MyBatis源码分析缓存第七章

mybatis缓存如何设计呢？

mybaits如何进行细粒度锁的控制
如何生存key的。
如何存入缓存

带着问题我们一步一步的分析源码

包结构

在这里插入图片描述
org.apache.ibatis.cache.Cache 这个接口是如何放入读、写、删操作类似于数据库的curd操作

public interface Cache {

  /**
   * @return The identifier of this cache
   */
  String getId();

  /**
   * @param key Can be any object but usually it is a {@link CacheKey}
   * @param value The result of a select.
   */
  void putObject(Object key, Object value);

  /**
   * @param key The key
   * @return The object stored in the cache.
   */
  Object getObject(Object key);

  /**
   * As of 3.3.0 this method is only called during a rollback
   * for any previous value that was missing in the cache.
   * This lets any blocking cache to release the lock that
   * may have previously put on the key.
   * A blocking cache puts a lock when a value is null
   * and releases it when the value is back again.
   * This way other threads will wait for the value to be
   * available instead of hitting the database.
   *
   *
   * @param key The key
   * @return Not used
   */
  Object removeObject(Object key);

  /**
   * Clears this cache instance.
   */
  void clear();

  /**
   * Optional. This method is not called by the core.
   *
   * @return The number of elements stored in the cache (not its capacity).
   */
  int getSize();

  /**
   * Optional. As of 3.2.6 this method is no longer called by the core.
   * <p>
   * Any locking needed by the cache must be provided internally by the cache provider.
   *
   * @return A ReadWriteLock
   */
  ReadWriteLock getReadWriteLock();

}

org.apache.ibatis.cache.CacheKey

public class CacheKey implements Cloneable, Serializable {

  private static final long serialVersionUID = 1146682552656046210L;

  public static final CacheKey NULL_CACHE_KEY = new NullCacheKey();

  private static final int DEFAULT_MULTIPLYER = 37;
  private static final int DEFAULT_HASHCODE = 17;

  private final int multiplier; //比列因子
  private int hashcode; //hashcode值
  private long checksum; //计算和
  private int count; //计算条数
  // 8/21/2017 - Sonarlint flags this as needing to be marked transient.  While true if content is not serializable, this is not always true and thus should not be marked transient.
  private List<Object> updateList; // 缓存数据

  public CacheKey() {
    this.hashcode = DEFAULT_HASHCODE;
    this.multiplier = DEFAULT_MULTIPLYER;
    this.count = 0;
    this.updateList = new ArrayList<>();
  }

  public CacheKey(Object[] objects) {
    this();
    updateAll(objects);
  }

  public int getUpdateCount() {
    return updateList.size();
  }

  public void update(Object object) {
    // 计算hashcode值
    int baseHashCode = object == null ? 1 : ArrayUtil.hashCode(object);

    count++;
    // 统计和
    checksum += baseHashCode;
    // baseHashCode = baseHashCode * count
    baseHashCode *= count;

    hashcode = multiplier * hashcode + baseHashCode;

    updateList.add(object);
  }

  public void updateAll(Object[] objects) {
    for (Object o : objects) {
      update(o);
    }
  }

  @Override
  public boolean equals(Object object) {
    if (this == object) {
      return true;
    }
    if (!(object instanceof CacheKey)) {
      return false;
    }

    final CacheKey cacheKey = (CacheKey) object;

    if (hashcode != cacheKey.hashcode) {
      return false;
    }
    if (checksum != cacheKey.checksum) {
      return false;
    }
    if (count != cacheKey.count) {
      return false;
    }

    for (int i = 0; i < updateList.size(); i++) {
      Object thisObject = updateList.get(i);
      Object thatObject = cacheKey.updateList.get(i);
      if (!ArrayUtil.equals(thisObject, thatObject)) {
        return false;
      }
    }
    return true;
  }

  @Override
  public int hashCode() {
    return hashcode;
  }

  @Override
  public String toString() {
    StringJoiner returnValue = new StringJoiner(":");
    returnValue.add(String.valueOf(hashcode));
    returnValue.add(String.valueOf(checksum));
    updateList.stream().map(ArrayUtil::toString).forEach(returnValue::add);
    return returnValue.toString();
  }

  @Override
  public CacheKey clone() throws CloneNotSupportedException {
    CacheKey clonedCacheKey = (CacheKey) super.clone();
    clonedCacheKey.updateList = new ArrayList<>(updateList);
    return clonedCacheKey;
  }

}

org.apache.ibatis.cache.decorators.BlockingCache

public class BlockingCache implements Cache {

  private long timeout;
  private final Cache delegate;
  private final ConcurrentHashMap<Object, ReentrantLock> locks;

  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) {
      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();
  }

  @Override
  public ReadWriteLock getReadWriteLock() {
    return null;
  }

  private ReentrantLock getLockForKey(Object key) {
    return locks.computeIfAbsent(key, k -> new ReentrantLock());
  }

  private void acquireLock(Object key) {
    Lock lock = getLockForKey(key);
    if (timeout > 0) {
      try {
        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();
    }
  }

  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;
  }
}

org.apache.ibatis.cache.impl.PerpetualCache

public class PerpetualCache implements Cache {

  private final String id;

  private Map<Object, Object> cache = new HashMap<>();

  public PerpetualCache(String id) {
    this.id = id;
  }

  @Override
  public String getId() {
    return id;
  }

  @Override
  public int getSize() {
    return cache.size();
  }

  @Override
  public void putObject(Object key, Object value) {
    cache.put(key, value);
  }

  @Override
  public Object getObject(Object key) {
    return cache.get(key);
  }

  @Override
  public Object removeObject(Object key) {
    return cache.remove(key);
  }

  @Override
  public void clear() {
    cache.clear();
  }

  @Override
  public ReadWriteLock getReadWriteLock() {
    return null;
  }

  @Override
  public boolean equals(Object o) {
    if (getId() == null) {
      throw new CacheException("Cache instances require an ID.");
    }
    if (this == o) {
      return true;
    }
    if (!(o instanceof Cache)) {
      return false;
    }

    Cache otherCache = (Cache) o;
    return getId().equals(otherCache.getId());
  }

  @Override
  public int hashCode() {
    if (getId() == null) {
      throw new CacheException("Cache instances require an ID.");
    }
    return getId().hashCode();
  }

}

下面以mybatis提供的缓存测试代码来分析缓存的使用。结合源码

org.apache.ibatis.cache.CacheKeyTest#shouldTestCacheKeysEqual

  void shouldTestCacheKeysEqual() {
    Date date = new Date();
    CacheKey key1 = new CacheKey(new Object[] { 1, "hello", null, new Date(date.getTime()) });
    CacheKey key2 = new CacheKey(new Object[] { 1, "hello", null, new Date(date.getTime()) });
    assertTrue(key1.equals(key2));
    assertTrue(key2.equals(key1));
    assertTrue(key1.hashCode() == key2.hashCode());
    assertTrue(key1.toString().equals(key2.toString()));
  }

org.apache.ibatis.cache.CacheKey#CacheKey()

 public CacheKey(Object[] objects) {
    this();
    updateAll(objects);
  }

org.apache.ibatis.cache.CacheKey#CacheKey()

 public CacheKey() {
    this.hashcode = DEFAULT_HASHCODE;
    this.multiplier = DEFAULT_MULTIPLYER;
    this.count = 0;
    this.updateList = new ArrayList<>();
  }

org.apache.ibatis.cache.CacheKey#updateAll

 public void updateAll(Object[] objects) {
    for (Object o : objects) {
      update(o);
    }
  }

org.apache.ibatis.cache.CacheKey#update


  public void update(Object object) {
    // 计算hashcode值
    int baseHashCode = object == null ? 1 : ArrayUtil.hashCode(object);

    count++;
    // 统计和
    checksum += baseHashCode;
    // baseHashCode = baseHashCode * count
    baseHashCode *= count;

    hashcode = multiplier * hashcode + baseHashCode;

    updateList.add(object);
  }

org.apache.ibatis.reflection.ArrayUtil#hashCode

 /**
   * Returns a hash code for {@code obj}.
   *
   * @param obj
   *          The object to get a hash code for. May be an array or <code>null</code>.
   * @return A hash code of {@code obj} or 0 if {@code obj} is <code>null</code>
   */
  public static int hashCode(Object obj) {
    if (obj == null) {
      // for consistency with Arrays#hashCode() and Objects#hashCode()
      return 0;
    }
    final Class<?> clazz = obj.getClass();
    if (!clazz.isArray()) {
      return obj.hashCode();
    }
    final Class<?> componentType = clazz.getComponentType();
    if (long.class.equals(componentType)) {
      return Arrays.hashCode((long[]) obj);
    } else if (int.class.equals(componentType)) {
      return Arrays.hashCode((int[]) obj);
    } else if (short.class.equals(componentType)) {
      return Arrays.hashCode((short[]) obj);
    } else if (char.class.equals(componentType)) {
      return Arrays.hashCode((char[]) obj);
    } else if (byte.class.equals(componentType)) {
      return Arrays.hashCode((byte[]) obj);
    } else if (boolean.class.equals(componentType)) {
      return Arrays.hashCode((boolean[]) obj);
    } else if (float.class.equals(componentType)) {
      return Arrays.hashCode((float[]) obj);
    } else if (double.class.equals(componentType)) {
      return Arrays.hashCode((double[]) obj);
    } else {
      return Arrays.hashCode((Object[]) obj);
    }
  }

总结：

这个两个为什么会相等呢？
1、 assertTrue(key1.equals(key2）、 assertTrue(key2.equals(key1));

 public boolean equals(Object object) {
    if (this == object) {
      return true;
    }
    if (!(object instanceof CacheKey)) {
      return false;
    }

    final CacheKey cacheKey = (CacheKey) object;
   // 因为这里是两个对象 做比较
    if (hashcode != cacheKey.hashcode) {
      return false;
    }
    if (checksum != cacheKey.checksum) {
      return false;
    }
    if (count != cacheKey.count) {
      return false;
    }

    for (int i = 0; i < updateList.size(); i++) {
    // 获取到缓存里面的值
      Object thisObject = updateList.get(i);
    // 比较
      Object thatObject = cacheKey.updateList.get(i);
      if (!ArrayUtil.equals(thisObject, thatObject)) {
        return false;
      }
    }
    return true;
  }

2、assertTrue(key1.hashCode() == key2.hashCode());
// hashcode肯定相等看了上面的源码就知道这个需要知道hashcode是怎么生成。

3、assertTrue(key1.toString().equals(key2.toString()));

把所传入的值拼接起来

  public String toString() {
    StringJoiner returnValue = new StringJoiner(":");
    // 添加
    returnValue.add(String.valueOf(hashcode));
    returnValue.add(String.valueOf(checksum));
    updateList.stream().map(ArrayUtil::toString).forEach(returnValue::add);
    return returnValue.toString();
  }

MyBatis源码分析缓存第七章

mybatis缓存如何设计呢？

包结构

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