }
}
class Singleton {
    private Singleton() {
    }
    private final static Singleton instance = new Singleton();
    public static Singleton getinstance() {
        return instance;
    }
}

饿汉式静态代码块

public class Singleton02 {
    public static void main(String[] args) {
        Singleton instance1 = Singleton.getinstance();
        Singleton instance2 = Singleton.getinstance();
        System.out.println(instance1 == instance2);
    }
}
class Singleton {
    private Singleton() {
    }
    private static Singleton instance;
    static{
        instance=new Singleton();
    }
    public static Singleton getinstance() {
        return instance;
    }
}

懒汉式（线程不安全）

public class Singleton3 {
    public static void main(String[] args) {
        Singleton instance1 = Singleton.getinstance();
        Singleton instance2 = Singleton.getinstance();
        System.out.println(instance1 == instance2);
    }
}
class Singleton {
    private  Singleton (){
    }
    private  static Singleton instance;
    public static Singleton getinstance(){
        if(instance==null){
            instance=new Singleton();
        }
        return instance;
    }
}

懒汉式（线程安全,同步，效率低下）

public class Singleton4 {
    public static void main(String[] args) {
        Singleton instance1 = Singleton.getinstance();
        Singleton instance2 = Singleton.getinstance();
        System.out.println(instance1 == instance2);
    }
}
class Singleton {
    private  Singleton (){
    }
    private static Singleton instance;
    public static synchronized Singleton getinstance(){
        if(instance==null){
            instance=new Singleton();
        }
        return instance;
    }
}

双重检查（线程安全,同步）

public class Singleton5 {
    //双重检查（线程安全,同步）
    public static void main(String[] args) {
        Singleton instance1 = Singleton.getinstance();
        Singleton instance2 = Singleton.getinstance();
        System.out.println(instance1 == instance2);
    }
}
class Singleton {
    private  Singleton(){
    }
    //volatile保证可见性，防止指令重排
    private static volatile Singleton instance;
    public static Singleton getinstance(){
        if(instance==null){
            synchronized (Singleton.class) {
                if(instance==null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

静态内部类

public class Singleton6 {
    public static void main(String[] args) {
        Singleton instance1 = Singleton.getinstance();
        Singleton instance2 = Singleton.getinstance();
        System.out.println(instance1 == instance2);
    }
}
class Singleton {
    private Singleton() {
    }
    //写一个静态内部类，该类有一个静态属性Instance
    private static class SingletonInstance {
        private final static Singleton Instance = new Singleton();
    }
    public static Singleton getinstance() {
        return SingletonInstance.Instance;
    }
}