提高性能
读写锁：分为读锁和写锁，多个读锁不互斥，读锁与写锁互斥，写锁与写锁互斥，这是由JVM控制的，我们只需要上好相应的锁即可。如果代码只读数据，可以很多人同时读，但不能同时写，那就上读锁；如果代码修改数据，只能有一个人在写，且不能同时读取，那就上写锁。总之，读的时候上读锁，写的时候上写锁。
import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class ReadWriteLockTest {
public static void main(String[] args) {
final Queue3 q3 = new Queue3();
for(int i=0;i<3;i++)
{
new Thread(){
public void run(){
while(true){
q3.get();
}
}
}.start();
new Thread(){
public void run(){
while(true){
q3.put(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该数据。
ReadWriteLock rwl = new ReentrantReadWriteLock();
public void get(){
rwl.readLock().lock();
try {
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{
rwl.readLock().unlock();
}
}
public void put(Object data){
rwl.writeLock().lock();
try {
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() + " have write data: " + data);
} catch (InterruptedException e) {
e.printStackTrace();
}finally{
rwl.writeLock().unlock();
}
}
}面试题缓存类实现
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class CacheDemo {
private Map<String, Object> cache = new HashMap<String, Object>();
public static void main(String[] args) {
// TODO Auto-generated method stub
}
private ReadWriteLock rwl = new ReentrantReadWriteLock();
public Object getData(String key){
rwl.readLock().lock();
Object value = null;
try{
value = cache.get(key);
if(value == null){
rwl.readLock().unlock();
rwl.writeLock().lock();
try{
if(value==null){
value = "aaaa";//实际是去queryDB();
}
}finally{
rwl.writeLock().unlock();
}
rwl.readLock().lock();
}
}finally{
rwl.readLock().unlock();
}
return value;
}
}
