GOF-23种设计模式-单例模式

1.单例模式

1）饿汉式

package com.bjsxt.singleton;

public class SingletonDemo1 {
    
    //类初始化时，立即加载这个对象（没有延时加载的优势）。加载类时，天然的是线程安全的！
    private static SingletonDemo1 instance = new SingletonDemo1();  
    
    private SingletonDemo1(){
    }
    
    //方法没有同步，调用效率高！
    public static SingletonDemo1  getInstance(){
        return instance;
    }
    
}

package com.bjsxt.singleton;

public class Client {
    
    public static void main(String[] args) {
        SingletonDemo4 s1 = SingletonDemo4.getInstance();
        SingletonDemo4 s2 = SingletonDemo4.getInstance();
        
        System.out.println(s1);
        System.out.println(s2);
    }
}

2）懒汉式

package com.bjsxt.singleton;

public class SingletonDemo2 {
    
    //类初始化时，不初始化这个对象（延时加载，真正用的时候再创建）。
    private static SingletonDemo2 instance;  
    
    private SingletonDemo2(){ //私有化构造器
    }
    
    //方法同步，调用效率低！
    public static  synchronized SingletonDemo2  getInstance(){
        if(instance==null){
            instance = new SingletonDemo2();
        }
        return instance;
    }
}

3）双重检查锁

package com.bjsxt.singleton;

public class SingletonDemo3 { 

  private static SingletonDemo3 instance = null; 

  public static SingletonDemo3 getInstance() { 
    if (instance == null) { 
      SingletonDemo3 sc; 
      synchronized (SingletonDemo3.class) { 
        sc = instance; 
        if (sc == null) { 
          synchronized (SingletonDemo3.class) { 
            if(sc == null) { 
              sc = new SingletonDemo3(); 
            } 
          } 
          instance = sc; 
        } 
      } 
    } 
    return instance; 
  } 

  private SingletonDemo3() { 
  } 
}

 4）静态内部类

package com.bjsxt.singleton;

public class SingletonDemo4 {
    
    private static class SingletonClassInstance {
        private static final SingletonDemo4 instance = new SingletonDemo4();
    }
    
    private SingletonDemo4(){
    }
    
    //方法没有同步，调用效率高！
    public static SingletonDemo4  getInstance(){
        return SingletonClassInstance.instance;
    } 
}

5）枚举式

package com.bjsxt.singleton;

public enum SingletonDemo5 {
    
    //这个枚举元素，本身就是单例对象！
    INSTANCE;
    
    //添加自己需要的操作！
    public void singletonOperation(){
    } 
}

其他：

测试懒汉式单例模式(如何防止反射和反序列化漏洞)

package com.bjsxt.singleton;

import java.io.ObjectStreamException;
import java.io.Serializable;

public class SingletonDemo6 implements Serializable {
    //类初始化时，不初始化这个对象（延时加载，真正用的时候再创建）。
    private static SingletonDemo6 instance;  
    
    private SingletonDemo6(){ //私有化构造器
        if(instance!=null){
            throw new RuntimeException();
        }
    }
    
    //方法同步，调用效率低！
    public static  synchronized SingletonDemo6  getInstance(){
        if(instance==null){
            instance = new SingletonDemo6();
        }
        return instance;
    }
    
    //反序列化时，如果定义了readResolve()则直接返回此方法指定的对象。而不需要单独再创建新对象！
    private Object readResolve() throws ObjectStreamException {
        return instance;
    }
    
}

测试反射和反序列化破解单例模式

package com.bjsxt.singleton;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class Client2 {
    
    public static void main(String[] args) throws Exception {
        SingletonDemo6 s1 = SingletonDemo6.getInstance();
        SingletonDemo6 s2 = SingletonDemo6.getInstance();
        
        System.out.println(s1);
        System.out.println(s2);
        
        //通过反射的方式直接调用私有构造器
//        Class<SingletonDemo6> clazz = (Class<SingletonDemo6>) Class.forName("com.bjsxt.singleton.SingletonDemo6");
//        Constructor<SingletonDemo6> c = clazz.getDeclaredConstructor(null);
//        c.setAccessible(true);
//        SingletonDemo6  s3 = c.newInstance();
//        SingletonDemo6  s4 = c.newInstance();
//        System.out.println(s3);
//        System.out.println(s4);
        
        //通过反序列化的方式构造多个对象 
        FileOutputStream fos = new FileOutputStream("d:/a.txt");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(s1);
        oos.close();
        fos.close();
        
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("d:/a.txt"));
        SingletonDemo6 s3 =  (SingletonDemo6) ois.readObject();
        System.out.println(s3);
        
        
    }
}

测试多线程环境下五种创建单例模式的效率

package com.bjsxt.singleton;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
import java.lang.reflect.Constructor;
import java.util.concurrent.CountDownLatch;

public class Client3 {

    public static void main(String[] args) throws Exception {

        long start = System.currentTimeMillis();
        int threadNum = 10;
        final CountDownLatch countDownLatch = new CountDownLatch(threadNum);

        for (int i = 0; i < threadNum; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {

                    for (int i = 0; i < 1000000; i++) {
//                        Object o = SingletonDemo4.getInstance();
                        Object o = SingletonDemo5.INSTANCE;
                    }

                    countDownLatch.countDown();
                }
            }).start();
        }

        countDownLatch.await(); // main线程阻塞，直到计数器变为0，才会继续往下执行！
        long end = System.currentTimeMillis();
        System.out.println("总耗时：" + (end - start));
    }
}