java/se: add padding long for baidu uid

Author: Alice52

java/javase/javase-jvm/src/main/java/cn/edu/ntu/javase/jvm/MemoryStruct.java

@@ -1,6 +1,7 @@
 package cn.edu.ntu.javase.jvm;
 
 import cn.edu.ntu.javase.common.model.Person;
+import cn.edu.ntu.javase.jvm.model.PaddedAtomicLong;
 import lombok.extern.slf4j.Slf4j;
 import org.junit.Test;
 import org.openjdk.jol.info.ClassLayout;
@@ -46,7 +47,7 @@ private static class T {
         // Integer a = new Integer(1);
         boolean b;
         // 对象则只占4个字节, 是指向对的一个指针
-        String s = "hello46541";
+        // String s = "hello46541";
     }
 
     @Test
@@ -56,4 +57,18 @@ public void testObjectStruct() {
         o.hashCode();
         log.info(ClassLayout.parseInstance(o).toPrintable());
     }
+
+    @Test
+    public void testObject() {
+        Object o = new Object();
+        log.info(ClassLayout.parseInstance(o).toPrintable());
+        o.hashCode();
+        log.info(ClassLayout.parseInstance(o).toPrintable());
+    }
+
+    @Test
+    public void testPaddedAtomicLong() {
+        PaddedAtomicLong o = new PaddedAtomicLong();
+        log.info(ClassLayout.parseInstance(o).toPrintable());
+    }
 }

java/javase/javase-jvm/src/main/java/cn/edu/ntu/javase/jvm/model/PaddedAtomicLong.java

@@ -0,0 +1,34 @@
+package cn.edu.ntu.javase.jvm.model;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.concurrent.atomic.AtomicLong;
+
+/**
+ * @author zack <br/>
+ * @create 2022-03-20 20:48 <br/>
+ */
+public class PaddedAtomicLong extends AtomicLong {
+    private static final long serialVersionUID = -3415778863941386253L;
+
+    /**
+     * todo: should fill 5*long? <br>
+     * Padded 6 long (48 bytes)
+     */
+    public volatile long p1, p2, p3, p4, p5 = 7L;
+
+    /** Constructors from {@link AtomicLong} */
+    public PaddedAtomicLong() {
+        super();
+    }
+
+    public PaddedAtomicLong(long initialValue) {
+        super(initialValue);
+    }
+
+    /** To prevent GC optimizations for cleaning unused padded references */
+    public long sumPaddingToPreventOptimization() {
+        return p1 + p2 + p3 + p4 + p5;
+    }
+}