fix bug

hutool-cache/src/test/java/cn/hutool/cache/LRUCacheTest.java

@@ -0,0 +1,52 @@
+package cn.hutool.cache;
+
+import cn.hutool.cache.impl.LRUCache;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.concurrent.CountDownLatch;
+
+/**
+ * 见：https://github.com/dromara/hutool/issues/1895<br>
+ * 并发问题测试，在5.7.15前，LRUCache存在并发问题，多线程get后，map结构变更，导致null的位置不确定，
+ * 并可能引起死锁。
+ */
+public class LRUCacheTest {
+
+	@Test
+	public void readWriteTest() throws InterruptedException {
+		LRUCache<Integer, Integer> cache = CacheUtil.newLRUCache(10);
+		for (int i = 0; i < 10; i++) {
+			cache.put(i, i);
+		}
+
+		CountDownLatch countDownLatch = new CountDownLatch(10);
+		// 10个线程分别读0-9 10000次
+		for (int i = 0; i < 10; i++) {
+			int finalI = i;
+			new Thread(() -> {
+				for (int j = 0; j < 10000; j++) {
+					cache.get(finalI);
+				}
+				countDownLatch.countDown();
+			}).start();
+		}
+		// 等待读线程结束
+		countDownLatch.await();
+		// 按顺序读0-9
+		StringBuilder sb1 = new StringBuilder();
+		for (int i = 0; i < 10; i++) {
+			sb1.append(cache.get(i));
+		}
+		Assert.assertEquals("0123456789", sb1.toString());
+
+		// 新加11，此时0最久未使用，应该淘汰0
+		cache.put(11, 11);
+
+		StringBuilder sb2 = new StringBuilder();
+		for (int i = 0; i < 10; i++) {
+			sb2.append(cache.get(i));
+		}
+		Assert.assertEquals("null123456789", sb2.toString());
+	}
+}