add README

README.md

+# 用Unsafe实现AtomicInteger类
+**原理**
+AtomicInteger可以在并发情况下达到原子化更新,避免使用了synchronized,而且性能非常高。
+![在这里插入图片描述](https://img-blog.csdnimg.cn/img_convert/7fa0b413ccc4eebf79a7d3f4fa8ba5e6.png#pic_center)
+
+AtomicInteger的底层是用Unsafe来实现的，这里我们手写一个AtomicInteger类，并且测试一下。
+**测试例子**
+启动 1000 个线程，每个线程做 -10 元 的操作，如果初始余额为 10000 那么正确的结果应当是 0。
+以下代码在JDK1.8通过测试。
+## 账户Account 类
+```java
+interface Account {
+    // 获取余额
+    Integer getBalance();
+
+    // 取款
+    void withdraw(Integer amount);
+
+    /**
+     * 方法内会启动 1000 个线程，每个线程做 -10 元 的操作
+     * 如果初始余额为 10000 那么正确的结果应当是 0
+     */
+    static void demo(Account account) {
+        List<Thread> ts = new ArrayList<>();
+        for (int i = 0; i < 1000; i++) {
+            ts.add(new Thread(() -> {
+                account.withdraw(10);
+            }));
+        }
+        long start = System.nanoTime();
+        ts.forEach(Thread::start);
+        ts.forEach(t -> {
+            try {
+                t.join();
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+        });
+        long end = System.nanoTime();
+        System.out.println(account.getBalance()
+                + " cost: " + (end-start)/1000_000 + " ms");
+    }
+}
+```
+## Account 实现类
+
+```java
+class DiyAccount implements Account {
+
+    private DiyAtomicInteger balance;
+
+    public DiyAccount(int balance) {
+        this.balance = new DiyAtomicInteger(balance);
+    }
+
+    @Override
+    public Integer getBalance() {
+        return balance.getValue();
+    }
+
+    @Override
+    public void withdraw(Integer amount) {
+        balance.decrement(amount);
+    }
+}
+```
+## DiyAtomicInteger 类
+
+```java
+class DiyAtomicInteger {
+    private volatile int value; //实例操作的变量
+    private static final long valueOffset;//偏移量
+    private static final Unsafe UNSAFE;// unsafe实例对象
+
+    static {
+        UNSAFE = UnsafeAccessor.getUnsafe();
+        try {
+            valueOffset = UNSAFE.objectFieldOffset(DiyAtomicInteger.class.getDeclaredField("value"));
+        } catch (NoSuchFieldException e) {
+            e.printStackTrace();
+            throw new RuntimeException(e);
+        }
+    }
+
+    public int getValue() {
+        return value;
+    }
+
+    public void decrement(int amount) {
+        while (true) {
+            int prev = this.value;
+            int next = prev - amount;
+            if (UNSAFE.compareAndSwapInt(this, valueOffset, prev, next)) {
+                break;
+            }
+        }
+    }
+
+    public DiyAtomicInteger(int value) {
+        this.value = value;
+    }
+
+}
+```
+## 测试
+
+```java
+public class Test {
+    public static void main(String[] args) {
+        Account account = new DiyAccount(10000);
+        Account.demo(account);
+    }
+}
+```
+输出
+
+```
+0 cost: 65 ms
+```
+## 其它——Unsafe工具类UnsafeAccessor
+我们知道`Unsafe `的构造方法是私有的，也没有`get`方法可以获取对象，所以我们只能通过反射的方式来实例化`Unsafe` 对象。
+
+```java
+public final class Unsafe {
+    private static final Unsafe theUnsafe;
+
+    private Unsafe() {
+    }
+    ...
+}
+```
+
+所以
+```java
+import sun.misc.Unsafe;
+import java.lang.reflect.Field;
+
+public class UnsafeAccessor {
+    private static final Unsafe unsafe;
+
+    static {
+        try {
+            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
+            theUnsafe.setAccessible(true);
+            unsafe = (Unsafe) theUnsafe.get(null);
+        } catch (NoSuchFieldException | IllegalAccessException e) {
+            throw new Error(e);
+        }
+    }
+
+    public static Unsafe getUnsafe() {
+        return unsafe;
+    }
+}
+```
\ No newline at end of file