fix dead lock bug

hutool-core/src/test/java/cn/hutool/core/lang/SingletonTest.java

@@ -3,6 +3,7 @@ package cn.hutool.core.lang;
 import cn.hutool.core.exceptions.UtilException;
 import cn.hutool.core.thread.ThreadUtil;
 import lombok.Data;
+import org.junit.Assert;
 import org.junit.Test;
 
 public class SingletonTest {
@@ -27,4 +28,24 @@ public class SingletonTest {
 		private String name;
 		private String age;
 	}
+
+	/**
+	 * 测试单例构建属性锁死问题
+	 * C构建单例时候，同时构建B，此时在SimpleCache中会有写锁竞争（写入C时获取了写锁，此时要写入B，也要获取写锁）
+	 */
+	@Test(timeout = 1000L)
+	public void reentrantTest(){
+		final C c = Singleton.get(C.class);
+		Assert.assertEquals("aaa", c.getB().getA());
+	}
+
+	@Data
+	static class B{
+		private String a = "aaa";
+	}
+
+	@Data
+	static class C{
+		private B b = Singleton.get(B.class);
+	}
 }