java的几种单例模式

1、赖汉式

public class LazySingleton {
private static LazySingleton instance = new LazySingleton();

private LazySingleton() {

}

public static LazySingleton getInstance() {
return instance;
}
}

2、饿汉式

public class HungrySingleton {
private static HungrySingleton instance;

private HungrySingleton() {

}

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

return instance;
}
}

存在线程同步问题

3、同步饿汉式

public class HungrySingleton {
private static HungrySingleton instance;

private HungrySingleton() {

}

public static synchronized HungrySingleton getInstance() {
if (null == instance) {
instance = new HungrySingleton();
}

return instance;
}
}

可以解决线程同步问题，但是程序中遇到大量调用该方法时，存在效率问题

4、内部类

public class InnerClassSingleton {

private InnerClassSingleton() {

}

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

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

既解决线程问题，同时也不存在效率问题