单例模式

单例模式主要包含以下几种，并且每种的优缺点如下：

1.饿汉模式

public class SingleObject {

    private final static SingleObject INSTANCE = new SingleObject();

    private SingleObject(){
    }
    public static SingleObject getInstance() {
        return INSTANCE;
    }
}

优点：实现简单
缺点：需要预先创建，不管是否需要

2.懒汉模式

public class SingleObject {

    private static SingleObject instance = null;

    private SingleObject(){

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

优点：不需要时不会加载创建
缺点：非线程安全并且获取实例慢

3.线程安全懒汉模式

public class SingleObject {
    private static SingleObject instance = null;

    private SingleObject(){
    }
    public synchronized static SingleObject getInstance() {
        if (instance == null) {
            instance = new SingleObject();
        }
        return instance;
    }
}

优点：线程安全
缺点：加了同步锁，并发量大时获取实例速度大大降低

4.双重校验锁

public class SingleObject {
    private volatile static SingleObject instance = null;
    private SingleObject(){
    }
    public static SingleObject getInstance() {
        if (instance == null) {
            synchronized (SingleObject.class){
                if (instance == null) {
                    instance = new SingleObject();
                }
            }
        }
        return instance;
    }
}

优点：线程安全+懒加载
缺点：实现复杂

5.静态内部类

public class SingleObject {
    private static class SingletonHolder{
        private final static SingleObject INSTANCE = new SingleObject();
    }

    private SingleObject(){
    }
    public static SingleObject getInstance() {
        return SingletonHolder.INSTANCE;
    }
}

优点：懒加载SingletonHolder类
缺点：实现复杂，需要多创建内部类

6.枚举

public enum  SingleObject {
    INSTANCE;
    private SingleObject(){
    }
    public static SingleObject getInstance() {
        return INSTANCE;
    }
}

完整方式：

public class SingleObject {

    private SingleObject(){
    }
    private enum SingletonEnum{
        INSTANCE;
        private SingleObject instance = null;

        private SingletonEnum(){
            instance = new SingleObject();
        }
        private SingleObject getInstance(){
            return instance;
        }
    }
    public static SingleObject getInstance() {
        return SingletonEnum.INSTANCE.getInstance();
    }
}

优点：线程安全，《effective java》推荐使用
缺点：实现较为繁琐