设计模式-单例模式(JAVA)

public class SingletonExample1 {
    private SingletonExample1() {
    }
 
    private static SingletonExample1 instance = null;
 
    public static SingletonExample1 getInsetance() {
        if (instance == null) {
            instance = new SingletonExample1();
        }
        return instance;
    }
}
 
public class SingletonExample2 {
    private SingletonExample2() {
    }
 
    private static SingletonExample2 instance = new SingletonExample2();
 
    public static SingletonExample2 getInstance() {
        return instance;
    }
}
 
public class SingletonExample3 {
    private SingletonExample3() {
    }
 
    private static SingletonExample3 instance = null;
 
    public static synchronized SingletonExample3 SingletonExample3() {
        if (instance == null) {
            instance = new SingletonExample3();
        }
        return instance;
    }
}
 
public class SingletonExample4 {
    private static SingletonExample4 instance = null;
 
    private SingletonExample4() {
    }
 
    public static SingletonExample4 SingletonExample4() {
        if (instance == null) {
            synchronized (SingletonExample4.class) {
                if (instance == null) {
                    instance = new SingletonExample4();
                }
            }
        }
        return instance;
    }
}
 
public class SingletonExample5 {
    private SingletonExample5() {
    }
 
    private static volatile SingletonExample5 instance = null;
 
    public static SingletonExample5 getInstance() {
        if (instance == null) {
            synchronized (SingletonExample5.class) {
                if (instance == null) {
                    instance = new SingletonExample5();
                }
            }
        }
        return instance;
    }
}
 
public class SingletonExample6 {
    private SingletonExample6() {
    }
 
    private static SingletonExample6 instance = null;
 
    static {
        instance = new SingletonExample6();
    }
 
    public static SingletonExample6 getInstance() {
        return instance;
    }
}
 
public class SingletonExample7 {
    private SingletonExample7() {
    }
 
    public static SingletonExample7 getInstance() {
        return Singleton.INSTANCE.instance;
    }
 
    private enum Singleton {
        INSTANCE;
        private SingletonExample7 instance;
 
        Singleton() {
            this.instance = new SingletonExample7();
        }
 
 
    }
}