Java线程:新特征-锁(上)
在Java5中,专门提供了锁对象,利用锁可以方便的实现资源的封锁,用来控制对竞争资源并发访问的控制,这些内容主要集中在java.util.concurrent.locks包下面,里面有三个重要的接口Condition、Lock、ReadWriteLock。
- Condition将Object监视器方法(wait、notify和 notifyAll)分解成截然不同的对象,以便通过将这些对象与任意Lock实现组合使用,为每个对象提供多个等待 set(wait-set)。
- Lock实现提供了比使用synchronized方法和语句可获得的更广泛的锁定操作。
- ReadWriteLock维护了一对相关的锁定,一个用于只读操作,另一个用于写入操作。
举个例子:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* Java线程:线程池-锁
*/
public class Test {
public static void main(String[] args){
//创建并发访问的账户
MyCount myCount=new MyCount("6215580000000000000",10000);
//创建一个锁对象
Lock lock=new ReentrantLock();
//创建一个线程池
ExecutorService pool = Executors.newCachedThreadPool();
//创建一些并发访问用户,一个信用卡,存的存,取的取
User u1 = new User("张三", myCount, -4000, lock);
User u2 = new User("张三他爹", myCount, 6000, lock);
User u3 = new User("张三他弟", myCount, -8000, lock);
User u4 = new User("张三", myCount, 800, lock);
//在线程池中执行各个用户的操作
pool.execute(u1);
pool.execute(u2);
pool.execute(u3);
pool.execute(u4);
//关闭线程池
pool.shutdown();
}
}
//信用卡用户
class User implements Runnable{
private String name; //用户名
private MyCount myCount; //所要操作的账户
private int iocash; //操作的金额,当然有正负之分了
private Lock myLock; //执行操作所需的锁对象
User(String name, MyCount myCount, int iocash, LockmyLock) {
this.name = name;
this.myCount = myCount;
this.iocash = iocash;
this.myLock = myLock;
}
@Override
public void run() {
//获取锁
myLock.lock();
//执行现金业务
System.out.println(name + "正在操作" + myCount +"账户,金额为" + iocash +",当前金额为" + myCount.getCash());
myCount.setCash(myCount.getCash() + iocash);
System.out.println(name + "操作" + myCount +"账户成功,金额为" + iocash +",当前金额为" + myCount.getCash());
//释放锁,否则别的线程没有机会执行了
myLock.unlock();
}
}
//信用卡账户,可随意透支
class MyCount {
private String oid; //账号
private int cash; //账户余额
MyCount(String oid, int cash) {
this.oid = oid;
this.cash = cash;
}
public String getOid() {
return oid;
}
public void setOid(String oid) {
this.oid = oid;
}
public int getCash() {
return cash;
}
public void setCash(int cash) {
this.cash = cash;
}
@Override
public String toString() {
return"MyCount{" +
"oid='" + oid + '\'' +
", cash=" + cash +
'}';
}
}
执行结果:
张三他爹正在操作MyCount{oid='6215580000000000000', cash=10000}账户,金额为6000,当前金额为10000
张三他爹操作MyCount{oid='6215580000000000000', cash=16000}账户成功,金额为6000,当前金额为16000
张三正在操作MyCount{oid='6215580000000000000', cash=16000}账户,金额为800,当前金额为16000
张三操作MyCount{oid='6215580000000000000',cash=16800}账户成功,金额为800,当前金额为16800
张三正在操作MyCount{oid='6215580000000000000', cash=16800}账户,金额为-4000,当前金额为16800
张三操作MyCount{oid='6215580000000000000',cash=12800}账户成功,金额为-4000,当前金额为12800
张三他弟正在操作MyCount{oid='6215580000000000000', cash=12800}账户,金额为-8000,当前金额为12800
张三他弟操作MyCount{oid='6215580000000000000', cash=4800}账户成功,金额为-8000,当前金额为4800
从上面的输出可以看到,利用锁对象太方便了,比直接在某个不知情的对象上用锁清晰多了。
但一定要注意的是,在获取了锁对象后,用完后应该尽快释放锁,以便别的等待该锁的线程有机会去执行
Java线程:新特征-锁(下)
在上面提到了Lock接口以及对象,使用它,很优雅的控制了竞争资源的安全访问,但是这种锁不区分读写,称这种锁为普通锁。为了提高性能,Java提供了读写锁,在读的地方使用读锁,在写的地方使用写锁,灵活控制,在一定程度上提高了程序的执行效率。
Java中读写锁有个接口java.util.concurrent.locks.ReadWriteLock,也有具体的实现ReentrantReadWriteLock,详细的API可以查看JavaAPI文档。
下面这个例子是在文例子的基础上,将普通锁改为读写锁,并添加账户余额查询的功能,代码如下:
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* Java线程:线程池-锁
*/
public class Test {
public static void main(String[] args){
//创建并发访问的账户
MyCount myCount=new MyCount("6215580000000000000",10000);
//创建一个锁对象
ReadWriteLock lock=new ReentrantReadWriteLock(false);
//创建一个线程池
ExecutorService pool = Executors.newCachedThreadPool();
//创建一些并发访问用户,一个信用卡,存的存,取的取
User u1 = new User("张三", myCount, -4000, lock, false);
User u2 = new User("张三他爹", myCount, 6000, lock, false);
User u3 = new User("张三他弟", myCount, -8000, lock, false);
User u4 = new User("张三", myCount, 800, lock,false);
User u5 = new User("张三他爹", myCount, 0, lock,true);
//在线程池中执行各个用户的操作
pool.execute(u1);
pool.execute(u2);
pool.execute(u3);
pool.execute(u4);
pool.execute(u5);
//关闭线程池
pool.shutdown();
}
}
//信用卡用户
class User implements Runnable{
private String name; //用户名
private MyCount myCount; //所要操作的账户
private int iocash; //操作的金额,当然有正负之分了
private ReadWriteLock myLock; //执行操作所需的锁对象
private boolean ischeck; //是否查询
User(String name, MyCount myCount, int iocash,ReadWriteLock myLock,boolean ischeck) {
this.name = name;
this.myCount = myCount;
this.iocash = iocash;
this.myLock = myLock;
this.ischeck = ischeck;
}
@Override
public void run() {
if (ischeck) {
//获取读锁
myLock.readLock().lock();
System.out.println("读:" + name +"正在查询" + myCount +"账户,当前金额为" + myCount.getCash());
//释放读锁
myLock.readLock().unlock();
} else {
//获取写锁
myLock.writeLock().lock();
//执行现金业务
System.out.println("写:" + name +"正在操作" + myCount +"账户,金额为" + iocash +",当前金额为" + myCount.getCash());
myCount.setCash(myCount.getCash() + iocash);
System.out.println("写:" + name +"操作" + myCount +"账户成功,金额为" + iocash +",当前金额为" + myCount.getCash());
//释放写锁
myLock.writeLock().unlock();
}
}
}
//信用卡账户,可随意透支
class MyCount {
private String oid; //账号
private int cash; //账户余额
MyCount(String oid, int cash) {
this.oid = oid;
this.cash = cash;
}
public String getOid() {
return oid;
}
public void setOid(String oid) {
this.oid = oid;
}
public int getCash() {
return cash;
}
public void setCash(int cash) {
this.cash = cash;
}
@Override
public String toString() {
return"MyCount{" +
"oid='" + oid + '\'' +
",cash=" + cash +
'}';
}
}
执行结果:
写:张三他爹正在操作MyCount{oid='6215580000000000000', cash=10000}账户,金额为6000,当前金额为10000
写:张三他爹操作MyCount{oid='6215580000000000000', cash=16000}账户成功,金额为6000,当前金额为16000
写:张三正在操作MyCount{oid='6215580000000000000', cash=16000}账户,金额为-4000,当前金额为16000
写:张三操作MyCount{oid='6215580000000000000', cash=12000}账户成功,金额为-4000,当前金额为12000
写:张三他弟正在操作MyCount{oid='6215580000000000000', cash=12000}账户,金额为-8000,当前金额为12000
写:张三他弟操作MyCount{oid='6215580000000000000', cash=4000}账户成功,金额为-8000,当前金额为4000
读:张三他爹正在查询MyCount{oid='6215580000000000000', cash=4000}账户,当前金额为4000
写:张三正在操作MyCount{oid='6215580000000000000', cash=4000}账户,金额为800,当前金额为4000
写:张三操作MyCount{oid='6215580000000000000', cash=4800}账户成功,金额为800,当前金额为4800
在实际开发中,最好在能用读写锁的情况下使用读写锁,而不要用普通锁,以求更好的性能。