图解设计模式(Creational)-Singleton 5/23

只有一个实例

示例代码

饿汉模式

public class Singleton1 {

    private static Singleton1 instance = new Singleton1();

    private Singleton1() {
    }

    public static Singleton1 getInstance() {
        return instance;
    }

}

懒汉模式（线程不安全）

public class Singleton2 {

    private static Singleton2 instance;

    private Singleton2() {

    }

    public Singleton2 getInstance() {
        if (instance == null) {
            instance = new Singleton2();
        }
        return instance;
    }

}

懒汉模式（双检锁、线程安全）

public class Singleton3 {

    private static volatile Singleton3 instance;

    private Singleton3() {

    }

    public Singleton3 getInstance() {
        if (instance == null) {
            synchronized (Singleton3.class) {
                if (instance == null) {
                    instance = new Singleton3();
                }
            }
        }
        return instance;
    }
}

枚举

public enum Singleton4 {

    instance;

}