1.面试题1：三个线程读，三个线程写同一个数据
public class ReadWriteLockTest { public static void main(String[] args) { final Queue3 q3 = new Queue3(); for(int i=0;i<3;i++){ //3个读线程 new Thread(new Runnable() { @Override public void run() { while(true){ q3.get(); } } }).start(); //3个写线程 new Thread(new Runnable() { @Override public void run() { q3.set(new Random().nextInt(10000)); } }).start(); } }}class Queue3{ private Object data = null; //共享数据，只能有一个线程能写该数据，但可以有多个线程同时读 ReadWriteLock lock = new ReentrantReadWriteLock(); public void get(){ try { lock.readLock().lock(); System.out.println(Thread.currentThread().getName()+" be ready to read data!"); Thread.sleep((long)(Math.random()*1000)); System.out.println(Thread.currentThread().getName()+" have read data: "+data); } catch (InterruptedException e) { e.printStackTrace(); }finally{ lock.readLock().unlock(); } } public void set(Object data){ try { lock.writeLock().lock(); System.out.println(Thread.currentThread().getName()+" be ready to write data!"); Thread.sleep((long)(Math.random()*1000)); this.data = data; System.out.println(Thread.currentThread().getName()+" hava write data: "+data); } catch (InterruptedException e) { e.printStackTrace(); }finally{ lock.writeLock().unlock(); } }}
2.
Hibernate中两者的区别
① User user = session.load(id,User.class);
② User user = session.get(id,User.class);
其中②直接从数据库查询，如果查询为空，user为null
其中①在查询时，无论数据路有没有都会得到：User$Proxy 代理类，是一个缓存的User
如果实际的realUser为空则查询数据库，如果从数据库查询出的为空，抛异常，如果不为空
直接返回realUser.getName()
User$Proxy extends User{ private Integer id = id; User realUser = null; getName(){ if(realUser == null){ realUser = session.get(id); if(realUser == null) throw exception //抛异常 } return realUser.getName(); } }
3.javaAPI 上的一段代码：
1 Sample usages. Here is a code sketch showing how to exploit reentrancy to perform lock downgrading after updating a cache (exception handling is elided for simplicity): 2 3 class CachedData { 4 Object data; 5 volatile boolean cacheValid; 6 ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); 7 8 void processCachedData() { //处理数据 9 rwl.readLock().lock(); //多个线程并发读，不冲突，只需要上一个读锁10 if (!cacheValid) { //检查缓存中有没有数据 if中的代码好比第一次获取数据11 // Must release read lock before acquiring write lock12 rwl.readLock().unlock(); //如果一个线程发现没有数据，释放读锁，获取写锁13 rwl.writeLock().lock();14 // Recheck state because another thread might have acquired15 // write lock and changed state before we did.16 if (!cacheValid) {17 data = ... //实际的写数据逻辑18 cacheValid = true; //缓存标志改为true，表示有缓存数据了19 }20 // Downgrade by acquiring read lock before releasing write lock21 rwl.readLock().lock();22 rwl.writeLock().unlock(); // Unlock write, still hold read23 }24 25 use(data); //使用数据26 rwl.readLock().unlock();27 }28 }
4.面试题：设计一个缓存系统
1 public class CacheDemo { 2 3 private Map cache = new HashMap<>(); 4 public static void main(String[] args) { 5 6 } 7 8 private ReadWriteLock rwl = new ReentrantReadWriteLock(); 9 public Object getData(String key){10 rwl.readLock().lock(); //获取读锁11 Object value = null;12 try {13 value = cache.get(key);14 if(value == null){15 rwl.readLock().unlock();16 rwl.writeLock().lock();17 try {18 if(value == null){19 value = "aaa"; //实际是去数据库查询20 }21 } finally{22 rwl.writeLock().unlock();23 }24 rwl.readLock().lock();25 }26 } catch (Exception e) {27 e.printStackTrace();28 }finally{29 rwl.readLock().unlock(); //释放读锁30 }31 return value;32 }33 34 }
