From 84e8456377127e301fc17887462a969370be2b3b Mon Sep 17 00:00:00 2001
From: Looly <loolly@gmail.com>
Date: Sun, 2 Apr 2023 21:37:50 +0800
Subject: [PATCH] =?UTF-8?q?=E4=BF=AE=E5=A4=8DSafeConcurrentHashMap.compute?=
 =?UTF-8?q?IfAbsent=E5=8F=AF=E8=83=BD=E5=AD=98=E5=9C=A8=E7=9A=84=E7=BB=93?=
 =?UTF-8?q?=E6=9E=9C=E4=B8=BAnull=E7=9A=84=E6=83=85=E5=86=B5?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 CHANGELOG.md                                  |  3 +-
 .../main/java/cn/hutool/core/map/MapUtil.java | 47 +++++++++++++++----
 .../core/map/SafeConcurrentHashMap.java       | 10 +---
 3 files changed, 41 insertions(+), 19 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a06cd3856..c6613efe5 100755
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 # 🚀Changelog
 
 -------------------------------------------------------------------------------------------------------------
-# 5.8.17.M1 (2023-04-01)
+# 5.8.17.M1 (2023-04-02)
 
 ### 🐣新特性
 * 【core  】      SerializeUtil.deserialize增加白名单类，避免RCE vulnerability（issue#3021@Github）
@@ -17,6 +17,7 @@
 ### 🐞Bug修复
 * 【core  】      CollUtil.split优化切割列表参数判断，避免OOM（pr#3026@Github）
 * 【core  】      修复FileUtil.move传入相同目录或子目录丢失源目录的问题（pr#3032@Github）
+* 【core  】      修复SafeConcurrentHashMap.computeIfAbsent可能存在的结果为null的情况（issue#I6RVMY@Gitee）
 
 -------------------------------------------------------------------------------------------------------------
 # 5.8.16 (2023-03-26)
diff --git a/hutool-core/src/main/java/cn/hutool/core/map/MapUtil.java b/hutool-core/src/main/java/cn/hutool/core/map/MapUtil.java
index 6d9ca97f4..2a7c79d08 100755
--- a/hutool-core/src/main/java/cn/hutool/core/map/MapUtil.java
+++ b/hutool-core/src/main/java/cn/hutool/core/map/MapUtil.java
@@ -1481,17 +1481,46 @@ public class MapUtil {
 	 */
 	public static <K, V> V computeIfAbsent(Map<K, V> map, K key, Function<? super K, ? extends V> mappingFunction) {
 		if (JdkUtil.IS_JDK8) {
-			V value = map.get(key);
-			if (null == value) {
-				map.putIfAbsent(key, mappingFunction.apply(key));
-				value = map.get(key);
-
-				// 判空后调用依旧无法解决死循环问题
-				//value = map.computeIfAbsent(key, mappingFunction);
-			}
-			return value;
+			return computeIfAbsentForJdk8(map, key, mappingFunction);
 		} else {
 			return map.computeIfAbsent(key, mappingFunction);
 		}
 	}
+
+	/**
+	 * 如果 key 对应的 value 不存在，则使用获取 mappingFunction 重新计算后的值，并保存为该 key 的 value，否则返回 value。<br>
+	 * 解决使用ConcurrentHashMap.computeIfAbsent导致的死循环问题。（issues#2349）<br>
+	 * A temporary workaround for Java 8 specific performance issue JDK-8161372 .<br>
+	 * This class should be removed once we drop Java 8 support.
+	 *
+	 * <p>
+	 *     注意此方法只能用于JDK8
+	 * </p>
+	 *
+	 * @param <K>             键类型
+	 * @param <V>             值类型
+	 * @param map             Map，一般用于线程安全的Map
+	 * @param key             键
+	 * @param mappingFunction 值计算函数
+	 * @return 值
+	 * @see <a href="https://bugs.openjdk.java.net/browse/JDK-8161372">https://bugs.openjdk.java.net/browse/JDK-8161372</a>
+	 */
+	public static <K, V> V computeIfAbsentForJdk8(final Map<K, V> map, final K key, final Function<? super K, ? extends V> mappingFunction) {
+		V value = map.get(key);
+		if (null == value) {
+			value = mappingFunction.apply(key);
+			final V res = map.putIfAbsent(key, value);
+			if(null != res){
+				// issues#I6RVMY
+				// 如果旧值存在，说明其他线程已经赋值成功，putIfAbsent没有执行，返回旧值
+				return res;
+			}
+			// 如果旧值不存在，说明赋值成功，返回当前值
+
+			// Dubbo的解决方式，判空后调用依旧无法解决死循环问题
+			// 见：Issue2349Test
+			//value = map.computeIfAbsent(key, mappingFunction);
+		}
+		return value;
+	}
 }
diff --git a/hutool-core/src/main/java/cn/hutool/core/map/SafeConcurrentHashMap.java b/hutool-core/src/main/java/cn/hutool/core/map/SafeConcurrentHashMap.java
index 4c043d117..ea0ef7758 100644
--- a/hutool-core/src/main/java/cn/hutool/core/map/SafeConcurrentHashMap.java
+++ b/hutool-core/src/main/java/cn/hutool/core/map/SafeConcurrentHashMap.java
@@ -72,15 +72,7 @@ public class SafeConcurrentHashMap<K, V> extends ConcurrentHashMap<K, V> {
 	@Override
 	public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
 		if (JdkUtil.IS_JDK8) {
-			V value = get(key);
-			if (null == value) {
-				putIfAbsent(key, mappingFunction.apply(key));
-				value = get(key);
-
-				// 判空后调用依旧无法解决死循环问题
-				//value = map.computeIfAbsent(key, mappingFunction);
-			}
-			return value;
+			return MapUtil.computeIfAbsentForJdk8(this, key, mappingFunction);
 		} else {
 			return super.computeIfAbsent(key, mappingFunction);
 		}