快速實(shí)現(xiàn)一個(gè)緩存

public class Cache<K,V> {final Map<K, V> m = new HashMap<>();final ReadWriteLock rwl = new ReentrantReadWriteLock();// 讀鎖final Lock r = rwl.readLock();// 寫鎖final Lock w = rwl.writeLock();// 讀緩存public V get(K key) {r.lock();try { return m.get(key); }finally { r.unlock(); }}// 寫緩存public V put(String key, Data v) {w.lock();try { return m.put(key, v); }finally { w.unlock(); }} }

實(shí)現(xiàn)緩存的按需加載

public class Cache<K,V> {final Map<K, V> m = new HashMap<>();final ReadWriteLock rwl = new ReentrantReadWriteLock();final Lock r = rwl.readLock();final Lock w = rwl.writeLock();public V getWithLoad(K key) {V v = get(key);// 緩存中存在，返回if(v != null) { return v;} // 緩存中不存在，查詢數(shù)據(jù)庫(kù)w.lock(); try {// 再次驗(yàn)證，其他線程可能已經(jīng)查詢過(guò)數(shù)據(jù)庫(kù)v = m.get(key); if(v == null){ // 查詢數(shù)據(jù)庫(kù)v= loadData(...);m.put(key, v);}} finally{w.unlock();}return v; }// 讀緩存public V get(K key) {r.lock();try { return m.get(key); }finally { r.unlock(); }}}

讀寫鎖的升級(jí)與降級(jí)

鎖的升級(jí)--禁止

// 讀緩存 r.lock(); try {v = m.get(key); if (v == null) {w.lock();try {// 再次驗(yàn)證并更新緩存// 省略詳細(xì)代碼} finally{w.unlock();}} } finally{r.unlock(); }

鎖的降級(jí)--支持

public class CachedData {Object data;volatile boolean cacheValid;final ReadWriteLock rwl = new ReentrantReadWriteLock();// 讀鎖 final Lock r = rwl.readLock();// 寫鎖final Lock w = rwl.writeLock();void processCachedData() {// 獲取讀鎖r.lock();if (!cacheValid) {// 釋放讀鎖，因?yàn)椴辉试S讀鎖的升級(jí)r.unlock();// 獲取寫鎖w.lock();try {// 再次檢查狀態(tài) if (!cacheValid) {data = ...cacheValid = true;}// 釋放寫鎖前，降級(jí)為讀鎖// 降級(jí)是可以的r.lock(); } finally {// 釋放寫鎖w.unlock(); }}// 此處仍然持有讀鎖try {use(data);} finally {r.unlock();}} }

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