五种单例

饿汉式_静态代码块

public class Singleton {
    private static Singleton singleton;
    static {
        singleton = new Singleton();
    }
    private Singleton() {
    }
    public static Singleton get() {
        return singleton;
    }
}

懒汉式_双重检查

public class Singleton {
    private static Singleton singleton;
    private Singleton() {
    }
    public static Singleton get() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}

懒汉式_静态内部类

public class Singleton {
    private static class SingletonInstance {
        private static final Singleton singleton = new Singleton();
    }
    private Singleton() {
    }
    public static Singleton get() {
        return SingletonInstance.singleton;
    }
}

饿汉式_枚举

public enum Singleton {
    INSTANCE;
    public void doProcess() {
        System.out.println("singleton");
    }
}