mybatis cache

docs/Mybatis/基础支持层/Mybatis-Cache.md

@@ -0,0 +1,319 @@
+# mybatis 缓存
+- Author: [HuiFer](https://github.com/huifer)
+- Description: 该文介绍 mybatis Cache 源码
+- `org.apache.ibatis.cache.Cache`
+```java
+public interface Cache {
+
+    String getId();
+
+    /**
+     * 放入数据
+     */
+    void putObject(Object key, Object value);
+
+    /**
+     * 获取数据
+     */
+    Object getObject(Object key);
+
+    /**
+     * 移除数据
+     */
+    Object removeObject(Object key);
+
+    /**
+     * 清空数据
+     */
+    void clear();
+
+    /**
+     * 有多少缓存数据
+     */
+    int getSize();
+
+    /**
+     * 重入锁
+     * @return A ReadWriteLock
+     */
+    default ReadWriteLock getReadWriteLock() {
+        return null;
+    }
+
+}
+```
+
+- BlockingCache: 阻塞的缓存
+- FifoCache: 按对象进入缓存的顺序来移除它们。
+- LruCache: 最近最少使用的：移除最长时间不被使用的对象。
+- SoftCache: 软引用：移除基于垃圾回收器状态和软引用规则的对象
+- WeakCache: 弱引用：更积极地移除基于垃圾收集器状态和弱引用规则的对象。
+
+## BlockingCache
+- BlockingCache 内部使用了`ReentrantLock`来进行加锁开锁这个操作.在插入缓存时上锁,插入缓存后释放.请求缓存值得时候同理
+```java
+public class BlockingCache implements Cache {
+
+    private final Cache delegate;
+    /**
+     * 线程安全的map
+     */
+    private final ConcurrentHashMap<Object, ReentrantLock> locks;
+    private long timeout;
+
+    public BlockingCache(Cache delegate) {
+        this.delegate = delegate;
+        this.locks = new ConcurrentHashMap<>();
+    }
+
+    @Override
+    public String getId() {
+        return delegate.getId();
+    }
+
+    @Override
+    public int getSize() {
+        return delegate.getSize();
+    }
+
+    @Override
+    public void putObject(Object key, Object value) {
+        try {
+            delegate.putObject(key, value);
+        } finally {
+            releaseLock(key);
+        }
+    }
+
+    @Override
+    public Object getObject(Object key) {
+        acquireLock(key);
+        Object value = delegate.getObject(key);
+        if (value != null) {
+            // 释放锁
+            releaseLock(key);
+        }
+        return value;
+    }
+
+    @Override
+    public Object removeObject(Object key) {
+        // despite of its name, this method is called only to release locks
+        releaseLock(key);
+        return null;
+    }
+
+    @Override
+    public void clear() {
+        delegate.clear();
+    }
+
+    private ReentrantLock getLockForKey(Object key) {
+        return locks.computeIfAbsent(key, k -> new ReentrantLock());
+    }
+
+    /**
+     * 请求锁
+     * @param key
+     */
+    private void acquireLock(Object key) {
+        Lock lock = getLockForKey(key);
+        if (timeout > 0) {
+            try {
+                // 上锁
+                boolean acquired = lock.tryLock(timeout, TimeUnit.MILLISECONDS);
+                if (!acquired) {
+                    throw new CacheException("Couldn't get a lock in " + timeout + " for the key " + key + " at the cache " + delegate.getId());
+                }
+            } catch (InterruptedException e) {
+                throw new CacheException("Got interrupted while trying to acquire lock for key " + key, e);
+            }
+        } else {
+            lock.lock();
+        }
+    }
+
+    /**
+     * 释放锁
+     * @param key
+     */
+    private void releaseLock(Object key) {
+        ReentrantLock lock = locks.get(key);
+        if (lock.isHeldByCurrentThread()) {
+            lock.unlock();
+        }
+    }
+
+    public long getTimeout() {
+        return timeout;
+    }
+
+    public void setTimeout(long timeout) {
+        this.timeout = timeout;
+    }
+}
+```
+
+## FifoCache
+- 存储结构是`java.util.LinkedList`
+```java
+public class FifoCache implements Cache {
+
+    private final Cache delegate;
+    /**
+     * 队列
+     */
+    private final Deque<Object> keyList;
+    private int size;
+
+    public FifoCache(Cache delegate) {
+        this.delegate = delegate;
+        this.keyList = new LinkedList<>();
+        this.size = 1024;
+    }
+
+    @Override
+    public String getId() {
+        return delegate.getId();
+    }
+
+    @Override
+    public int getSize() {
+        return delegate.getSize();
+    }
+
+    public void setSize(int size) {
+        this.size = size;
+    }
+
+    @Override
+    public void putObject(Object key, Object value) {
+        cycleKeyList(key);
+        delegate.putObject(key, value);
+    }
+
+    @Override
+    public Object getObject(Object key) {
+        return delegate.getObject(key);
+    }
+
+    @Override
+    public Object removeObject(Object key) {
+        return delegate.removeObject(key);
+    }
+
+    @Override
+    public void clear() {
+        delegate.clear();
+        keyList.clear();
+    }
+
+    /**
+     * 添加 key 删除最开始的一个
+     *
+     * @param key
+     */
+    private void cycleKeyList(Object key) {
+        keyList.addLast(key);
+        if (keyList.size() > size) {
+            Object oldestKey = keyList.removeFirst();
+            delegate.removeObject(oldestKey);
+        }
+    }
+
+}
+```
+
+## LruCache
+- 存储结构是`java.util.LinkedHashMap`
+```java
+/**
+ * Lru (least recently used) cache decorator.
+ * LRU  緩存策略 最近最少使用的：移除最长时间不被使用的对象。
+ *
+ * @author Clinton Begin
+ */
+public class LruCache implements Cache {
+
+    private final Cache delegate;
+    /**
+     * {@link  LinkedHashMap}
+     */
+    private Map<Object, Object> keyMap;
+    private Object eldestKey;
+
+    public LruCache(Cache delegate) {
+        this.delegate = delegate;
+        setSize(1024);
+    }
+
+    @Override
+    public String getId() {
+        return delegate.getId();
+    }
+
+    @Override
+    public int getSize() {
+        return delegate.getSize();
+    }
+
+    /**
+     * 设置大小
+     *
+     * @param size
+     */
+    public void setSize(final int size) {
+        keyMap = new LinkedHashMap<Object, Object>(size, .75F, true) {
+            private static final long serialVersionUID = 4267176411845948333L;
+
+            @Override
+            protected boolean removeEldestEntry(Map.Entry<Object, Object> eldest) {
+                // 数量超出预设值 执行
+                boolean tooBig = size() > size;
+                if (tooBig) {
+//                    获取被移除的key
+                    eldestKey = eldest.getKey();
+                }
+                return tooBig;
+            }
+        };
+    }
+
+    @Override
+    public void putObject(Object key, Object value) {
+        delegate.putObject(key, value);
+        cycleKeyList(key);
+    }
+
+    @Override
+    public Object getObject(Object key) {
+        keyMap.get(key); //touch
+        return delegate.getObject(key);
+    }
+
+    @Override
+    public Object removeObject(Object key) {
+        return delegate.removeObject(key);
+    }
+
+    @Override
+    public void clear() {
+        delegate.clear();
+        keyMap.clear();
+    }
+
+    /**
+     * 删除最早的一个key
+     * @param key
+     */
+    private void cycleKeyList(Object key) {
+        keyMap.put(key, key);
+        if (eldestKey != null) {
+            delegate.removeObject(eldestKey);
+            eldestKey = null;
+        }
+    }
+
+}
+```
+