From 4fc72f3d5e36b8c934ab320381157f8e6bacfc72 Mon Sep 17 00:00:00 2001
From: ZhouXY108 <luquanlion@outlook.com>
Date: Sun, 6 Aug 2023 03:43:44 +0800
Subject: [PATCH] =?UTF-8?q?=E4=BF=AE=E6=94=B9=20SafeConcurrentHashMap=20?=
 =?UTF-8?q?=E7=9A=84=E9=94=99=E8=AF=AF=E3=80=82?=
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Content-Transfer-Encoding: 8bit

---
 .../xyz/zhouxy/plusone/commons/base/JRE.java  | 58 +++++++++++++++++++
 .../collection/SafeConcurrentHashMap.java     | 29 ++++++++--
 2 files changed, 82 insertions(+), 5 deletions(-)
 create mode 100644 src/main/java/xyz/zhouxy/plusone/commons/base/JRE.java

diff --git a/src/main/java/xyz/zhouxy/plusone/commons/base/JRE.java b/src/main/java/xyz/zhouxy/plusone/commons/base/JRE.java
new file mode 100644
index 0000000..a4ae6cf
--- /dev/null
+++ b/src/main/java/xyz/zhouxy/plusone/commons/base/JRE.java
@@ -0,0 +1,58 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package xyz.zhouxy.plusone.commons.base;
+
+import org.apache.commons.lang3.StringUtils;
+import org.apache.commons.lang3.reflect.MethodUtils;
+
+/**
+ * JRE version
+ */
+public class JRE {
+
+    private static final int JAVA_8 = 8;
+
+    public static final int CURRENT_VERSION = getJre();
+
+    public static boolean isJava8() {
+        return CURRENT_VERSION == JAVA_8;
+    }
+
+    private static int getJre() {
+        String version = System.getProperty("java.version");
+        boolean isBlank = StringUtils.isBlank(version);
+        if (!isBlank && version.startsWith("1.8")) {
+            return JAVA_8;
+        }
+        // if JRE version is 9 or above, we can get version from
+        // java.lang.Runtime.version()
+        try {
+            Object javaRunTimeVersion = MethodUtils.invokeMethod(Runtime.getRuntime(), "version");
+            return (int) MethodUtils.invokeMethod(javaRunTimeVersion, "major");
+        } catch (Exception e) {
+            // Can't determine current JRE version (maybe java.version is null),
+            // assuming that JRE version is 8.
+        }
+        // default java 8
+        return JAVA_8;
+    }
+
+    private JRE() {
+        throw new IllegalStateException("Utility class");
+    }
+}
diff --git a/src/main/java/xyz/zhouxy/plusone/commons/collection/SafeConcurrentHashMap.java b/src/main/java/xyz/zhouxy/plusone/commons/collection/SafeConcurrentHashMap.java
index f76e916..97e01bb 100644
--- a/src/main/java/xyz/zhouxy/plusone/commons/collection/SafeConcurrentHashMap.java
+++ b/src/main/java/xyz/zhouxy/plusone/commons/collection/SafeConcurrentHashMap.java
@@ -1,11 +1,14 @@
 package xyz.zhouxy.plusone.commons.collection;
 
 import java.util.Map;
+import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Function;
 
 import javax.annotation.concurrent.ThreadSafe;
 
+import xyz.zhouxy.plusone.commons.base.JRE;
+
 @ThreadSafe
 public class SafeConcurrentHashMap<K, V> extends ConcurrentHashMap<K, V> {
 
@@ -83,11 +86,27 @@ public class SafeConcurrentHashMap<K, V> extends ConcurrentHashMap<K, V> {
     /** {@inheritDoc} */
     @Override
     public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
-        V v = get(key);
-        if (null == v) {
-            putIfAbsent(key, mappingFunction.apply(key));
-            v = get(key);
+        Objects.requireNonNull(mappingFunction);
+        if (JRE.isJava8()) {
+            V v = get(key);
+            if (null == v) {
+                // this bug fix methods maybe cause `mappingFunction.apply` multiple calls.
+                v = mappingFunction.apply(key);
+                if (null == v) {
+                    return null;
+                }
+                final V res = putIfAbsent(key, v);
+                if (null != res) {
+                    // if pre value present, means other thread put value already,
+                    // and putIfAbsent not effect
+                    // return exist value
+                    return res;
+                }
+                // if pre value is null, means putIfAbsent effected, return current value
+            }
+            return v;
+        } else {
+            return computeIfAbsent(key, mappingFunction);
         }
-        return v;
     }
 }