JAVA中的锁
JAVA中的锁
一、重入锁
重入锁,也叫做递归锁,指的是同一线程 外层函数获得锁之后 ,内层递归函数仍然有获取该锁的代码,但不受影响。在JAVA环境下 ReentrantLock 和synchronized 都是 可重入锁。这里主要来看ReentrantLock。
ReentrantLock: 在需要进行同步的代码部分加上锁定,但不要忘记最后一定要释放锁定, 不然会造成锁永远无法释放,其他线程永远进不来的结果。
简单使用:
public class UseReentrantLock {
private Lock lock = new ReentrantLock();
public void method1() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入method1..");
Thread.sleep(1000);
System.out.println("当前线程:" + Thread.currentThread().getName() + "退出method1..");
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void method2() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入method2..");
Thread.sleep(2000);
System.out.println("当前线程:" + Thread.currentThread().getName() + "退出method2..");
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
final UseReentrantLock ur = new UseReentrantLock();
Thread t1 = new Thread(() -> {
ur.method1();
ur.method2();
}, "t1");
t1.start();
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
在使用synchronized的时候,如果需要多线程间进行协调工作则需要Object的wait()和notify()、notifyAll()方法配合工作。那么同样,在使用Lock的时候,可以使用一个新的等待/通知的类,他就是Condition。这个Condition一定是针对具体某一把锁的。也就是在只有锁的基础上才会产生Condition。
使用单个Condition:
public class UseCondition {
private Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
public void method1() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入等待状态..");
Thread.sleep(3000);
System.out.println("当前线程:" + Thread.currentThread().getName() + "释放锁..");
condition.await(); // 类似于 wait
System.out.println("当前线程:" + Thread.currentThread().getName() + "继续执行...");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void method2() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入..");
Thread.sleep(3000);
System.out.println("当前线程:" + Thread.currentThread().getName() + "发出唤醒..");
condition.signal(); // 类似于 notify
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
final UseCondition uc = new UseCondition();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
uc.method1();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
uc.method2();
}
}, "t2");
t1.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
使用多个Condition:可以通过一个Lock对象产生多个Condition进行多线程间的交互,非常的灵活。可以使得部分需要唤醒的线程唤醒,其他线程则继续等待通知
public class UseManyCondition {
private ReentrantLock lock = new ReentrantLock();
private Condition c1 = lock.newCondition();
private Condition c2 = lock.newCondition();
public void m1() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入方法m1等待..");
c1.await();
System.out.println("当前线程:" + Thread.currentThread().getName() + "方法m1继续..");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void m2() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入方法m2等待..");
c1.await();
System.out.println("当前线程:" + Thread.currentThread().getName() + "方法m2继续..");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void m3() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入方法m3等待..");
c2.await();
System.out.println("当前线程:" + Thread.currentThread().getName() + "方法m3继续..");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void m4() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "唤醒..");
c1.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void m5() {
try {
lock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "唤醒..");
c2.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
final UseManyCondition umc = new UseManyCondition();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
umc.m1();
}
}, "t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
umc.m2();
}
}, "t2");
Thread t3 = new Thread(new Runnable() {
@Override
public void run() {
umc.m3();
}
}, "t3");
Thread t4 = new Thread(new Runnable() {
@Override
public void run() {
umc.m4();
}
}, "t4");
Thread t5 = new Thread(new Runnable() {
@Override
public void run() {
umc.m5();
}
}, "t5");
t1.start(); // c1
t2.start(); // c1
t3.start(); // c2
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
t4.start(); // c1 signalAll
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
t5.start(); // c2
}
}
支持重入:
public class TestReentrant {
private ReentrantLock lock = new ReentrantLock();
public void m1() {
try {
lock.lock();
System.out.println("进入m1方法");
// 调用m2方法
m2();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void m2() {
try {
lock.lock();
System.out.println("进入m2方法");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public static void main(String[] args) {
TestReentrant thc = new TestReentrant();
thc.m1();
}
}
二、读写锁
ReentrantReadWriteLock,其核心就是实现读写分离的锁。在高并发访问下,尤其时读多写少的情况下,性能要远高于重入锁。synchronized、ReentrantLock,同一时间内,只能有一个线程进行访问被锁定的代码,那么读写锁则不同,其本质时分成两个锁,即读锁、写锁。在读锁下,多个线程可以并发的进行访问,但是在写锁的时候,只能一个一个的顺序访问。
public class UseReentrantReadWriteLock {
private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
private ReadLock readLock = rwLock.readLock();
private WriteLock writeLock = rwLock.writeLock();
public void read() {
try {
readLock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入...");
Thread.sleep(3000);
System.out.println("当前线程:" + Thread.currentThread().getName() + "退出...");
} catch (Exception e) {
e.printStackTrace();
} finally {
readLock.unlock();
}
}
public void write() {
try {
writeLock.lock();
System.out.println("当前线程:" + Thread.currentThread().getName() + "进入...");
Thread.sleep(3000);
System.out.println("当前线程:" + Thread.currentThread().getName() + "退出...");
} catch (Exception e) {
e.printStackTrace();
} finally {
writeLock.unlock();
}
}
public static void main(String[] args) {
final UseReentrantReadWriteLock urrw = new UseReentrantReadWriteLock();
Thread t1 = new Thread(() -> {
urrw.read();
}, "t1");
Thread t2 = new Thread(() -> {
urrw.read();
}, "t2");
Thread t3 = new Thread(() -> {
urrw.write();
}, "t3");
Thread t4 = new Thread(() -> {
urrw.write();
}, "t4");
// t1.start(); // R
// t2.start(); // R
t1.start(); // R
t3.start(); // W
// t3.start();// W
// t4.start();// W
}
}
三、乐观锁
总是认为不会产生并发问题,每次去取数据的时候总认为不会有其他线程对数据进行修改,因此不会上锁,但是在更新时会判断其他线程在这之前有没有对数据进行修改,一般会使用版本号机制或CAS操作实现。
version方式:一般是在数据表中加上一个数据版本号version字段,表示数据被修改的次数,当数据被修改时,version值会加一。当线程A要更新数据值时,在读取数据的同时也会读取version值,在提交更新时,若刚才读取到的version值为当前数据库中的version值相等时才更新,否则重试更新操作,直到更新成功。
核心SQL语句
update table set x=x+1, version=version+1 where id=#{id} and version=#{version};
CAS操作方式:即compare and swap 或者 compare and set,涉及到三个操作数,数据所在的内存值,预期值,新值。当需要更新时,判断当前内存值与之前取到的值是否相等,若相等,则用新值更新,若失败则重试,一般情况下是一个自旋操作,即不断的重试。
四、悲观锁
总是假设最坏的情况,每次取数据时都认为其他线程会修改,所以都会加锁(读锁、写锁、行锁等),当其他线程想要访问数据时,都需要阻塞挂起。可以依靠数据库实现,如行锁、读锁和写锁等,都是在操作之前加锁,在Java中,synchronized的思想也是悲观锁。
转载于:https://my.oschina.net/caofanqi/blog/3005752
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