应用

1. 各种Manager
2. 各种Factory

单例模式

饿汉

public class Singleton {

    private static final Singleton INSTANCE = new Singleton();

    private Singleton() {
    }

    private static Singleton getInstance() {
        return INSTANCE;
    }
}

懒汉

public class Singleton {

    private static Singleton INSTANCE;

    private Singleton() {
    }

    private static synchronized Singleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Singleton();
        }
        return INSTANCE;
    }
}

懒汉（双重检锁）

public class Singleton {

    private static volatile Singleton INSTANCE;

    private Singleton() {
    }

    private static Singleton getInstance() {
        if (INSTANCE == null) {
            synchronized (Singleton.class) {
                if (INSTANCE == null) {
                    INSTANCE = new Singleton();
                }
            }
        }
        return INSTANCE;
    }
}

静态内部类

public class Singleton {

    private static class Holder {
        private static final Singleton INSTANCE = new Singleton();
    }

    private Singleton() {
    }

    private static Singleton getInstance() {
        return Holder.INSTANCE;
    }
}

枚举

public enum Singleton {

    INSTANCE;

    private static Singleton getInstance() {
        return INSTANCE;
    }
}

保护单例

防止序列化

private Object readResolve() {
      return getInstance();
}

防止反射

构造器中加入反射拦截代码