feat

Author: CodePrometheus

.gitignore

@@ -1,2 +1,87 @@
-/out/
-.idea\
+.idea
+### Maven template
+target/
+pom.xml.tag
+pom.xml.releaseBackup
+pom.xml.versionsBackup
+pom.xml.next
+release.properties
+dependency-reduced-pom.xml
+buildNumber.properties
+.mvn/timing.properties
+# https://github.com/takari/maven-wrapper#usage-without-binary-jar
+.mvn/wrapper/maven-wrapper.jar
+
+### JetBrains template
+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio, WebStorm and Rider
+# Reference: https://intellij-support.jetbrains.com/hc/en-us/articles/206544839
+
+# User-specific stuff
+.idea/**/workspace.xml
+.idea/**/tasks.xml
+.idea/**/usage.statistics.xml
+.idea/**/dictionaries
+.idea/**/shelf
+
+# Generated files
+.idea/**/contentModel.xml
+
+# Sensitive or high-churn files
+.idea/**/dataSources/
+.idea/**/dataSources.ids
+.idea/**/dataSources.local.xml
+.idea/**/sqlDataSources.xml
+.idea/**/dynamic.xml
+.idea/**/uiDesigner.xml
+.idea/**/dbnavigator.xml
+
+# Gradle
+.idea/**/gradle.xml
+.idea/**/libraries
+
+# Gradle and Maven with auto-import
+# When using Gradle or Maven with auto-import, you should exclude module files,
+# since they will be recreated, and may cause churn.  Uncomment if using
+# auto-import.
+# .idea/artifacts
+# .idea/compiler.xml
+# .idea/jarRepositories.xml
+# .idea/modules.xml
+# .idea/*.iml
+# .idea/modules
+# *.iml
+# *.ipr
+
+# CMake
+cmake-build-*/
+
+# Mongo Explorer plugin
+.idea/**/mongoSettings.xml
+
+# File-based project format
+*.iws
+
+# IntelliJ
+out/
+
+# mpeltonen/sbt-idea plugin
+.idea_modules/
+
+# JIRA plugin
+atlassian-ide-plugin.xml
+
+# Cursive Clojure plugin
+.idea/replstate.xml
+
+# Crashlytics plugin (for Android Studio and IntelliJ)
+com_crashlytics_export_strings.xml
+crashlytics.properties
+crashlytics-build.properties
+fabric.properties
+
+# Editor-based Rest Client
+.idea/httpRequests
+
+# Android studio 3.1+ serialized cache file
+.idea/caches/build_file_checksums.ser
+

JMM/src/singleton/lazyman/Singleton.java

@@ -0,0 +1,66 @@
+package singleton.lazyman;
+
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+import java.util.concurrent.*;
+
+/**
+ * @Author: zzStar
+ * @Date: 12-08-2021 14:03
+ */
+public class Singleton {
+
+    private static Singleton singleton = null;
+
+    private Integer x = 1;
+
+    public static Singleton getInstance() {
+        if (singleton == null) {
+            synchronized (Singleton.class) {
+                if (singleton == null) {
+                    singleton = new Singleton();
+                }
+            }
+        }
+        return singleton;
+    }
+
+    private static void removeInstance() {
+        singleton = null;
+    }
+
+    public static void main(String[] args) throws InterruptedException, ExecutionException {
+        ExecutorService pool = Executors.newCachedThreadPool();
+        // 500个线程并发，执行1000_000次
+        CyclicBarrier barrier = new CyclicBarrier(500);
+        for (int i = 0; i < 1000_000; i++) {
+            barrier.reset();
+            System.out.println("i = " + i);
+            List<Callable<Singleton>> list = new ArrayList<>();
+            for (int j = 0; j < 500; j++) {
+                list.add(() -> {
+                    barrier.await();
+                    Singleton instance = Singleton.getInstance();
+                    if (instance.x == null) {
+                        throw new RuntimeException("拿到了未被初始化的对象");
+                    }
+                    return instance;
+                });
+            }
+            List<Future<Singleton>> futures = pool.invokeAll(list);
+            Set<Singleton> set = new HashSet<>();
+            for (Future<Singleton> future : futures) {
+                set.add(future.get());
+            }
+            if (set.size() > 1) {
+                System.out.println("并发生成多实例");
+                throw new RuntimeException();
+            }
+            Singleton.removeInstance();
+        }
+        System.out.println("正常结束");
+    }
+
+}

JMM/src/singleton/lazyman/SingletonDoubleCheck.java

@@ -10,6 +10,8 @@
 public class SingletonDoubleCheck {
 
     // 新建对象中的三个步骤，重排序会带来的NPE，防止重排序
+    // NPE: 由于重排序，可能会有线程获取到成员未初始化好的对象
+    // new不是JVM原子指令，初始化与变量的引用绑定存储地址可能乱序，导致并发下会获取未初始化实例
     private volatile static SingletonDoubleCheck instance;
 
     private SingletonDoubleCheck() {
@@ -18,6 +20,7 @@ private SingletonDoubleCheck() {
     public static SingletonDoubleCheck getInstance() {
 
         if (instance == null) {
+            // 锁instance本身会出现空指针
             synchronized (SingletonDoubleCheck.class) {
                 // 第二个线程进来做一个判断检查，依然是空，才创建实例
                 if (instance == null) {