小白都能懂的设计模式 java版 单例模式

单例模式是每个程序员都很清楚的设计模式 这里先介绍三种，饿汉式单例，懒汉式单例，静态内部类，直接code，比较清晰~

饿汉式单例：

package shejimoshi; //饿汉式单例 public class HungrySingle { //可能会浪费空间 private byte[] data1 = new byte[2024*2024]; private byte[] data2 = new byte[2024*2024]; private byte[] data3 = new byte[2024*2024]; private byte[] data4 = new byte[2024*2024]; private HungrySingle(){ } private final static HungrySingle HUNGRY_SINGLE = new HungrySingle(); public static HungrySingle getInstance() { return HUNGRY_SINGLE; } }

懒汉式单例：

package shejimoshi; //懒汉式单例 public class LazyMan { private LazyMan() { System.out.println(Thread.currentThread().getName() + "ok"); } private volatile static LazyMan lazyMan; //双重检测锁模式的 懒汉式单例 DCL 懒汉式 public static LazyMan getInstance() { if(lazyMan == null) { synchronized (LazyMan.class) { if (lazyMan == null) { lazyMan = new LazyMan(); //不是一个原子性操作 /* * 1. 分布内存空间 * 2. 执行构造方法，初始化对象 * 3. 把整个对象指向整个空间 * * 123执行顺序可能是132，可能有问题， * */ } } } return lazyMan; } //多线程并发 public static void main(String[] args) { for (int i = 0; i < 10; i++) { new Thread(()->{ LazyMan.getInstance(); }).start(); } } }

使用静态内部类：

package shejimoshi; //静态内部类 public class Holder { private Holder() { } public static Holder getInstance() { return InnerClass.HOLDER; } public static class InnerClass { private static final Holder HOLDER = new Holder(); } }

反射可以通过setAccessible = true，破坏单例模式的私有性，所以上述都是不安全的

public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException { LazyMan instance1 = LazyMan.getInstance(); Constructor<LazyMan> declaredConstructor = LazyMan.class.getDeclaredConstructor(null); //破坏构造器私有性 declaredConstructor.setAccessible(true); LazyMan instance2 = declaredConstructor.newInstance(); System.out.println("instance1 = " + instance1); System.out.println("instance2 = " + instance2); }

解决方式后续补充~