1 package singleInstance;
 2 /**饿汉式，类加载时候创建实例，浪费资源，线程是安全的*/
 3 public class SingleDome01 {
 4     //私有构造方法，防止直接new对象
 5     private SingleDome01() {
 6     }
 7     //类加载的时候，创建对象
 8     private static SingleDome01 instance = new SingleDome01();
 9     //提供getter，方便外界访问私有对象
10     public static SingleDome01 getInstance() {
11         return instance;
12     }
13 }

 1 package singleInstance;
 2 /**懒汉式，被调用时候才会实例化，节约资源，高并发线程时不安全*/
 3 public class SingleDome02 {
 4     //私有构造方法，防止直接new对象
 5     private SingleDome02(){
 6     }
 7     private static SingleDome02 instance;
 8     //提供getter,以便外界调用私有对象，返回SingleDome02对象
 9     public static SingleDome02 getInstance() {
10         if(instance==null){
11             instance = new SingleDome02();
12         }
13         return instance;
14     }
15 }

 1 package singleInstance;
 2 /**采用内部类实例化对象，在调用时候才会加载，节约资源，还能保证线程是安全的*/
 3 public class SingleDome03 {
 4     //私有构造方法，防止直接new对象
 5     private SingleDome03() {
 6     }
 7     //静态内部类只有在调用时候才会加载
 8     private static class InnerSingleDome03{
 9         private static SingleDome03 instance = new SingleDome03();
10     }
11     public static SingleDome03 getInstance(){
12         return InnerSingleDome03.instance;
13     }
14 }

 1 package singleInstance;
 2
 3 import java.util.HashMap;
 4 import java.util.Map;
 5
 6 /**采用spring的用法，利用反射创建对象，讲实例注册到map里面，下次用的时候直接取*/
 7 public class SingleDome04 {
 8     //私有构造方法，防止外部直接new对象
 9     private SingleDome04() {
10     }
11     private static Map<String,SingleDome04> map = new HashMap<String,SingleDome04>();
12     //静态代码块,类加载时候实例化对象和map，并将对象放入map里
13     static{
14         SingleDome04 instance = new SingleDome04();
15         map.put(instance.getClass().getName(), instance);
16     }
17     public static SingleDome04 getInstance(String name){
18         if(name==null){
19             name="singleInstance.SingleDome04";
20         }
21         if(!map.containsKey(name)){
22             try {
23                 map.put(name, (SingleDome04)Class.forName(name).newInstance());
24             } catch (InstantiationException e) {
25                 e.printStackTrace();
26             } catch (IllegalAccessException e) {
27                 e.printStackTrace();
28             } catch (ClassNotFoundException e) {
29                 e.printStackTrace();
30             }
31         }
32         return map.get(name);
33     }
34 }

 1 package singleInstance;
 2
 3 public class TestSingle {
 4     public static void main(String[] args) {
 5         SingleDome01 s1 = SingleDome01.getInstance();
 6         SingleDome01 s2 = SingleDome01.getInstance();
 7         if(s1==s2){
 8             System.out.println("s1=s2");
 9         }else{
10             System.out.println("s1!=s2");
11         }
12         SingleDome02 s3 = SingleDome02.getInstance();
13         SingleDome02 s4 = SingleDome02.getInstance();
14         if(s3==s4){
15             System.out.println("s3=s4");
16         }else{
17             System.out.println("s3!=s4");
18         }
19         SingleDome03 s5 = SingleDome03.getInstance();
20         SingleDome03 s6 = SingleDome03.getInstance();
21         if(s5==s6){
22             System.out.println("s5=s6");
23         }else{
24             System.out.println("s5!=s6");
25         }
26         SingleDome04 s7 = SingleDome04.getInstance("singleInstance.SingleDome04");
27         SingleDome04 s8 = SingleDome04.getInstance("singleInstance.SingleDome04");
28         if(s7==s8){
29             System.out.println("s7=s8");
30         }else{
31             System.out.println("s7!=s8");
32         }
33     }
34 }