synchronized的功能拓展:重入锁(读书笔记)
2024-09-03 20:09:54
重入锁可以完全代替synchronized关键字.在JDK5.0的早期版本中,重入锁的性能远远好于synchronized,但是从JDK6.0开始.JDK在synchronized上做了大量的优化.使得两者的性能差距不大,
public class ReenterLock implements Runnable {
public static ReentrantLock lock = new ReentrantLock();
public static int i = 0;
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see Thread#run()
*/
@Override
public void run() {
for (int j = 0; j < 10000000; j++) {
lock.lock();//加锁
try {
i++;
} finally {
lock.unlock();//释放锁
}
}
}
public static void main(String[] args) throws InterruptedException {
ReenterLock t1 = new ReenterLock();//线程实例
Thread th1 = new Thread(t1);
Thread th2 = new Thread(t1);
th1.start();
th2.start();
th1.join();
th2.join();
System.out.println("i = " + i);
}
}
这里 我们使用重入锁保护临界区资源i 确保多线程对i操作的安全性,我们可以看出,与synchronized相比,重入锁有着显示的操作过程,开发人员必须手动指定何时加锁,何时释放锁,也正因为这样,重入锁对于逻辑控制的灵活性要远远好于synchronized,但是要必须释放锁,否则 其他线程就没有机会现在方法临界区资源了!另外重入锁允许一个线程连续几次获取同一把锁,但是释放锁的时候也要释放相同次数.
- 中断响应 与synchronized相比,如果一个线程在等待锁,那么结果只要两种情况,1 获得这把锁执行, 2 他保持等待状态,而使用重入锁,则提供了另外一种可能性,那就是线程可以被中断,也就是在等待过程中,程序可以根据需要取消对锁的请求.
下面代码产生了一个死锁,但是得益于锁的中断,我们可以很轻松的解决死锁.
public class IntLock implements Runnable {
public static ReentrantLock lock1 = new ReentrantLock();
public static ReentrantLock lock2 = new ReentrantLock();
int lock;
public IntLock(int lock) {
this.lock = lock;
}
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see Thread#run()
*/
@Override
public void run() {
try {
if (lock == 1) {
lock1.lockInterruptibly();//锁1 这是一个可以对中断进行想要的锁申请动作!
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
lock2.lockInterruptibly();//锁2 加锁
} else {
lock2.lockInterruptibly();
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
lock1.lockInterruptibly();//锁1 加锁
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
if (lock1.isHeldByCurrentThread()) { //判断持有自己锁的线程是否是当前线程
lock1.unlock();
}
if (lock2.isHeldByCurrentThread()) {
lock2.unlock();
}
System.out.println(Thread.currentThread().getId() + ":线程退出");
}
}
public static void main(String[] args) throws InterruptedException {
IntLock r1 = new IntLock(1);//线程实例 1
IntLock r2 = new IntLock(2);//线程实例 2
Thread t1 = new Thread(r1);//线程1
Thread t2 = new Thread(r2);//线程2
t1.start();
t2.start();
Thread.sleep(1000);//Main线程 休眠1s
t2.interrupt();//中断其中一个线程
}
}
线程t1和t2启动后,t1占用rock1 在占用rock2 t2先占用rock2 然后请求rock1 因此很容易形成互相等待, 当我们让t2中断时,他放弃了申请lock1 然后释放了lock2 实际上是 t1线程 完成任务正常退出,而t2 是中断的,
- 锁申请等待限时 除了等待通知之外,要避免死锁还有另外一种方法,那就是限时等待, 就是规定一个时间,超出时间没有拿到锁 就退出
public class TimeLock implements Runnable {
public static ReentrantLock lock = new ReentrantLock();
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see Thread#run()
*/
@Override
public void run() {
try {
if (lock.tryLock(5, TimeUnit.SECONDS)) {//试图获取锁,等待5秒 如果超时那就false
Thread.sleep(6000);
} else {
System.out.println("get lock failed");
}
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
}
public static void main(String[] args) {
TimeLock r1 = new TimeLock();
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r1);
t1.start();
t2.start();
}
}
在本例中,由于占用锁的线程会持有锁长达6s,故另一个线程无法在5s的等待时间内获得锁,因此,请求锁会失败!
ReentrantLock.tryLock()方法可以不带参数直接运行,在这种情况下,当前线程会尝试获得锁,如果锁并未被其他线程占用,则申请会成功!并立即返回true,如果锁被其他线程占用,则档期你先吃不会进行任何等待,而是立即返回false.
public class TryLock implements Runnable {
public static ReentrantLock lock1 = new ReentrantLock();
public static ReentrantLock lock2 = new ReentrantLock();
int lock;
public TryLock(int lock) {
this.lock = lock;
}
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see Thread#run()
*/
@Override
public void run() {
if (lock == 1) {
while (true) {
if (lock1.tryLock()) {
try {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (lock2.tryLock()) {
try {
System.out.println(Thread.currentThread().getId() + ":My Job done");
return;
} finally {
lock2.unlock();
}
}
} finally {
lock1.unlock();
}
}
}
} else {
while (true) {
if (lock2.tryLock()) {
try {
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (lock1.tryLock()) {
try {
System.out.println(Thread.currentThread().getId() + ":My Job done");
return;
} finally {
lock1.unlock();
}
}
} finally {
lock2.unlock();
}
}
}
}
}
public static void main(String[] args) {
TryLock r1 = new TryLock(1);
TryLock r2 = new TryLock(2);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
}
上述代码中.采用了非常容易死锁的加锁顺序,在一般情况下,这会导致t1和t2互相等待. 但是使用tryLock()后,就会得到大大改善,线程不会傻傻的等待,而是不停的尝试.因此只要时间足够的长,线程总是会得到所有需要的资源.
- 公平锁 在大多情况下.锁的申请都是非公平的.两个线程同时申请锁a,谁先获得锁a呢 这是不一定的,系统只是会从这个锁的等待队列中随机挑选一个.而公平的锁,则不是这样的,他会按照时间的先后顺序,保证先到先得,后到后的,公平锁的一大特点就是,不会产生饥饿现象,只要你排队,最终还是可以得到资源的, 如果我们使用synchronized关键字进行锁控制,那么产生的锁就是非公平的,而重入锁循序我们对其公平性进行设置,下面是他的构造器
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
sync = fair ? new FairSync() : new NonfairSync();
}
当参数fair为true时,表示锁是公平的,公平锁必须要求系统维护一个有序的队列,因此实现成本比较高,性能相对低下,因此默认锁是非公平的.
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