【GOF23设计模式】单例模式

来源：http://www.bjsxt.com/ 
一、【GOF23设计模式】_单例模式、应用场景、饿汉式、懒汉式

1、GOF23设计模式 

2、单例模式 

3、饿汉式 

 1 package com.test.singleton;
 2 /**
 3  * 测试饿汉式单例模式
 4  */
 5 public class SingletonDemo01 {
 6
 7     //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！
 8     private static SingletonDemo01 instance = new SingletonDemo01();
 9
10     private SingletonDemo01(){//私有化构造器
11     }
12
13     //方法不用同步，调用效率高！
14     public static SingletonDemo01 getInstance(){
15         return instance;
16     }
17 }

4、懒汉式 

 1 package com.test.singleton;
 2 /**
 3  * 测试懒汉式单例模式
 4  */
 5 public class SingletonDemo02 {
 6
 7     //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)
 8     private static SingletonDemo02 instance;
 9
10     private SingletonDemo02(){//私有化构造器
11     }
12
13     //方法同步，调用效率低！
14     public static synchronized SingletonDemo02 getInstance(){
15         if(null==instance){
16             instance = new SingletonDemo02();
17         }
18         return instance;
19     }
20 }

二、【GOF23设计模式】_单例模式、双重检查锁式、静态内部类式、枚举式、UML类图

双重检测锁实现 

 1 package com.test.singleton;
 2 /**
 3  * 测试双重检测锁式单例模式
 4  */
 5 public class SingletonDemo03 {
 6
 7     //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)
 8     private static SingletonDemo03 instance;
 9
10     private SingletonDemo03(){//私有化构造器
11     }
12
13     //调用效率低！
14     public static SingletonDemo03 getInstance(){
15         if(null==instance){
16             SingletonDemo03 sc;
17             synchronized(SingletonDemo03.class){
18                 sc = instance;
19                 if(null==sc){
20                     synchronized (SingletonDemo03.class) {
21                         if(null==sc){
22                             sc = new SingletonDemo03();
23                         }
24                     }
25                     instance = sc;
26                 }
27             }
28         }
29         return instance;
30     }
31 }

静态内部类实现 

 1 package com.test.singleton;
 2 /**
 3  * 测试静态内部类实现单例模式
 4  * 这种方式：线程安全，调用效率高，并且实现了延时加载！
 5  */
 6 public class SingletonDemo04 {
 7
 8     private static class SingletonClassInstance {
 9         private static final SingletonDemo04 instance = new SingletonDemo04();
10     }
11
12     private SingletonDemo04(){//私有化构造器
13     }
14
15     //方法没有同步，调用效率高！
16     public static SingletonDemo04 getInstance(){
17         return SingletonClassInstance.instance;
18     }
19 }

枚举实现 

 1 package com.test.singleton;
 2 /**
 3  * 测试枚举式实现单例模式(没有延时加载)
 4  */
 5 public enum SingletonDemo05 {
 6
 7     //这个枚举元素，本身就是单例对象！
 8     INSTANCE;
 9
10     //添加自己需要的操作！
11     public void singletonOperation(){
12     }
13 }

 1 package com.test.singleton;
 2
 3 public class Client {
 4     public static void main(String[] args) {
 5         SingletonDemo01 s1 = SingletonDemo01.getInstance();
 6         SingletonDemo01 s2 = SingletonDemo01.getInstance();
 7         System.out.println(s1==s2);//true
 8
 9         SingletonDemo03 s3 = SingletonDemo03.getInstance();
10         SingletonDemo03 s4 = SingletonDemo03.getInstance();
11         System.out.println(s3==s4);//true
12
13         System.out.println(SingletonDemo05.INSTANCE==SingletonDemo05.INSTANCE);//true
14     }
15 }

五种单例模式实现 

三、【GOF23设计模式】_单例模式、反射和反序列化漏洞和解决方案、多线程环境测试、CountDownLatch同步类的使用

 1 package com.test.singleton;
 2
 3 import java.io.FileInputStream;
 4 import java.io.FileOutputStream;
 5 import java.io.ObjectInputStream;
 6 import java.io.ObjectOutputStream;
 7 import java.lang.reflect.Constructor;
 8
 9 /**
10  * 测试反射和反序列化破解单例模式
11  */
12 public class Client2 {
13     public static void main(String[] args) throws Exception {
14         SingletonDemo06 s1 = SingletonDemo06.getInstance();
15         SingletonDemo06 s2 = SingletonDemo06.getInstance();
16         System.out.println(s1);
17         System.out.println(s2);
18
19         //通过反射的方式直接调用私有构造器
20 /*      Class<SingletonDemo06> clazz = (Class<SingletonDemo06>) Class.forName("com.test.singleton.SingletonDemo06");
21         Constructor<SingletonDemo06> c = clazz.getDeclaredConstructor(null);
22         c.setAccessible(true);//访问私有，破解单例
23         SingletonDemo06 s3 = c.newInstance();
24         SingletonDemo06 s4 = c.newInstance();
25         System.out.println(s3);//SingletonDemo06的私有构造器中加了代码防止破解
26         System.out.println(s4);*/
27
28         //通过反序列化的方式构造多个对象
29         FileOutputStream fos = new FileOutputStream("g:/java/test/a.txt");
30         ObjectOutputStream oos = new ObjectOutputStream(fos);
31         oos.writeObject(s1);
32         oos.close();
33         fos.close();
34
35         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("g:/java/test/a.txt"));
36         SingletonDemo06 s3 = (SingletonDemo06) ois.readObject();
37         System.out.println(s3);//SingletonDemo06定义了readResolve()防止破解
38     }
39 }

 1 package com.test.singleton;
 2
 3 import java.io.ObjectStreamException;
 4 import java.io.Serializable;
 5
 6 /**
 7  * 测试懒汉式单例模式(如何防止反射和反序列化漏洞)
 8  */
 9 public class SingletonDemo06 implements Serializable{
10
11     //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)
12     private static SingletonDemo06 instance;
13
14     private SingletonDemo06(){//私有化构造器
15         if(instance!=null){//防止反射破解单例(对象.setAccessible(true);)
16             throw new RuntimeException();
17         }
18     }
19
20     //方法同步，调用效率低！
21     public static synchronized SingletonDemo06 getInstance(){
22         if(null==instance){
23             instance = new SingletonDemo06();
24         }
25         return instance;
26     }
27
28     //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！
29     private Object readResolve() throws ObjectStreamException{
30         return instance;
31     }
32 }

 1 package com.test.singleton;
 2
 3 import java.util.concurrent.CountDownLatch;
 4
 5 /**
 6  * 测试多线程环境下五种创建单例模式的效率
 7  */
 8 public class Client3 {
 9     public static void main(String[] args) throws Exception{
10
11         int threadNum = 10;
12         final CountDownLatch countDownLatch = new CountDownLatch(threadNum);
13
14         long start = System.currentTimeMillis();
15         for(int i=0;i<10;i++){
16             new Thread(new Runnable(){
17                 @Override
18                 public void run() {
19                     for (int i = 0; i < 1000000; i++) {
20                         Object o = SingletonDemo01.getInstance();//分别测试几种单例模式的效率
21                         //Object o = SingletonDemo05.INSTANCE;
22                     }
23
24                     countDownLatch.countDown();//计数器减1
25                 }
26             }).start();
27         }
28
29         countDownLatch.await();//main线程阻塞，直到计数器变为0，才会继续往下执行！
30
31         long end = System.currentTimeMillis();
32         System.out.println("总耗时：" + (end-start));
33     }
34 }