From f975b3c9fe0a8c214c7ff0b214ffb025cbb3183e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?F=E5=98=89=E9=98=B3?= <fjy8018@qq.com>
Date: Mon, 24 Sep 2018 13:08:48 +0800
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---
 .../singleton/LazyDoubleCheckSingleton.java   | 66 +++++++++++++++++++
 .../LazyDoubleCheckSingletonTest.java         | 40 +++++++++++
 2 files changed, 106 insertions(+)
 create mode 100644 src/main/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingleton.java
 create mode 100644 src/test/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingletonTest.java

diff --git a/src/main/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingleton.java b/src/main/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingleton.java
new file mode 100644
index 0000000..3956e2a
--- /dev/null
+++ b/src/main/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingleton.java
@@ -0,0 +1,66 @@
+package top.fjy8018.designpattern.pattern.creational.singleton;
+
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * 懒汉式单例：延迟加载（双重校验锁保证线程安全）
+ *
+ * @author F嘉阳
+ * @date 2018-09-24 10:29
+ */
+@Slf4j
+public class LazyDoubleCheckSingleton {
+    /**
+     * volatile确保内存可见性，同时禁止指令重排
+     * volatile会增加一定的汇编代码：
+     * 1. 将当前处理器缓存行的数据写回系统内存
+     * 2. 写回系统内存后使其他处理器缓存的地址无效，迫使其他处理器从共享内存中同步最新数据
+     */
+    private volatile static LazyDoubleCheckSingleton lazySingleton = null;
+
+    /**
+     * 单例模式构造器必须是屏蔽的
+     */
+    private LazyDoubleCheckSingleton() {
+    }
+
+    /**
+     * 双重校验锁确保线程安全
+     * 存在指令重排序导致的隐患
+     *
+     * @return
+     */
+    public static LazyDoubleCheckSingleton getInstance() {
+        // 首次检查
+        if (lazySingleton == null) {
+            // 再次检查，在此处加锁比对方法加锁开销更小，因为如果首次检查不为空则直接返回
+            synchronized (LazyDoubleCheckSingleton.class) {
+                log.debug("LazySingletonSynchronized实例化");
+                lazySingleton = new LazyDoubleCheckSingleton();
+                // 实例化包含三个过程，这三个过程可能由于指令重排序导致执行顺序打乱
+                // 但Java底层只保证三个过程在单线程内顺序一致
+                // 1.分配内存
+                // 2.初始化
+                // 3.LazyDoubleCheckSingleton对象指向刚分配的内存地址
+            }
+        }
+        return lazySingleton;
+    }
+
+    /**
+     * 线程安全验证
+     *
+     * @return
+     */
+    public static LazyDoubleCheckSingleton getInstance2() throws InterruptedException {
+        if (lazySingleton == null) {
+            // 使用线程等待模拟复杂实例化过程，让多线程同时进入该方法
+            Thread.sleep(1000);
+            synchronized (LazyDoubleCheckSingleton.class) {
+                log.debug("LazySingletonSynchronized实例化");
+                lazySingleton = new LazyDoubleCheckSingleton();
+            }
+        }
+        return lazySingleton;
+    }
+}
diff --git a/src/test/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingletonTest.java b/src/test/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingletonTest.java
new file mode 100644
index 0000000..21690f5
--- /dev/null
+++ b/src/test/java/top/fjy8018/designpattern/pattern/creational/singleton/LazyDoubleCheckSingletonTest.java
@@ -0,0 +1,40 @@
+package top.fjy8018.designpattern.pattern.creational.singleton;
+
+import lombok.extern.slf4j.Slf4j;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@Slf4j
+class LazyDoubleCheckSingletonTest {
+
+    @Test
+    void getInstance() {
+        LazyDoubleCheckSingleton instance = LazyDoubleCheckSingleton.getInstance();
+    }
+
+    @Test
+    void getInstance2() throws InterruptedException {
+        Thread thread1 = new Thread(new LazyDoubleCheckSingletonTest.MyRunnable());
+        Thread thread2 = new Thread(new LazyDoubleCheckSingletonTest.MyRunnable());
+
+        thread1.start();
+        thread2.start();
+
+        Thread.sleep(3000);
+        log.debug("finish");
+    }
+
+    private class MyRunnable implements Runnable {
+        @Override
+        public void run() {
+            LazySingletonSynchronized lazySingleton = null;
+            try {
+                lazySingleton = LazySingletonSynchronized.getInstance2();
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            log.info(Thread.currentThread().getName() + " " + lazySingleton);
+        }
+    }
+}
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