单例模式

1. 饿汉式（静态常量或静态代码块）

public class StaticSingleton {
    private static StaticSingleton singleton = new StaticSingleton();

    private StaticSingleton(){

    }

    public static StaticSingleton getSingleton(){
        return singleton;
    }
}

2. 懒汉式(线程安全，同步方法)

public class SynSingleton {
    private static SynSingleton singleton;

    private SynSingleton() {

    }

    public static synchronized SynSingleton getSingleton() {
        if (singleton == null) {
            singleton = new SynSingleton();
        }
        return singleton;
    }
}

3. 懒汉式(线程安全，双重检查，同步代码块)

public class DoubleCheckSingleton {

    private static volatile DoubleCheckSingleton singleton;

    private DoubleCheckSingleton() {

    }

    public static DoubleCheckSingleton getSingleton() {
        if (singleton == null) {
            synchronized (DoubleCheckSingleton.class) {
                if (singleton == null) {
                    singleton = new DoubleCheckSingleton();
                }
            }
        }
        return singleton;
    }
}

4. 静态内部类

public class InnerClassSingleton {
    private static InnerClassSingleton singleton;

    private InnerClassSingleton() {

    }

    private static class SingletonInstance {
        private static InnerClassSingleton instance = new InnerClassSingleton();
    }

    public static InnerClassSingleton getInstance() {
        return SingletonInstance.instance;
    }
}