双重检查锁测试

concurrency-test/src/main/java/xyz/xuminghai/test/CompareDoubleCheckLockTest.java

@@ -0,0 +1,68 @@
+package xyz.xuminghai.test;
+
+import org.openjdk.jcstress.annotations.*;
+import org.openjdk.jcstress.infra.results.L_Result;
+
+import java.io.Serializable;
+
+/**
+ * 2022/5/8 15:01 星期日<br/>
+ * 和{@link DoubleCheckLockTest}比较
+ * @author xuMingHai
+ */
+@JCStressTest
+@Outcome(id = "Instance", expect = Expect.ACCEPTABLE, desc = "OK")
+@Outcome(expect = Expect.ACCEPTABLE_INTERESTING, desc = "INTERESTING!!!")
+@State
+public class CompareDoubleCheckLockTest {
+
+    private volatile static Instance instance;
+
+    public static Instance getInstance() {
+        Instance result = instance;
+        if (result == null) {
+            synchronized (CompareDoubleCheckLockTest.class) {
+                result = instance;
+                if (result == null) {
+                    instance = new Instance();
+                }
+            }
+        }
+        return instance;
+    }
+
+    private static class Instance implements Serializable {
+        private static final long serialVersionUID = -3475949357133835998L;
+
+        @Override
+        public String toString() {
+            return "Instance";
+        }
+    }
+
+    /*
+      使用4个线程测试
+     */
+
+    @Actor
+    public void actor1(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+    @Actor
+    public void actor2(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+    @Actor
+    public void actor3(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+    @Actor
+    public void actor4(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+
+}

concurrency-test/src/main/java/xyz/xuminghai/test/DoubleCheckLockTest.java

@@ -0,0 +1,89 @@
+package xyz.xuminghai.test;
+
+import org.openjdk.jcstress.annotations.*;
+import org.openjdk.jcstress.infra.results.L_Result;
+
+import java.io.Serializable;
+
+/**
+ * 2022/5/8 14:47 星期日<br/>
+ * 双重检查锁单例<br/>
+ * <p>在维基百科中的<a href="https://zh.m.wikipedia.org/zh-cn/%E5%8F%8C%E9%87%8D%E6%A3%80%E6%9F%A5%E9%94%81%E5%AE%9A%E6%A8%A1%E5%BC%8F">双重检查锁</a>，使用了局部变量代替直接比较 volatile 修饰的变量</p>
+ * 以下来自 Joshua Bloch "Effective Java, Second Edition", p. 283
+ *  <pre>
+ * //Double-check idiom for lazy initialization of instance fields
+ * private volatile FieldType field;
+ * Fie1dType getFie1d() {
+ *             FieldType result= field;
+ *             if(result == nu11) { // First check (no locking)
+ *                 synchronized(this) {
+ *                     result = field;
+ *                     if (result == nul1) // Second check (with locking)
+ *                         field = result = computeFieldValue();
+ *                 }
+ *             }
+ *             return result;
+ *         }
+ *  </pre>
+ *  这段代码可能看起来似乎有些费解。尤其对于需要用到局部变量result可能有点不解。
+ *  这个变量的作用是确保field只在已经被初始化的情况下读取一次。虽然这不是严格需要，但是可以提升性能，并且因为给低级的并发编程应用了一些标准，因此更加优雅。
+ *  在我的机器上，上述的方法比没用局部变量的方法快了大约25%。
+ * @author xuMingHai
+ */
+@JCStressTest
+@Outcome(id = "Instance", expect = Expect.ACCEPTABLE, desc = "OK")
+@Outcome(expect = Expect.ACCEPTABLE_INTERESTING, desc = "INTERESTING!!!")
+@State
+public class DoubleCheckLockTest {
+
+    private volatile static Instance instance;
+
+    public static Instance getInstance() {
+        Instance result = instance;
+        if (result == null) {
+            synchronized (DoubleCheckLockTest.class) {
+                result = instance;
+                if (result == null) {
+                    instance = new Instance();
+                }
+            }
+        }
+        return instance;
+    }
+
+    private static class Instance implements Serializable {
+
+        private static final long serialVersionUID = -3949564844175144101L;
+
+        @Override
+        public String toString() {
+            return "Instance";
+        }
+    }
+
+        /*
+      使用4个线程测试
+     */
+
+    @Actor
+    public void actor1(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+    @Actor
+    public void actor2(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+    @Actor
+    public void actor3(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+    @Actor
+    public void actor4(L_Result r) {
+        r.r1 = getInstance();
+    }
+
+
+}