版权声明:转载请注名出处 https://blog.csdn.net/meism5/article/details/91366723
ReadWriteLock 如何使用?
ReadWriteLock,读写锁。
ReentrantReadWriteLock 是 ReadWriteLock 的一种实现。
特点:
- 包含一个 ReadLock 和 一个 WriteLock 对象
- 读锁与读锁不互斥;读锁与写锁,写锁与写锁互斥
- 适合对共享资源有读和写操作,写操作很少,读操作频繁的场景
- 可以从写锁降级到读锁。获取写锁->获取读锁->释放写锁
- 无法从读锁升级到写锁
- 读写锁支持中断
- 写锁支持Condition;读锁不支持Condition
示例1--根据 key 获取 value 值
private ReadWriteLock lock = new ReentrantReadWriteLock();//定义读写锁
//根据 key 获取 value 值
public Object getValue(String key){
//使用读写锁的基本结构
lock.readLock().lock();//加读锁
Object value = null;
try{
value = cache.get(key);
if(value == null){
lock.readLock().unlock();//value值为空,释放读锁
lock.writeLock().lock();//加写锁,写入value值
try{
//重新检查 value值是否已经被其他线程写入
if(value == null){
value = "value";//写入数据
}
}finally{
lock.writeLock().unlock();
}
lock.readLock().lock();
}
}finally{
lock.readLock().unlock();
}
return value;
}
示例2--多线程环境下的读写锁使用
package constxiong.interview;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* 测试可重入 读写锁
* @author ConstXiong
* @date 2019-06-10 11:19:42
*/
public class TestReentrantReadWriteLock {
private Map<String, Object> map = new HashMap<String, Object>();
private ReadWriteLock lock = new ReentrantReadWriteLock();
/**
* 根据 key 获取 value
* @param key
* @return
*/
public Object get(String key) {
Object value = null;
lock.readLock().lock();
try {
Thread.sleep(50L);
value = map.get(key);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.readLock().unlock();
}
return value;
}
/**
* 设置key-value
* @param key
* @return
*/
public void set(String key, Object value) {
lock.writeLock().lock();
try {
Thread.sleep(50L);
map.put(key, value);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.writeLock().unlock();
}
}
//测试5个线程读数据,5个线程写数据
public static void main(String[] args) {
final TestReentrantReadWriteLock test = new TestReentrantReadWriteLock();
final String key = "lock";
final Random r = new Random();
for (int i = 0; i < 5; i++) {
new Thread(){
@Override
public void run() {
for (int j = 0; j < 10; j++) {
System.out.println(Thread.currentThread().getName() + " read value=" + test.get(key));
}
}
}.start();
new Thread(){
@Override
public void run() {
for (int j = 0; j < 10; j++) {
int value = r.nextInt(1000);
test.set(key, value);
System.out.println(Thread.currentThread().getName() + " write value=" + value);
}
}
}.start();
}
}
}