vault backup: 2023-03-19 10:30:07

Affected files:
inbox/使用 Unsafe 原子性更新对象成员值.md

inbox/使用 Unsafe 原子性更新对象成员值.md

@@ -0,0 +1,59 @@
+---
+date created: 2023-03-19, 10:22:08
+date modified: 2023-03-19, 10:22:16
+---
+
+# Meta
+
+- alias:
+- parent ::
+- siblings ::
+- child ::
+- refs:
+    - https://stackoverflow.com/questions/13003871/how-do-i-get-the-instance-of-sun-misc-unsafe
+    - AQS 中的代码
+
+---
+
+```java
+public class ReflectTest {
+    public static void main(String[] args) {
+        Student student = new Student();
+        student.setState(123);
+
+        System.out.println(student.compareAndSetState(123, 99));
+        System.out.println(student.getState()); // 99
+    }
+}
+
+class Student {
+    private int state;
+
+    private static final long stateOffset;
+
+    private static final Unsafe unsafe;
+
+    static {
+        try {
+            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
+            theUnsafe.setAccessible(true);
+            unsafe = (Unsafe) theUnsafe.get(null);
+            stateOffset = unsafe.objectFieldOffset(Student.class.getDeclaredField("state"));
+        } catch (Exception ex) {
+            throw new Error(ex);
+        }
+    }
+
+    public void setState(int state) {
+        this.state = state;
+    }
+
+    public int getState() {
+        return state;
+    }
+
+    public final boolean compareAndSetState(int expected, int update) {
+        return unsafe.compareAndSwapInt(this, stateOffset, expected, update);
+    }
+}
+```