From c793370441cd0607142a51a8c362e41ae53fcbfc Mon Sep 17 00:00:00 2001
From: maxy19 <315802767@qq.com>
Date: Wed, 1 Jan 2020 16:00:02 +0800
Subject: [PATCH] =?UTF-8?q?doc=EF=BC=9A=E5=A2=9E=E5=8A=A0=E4=B8=8E?=
 =?UTF-8?q?=E4=BF=AE=E6=94=B9dcl=E7=9B=B8=E5=85=B3=E6=B3=A8=E9=87=8A?=
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---
 pom.xml                                       |  5 +++++
 .../mxy/design/singleton/dcl/Singleton.java   | 22 ++++++++++++++++---
 .../mxy/design/singleton/SingletonTest.java   | 17 ++++++--------
 3 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/pom.xml b/pom.xml
index 504974e..ad04c1e 100644
--- a/pom.xml
+++ b/pom.xml
@@ -90,6 +90,11 @@
             <artifactId>cglib</artifactId>
             <version>3.3.0</version>
         </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>1.18.10</version>
+        </dependency>
 
 
     </dependencies>
diff --git a/src/main/com/mxy/design/singleton/dcl/Singleton.java b/src/main/com/mxy/design/singleton/dcl/Singleton.java
index e322be4..4a5dc1b 100644
--- a/src/main/com/mxy/design/singleton/dcl/Singleton.java
+++ b/src/main/com/mxy/design/singleton/dcl/Singleton.java
@@ -1,20 +1,36 @@
 package com.mxy.design.singleton.dcl;
 
+import lombok.SneakyThrows;
+
 /**
  * DCL模式
  */
 public class Singleton {
-    //重点:volatile 增加线程间的可见性 线程安全
+    /*
+      重点:volatile 利用有序性保证变量不参与重排序
+      synchronized 保证代码块里面有序性，不能保证未进入代码块的变量有序，jvm依然可以重排序
+      dcl问题点：
+      需要知道的是 singleton = new Singleton();这句代码并不是一个原子操作,他的操作大体上可以被拆分为三步
+        1.分配内存空间
+        2.实例化对象instance
+        3.把instance引用指向已分配的内存空间,此时instance有了内存地址,不再为null了
+        java是允许对指令进行重排序, 那么以上的三步的执行顺序就有可能是1-3-2. 在这种情况下,
+        如果线程A执行完1-3之后被阻塞了, 而恰好此时线程B进来了 此时的 singleton 已经不为空了所以线程B走完代码
+        在[第一步]以后就直接返回了这个还没有实例化好的instance, 所以在调用其后续的实例方法时就会得不到预期的结果
+    */
     private static volatile Singleton singleton;
 
-    private Singleton() {}
+    private Singleton() {
+    }
 
+    @SneakyThrows
     public static Singleton getInstance() {
-        if (singleton == null) {
+        if (singleton == null) { //第一步，这一行在临界区，不受锁保护
             synchronized (Singleton.class) {
                 if (singleton == null) {
                     singleton = new Singleton();
                 }
+                //同步代码块的unlock之前会把线程中的最新状态刷回主存
             }
         }
         return singleton;
diff --git a/src/test/com/mxy/design/singleton/SingletonTest.java b/src/test/com/mxy/design/singleton/SingletonTest.java
index 6493e57..9189fda 100644
--- a/src/test/com/mxy/design/singleton/SingletonTest.java
+++ b/src/test/com/mxy/design/singleton/SingletonTest.java
@@ -1,11 +1,11 @@
 package com.mxy.design.singleton;
 
 import com.mxy.design.singleton.enum_.Singleton;
+import lombok.SneakyThrows;
 import org.junit.Test;
 
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
-import java.util.concurrent.TimeUnit;
 
 /**
  * 单利模式:单例对象的类只能允许一个实例存在。
@@ -25,7 +25,7 @@ public class SingletonTest {
 
     @Test
     public void logicTest1() {
-        for (int i = 0; i <10 ; i++) {
+        for (int i = 0; i < 10; i++) {
             System.out.println(com.mxy.design.singleton.lazy.Singleton.getInstance());
         }
         System.out.println("懒汉模式");
@@ -33,7 +33,7 @@ public void logicTest1() {
 
     @Test
     public void logicTest2() {
-        for (int i = 0; i <10 ; i++) {
+        for (int i = 0; i < 10; i++) {
             System.out.println(com.mxy.design.singleton.hungry.Singleton.getInstance());
         }
         System.out.println("饿汉模式");
@@ -59,7 +59,7 @@ public void logicTest4() {
             threadPool.execute(new Runnable() {
                 @Override
                 public void run() {
-                    System.out.println(com.mxy.design.singleton.dcl.Singleton.getInstance());
+                    System.out.println(com.mxy.design.singleton.dcl.Singleton.getInstance().hashCode());
                 }
             });
         }
@@ -95,14 +95,11 @@ public void run() {
         System.out.println("静态内部类模式");
     }
 
+    @SneakyThrows
     private void shutDownThreadPool() {
         threadPool.shutdown();
-        try {
-            if (!threadPool.awaitTermination(5, TimeUnit.SECONDS)) {
-                threadPool.shutdownNow();
-            }
-        } catch (InterruptedException e) {
-            threadPool.shutdownNow();
+        while (!threadPool.isTerminated()) {
+            Thread.sleep(100);
         }
     }