fix weak bug

hutool-core/src/test/java/cn/hutool/core/util/ReferenceUtilTest.java

@@ -1,6 +1,9 @@
 package cn.hutool.core.util;
 
+import cn.hutool.core.lang.Console;
+import cn.hutool.core.lang.mutable.MutableObj;
 import org.junit.Assert;
+import org.junit.Ignore;
 import org.junit.Test;
 
 import java.lang.ref.PhantomReference;
@@ -31,4 +34,23 @@ public class ReferenceUtilTest {
 		// get方法永远都返回null，PhantomReference只能用来监控对象的GC状况
 		Assert.assertNull(integerReference.get());
 	}
+
+	@Test
+	@Ignore
+	public void gcTest(){
+		// https://blog.csdn.net/zmx729618/article/details/54093532
+		// 弱引用的对象必须使用可变对象，不能使用常量对象（比如String）
+		WeakReference<MutableObj<String>> reference = new WeakReference<>(new MutableObj<>("abc"));
+		int i=0;
+		while(true){
+			if(reference.get()!=null){
+				i++;
+				Console.log("Object is alive for {} loops - ", i);
+				System.gc();
+			}else{
+				Console.log("Object has been collected.");
+				break;
+			}
+		}
+	}
 }