1. 概述
在优化系统性能时,优化数据库性能是非常重要的一个环节,而添加缓存则是优化数据库时最有效的手段之一。正确、合理地使用缓存可以将一部分数据库请求拦截在缓存这一层。
MyBatis 中提供了一级缓存和二级缓存,而这两级缓存都是依赖于基础支持层中的缓 存模块实现的。这里需要读者注意的是,MyBatis 中自带的这两级缓存与 MyBatis 以及整个应用是运行在同一个 JVM 中的,共享同一块堆内存。如果这两级缓存中的数据量较大, 则可能影响系统中其他功能的运行,所以当需要缓存大量数据时,优先考虑使用 Redis、Memcache 等缓存产品
2. Cache
org.apache.ibatis.cache.Cache ,缓存容器接口。注意,它是一个容器,有点类似 HashMap ,可以往其中添加各种缓存。代码如下:
public interface Cache {
/**
* @return The identifier of this cache 标识
*/
String getId();
/**
* 添加指定键的值
*
* @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);
/**
* 获得指定键的值
*
* @param key The key
* @return The object stored in the cache.
*/
Object getObject(Object key);
/**
* 移除指定键的值
*
* 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);
/**
* 清空缓存
*
* Clears this cache instance
*/
void clear();
/**
* 获得容器中缓存的数量
*
* Optional. This method is not called by the core.
*
* @return The number of elements stored in the cache (not its capacity).
*/
int getSize();
/**
* 获得读取写锁。该方法可以忽略了已经。
*
* Optional. As of 3.2.6 this method is no longer called by the core.
*
* Any locking needed by the cache must be provided internally by the cache provider.
*
* @return A ReadWriteLock
*/
@Deprecated
ReadWriteLock getReadWriteLock();
}
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Cache 基于不同的缓存过期策略、特性,有不同的实现类。下面,我们来逐个来看。可以组合多种 Cache ,实现特性的组合。这种方式,就是常见的设计模式
2.1 PerpetualCache
org.apache.ibatis.cache.impl.PerpetualCache ,实现 Cache 接口,永不过期的 Cache 实现类,基于 HashMap 实现类。代码如下:
// PerpetualCache.java
public class PerpetualCache implements Cache {
/**
* 标识
*/
private final String id;
/**
* 缓存容器
*/
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();
}
// ... 省略 equals 和 hashCode 方法
}
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2.2 LoggingCache
org.apache.ibatis.cache.decorators.LoggingCache ,实现 Cache 接口,支持打印日志的 Cache 实现类。代码如下:
public class LoggingCache implements Cache {
/**
* MyBatis Log 对象
*/
private final Log log;
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
/**
* 统计请求缓存的次数
*/
protected int requests = 0;
/**
* 统计命中缓存的次数
*/
protected int hits = 0;
public LoggingCache(Cache delegate) {
this.delegate = delegate;
this.log = LogFactory.getLog(getId());
}
@Override
public String getId() {
return delegate.getId();
}
@Override
public int getSize() {
return delegate.getSize();
}
@Override
public void putObject(Object key, Object object) {
delegate.putObject(key, object);
}
@Override
public Object getObject(Object key) {
// 请求次数 ++
requests++;
// 获得缓存
final Object value = delegate.getObject(key);
// 如果命中缓存,则命中次数 ++
if (value != null) {
hits++;
}
if (log.isDebugEnabled()) {
log.debug("Cache Hit Ratio [" + getId() + "]: " + getHitRatio());
}
return value;
}
@Override
public Object removeObject(Object key) {
return delegate.removeObject(key);
}
@Override
public void clear() {
delegate.clear();
}
/**
* @return 命中比率
*/
private double getHitRatio() {
return (double) hits / (double) requests;
}
// ... 省略 equals 和 hashCode 方法
}
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2.3 BlockingCache
org.apache.ibatis.cache.decoratorsBlockingCache ,实现 Cache 接口,阻塞的 Cache 实现类。
这里的阻塞比较特殊,当线程去获取缓存值时,如果不存在,则会阻塞后续的其他线程去获取该缓存。
为什么这么有这样的设计呢?因为当线程 A 在获取不到缓存值时,一般会去设置对应的缓存值,这样就避免其他也需要该缓存的线程 B、C 等,重复添加缓存。
代码如下:
public class BlockingCache implements Cache {
/**
* 阻塞等待超时时间
*/
private long timeout;
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
/**
* 缓存键与 ReentrantLock 对象的映射
*/
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 {
// <2.1> 添加缓存
delegate.putObject(key, value);
} finally {
// <2.2> 释放锁
releaseLock(key);
}
}
@Override
public Object getObject(Object key) {
// <1.1> 获得锁
acquireLock(key);
// <1.2> 获得缓存值
Object value = delegate.getObject(key);
// <1.3> 释放锁
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
// 释放锁
releaseLock(key);
return null;
}
@Override
public void clear() {
delegate.clear();
}
@Override
public ReadWriteLock getReadWriteLock() {
return null;
}
/**
* 获得 ReentrantLock 对象。如果不存在,进行添加
*
* @param key 缓存键
* @return ReentrantLock 对象
*/
private ReentrantLock getLockForKey(Object key) {
ReentrantLock lock = new ReentrantLock();
ReentrantLock previous = locks.putIfAbsent(key, lock);
return previous == null ? lock : previous;
}
private void acquireLock(Object key) {
// 获得 ReentrantLock 对象。
Lock lock = getLockForKey(key);
// 获得锁,直到超时
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 {
// 释放锁
lock.lock();
}
}
private void releaseLock(Object key) {
// 获得 ReentrantLock 对象
ReentrantLock lock = locks.get(key);
// 如果当前线程持有,进行释放
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
public long getTimeout() {
return timeout;
}
public void setTimeout(long timeout) {
this.timeout = timeout;
}
}
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2.4 SynchronizedCache
org.apache.ibatis.cache.decorators.SynchronizedCache ,实现 Cache 接口,同步的 Cache 实现类。代码如下:
public class SynchronizedCache implements Cache {
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
public SynchronizedCache(Cache delegate) {
this.delegate = delegate;
}
@Override
public String getId() {
return delegate.getId();
}
@Override // 同步
public synchronized int getSize() {
return delegate.getSize();
}
@Override // 同步
public synchronized void putObject(Object key, Object object) {
delegate.putObject(key, object);
}
@Override // 同步
public synchronized Object getObject(Object key) {
return delegate.getObject(key);
}
@Override // 同步
public synchronized Object removeObject(Object key) {
return delegate.removeObject(key);
}
@Override // 同步
public synchronized void clear() {
delegate.clear();
}
// ... 省略 equals 和 hashCode 方法
}
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2.5 SerializedCache
org.apache.ibatis.cache.decorators.SerializedCache ,实现 Cache 接口,支持序列化值的 Cache 实现类。代码如下:
public class SerializedCache implements Cache {
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
public SerializedCache(Cache delegate) {
this.delegate = delegate;
}
@Override
public String getId() {
return delegate.getId();
}
@Override
public int getSize() {
return delegate.getSize();
}
@Override
public void putObject(Object key, Object object) {
if (object == null || object instanceof Serializable) {
delegate.putObject(key, serialize((Serializable) object)); // 序列化
} else {
throw new CacheException("SharedCache failed to make a copy of a non-serializable object: " + object);
}
}
@Override
public Object getObject(Object key) {
Object object = delegate.getObject(key);
return object == null ? null : deserialize((byte[]) object); // 反序列化
}
@Override
public Object removeObject(Object key) {
return delegate.removeObject(key);
}
@Override
public void clear() {
delegate.clear();
}
// ... 省略 equals 和 hashCode 方法
private byte[] serialize(Serializable value) {
try (ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos)) {
oos.writeObject(value);
oos.flush();
return bos.toByteArray();
} catch (Exception e) {
throw new CacheException("Error serializing object. Cause: " + e, e);
}
}
private Serializable deserialize(byte[] value) {
Serializable result;
try (ByteArrayInputStream bis = new ByteArrayInputStream(value);
ObjectInputStream ois = new CustomObjectInputStream(bis)) {
result = (Serializable) ois.readObject();
} catch (Exception e) {
throw new CacheException("Error deserializing object. Cause: " + e, e);
}
return result;
}
public static class CustomObjectInputStream extends ObjectInputStream {
public CustomObjectInputStream(InputStream in) throws IOException {
super(in);
}
@Override
protected Class<?> resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException {
return Resources.classForName(desc.getName()); // 解析类
}
}
}
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2.6 ScheduledCache
org.apache.ibatis.cache.decorators.ScheduledCache ,实现 Cache 接口,定时清空整个容器的 Cache 实现类。代码如下:
public class ScheduledCache implements Cache {
/**
* 被装饰的 Cache 对象
*/
private final Cache delegate;
/**
* 清空间隔,单位:毫秒
*/
protected long clearInterval;
/**
* 最后清空时间,单位:毫秒
*/
protected long lastClear;
public ScheduledCache(Cache delegate) {
this.delegate = delegate;
this.clearInterval = 60 * 60 * 1000; // 1 hour
this.lastClear = System.currentTimeMillis();
}
public void setClearInterval(long clearInterval) {
this.clearInterval = clearInterval;
}
@Override
public String getId() {
return delegate.getId();
}
@Override
public int getSize() {
// 判断是否要全部清空
clearWhenStale();
return delegate.getSize();
}
@Override
public void putObject(Object key, Object object) {
// 判断是否要全部清空
clearWhenStale();
delegate.putObject(key, object);
}
@Override
public Object getObject(Object key) {
// 判断是否要全部清空
return clearWhenStale() ? null : delegate.getObject(key); // 获得值
}
@Override
public Object removeObject(Object key) {
// 判断是否要全部清空
clearWhenStale();
return delegate.removeObject(key);
}
@Override
public void clear() {
// 记录清空时间
lastClear = System.currentTimeMillis();
// 全部清空
delegate.clear();
}
@Override
public ReadWriteLock getReadWriteLock() {
return null;
}
@Override
public int hashCode() {
return delegate.hashCode();
}
@Override
public boolean equals(Object obj) {
return delegate.equals(obj);
}
/**
* 判断是否要全部清空
*
* @return 是否全部清空
*/
private boolean clearWhenStale() {
// 判断是否要全部清空
if (System.currentTimeMillis() - lastClear > clearInterval) {
// 清空
clear();
return true;
}
return false;
}
}
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2.7 FifoCache
org.apache.ibatis.cache.decorators.FifoCache ,实现 Cache 接口,基于先进先出的淘汰机制的 Cache 实现类。代码如下:
// FifoCache.java
public class FifoCache implements Cache {
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
/**
* 双端队列,记录缓存键的添加
*/
private final Deque<Object> keyList;
/**
* 队列上限
*/
private int size;
public FifoCache(Cache delegate) {
this.delegate = delegate;
this.keyList = new LinkedList<>(); // 使用了 LinkedList
this.size = 1024; // 默认为 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) {
// 循环 keyList
cycleKeyList(key);
delegate.putObject(key, value);
}
@Override
public Object getObject(Object key) {
return delegate.getObject(key);
}
@Override
public Object removeObject(Object key) {
// <2> 此处,理论应该也要移除 keyList 呀
return delegate.removeObject(key);
}
@Override
public void clear() {
delegate.clear();
keyList.clear();
}
@Override
public ReadWriteLock getReadWriteLock() {
return null;
}
private void cycleKeyList(Object key) {
// <1> 添加到 keyList 对位
keyList.addLast(key);
// 超过上限,将队首位移除
if (keyList.size() > size) {
Object oldestKey = keyList.removeFirst();
delegate.removeObject(oldestKey);
}
}
}
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2.8 LruCache
org.apache.ibatis.cache.decorators.LruCache ,实现 Cache 接口,基于最少使用的淘汰机制的 Cache 实现类。代码如下:
public class LruCache implements Cache {
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
/**
* 基于 LinkedHashMap 实现淘汰机制
*/
private Map<Object, Object> keyMap;
/**
* 最老的键,即要被淘汰的
*/
private Object eldestKey;
public LruCache(Cache delegate) {
this.delegate = delegate;
// 初始化 keyMap 对象
setSize(1024);
}
@Override
public String getId() {
return delegate.getId();
}
@Override
public int getSize() {
return delegate.getSize();
}
public void setSize(final int size) {
// LinkedHashMap的一个构造函数,当参数accessOrder为true时,即会按照访问顺序排序,最近访问的放在最前,最早访问的放在后面
keyMap = new LinkedHashMap<Object, Object>(size, .75F, true) { //
private static final long serialVersionUID = 4267176411845948333L;
// LinkedHashMap自带的判断是否删除最老的元素方法,默认返回false,即不删除老数据
// 我们要做的就是重写这个方法,当满足一定条件时删除老数据
@Override
protected boolean removeEldestEntry(Map.Entry<Object, Object> eldest) {
boolean tooBig = size() > size;
if (tooBig) {
eldestKey = eldest.getKey();
}
return tooBig;
}
};
}
@Override
public void putObject(Object key, Object value) {
// 添加到缓存
delegate.putObject(key, value);
// 循环 keyMap
cycleKeyList(key);
}
@Override
public Object getObject(Object key) {
// 刷新 keyMap 的访问顺序
keyMap.get(key); // touch
// 获得缓存值
return delegate.getObject(key);
}
@Override
public Object removeObject(Object key) {
return delegate.removeObject(key);
}
@Override
public void clear() {
delegate.clear();
keyMap.clear();
}
@Override
public ReadWriteLock getReadWriteLock() {
return null;
}
private void cycleKeyList(Object key) {
// 添加到 keyMap 中
keyMap.put(key, key);
// 如果超过上限,则从 delegate 中,移除最少使用的那个
if (eldestKey != null) {
delegate.removeObject(eldestKey);
eldestKey = null; // 置空
}
}
}
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2.9 WeakCache
org.apache.ibatis.cache.decorators.WeakCache ,实现 Cache 接口,基于 java.lang.ref.WeakReference 的 Cache 实现类。代码如下:
public class WeakCache implements Cache {
/**
* 强引用的键的队列
*/
private final Deque<Object> hardLinksToAvoidGarbageCollection;
/**
* {@link #hardLinksToAvoidGarbageCollection} 的大小
*/
private int numberOfHardLinks;
/**
* 被 GC 回收的 WeakEntry 集合,避免被 GC。
*/
private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
public WeakCache(Cache delegate) {
this.delegate = delegate;
this.numberOfHardLinks = 256;
this.hardLinksToAvoidGarbageCollection = new LinkedList<>();
this.queueOfGarbageCollectedEntries = new ReferenceQueue<>();
}
@Override
public String getId() {
return delegate.getId();
}
@Override
public int getSize() {
// 移除已经被 GC 回收的 WeakEntry
removeGarbageCollectedItems();
return delegate.getSize();
}
public void setSize(int size) {
this.numberOfHardLinks = size;
}
@Override
public void putObject(Object key, Object value) {
// 移除已经被 GC 回收的 WeakEntry
removeGarbageCollectedItems();
// 添加到 delegate 中
delegate.putObject(key, new WeakEntry(key, value, queueOfGarbageCollectedEntries));
}
@Override
public Object getObject(Object key) {
Object result = null;
@SuppressWarnings("unchecked") // assumed delegate cache is totally managed by this cache
// 获得值的 WeakReference 对象
WeakReference<Object> weakReference = (WeakReference<Object>) delegate.getObject(key);
if (weakReference != null) {
// 获得值
result = weakReference.get();
// 为空,从 delegate 中移除 。为空的原因是,意味着已经被 GC 回收
if (result == null) {
delegate.removeObject(key);
// 非空,添加到 hardLinksToAvoidGarbageCollection 中,避免被 GC
} else {
// 添加到 hardLinksToAvoidGarbageCollection 的队头
hardLinksToAvoidGarbageCollection.addFirst(result);
// 超过上限,移除 hardLinksToAvoidGarbageCollection 的队尾
if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) {
hardLinksToAvoidGarbageCollection.removeLast();
}
}
}
return result;
}
@Override
public Object removeObject(Object key) {
// 移除已经被 GC 回收的 WeakEntry
removeGarbageCollectedItems();
// 移除出 delegate
return delegate.removeObject(key);
}
@Override
public void clear() {
// 清空 hardLinksToAvoidGarbageCollection
hardLinksToAvoidGarbageCollection.clear();
// 移除已经被 GC 回收的 WeakEntry
removeGarbageCollectedItems();
// 清空 delegate
delegate.clear();
}
@Override
public ReadWriteLock getReadWriteLock() {
return null;
}
/**
* 移除已经被 GC 回收的键
*/
private void removeGarbageCollectedItems() {
WeakEntry sv;
while ((sv = (WeakEntry) queueOfGarbageCollectedEntries.poll()) != null) {
delegate.removeObject(sv.key);
}
}
private static class WeakEntry extends WeakReference<Object> {
/**
* 键
*/
private final Object key;
private WeakEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
super(value, garbageCollectionQueue);
this.key = key;
}
}
}
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2.10 SoftCache
org.apache.ibatis.cache.decorators.SoftCache ,实现 Cache 接口,基于 java.lang.ref.SoftReference 的 Cache 实现类。代码如下:
public class SoftCache implements Cache {
/**
* 强引用的键的队列
*/
private final Deque<Object> hardLinksToAvoidGarbageCollection;
/**
* {@link #hardLinksToAvoidGarbageCollection} 的大小
*/
private int numberOfHardLinks;
/**
* 被 GC 回收的 WeakEntry 集合,避免被 GC。
*/
private final ReferenceQueue<Object> queueOfGarbageCollectedEntries;
/**
* 装饰的 Cache 对象
*/
private final Cache delegate;
public SoftCache(Cache delegate) {
this.delegate = delegate;
this.numberOfHardLinks = 256;
this.hardLinksToAvoidGarbageCollection = new LinkedList<>();
this.queueOfGarbageCollectedEntries = new ReferenceQueue<>();
}
@Override
public String getId() {
return delegate.getId();
}
@Override
public int getSize() {
removeGarbageCollectedItems();
return delegate.getSize();
}
public void setSize(int size) {
this.numberOfHardLinks = size;
}
@Override
public void putObject(Object key, Object value) {
// 移除已经被 GC 回收的 SoftEntry
removeGarbageCollectedItems();
delegate.putObject(key, new SoftEntry(key, value, queueOfGarbageCollectedEntries));
}
@Override
public Object getObject(Object key) {
Object result = null;
@SuppressWarnings("unchecked") // assumed delegate cache is totally managed by this cache
// 获得值的 WeakReference 对象
SoftReference<Object> softReference = (SoftReference<Object>) delegate.getObject(key);
if (softReference != null) {
// 获得值
result = softReference.get();
// 为空,从 delegate 中移除 。为空的原因是,意味着已经被 GC 回收
if (result == null) {
delegate.removeObject(key);
// 非空,添加到 hardLinksToAvoidGarbageCollection 中,避免被 GC
} else {
// See #586 (and #335) modifications need more than a read lock
// 添加到 hardLinksToAvoidGarbageCollection 的队头
synchronized (hardLinksToAvoidGarbageCollection) {
hardLinksToAvoidGarbageCollection.addFirst(result);
// 超过上限,移除 hardLinksToAvoidGarbageCollection 的队尾
if (hardLinksToAvoidGarbageCollection.size() > numberOfHardLinks) {
hardLinksToAvoidGarbageCollection.removeLast();
}
}
}
}
return result;
}
@Override
public Object removeObject(Object key) {
// 移除已经被 GC 回收的 SoftEntry
removeGarbageCollectedItems();
// 移除出 delegate
return delegate.removeObject(key);
}
@Override
public void clear() {
// 清空 hardLinksToAvoidGarbageCollection
synchronized (hardLinksToAvoidGarbageCollection) {
hardLinksToAvoidGarbageCollection.clear();
}
// 移除已经被 GC 回收的 WeakEntry
removeGarbageCollectedItems();
// 清空 delegate
delegate.clear();
}
@Override
public ReadWriteLock getReadWriteLock() {
return null;
}
private void removeGarbageCollectedItems() {
SoftEntry sv;
while ((sv = (SoftEntry) queueOfGarbageCollectedEntries.poll()) != null) {
delegate.removeObject(sv.key);
}
}
private static class SoftEntry extends SoftReference<Object> {
/**
* 键
*/
private final Object key;
SoftEntry(Object key, Object value, ReferenceQueue<Object> garbageCollectionQueue) {
super(value, garbageCollectionQueue);
this.key = key;
}
}
}
复制代码
3. CacheKey
org.apache.ibatis.cache.CacheKey ,实现 Cloneable、Serializable 接口,缓存键。
因为 MyBatis 中的缓存键不是一个简单的 String ,而是通过多个对象组成。所以 CacheKey 可以理解成将多个对象放在一起,计算其缓存键。
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