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add singleton readme in gof example

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GOF/gof-example/src/main/java/com/ipipman/gof/example/singleton
- GOF-Singleton单例模式.md
- IdGenerator3.java

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`@@ -75,5 +75,37 @@ public class IdGenerator2 {`
`75`	`75`
`76`	`76`
`77`	`77`
	`78`	`+#### 3. 双重检测`
	`79`	`+`
	`80`	`+饿汉式不支持延迟加载,懒汉式有性能问题,不支持高并发;`
	`81`	`+`
	`82`	`+双重检测兼顾了性能和延迟加载问题;`
	`83`	`+`
	`84`	`+在实现方式中,只要 instance 被创建后,即便再调用 getInstance() 函数也不会再进入到加锁逻辑中了;`
	`85`	`+`
	`86`	+```java
	`87`	`+public class IdGenerator3 {`
	`88`	`+ private AtomicLong id = new AtomicLong(0);`
	`89`	`+ private static volatile IdGenerator3 instance;`
	`90`	`+ private IdGenerator3() {`
	`91`	`+ }`
	`92`	`+ public static IdGenerator3 getInstance() {`
	`93`	`+ if (instance == null) {`
	`94`	`+ synchronized (IdGenerator3.class) {`
	`95`	`+ if (instance == null) {`
	`96`	`+ instance = new IdGenerator3();`
	`97`	`+ }`
	`98`	`+ }`
	`99`	`+ }`
	`100`	`+ return instance;`
	`101`	`+ }`
	`102`	`+ public Long getId() {`
	`103`	`+ return id.incrementAndGet();`
	`104`	`+ }`
	`105`	`+}`
	`106`	+```
	`107`	`+`
	`108`	`+`
	`109`	`+`
`78`	`110`
`79`	`111`

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`@@ -0,0 +1,35 @@`
	`1`	`+package com.ipipman.gof.example.singleton;`
	`2`	`+`
	`3`	`+import java.util.concurrent.atomic.AtomicLong;`
	`4`	`+`
	`5`	`+// 3.双重检测`
	`6`	`+// 饿汉式不支持延迟加载,懒汉式有性能问题,不支持高并发;`
	`7`	`+// 双重检测兼顾了性能和延迟加载问题;`
	`8`	`+// 在实现方式中,只要 instance 被创建后,即便再调用 getInstance() 函数也不会再进入到加锁逻辑中了;`
	`9`	`+`
	`10`	`+public class IdGenerator3 {`
	`11`	`+ private AtomicLong id = new AtomicLong(0);`
	`12`	`+ private static volatile IdGenerator3 instance;`
	`13`	`+ private IdGenerator3() {`
	`14`	`+ }`
	`15`	`+ public static IdGenerator3 getInstance() {`
	`16`	`+ if (instance == null) {`
	`17`	`+ synchronized (IdGenerator3.class) {`
	`18`	`+ if (instance == null) {`
	`19`	`+ instance = new IdGenerator3();`
	`20`	`+ }`
	`21`	`+ }`
	`22`	`+ }`
	`23`	`+ return instance;`
	`24`	`+ }`
	`25`	`+`
	`26`	`+ public Long getId() {`
	`27`	`+ return id.incrementAndGet();`
	`28`	`+ }`
	`29`	`+`
	`30`	`+ // TODO Testing...`
	`31`	`+ public static void main(String[] args) {`
	`32`	`+ IdGenerator3 idGenerator3 = IdGenerator3.getInstance();`
	`33`	`+ System.out.println(idGenerator3.getId());`
	`34`	`+ }`
	`35`	`+}`

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